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Hydrogen-Bond-Directed Giant Unilamellar Vesicles of Guanosine Derivative: Preparation, Properties, and Fusion
Abstract
By mixing a small volume of THF containing guanosine derivative 1 and tetraethylenegrycol dodecyl ether (TEGDE) with water and subsequently removing TEGDE by gel permeation chromatography, micrometer-sized giant unilamellar vesicles (GUV) of 1 were successfully prepared. The vesicle membrane was a 2-D sheet assembly of thickness 2.5 nm, composed of a 2-D inter-guanine hydrogen-bond network. The GUV dispersion showed high stability because of a large negative zeta potential, which allowed repeated sedimentation and redispersion by centrifugation and subsequent gentle agitation. TEGDE-triggered fusion of GUVs took place within 350 ms, which proceeded by fusion of the vesicle membranes in contact. These unique static and dynamic properties of the GUV membrane assembled by the 2-D hydrogen-bond network are discussed.
... Beyond polymeric systems, low-molecular-weight compounds based on guanosine are known to arrange themselves into higher-order structures simply through the strong donor-acceptor interactions between carbonyl groups and protonated amines 18 . Vesicles based on the self-assembly of guanosine 26,27 , but also other strong hydrogen-bonding motifs, have been mentioned previously 28,29 . Complementing this exploitation of hydrogen bonding are amphiphiles that are generated in situ by ionic interactions with small molecules bearing opposite charges [30][31][32][33][34] . ...
In nature, self-assembly processes based on amphiphilic molecules play an integral part in the design of structures of higher order such as cells. Among them, amphiphilic glycoproteins or glycolipids take on a pivotal role due to their bioactivity. Here we show that sugars, in particular, fructose, are capable of directing the self-assembly of highly insoluble curcumin resulting in the formation of well-defined capsules based on non-covalent forces. Simply by mixing an aqueous solution of fructose and curcumin in an open vessel leads to the generation of capsules with sizes ranging between 100 and 150 nm independent of the initial concentrations used. Our results demonstrate that hydrogen bonding displayed by fructose can induce the self-assembly of hydrophobic molecules such as curcumin into well-ordered structures, and serving as a simple and virtually instantaneous way of making nanoparticles from curcumin in water with the potential for template polymerization and nanocarriers.
... As a result, the development of artificial platforms for driving membrane fusion has gained significant interest over the years. Thus, a variety of targeting groups have been used to instigate fusion, 3,4 including small molecule based molecular recognition, [5][6][7][8][9][10][11][12][13] complementary DNA strands, [14][15][16][17][18][19][20][21][22][23][24] peptides, [25][26][27][28][29][30][31][32][33] metal complex formation, [34][35][36][37] and complementary reactive functional groups. [38][39][40][41][42][43][44][45][46] This process typically involves the incorporation of separate interacting functional groups into different vesicles to drive association as well as manipulation of the lipid content to dictate the stability of membrane bilayer and drive fusion. ...
Artificial systems for controlled membrane fusion applicable for drug delivery would ideally use triggers that are orthogonal to biology. To apply the strain-promoted alkyne-azide cycloaddition (SPAAC) to drive membrane fusion, ODIBO-lipid 1 was designed, synthesized and studied alongside ADIBO-lipids 2-4 to assess fusion with liposomes containing azido-lipid 5. Lipids 1-2 were first shown to be effective for liposome derivatization. Next, fusion was evaluated using liposomes containing 1 and varying ratios of PC and PE via a FRET dilution fusion assay, and a 1/1 PC/PE ratio yielded the greatest signal change attributed to fusion. Finally, lipids 1-4 were compared, and 1 yielded the greatest triggering of fusion, while 2-4 yielded varying efficacies depending on the structural features of each lipid. Fusion was further validated through STEM studies showing larger multilamellar assemblies after liposome mixing. This work provides a platform for triggered fusion towards drug delivery applications and an understanding of the effects of lipid structure and membrane composition on fusion.
As one of the natural nucleobases, guanine has attracted increasing interest in molecular self-assembly science because of its abundant interaction sites and high electron cloud density. Guanines, guanine derivatives, and guanine-rich DNA sequence are able to self-assemble into versatile aggregate structures by the means of hydrogen bonds and π-π, ion-dipole, solvophobic, and electrostatic interactions. Recent advances have shown that many guanine analogue-based (G-based) luminescent aggregates exhibit promising applications for fluorescent and chemiluminescent sensing and circularly polarized luminescence (CPL). This perspective summarizes the state-of-art strategies for constructing G-based assemblies and presents representative examples for luminescence functions. Finally, the inspirations are provided for exploiting unique G-based systems and luminescent G-based assemblies.
The maintenance of an orderly and controllable multicellular society depends on the communication and signal regulation between various types of biological cells. How to replicate complicated signal transduction pathways in synthetic protocellular communities remains a key challenge in bottom-up synthetic biology. Herein, we review recent advances in the design and construction of interactive protocell communities, or protocell communities and biological communities, and explore the ways of designing and constructing artificial paracrine-like signaling pathways and juxtacrine-like signaling pathways. Key molecules involved in the signaling pathways that can be used to connect two or more spatially separated communities, and diverse signal outputs generated by the communication are summarized. We also propose the limitations, challenges and opportunities in this field.
There have been several attempts to construct supramolecular chemical systems that mimic the phase transitions in living systems. However, most of these phase transitions are one‐to‐one and induced by one stimulus or chemical; there have been few reports on the pathway‐dependent phase transition of supramolecular self‐assemblies in multi‐step. To induce multistep phase transitions, we prepared molecular crystals containing a cationic amphiphile having azobenzene and disulfide groups. A reducing agent caused the crystals to become vesicles, and adjacent, non‐touching vesicles fused under UV and subsequent visible light. Adding a reducing agent to the worm‐like aggregates that were generated after UV irradiation of the original crystals resulted in the growth of sheet‐like aggregates. 1H NMR and fluorescence anisotropy measurements showed that a series of phase transitions was induced by changes in the phase structures from molecular conversions of the reactive amphiphiles. The multiple pathway‐dependent phase transitions of supramolecular self‐assemblies can provide a methodology for developing new stimuli‐responsive materials that exhibit the desirable properties under specific circumstances from Systems Chemistry viewpoint.
Polyion complex vesicles (PICsomes) formed from a self-assembly of an oppositely charged pair of block- and homo-polyelectrolytes have shown exceptional features for functional loading of bioactive agents. Nevertheless, the stability of PICsomes is often jeopardized in physiological environment, and only PICsomes having chemically crosslinked membranes have endured in harsh in vivo conditions, such as in blood stream. Herein, we developed versatile PICsomes aimed to last in in vivo settings by stabilizing their membrane through a combination of ionic and hydrogen bonding, which is widely found in natural proteins as salt bridge, by controlled introduction of guanidinium groups in the polycation fraction toward concurrent polyion complexation and hydrogen bonding. The guanidinylated PICsomes were successfully assembled in physiological salt condition, with precise control of their morphology by tuning the guanidinium content, and the ratio of anionic and cationic components. Guanidinylated PICsomes with 100 nm diameter, which are relevant to nanocarrier development, were stable in high urea concentration, physiological temperature, and under serum incubation, persisting in blood circulation in vivo.
Covalent or non-covalent surface functionalization of soft matter structures is an important tool for tailoring their function and stability. Functionalized surfaces and nanoparticles have found numerous applications in drug delivery and diagnostics and new functionalization chemistry is continuously being developed in the discipline of bottom-up systems chemistry. The association of polar functional molecules, e.g. molecular recognition agents, with soft matter structures can be achieved by derivatization with alkyl chains, allowing non-covalent anchoring into amphiphilic membranes. Here, we report the synthesis of five new guanine-N9-derivatives bearing alkyl chains with different attachment chemistries, exploiting a synthesis pathway that allows a flexible choice of hydrophobic anchor moiety. In this study, these guanine derivatives were functionalized with C10-chains for insertion into decanoic acid bilayer structures, where both alkyl chain-length and attachment chemistry determined their interaction with the membrane. Incubation of these guanine conjugates, as solids, with a decanoic acid vesicle suspension showed that ether- and triazole-linked C10-anchors yielded an increased partitioning of the guanine derivative into the membranous phase compared to directly N9-linked saturated alkyl anchors. Decanoic acid vesicle membranes could be loaded with up to 5.5 mol% guanine-derivative, a six-fold increase over previous limits. Thus, anchor chemistries exhibiting favorable interactions with a bilayer’s hydrophilic surface can significantly increase the degree of structure functionalization.
Controlled membrane fusion of proteinosome-based protocells is achieved via a hydrogel-mediated process involving dynamic covalent binding, self-healing, and membrane reconfiguration at the contact interface. The rate of proteinosome fusion is dependent on dynamic Schiff base covalent interchange, and is accelerated in the presence of encapsulated glucose oxidase and glucose, or inhibited with cinnamyl aldehyde due to enzyme-mediated decreases in pH or competitive covalent binding, respectively. The coordinated fusion of the proteinosomes leads to the concomitant transportation and redistribution of entrapped payloads such as DNA and dextran. Silica colloids with amino-functionalized surfaces undergo partial fusion with the proteinosomes via a similar dynamic hydrogel-mediated mechanism. Overall, the strategy provides opportunities for the development of interacting colloidal objects, control of collective behavior in soft matter microcompartmentalized systems, and increased complexity in synthetic protocell communities.
DNA based spherical nanostructures is one of the promising nanocarriers for drug delivery applications because of their excellent biocompatibility and unique structural features of DNA. Herein, we report the amphiphilicity driven self-assembly of DNA-phenyleneethynylene based amphiphile into luminescent vesicles and their cellular uptake toward HeLa cell. Spectroscopic and microscopic analyses reveal the self-assembly of this class of amphiphiles into vesicular nanostructures with corona made of DNA and a functional hydrophobic membrane. Vesicles exhibit promising cell permeability.
Herein we develop a quantitative dye dequenching technique for the measurement of polymersome fusion, using it to characterize the salt mediated, mechanically-induced fusion of polymersomes with polymer, lipid, and so-called stealth lipid vesicles. While dye dequenching has been used to quantitatively explore liposome fusion in the past, this is the first use of dye dequenching to measure polymersome fusion of which we are aware. In addition to providing quantitative results, dye dequenching is ideal for detecting fusion in instances where DLS results would be ambiguous, such as low yield levels and size ranges outside the capabilities of DLS. The dye chosen for this study was a cyanine derivative, 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindotricarbocyanine iodide (DiR), which proved to provide excellent data on the extent of polymersome fusion. Using this technique, we have shown the limited fusion capabilities of polymersome/liposome heterofusion, notably DOPC vesicles fusing with polymersomes at half the efficiency of polymersome homofusion and DPPC vesicles showing virtually no fusion. In addition to these key heterofusion experiments, we determined the broad applicability of dye dequenching in measuring kinetic rates of polymersome fusion; and showed that even at elevated temperatures or over multiple weeks' time, no polymersome fusion occurred without agitation. Stealth liposomes formed from DPPC and PEO-functionalized lipid, however, fused with polymersomes and stealth liposomes with relatively high efficiency, lending support to our hypothesis that the response of the PEO corona to salt is a key factor in the fusion process. This last finding suggests that although the conjugation of PEO to lipids increases vesicle biocompatibility and enables their longer circulation times, it also renders the vesicles subject to destabilization under high salt and shear (e.g. in the circulatory system) that may lead to, in this case, fusion.
A series of 1,3-diamido phosphocholines was synthesized, and their potential to form stable bilayers was investigated. Large and giant unilamellar vesicles produced from these new lipids form a wide variety of faceted liposomes. Factors such as cooling rates and the careful choice of the liposome preparation method influence the formation of facets. Interdigitation was hypothesized as a main factor for the stabilization of facets and effectively monitored by small-angle X-ray scattering measurements.
Previously, it was found that extruded (200 nm) polymer vesicles are capable of fusion into giant polymersomes using agitation in the presence of salt. In this study, several factors contributing to this phenomenon, including the effects of (i) polymer vesicle concentration, (ii) agitation speed and duration, and (iii) variation of the salt and its concentration are investigated. To accomplish these goals dynamic light scattering is used in conjunction with fluorescence microscopy, which provides insight into vesicles above the practical limit for DLS characterization. Increasing the concentration of the polymer dramatically increases the production of giant vesicles through the increased collisions of polymersomes. Likewise, increasing the frequency of agitation increases the efficiency of fusion, although ultimately the size of vesicle that could be produced is limited due to the high shear involved. Finally, salt-mediation of the fusion process was not limited to NaCl, but is instead a general effect facilitated by the presence of solvated ionic compounds, albeit with different salts initiating fusion at different concentrations. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014
In this paper we report how the confining surfaces and the ionic effects of different concentration of guanosine solution can be used to vary the alignment of liquid crystal phases of guanosine nucleotides. Liquid crystal phases of guanosine 5'-monophosphate ammonium salt and guanosine 5'-monophosphate free acid in pure water, with and without silver sulphate, were studied by polarized optical microscope. A periodic modulation of the texture was observed. This modulation depends on both on the concentration and on the presence of silver ions in the liquid crystal phase. We demonstrate that, according to the surface energy of the alignment layers, it is possible to homeotropically align the guanosine chromonic phase without applying any external magnetic field. Finally, we report the formation of spherical, vesicle-like guanosine 5'-monophosphate aggregates, when the solution was confined between two hydrophobic surfaces containing exposed Si groups.
Large (200 nm) poly(ethylene oxide)-b-poly(butadiene) polymer vesicles fuse into giant (>1 μm) vesicles with mild agitation in dilute aqueous NaCl solutions. This unusual effect is attributed to the salt-induced contraction of the poly(ethylene oxide) corona, reducing steric resistance between vesicles and, with agitation, increasing the probability of contact between the hydrophobic cores of adjacent membranes. In addition, NaCl and agitation facilitated the creation of giant hybrid vesicles from much smaller homogeneous polymersomes and liposomes. Whereas lipid vesicles do not readily fuse with each other under the same circumstances, they did fuse with polymersomes to produce hybrid polymer/lipid vesicles.
Thymidine derivatives with carboxylic acid and pyridyl groups were synthesized for constructing one-dimensional network structure based on hydrogen bonding in crystalline state. The solid sate structures and hydrogen bonding networks of the thymidine derivatives were characterized by single X-ray diffraction analysis. The thymidine derivatives formed a zwitterion structure with a pyridinium proton and a carboxylate moiety in a crystalline state due to transfer of a proton from the carboxylic acid to the pyridyl moiety. Strong hydrogen bonds between the pyridinium proton and the carboxylate moiety connected the thymidine units, resulting in a one-dimensional polymeric structure with a uniform direction reminiscent of the structure of single-strand polythymidine. The chemical structure of the pyridyl group affects the hydrogen-bonding networks. The well-designed hydrogen-bonding interaction served as connecting parts for polythymidine mimics even in the presence of other hydrogen-bonding motifs such as nucleobases.
We designed and synthesized the Janus-type TA nucleosides () by using a transglycosylation protocol. Surprisingly, the subtle translocation of the ribose from N8 to N1 by about 230 pm in space leads to the formation of entirely different shaped superstructures, micro-flowers for J-AT and nano-bundles for J-TA.
Two guanosine analogues have been designed and synthesized by connecting one () or three adamantane branches (). The compound containing a single adamantane branch formed G-quartets in acetonitrile solution, and was then transformed into a G-ribbon gel at concentrations higher than the critical gelation concentration. In contrast, the compound with three adamantane branches precipitated after a heating-cooling process. By means of circular dichroism and UV/visible spectra, NMR, SEM, and structural studies, the mechanism of the formation of the G-quartets and G-ribbon gel, as well as the difference in the self-assembly modes of the two compounds, have been fully elucidated. Compound firstly self-assembled into G-quartets in solutions in the concentration range 5.0 × 10(-4) to 1.0 × 10(-2) M, and these G-quartets were transformed into a G-ribbon on further increasing the concentration. Gelation occurred when the G-ribbon self-assembled into a hexagonal columnar structure with the help of intermolecular hydrogen-bonding and hydrophobic interactions. This gel was sensitive to sonication and underwent a morphology change from a columnar structure to a flower-like structure composed of flakes. In contrast, due to steric hindrance, compound only assembled into a spherical structure based on hydrophobic interactions.
Guanosine derivative 1 forms hydrogen-bond-directed giant vesicles. On a silicon substrate, the vesicles retain their shape and internal water phase even after removal of external water under vacuum. Dry manipulation of the micrometer-sized vesicles was carried out via AFM-tip-induced partition and fusion of the vesicles. For larger vesicles (5-10 μm), external solutions were successfully injected through a microcapillary inserted into the vesicle in air.
Biological membranes have properties and behavior that emerge from the propagation of the molecular characteristics of their components across many scales. Artificial smart materials, such as drug delivery vehicles and nanoparticles, often rely on modifying naturally-occurring soft matter, such as polymers and lipid vesicles, so that they possess useful behavior. Mesoscopic simulations allow in silico experiments to be easily and cheaply performed on complex, soft materials requiring as input only the molecular structure of the constituents at a coarse-grained level. They can therefore act as a guide to experimenters prior to performing costly assays. Additionally, mesoscopic simulations provide the only currently feasible window on the length and time scales relevant to important biophysical processes such as vesicle fusion. We describe here recent work using Dissipative Particle Dynamics simulations to explore the structure and behavior of amphiphilic membranes, the fusion of vesicles, and the interactions between rigid nanoparticles and soft surfaces.
Here we report a remarkable enhancement in the adhesion strength of transmembrane cell receptors, human platelet integrin, in a new class of supported lipid membranes, which are separated from the solid substrates by linear polymer spacers. The amphiphilic polymer tether consists of linear hydrophilic poly(2-oxazoline) chains of defined length ( degree of polymerization n = 104, M-W/M-n = 1.30), whose chain termini are functionalized with the tri-functional silane surface coupling group and hydrophobic n-alkyl chains as membrane anchors (lipopolymers). As a model of test cells, giant lipid vesicles were functionalized with synthetic ligand molecules containing the RGD sequence, and the free energy of adhesion Delta g(ad) between the integrin-doped tethered membrane and the vesicle was measured using a micro-interferometry technique. It has been demonstrated that the adhesion function of integrin receptors in these polymer-tethered membranes is 30 times stronger than those incorporated into membranes directly deposited onto solid substrates (solid-supported membranes). The obtained results demonstrate that linear lipopolymer spacers provide a fluid and non-denaturing environment for the incorporated cell receptors and allow quantitative modelling of cell adhesion processes.
Research on giant vesicles is becoming increasingly popular. Giant vesicles provide model biomembrane systems for systematic measurements of mechanical and rheological properties of bilayers as a function of membrane composition and temperature, as well as hydrodynamic interactions. Membrane response to external factors (for example electric fields, ions and amphiphilic molecules) can be directly visualized under the microscope. In this paper we review our current understanding of lipid bilayers as obtained from studies on giant unilamellar vesicles. Because research on giant vesicles increasingly attracts the interest of scientists from various backgrounds, we also try to provide a concise introduction for newcomers in the field. Finally, we summarize some recent developments on curvature effects induced by polymers, domain formation in membranes and shape transitions induced by electric fields.
Membrane fusion has an overarching influence on living organisms. The fusion of sperm and egg membranes initiates the life of a sexually reproducing organism. Intracellular membrane fusion facilitates molecular trafficking within every cell of the organism during its entire lifetime, and virus-cell membrane fusion may signal the end of the organism's life. Considering its importance, surprisingly little is known about the molecular-level mechanism of membrane fusion. Due to the complexity of a living cell, observations often leave room for ambiguity in interpretation. Therefore artificial model systems composed of only a few components are being used to further our understanding of controlled fusion processes. In this critical review we first give an overview of the hypothesized mechanism of membrane fusion and the techniques that are used to investigate it, and then present a selection of non-targeted and targeted model systems, finishing with current applications and predictions on future developments (85 references).
We have developed a method to evaluate the fusion process of giant vesicles using a fluorescence-activated cell sorter (FACS). Three fluorescent markers and FACS technology were used to evaluate the extent of association and fusion of giant vesicles. Two fluorescent markers encapsulated in different vesicle populations were used as association markers; when these vesicles associate, the two independent markers should be observed simultaneously in a single detection event. The quenched fluorescent marker and the dequencher, which were encapsulated in separate vesicle populations, were used as the fusion marker. When the internal aqueous solutions mix, the quenched marker is liberated by the dequencher and emits the third fluorescent signal. Although populations of pure POPC vesicles showed no detectable association or fusion, the same populations, oppositely charged by the exogenous addition of charged amphiphiles, showed up to 50% association and 30% fusion upon population analysis of 100,000 giant vesicles. Although a substantial fraction of the vesicles associated in response to a small amount of the charged amphiphiles (5% mole fraction compared to POPC alone), a larger amount of the charged amphiphiles (25%) was needed to induce vesicle fusion. The present methodology also revealed that the association and fusion of giant vesicles was dependent on size, with larger giant vesicles associating and fusing more frequently.
The impact of the spatial confinement of polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer (BCP) vesicles on the reactivity of encapsulated bovine pancreas trypsin is studied. Enzymes, as well as small molecules, are encapsulated with loading efficiencies up to 30% in BCP vesicles with variable internal volumes between 0.014 aL (internal vesicle diameter, d(in) = 30 nm) and 8 aL (d(in) = 250 nm), obtained by manipulating the vesicle preparation conditions. The kinetics of the trypsin-catalyzed reaction of a fluorogenic substrate inside and outside the vesicles is quantitatively estimated using fluorescence spectroscopic analyses in conjunction with the use of NaNO(2) as selective quencher for non-encapsulated fluorophores. The values of the catalytic turnover number obtained for reactions in differently sized nanoscale reactors show a significant increase (up to approximately 5x) with decreasing BCP vesicle volume, while the values of the Michaelis-Menten constant decrease. The observed increase in enzyme efficiency by two orders of magnitude compared to bulk solution is attributed to an enhanced rate of enzyme-substrate and molecule-wall collisions inside the nanosized reactors, as predicted in the literature on the basis of Monte Carlo simulations.
Vesicles were made from amphiphilic diblock copolymers and characterized by micromanipulation. The average molecular weight of the specific polymer studied, polyethyleneoxide-polyethylethylene (EO40-EE37), is several times greater than that of typical phospholipids in natural membranes. Both the membrane bending and area expansion moduli of electroformed polymersomes (polymer-based liposomes) fell within the range of lipid membrane measurements, but the giant polymersomes proved to be almost an order of magnitude tougher and sustained far greater areal strain before rupture. The polymersome membrane was also at least 10 times less permeable to water than common phospholipid bilayers. The results suggest a new class of synthetic thin-shelled capsules based on block copolymer chemistry.
A dissipative particle dynamics model is applied to probe the lipidic membrane fusion. This model is verified by reproducing the lipid phase behavior. The classical stalk model has been visited and modified. The tilt deformation of the lipids and the noncircular shape of the stalk are supported. The stalk is shown to undergo asymmetric expansion to form the trans-monolayers contact (TMC). Unlike previous models, an energy barrier between the stalk and the TMC has been identified, implying that the TMC should be a metastable formation. This shows good agreement with the fusion experiments. Two typical elastic continuum models are compared with our result and possible modifications to the two elastic models are suggested. The effect of spontaneous curvature of lipid on selection of fusion pathway is also examined. It is observed that a bent stalk with pore or an inverted micellar intermediate will have more chance to occur than traditional stalk when the spontaneous curvature of the lipid becomes more negative.
Membrane fusion is a vital process of life involved, for example, in cellular secretion via exocytosis, signaling between nerve cells, and virus infection. In both the life sciences and bioengineering, controlled membrane fusion has many possible applications, such as drug delivery, gene transfer, chemical microreactors, or synthesis of nanomaterials. Until now, the fusion dynamics has been elusive because direct observations have been limited to time scales that exceed several milliseconds. Here, the fusion of giant lipid vesicles is induced in a controlled manner and monitored with a temporal resolution of 50 μs. Two different fusion protocols are used that are based on synthetic fusogenic molecules and electroporation. For both protocols, the opening of the fusion necks is very fast, with an average expansion velocity of centimeters per second. This velocity indicates that the initial formation of a single fusion neck can be completed in a few hundred nanoseconds.
• electrofusion
• fast digital microscopy
• liposomes
• membrane biophysics
• molecular recognition
A novel asymmetrically substituted sulfamide forms hydrogen bond-directed amphiphilic 2-D sheet assemblies and shows high ability to induce gelation of a variety of solvents including polar water, nonpolar dodecane and aqueous/organic biphasic systems, and this high and wide gelation ability is realized by adjusting the mode of association of the amphiphilic sheet assemblies.
2′-Deoxy-3′,5′-bis-O-(octyldiisopropylsilyl)guanosine (1) showed a good gelation ability for non-polar solvents like alkanes. Atomic force micrography of the organogel showed the formation of mesoscopic-scale sheet-like assemblies, whose width and height were 50–300 and 2.5 nm, respectively. From the IR spectral analysis and crystallographic data of reference compounds, the gel was shown to have a structural hierarchy with different levels of hydrogen bonds, i.e., a one-dimensional tape motif with double hydrogen bonds between guanine units and a mesoscopic-scale sheet-like assembly with hydrogen bonds between adjacent tape motifs. Formation of the sheet structure was shown to be essential for the gelation process. On heating, the concentrated 1–dodecane (37–63 wt%) gel showed gel (G)–lamellar
liquid crystal (L)–isotropic fluid (I) phase, and the G–L transition was ascribed to the cleavage of the inter-tape hydrogen bonds.
Over the last ten years there has been a strong (bio)technological drive for the development of miniaturised reaction systems, motivated mainly by the need to reduce sample consumption and parallelise. Self-assembled soft-matter containers have naturally evolved to handle small volumes and could provide viable fluidic solutions especially in niche areas where ultra-miniaturisation, biocompatibility or cost are of critical importance. Here we focus on nanocontainers that are made of lipids and are immobilised on surfaces. We will highlight the most prominent contributions to date on the fabrication and the applications of surface-based vesicle systems as miniaturised reactors. Emphasis will be put on single-vesicle experiments.
In materials science the concept of function through organization led to the development of new liquid-crystalline materials. From the point of view of macromolecular chemistry, this review tries to combine these two different fields and especially hopes to stimulate their interaction and joint treatment. To exemplify this, the molecular architecture of polymeric organized systems will be discussed. The intention of this review article is to show that polymer science is able to contribute to the simulation of cellular processes such as the stabilization of biomembranes, specific surface recognition, or even the 'uncorking' of cells.
Membrane fusions of vesicles of biomembranes play various important roles in cells, but their mechanisms are unclear and controversial. In the present study, we found that 30 muM to 1 mM La3+ induced membrane fusion of two giant unilamellar vesicles (GUVs) composed of a mixture of dioleoylphosphatidylcholine (DOPC) and dipalmitoleoylphosphatidylethanolamine (DPOPE). We succeeded in observing a process of this membrane fusion in detail. First, two GUVs became strongly associated, with a partition membrane between them composed of two bilayers, one from each GUV. Then, the partition membrane was suddenly broken at one site on its edge. The area of this breakage site gradually spread, until it was completely separated from the GUV to complete the membrane fusion. Here, we propose a new model (i.e., the partition breakage model) for the mechanism of La3+-induced membrane fusion of GUVs.
A bio-nanocapsule (BNC) is an ~50-nm hepatitis B virus (HBV) subviral particle comprising HBV envelope L proteins and a lipid bilayer, and is synthesized in recombinant Saccharomyces cerevisiae. When BNCs are administered intravenously in a mouse xenograft model, they can accumulate specifically in human liver-derived tissues and enter cells efficiently by the HBV-derived human liver-specific infection machinery, localized at the outer-membrane pre-S region of the L protein. BNC specificity for the human liver can be altered to other tissues by substituting the pre-S region using targeting molecules (e.g., antibodies, lectins, cytokines). BNCs can spontaneously form complexes with liposomes (LPs) by the membrane fusogenic activity of the pre-S region. LPs containing various therapeutic materials (e.g., chemicals, proteins, DNA, RNA) can therefore be covered with BNCs to form an ~150-nm BNC–LP conjugate. BNC–LP conjugates injected intravenously can deliver incorporated materials to target tissues specifically and efficiently by utilizing the HBV-derived infection machinery. The stability of BNC–LP conjugates in the blood circulation is similar to that of PEGylated LPs. In this chapter, we describe the preparation and in vivo application of BNC–LP conjugates, and the potential of BNC–LP conjugates as in vivo pinpoint drug delivery systems.
Recent developments in lipid membrane models for simulations are
reviewed. To reduce computational costs, various coarse-grained
molecular models have been proposed. Among them, implicit solvent
(solvent-free) molecular models are relatively more coarse-grained and
efficient for simulating large bilayer membranes. On a μm scale, the
molecular details are typically negligible and the membrane can be
described as a continuous curved surface. The theoretical models for
fluid and elastic membranes with mesh or meshless discretizations are
presented. As examples of applications, the dynamics of vesicles in
flows, vesicle formation, and membrane fusion are presented.
Recent developments in the area of H-bonded supramolecular assemblies of π-conjugated systems, that is, oligomers and polymers, are described. The state-of-the-art summary of the recent developments in the design of discrete systems and functional materials is presented.
Nucleotide-appended bolaamphiphiles 1−7, in which two 3‘-phosphorylated thymidine moieties are connected to both ends of a long oligomethylene spacer, have been first synthesized. Their self-assembling behavior in aqueous solutions was investigated in terms of gelling ability of water molecules. The longer homologues 6 and 7 with the C18 and C20 oligomethylene spacers, respectively, proved to be capable of gelling water very effectively (>25 000 water molecules per molecule) through spontaneous formation of a fibrous network. Gelation behavior of both bolaamphiphiles strongly depended on the pH and temperature of the aqueous solutions used. The gel-to-sol transition temperature (TGS) of 7 was determined to be approximately 85 °C. XRD measurement of a freeze-dried hydrogel from 7 suggested the presence of lamellar organization consisting of monolayer sheets. Hydrogen bonds involving the 5‘-hydroxyl group of the deoxyribose moiety, hydrophobic interaction between the long oligomethylene chains, and π−π stacking of the thymine residues are responsible for the effective hydrogel formation.
By means of STM, direct evidence on the sub-nanometer scale of a dynamer operating at surfaces has been provided. Octadecyl guanine (G) was reversibly interconverted at the solid–liquid interface between two highly ordered supramolecular motifs, that is, hydrogenbonded ribbons and G4-based architectures, upon subsequent addition of [2.2.2]cryptand, potassium picrate (K+(pic)-), and trifluoromethanesulfonic acid. The visualization of such supramolecular interconversion at the solid–liquid interface opens new avenues towards understanding the mechanism of formation and function of complex nucleobase architectures such as DNA or RNA. Furthermore, the in situ reversible assembly and reassembly between two highly ordered supramolecular structures at the surfaces represents the first step towards the generation of nanopatterned responsive architectures.
The electrophoretic mobility of oil droplets, dispersed without any surfactant in the aqueous phase, was measured. Four different oils were studied: xylene, dodecane, hexadecane, and perfluoromethyldecalin. Special precautions were undertaken to avoid artifacts caused by the presence of surfactant impurities. The results show that the oil droplets are negatively charged and the magnitude of their -potential strongly depends on pH and the ionic strength of the aqueous phase. The electrophoretic mobility is almost independent of the type of specific nonpolar oil. Series of experiments were performed to check different hypotheses about the origin of the spontaneous charging of the oil-water interfaces. The results lead to the conclusion that hydroxyl ions, released by the dissociation-association equilibrium of the water molecules, adsorb at the oil-water interface. The specific adsorption energy was estimated to be 25kT per ion (kT is the thermal energy). The molecular origin and the implications of this phenomenon are discussed. The -potential decreases in magnitude when poly(oxyethylene) chain nonionic surfactants are adsorbed at the interface.
Crystal engineering is the branch of supramolecular chemistry concerned with the design and synthesis of extended structures with predictable form and function. In this chapter, the use of hydrogen bonds to generate one-, two- and three-dimensional structures is discussed, with the different strategies employed compared. The review concentrates on systems in which two or more hydrogen bonds link components together, and extended structures based on both one and two components are highlighted. Parallels are drawn between crystal engineering using purely organic components, and the more recent extension to the inclusion of coordination and organometallic complexes.
This review introduces recent research in the area of self-assembled nanostructures from rod molecules
in solutions; in particular one-dimensional nano-aggregates such as fibers, ribbons, tubules are highlighted.
Self-assembled nanostructures are well-known to be strongly dependent upon the architecture of molecular
building blocks; thus, in this review we discuss assembling behavior and the related functions associated
with molecular shapes such as simple rod–coils, macrocycles, dendron–rod–coils, dumbbells,
wedges and conjugated rods with lateral chains. In addition, biomimetic or bioconjugate amphiphilic rod
systems are described.
Potential technological applications for artificial lipid and proteo-lipid membranes, such as capsules for controlled delivery
of drugs and coatings for biosensors and biomaterials, are in many cases limited by the inherent instability of lipid lamellar
phases. Development of methods to stabilize artificial lipid membranes has therefore been a focus of research efforts since
the 1970s. Linear and cross-linking polymerization of synthetic lipid monomers is a well-studied strategy. Several comprehensive
reviews on polymerizable lipids and supramolecular structures derived from them appeared between 1985 and 2002. Consequently,
this review focuses on significant developments in this field during 2000–2008. These include synthesis of new types of polymerizable
lipids, creation and characterization of novel poly(lipid) membrane systems, and applications of polymerized vesicles and
membranes in chemical sensing, separations science, drug delivery, materials biocompatibility, and energy storage. Polymerization
of membranes to achieve stability and their functionalization for technological applications are emphasized.
The zeta-potentials of microbubbles in the 40- to 80-μm size range have been determined by means of a simple microelectrophoresis technique. In general, the sign of the bubble charge is determined by the polar head of the surfactant when an ionic surfactant is used to produce the bubbles. When using hydrolyzable ionic surfactants, however, the bubble charge is significantly affected by the hydrolysis products. In the presence of nonionic surfactants, bubbles can be charged either positively or negatively depending on the pH, and the isoelectric points appear to be related to the oxygen-to-carbon ratio of the surfactant molecule. With ionic surfactants, an increase in concentration results in an increase in bubble charge, while with nonionic surfactants the zeta-potentials change little in the concentration range studied. The negative charges observed with air bubbles and oil droplets in the absence of surfactants can be explained by the differences in the hydration energies of H+ and OH− ions.
Dispersal of dioctadecyldimethylammonium methacrylate (DODAM) in water via ultrasonic irradiation yielded small-diameter vesicles having a phase transition at ca. 42-46°C. Photopolymerization (254 nm) at 30 and 60°C resulted in the formation of polymer-encased vesicles which retained phase-transition behavior. Combination of dynamic light scattering, electron microscopy, and captured volume data provides strong evidence for vesicle shrinkage when polymerization is carried out at 60°C; shrinkage occurring during photopolymerization at 30°C is less certain. Poly(methacrylic acid), derived from 30°-polymerized vesicles (30°-polymerized means polymerized at 30°C in this paper), was 75.4% syndiotactic, 22.3% heterotactic, and 2.3% isotactic and was significantly more soluble in DMF than poly(methacrylic acid) derived from 60°-polymerized vesicles; the latter polymer was predominantly syndiotactic. At 25°C, nonpolymerized, 30°-polymerized, and 60°-polymerized vesicles showed similar permeability toward sucrose; at 60âC, the 60°-polymerized vesicles were less permeable. Storage of the 60°-polymerized and the nonpolymerized DODAM vesicles for 2 months at room temperature revealed the former to be more stable. The monolayer properties and photopolymerization behavior of DODAM have been investigated at the air-water interface.
We describe polymersomes with ionic liquid interiors dispersed in water. The vesicles are prepared via a simple and spontaneous migration of poly(butadiene-b-ethylene oxide) (PB-PEO) block copolymer vesicles from a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), to water at room temperature. As PB is insoluble in both water and [EMIM][TFSI] and PEO is well solvated in both media, the vesicles feature a PB membrane with PEO brushes forming both interior and exterior coronas. The robust and stable PB-PEO vesicles migrate across the liquid-liquid interface with their ionic liquid interiors intact and form a stabilized aqueous dispersion of vesicles enclosing microscopic ionic liquid pools. The nanostructure of the vesicles with ionic liquid interiors dispersed in water is characterized by direct visualization using cryogenic transmission electron microscopy. Upon heating, the vesicles can be quantitatively transferred back to [EMIM][TFSI], thus enabling facile recovery. The reversible transport capability of the shuttle system is demonstrated by the use of distinct hydrophobic dyes, which are selectively and simultaneously loaded in the vesicle membrane and interior. Furthermore, the fluorescence of the loaded dyes in the vesicles enables probing of the microenvironment of the vesicular ionic liquid interior through solvatochromism and direct imaging of the vesicles using laser scanning confocal microscopy. This vesicle system is of particular interest as a nanocarrier or nanoreactor for reactions, catalysis, and separations using ionic liquids.
Liposomes are structurally and functionally some of the most versatile supramolecular assemblies in existence. Since the beginning of active research on lipid vesicles in 1965, the field has progressed enormously and applications are well established in several areas, such as drug and gene delivery. In the analytical sciences, liposomes serve a dual purpose: Either they are analytes, typically in quality-assessment procedures of liposome preparations, or they are functional components in a variety of new analytical systems. Liposome immunoassays, for example, benefit greatly from the amplification provided by encapsulated markers, and nanotube-interconnected liposome networks have emerged as ultrasmall-scale analytical devices. This review provides information about new developments in some of the most actively researched liposome-related topics.
There is considerable interest in preparing cell-sized giant unilamellar vesicles from natural or nonnatural amphiphiles because a giant vesicle membrane resembles the self-closed lipid matrix of the plasma membrane of all biological cells. Currently, giant vesicles are applied to investigate certain aspects of biomembranes. Examples include lateral lipid heterogeneities, membrane budding and fission, activities of reconstituted membrane proteins, or membrane permeabilization caused by added chemical compounds. One of the challenging applications of giant vesicles include gene expressions inside the vesicles with the ultimate goal of constructing a dynamic artificial cell-like system that is endowed with all those essential features of living cells that distinguish them from the nonliving form of matter. Although this goal still seems to be far away and currently difficult to reach, it is expected that progress in this and other fields of giant vesicle research strongly depend on whether reliable methods for the reproducible preparation of giant vesicles are available. The key concepts of currently known methods for preparing giant unilamellar vesicles are summarized, and advantages and disadvantages of the main methods are compared and critically discussed.
Giant Unilamellar Vesicles (GUVs) provide a key model membrane system to study lipid-lipid and lipid-protein interactions, which are relevant to vital cellular processes, by (single-molecule) optical microscopy. Here, we review the work on reconstitution techniques for membrane proteins and other preparation methods for developing GUVs towards most suitable close-to-native membrane systems. Next, we present a few applications of protein-containing GUVs to study domain assembly and protein partitioning into raft-like domains.
Hydrogen-bond-directed giant supramolecular vesicles (diameter 1.20 +/- 0.30 microm (SD)) of an alkylsilylated deoxyguanosine derivative, 2a, were prepared faciley by mixing a small volume of a 2a/THF solution with water. The formation of 2-D inter-guanine hydrogen-bond networks of 2a within the vesicles was indicated by IR spectra. The vesicle solution was stable enough for more than 30 days, in a wide range of temperatures, and between pH 4 and 10 without showing lysis, fusion, precipitation, or leakage of the encapsulated fluorescent probe. In a typical micrometer-sized vesicle, a sufficiently large internal water phase for encapsulating water-soluble substances was surrounded by a multilamellar membrane 15-20 nm in thickness, which was composed of 6-9 layers of 2-D hydrogen-bond-directed sheet assemblies. AC-mode AFM observation of the vesicle on a silicon substrate further demonstrated the high stability and deformable properties of the vesicle membrane under vacuum or mechanical stress. The formation and properties of the vesicle membrane in water were analyzed from the viewpoint of the 2-D hydrogen-bond-directed sheet assemblies, and the scope of the design principle to use nonpolar soft segments as the shielding units of the hydrogen-bond networks in water is discussed.
Cyanuric acid (CA) and melamine (M) functionalized lipids can form membranes that exhibit robust hydrogen-bond driven surface recognition in water, facilitated by multivalent surface clustering of recognition groups and variable hydration at the lipid-water interface. Here we describe a minimal lipid recognition cluster: three CA or M recognition groups are forced into proximity by covalent attachment to a single lipid headgroup. This trivalent lipid system guides recognition at the lipid-water interface using cyanurate-melamine hydrogen bonding when incorporated at 0.1-5 mol percent in fluid phospholipid membranes, inducing both vesicle-vesicle binding and membrane fusion. Fusion was accelerated when the antimicrobial peptide magainin was used to anchor trivalent recognition, or when added exogenously to a preassembled lipid vesicle complex, underscoring the importance of coupling recognition with membrane disruption in membrane fusion. Membrane apposition and fusion were studied in vesicle suspensions using light scattering, FRET assays for lipid mixing, surface plasmon resonance, and cryo-electron microscopy. Recognition was found to be highly spatially selective as judged by vesicular adhesion to surface patterned supported lipid bilayers (SLBs). Fusion to SLBs was also readily observed by fluorescence microscopy. Together, these studies indicate effective and functional recognition of trivalent phospholipids, despite low mole percentage concentration, solvent competition for hydrogen bond donor/acceptor sites, and simplicity of structure. This novel designed molecular recognition motif may be useful for directing aqueous-phase assembly and biomolecular interactions.
An overview of the wide range of polymer-based capsules that have been constructed from synthetic and biological building blocks or from biological building blocks that are taken out of their natural environment, using both hyperbranched and self-assembly approaches, was reviewed. The capsules that are discussed can be considered as the simplest mimics of an organelle or cell and contain a cavity in which chemical reactions can take place or cargo can be stored. The chemical tool box available for constructing polymer micelles and polymersomes is much larger, and natural motifs have been actively incorporated into their designs. Regarding LbL, polymersome, or polymeric micelle nanoreactors, it can be predicted that nature's biocatalysts will be increasingly used for encapsulation in these synthetic systems, holding promise for future nanoscale diagnostic devices. One of the main challenges in this field will be the effective stimulation of responsive polymersomes since most stimuli reported thus far cannot be applied in living organisms.
Over the last two decades, guanosine-related molecules have been of interest in different areas, ranging from structural biology to medicinal chemistry, supramolecular chemistry and nanotechnology. The guanine base is a multiple hydrogen-bonding unit, capable also of binding to cations, and fits very well with contemporary studies in supramolecular chemistry, self-assembly and non-covalent synthesis. This Concepts article, after reviewing on the diversification of self-organised assemblies from guanosine-based low-molecular-weight molecules, will mainly focus on the use of guanine moiety as a potential scaffold for designing functional materials of tailored physical properties.
„Superfilm“: Zweidimensionale Blattstrukturen aus alkylsilylierten Desoxyguanosin-Derivaten wurden aus wasserstoffverbrückten eindimensionalen Bändern erhalten. Durch Einführen polarer Oxyethylen-Einheiten auf der Oberfläche der Blattstrukturen konnten elastische stabile supramolekulare Filme mit definiertem hierarchischem Aufbau konstruiert werden.
Methods based on self-assembly, self-organization, and forced shape transformations to form synthetic or semisynthetic enclosed lipid bilayer structures with several properties similar to biological nanocompartments are reviewed. The procedures offer unconventional micro- and nanofabrication routes to yield complex soft-matter devices for a variety of applications for example, in physical chemistry and nanotechnology. In particular, we describe novel micromanipulation methods for producing fluid-state lipid bilayer networks of nanotubes and surface-immobilized vesicles with controlled geometry, topology, membrane composition, and interior contents. Mass transport in nanotubes and materials exchange, for example, between conjugated containers, can be controlled by creating a surface tension gradient that gives rise to a moving boundary or by induced shape transformations. The network devices can operate with extremely small volume elements and low mass, to the limit of single molecules and particles at a length scale where a continuum mechanics approximation may break down. Thus, we also describe some concepts of anomalous fluctuation-dominated kinetics and anomalous diffusive behaviours, including hindered transport, as they might become important in studying chemistry and transport phenomena in these confined systems. The networks are suitable for initiating and controlling chemical reactions in confined biomimetic compartments for rationalizing, for example, enzyme behaviors, as well as for applications in nanofluidics, bioanalytical devices, and to construct computational and complex sensor systems with operations building on chemical kinetics, coupled reactions and controlled mass transport.
The long-chain alkyl derivatives of a nucleoside analogue-acyclovir were prepared in the paper. One is stearyl-glycero-succinyl-acyclovir (SGSA) with a single 18-carbon length (C18) alkyl chain. Another is dioctadecyl-aspartate-succinyl-acyclovir (DASA) with double C18 alkyl chains. They were prepared by the esterification of succinyl-acyclovir with the lipids, and sodium salts of them were also prepared. Guanine moieties and alkyl moieties bring the derivatives intermolecular hydrogen bonding and hydrophobic interaction in water separately. The forces are influenced by the number of alkyl chains and the charged state, and determine the solubility and the self-assembly behavior of the derivatives. The double alkyl-chain derivatives (DASA and DASA-Na) formed rigid Langmuir monolayers on air/water surface, while the single alkyl chain derivatives (SGSA and SGSA-Na) did not. However, cholesterol (Chol) could assist SGSA to form rigid monolayers through inserting into the alkyl chains of SGSA to mimic the second alkyl chain. SGSA self-aggregates in water were prepared by the injection method with tetrahydrofuran as solvent. Cuboid-like shape and nanoscale size demonstrated that SGSA self-aggregates were self-assembled nanoparticles. Shape, particle size, zeta potential and phase transition of the nanoparticles were characterized. And they showed an average size of 83.2 nm, a negative surface charge of -31.3-mV zeta potential and a gel-liquid crystalline phase transition of 50.38 degrees C. The formation mechanism of self-assembled nanoparticles was analyzed. Hydrophobic interaction of alkyl chains improves SGSA molecules to form bilayers, and then cuboid-like nanoparticles were obtained by layer-by-layer aggregation based on inter-bilayers hydrogen bonding. However, the charged guanine moieties make SGSA-Na lose the function of hydrogen bonding so that SGSA-Na only forms vesicles in water based on hydrophobic interaction. Strong hydrophobicity and wide-open rigid double alkyl chains of DASA and DASA-Na restrict self-assembly in water media, and no homogeneous suspensions were obtained. Therefore, the molecular self-assembly behavior of the long-chain alkyl derivatives of nucleoside analogues on water surface or in water media is determined by the number of alkyl chains and the charged state.
For Abstract see ChemInform Abstract in Full Text.
Membrane fusion is very important for the formation of many complex organs in metazoans throughout evolution, such as muscles, bones, and placentae. Lipid vesicles (liposomes) are frequently used as model membranes to study the fusion process. This work demonstrates for the first time the real-time membrane fusion of giant polymer vesicles by directly displaying a series of high-resolution and real-time transformation images of individual vesicles. The fusion process includes the sequential steps of membrane contact, forming the center wall, symmetric expansion of fusion pore and complete fusion, undergoing the intermediates of "8" shape with a protruding rim at the contact site, peanut (pear) shape, and oblate sphere. The vesicle swells during fusion, and the fusing vesicle only deforms in the neck domain around the fusion pore in the lateral direction, which verifies the importance of the lateral tension on the fusion pore at the vesicle deformation level. The successful fusion of the synthetic and protein-free polymer vesicles reported here also supports that vesicle proximity combined with membrane perturbation suffices to induce membrane fusion, and that the protein is not necessary for the fusion process.
Polymersomes are self-assembled polymer shells composed of block copolymer amphiphiles. These synthetic amphiphiles have amphiphilicity similar to lipids, but they have much larger molecular weights, so for this reason--along with others reviewed here--comparisons of polymersomes with viral capsids composed of large polypeptide chains are highly appropriate. We summarize the wide range of polymers used to make polymersomes along with descriptions of physical properties such as stability and permeability. We also elaborate on emerging studies of in vivo stealthiness, programmed disassembly for controlled release, targeting in vitro, and tumor-shrinkage in vivo. Comparisons of polymersomes with viral capsids are shown to encompass and inspire many aspects of current designs.
Three amino acid-derived chiral surfactants, sodium N-[4-(n-dodecyloxy)benzoyl]-L-leucinate (SDBL), sodium N-[4-(n-dodecyloxy)benzoyl]-L-isoleucinate (SDBIL), and sodium N-[4-(n-dodecyloxy)benzoyl]-L-threoninate (SDBT), were synthesized, and their aggregation behavior was studied in aqueous solution. Surface tension, fluorescence probe, dynamic light scattering, nuclear magnetic resonance (NMR), gel permeation chromatography, circular dichroism, and optical as well as transmission electron microscopic techniques were utilized to characterize the self-assemblies formed by the amphiphiles. Results of these studies reveal that the surfactants have a very low critical aggregation concentration (cac) and they form spherical vesicles spontaneously in dilute aqueous solution. The mean diameters of the vesicles were measured to be in the range of 130-190 nm. 1H NMR spectra indicated hydrogen bonding between the amide groups near the surfactant headgroup, which is one of the driving forces for vesicle formation. The vesicle formation is more favored at a pH of about 7.0. The amphiphiles also form chiral helical aggregates at relatively higher concentrations as indicated by circular dichroism spectra. The stability of the vesicles was also evaluated with respect to the surfactant concentration, pH, temperature, and aging. The vesicles have a tendency to transform into elongated vesicles (closed tubules) or rodlike micelles with an increase of the surfactant concentration and/or pH. On the basis of the results obtained from different studies, phase diagrams for all three water/amphiphile systems have been constructed. The studies have further shown that the stereogenic center at the amino acid side chain has a significant effect on the aggregation properties of the amphiphiles and on the stability of the self-assemblies.
(Figure Presented) Stabilized net: A molecular design approach to protect a 2D hydrogen-bonding network with nonpolar shielding layers has allowed the fabrication of micrometer-scale supramolecular vesicles in water (see picture). These hydrogen-bond-directed giant vesicles and their dispersions in aqueous media show high stability under various conditions.
Novel supramolecular coatings that make use of low-molecular weight ditopic monomers with guanine end groups are studied using fluid tapping AFM. These molecules assemble on highly oriented pyrolytic graphite (HOPG) from aqueous solutions to form nanosized banding structures whose sizes can be systematically tuned at the nanoscale by tailoring the molecular structure of the monomers. The nature of the self-assembly in these systems has been studied through a combination of the self-assembly of structural derivatives and molecular modeling. Furthermore, we introduce the concept of using these molecular assemblies as scaffolds to organize functional groups on the surface. As a first demonstration of this concept, scaffold monomers that contain a monomethyl triethyleneglycol branch were used to organize these "functional" units on a HOPG surface. These supramolecular grafted assemblies have been shown to be stable at biologically relevant temperatures and even have the ability to significantly reduce static platelet adhesion.
- Everett D. H.