Randolph S. Duran

Louisiana State University, Baton Rouge, Louisiana, United States

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Publications (61)180.47 Total impact

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    ABSTRACT: MscL is a bacterial mechanosensitive channel that protects cells from lysis upon acute decrease in external osmotic environment. It is one of the best characterized mechanosensors known, thus serving as a paradigm of how such molecules sense and respond to stimuli. In addition, the fact that it can be genetically modified, expressed, isolated, and manipulated has led to its proposed use as a triggered nanovalve for various functions including sensors within microelectronic array chips, as well as vesicular-based targeted drug release. X-ray crystallography reveals a homopentameric complex with each subunit containing two transmembrane α-helices (TM1 and TM2) and a single carboxyl terminal α-helix arranging within the complex to form a 5-fold cytoplasmic bundle (CB), whose function and stability remain unclear. In this study, we show three routes that throttle the open channel conductance. When the linker between the TM2 and CB domain is shortened by deletions or constrained by either cross-linking or heavy metal coordination, the conductance of the channel is reduced; in the later two cases, even reversibly. While they have implications for the stability of the CB, these data also provide routes for engineering MscL sensors that are more versatile for potential nanotech devices.
    ACS Nano 12/2011; 6(2):1134-41. · 12.03 Impact Factor
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    ABSTRACT: The effect of filler loading on the mechanical properties of crosslinked triazole polymers obtained by polymerization of E300 dipropiolate (1) with diazide (2) obtained from tetraethylene glycol using tetraacetylene functionalized crosslinker (3) was studied systematically. Aluminum (10–14 μm) was used as the primary filler during the formulations; the effect of secondary fillers such as aluminum (<75 μm), NaCl (45–50 and 83–105 μm) was studied with the increase in the total filler loading. The modulus of the aluminum-filled crosslinked triazole polymers increases with the increase in the filler content while using either particle sized aluminum powder. The use of Al (particle size <75 μm) and NaCl (particle size 45–50 μm and 83–105 μm) as secondary or additional fillers while using aluminum (10–14 μm) as the main filler, has a diminishing effect on the modulus and strain of the crosslinked triazole polymers. Triazole polymers described herein have the ability to wet and adhere to large quantities of these inorganic salts and thus maintain mechanical properties of the composite comparable to typical polyurethane elastomeric matrices, regardless of the chemistry of the particulate filler, which imparts an important and necessary binder characteristic for energetic composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
    Journal of Applied Polymer Science 10/2010; 118(1):121 - 127. · 1.40 Impact Factor
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    ABSTRACT: Lipid bilayers are of interest in applications where a cell membrane mimicking environment is desired. The performance of the lipid bilayer is largely dependent on the physical and chemical properties of the component lipids. Lipid bilayers consisting of phytanoyl lipids have proven to be appropriate choices since they exhibit high mechanical and chemical stability. In addition, such bilayers have high electrical resistances. Two different phytanoyl lipids, 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) and 1,2-diphytanoyl-sn-glycero-3-phosphoethanolamine (DPhPE), and various combinations of the two have been investigated with respect to their behavior in aqueous solutions, their interactions with solid surfaces, and their electrical stability. Dynamic light scattering, nuclear magnetic resonance diffusion, and cryogenic transmission electron microscopy measurements showed that pure DPhPC as well as mixtures of DPhPC and DPhPE consisting of greater than 50% (mol%) DPhPC formed unilamellar vesicles. If the total lipid concentration was greater than 0.15g/l, then the vesicles formed solid-supported bilayers on plasma-treated gold and silica surfaces by the process of spontaneous vesicle adsorption and rupture, as determined by quartz crystal microbalance with dissipation monitoring and atomic force microscopy. The solid-supported bilayers exhibited a high degree of viscoelasticity, probably an effect of relatively high amounts of imbibed water or incomplete vesicle fusion. Lipid compositions consisting of greater than 50% DPhPE formed small flower-like vesicular structures along with discrete liquid crystalline structures, as evidenced by cryogenic transmission electron microscopy. Furthermore, electrophysiology measurements were performed on bilayers using the tip-dip methodology and the bilayers' capacity to retain its electrical resistance towards an applied potential across the bilayer was evaluated as a function of lipid composition. It was shown that the lipid ratio significantly affected the bilayer's electrical stability, with pure DPhPE having the highest stability followed by 3DPhPC:7DPhPE and 7DPhPC:3DPhPE in decreasing order. The bilayer consisting of 5DPhPC:5DPhPE had the lowest stability towards the applied electrical potential.
    Colloids and surfaces B: Biointerfaces 10/2010; 82(2):550-61. · 4.28 Impact Factor
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    ABSTRACT: Thirteen triazole polymers were prepared as potential rocket propellant binders by the reactions of various diacetylenes and diazides. The reaction of E300 dipropiolate (1) with diazide (2) obtained from tetraethylene glycol was selected to study the effects of concentration of the tetraacetylene functionalized crosslinker (3) on the mechanical properties of resulting triazole polymers. The modulus of the polymers increased, whereas the strain (% elongation at failure) decreased with increasing percentage of crosslinker. The resulting triazole polymers also showed that the desired mechanical properties could be obtained by adjusting the crosslinker concentration during the polymerization. Addition of 43 wt % aluminum filler did not significantly affect the strain. The modulus of these triazole polymers was comparable with typical polyurethane elastomeric matrices for rocket propellants. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
    Journal of Applied Polymer Science 09/2010; 117(5):2612 - 2621. · 1.40 Impact Factor
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    ABSTRACT: The mechanical properties of crosslinked polymers depend on their structural features, one of which is the functionality of the crosslinks in a polymer network. To study the effect of crosslink functionality (ϕ) on the mechanical properties of 1,2,3-triazole polymers for potential application as rocket propellant binders, crosslinkers with different ϕ's (3, 4, 6, 16, 32, and 64) were used in the polymerization. As the percentage of acetylenic groups provided by crosslinker was kept constant and the functionality of the crosslinker increased, the resulting polymer showed a higher modulus but a lower strain. Compared to traditional polyurethane binders, 1,2,3-triazole polymers showed comparable mechanical properties, although the stress and modulus tended to be lower and the strain capability tended to be greater for the triazole-linked rubbers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
    Journal of Applied Polymer Science 07/2010; 117(1):473 - 478. · 1.40 Impact Factor
  • Jorge L Chávez, Hui Jiang, Randolph S Duran
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    ABSTRACT: Hybrid organic-inorganic templates and core-shell nanoparticles were used as models to study the communication between fluorescent probes placed inside nanoparticles. The hybrid templates were prepared on the basis of a mixed-surfactant system using octadecyltrimethoxysilane as a reactive amphiphile. The core-shell particles were obtained after coating of the templates with a siloxane shell, using the silanol groups on their surface. Atomic force microscopy imaging showed that the templates were made of a flexible material that flattened significantly after deposition on a substrate and evaporation of the solvent. Pyrene was sequestered by the templates in an aqueous suspension, which placed it in a nonpolar environment, as observed by its fluorescence response. Subsequently, double-doped templates were prepared by sequestering coumarin 153 (C153), with pyrene-doped hybrid templates. The communication between these probes was studied on the basis of their spectral properties, by means of fluorescence resonance energy transfer (FRET). Energy transfer between the dyes with efficiencies up to 55% was observed. Similarly, double-doped core-shell particles prepared on the basis of the hybrid templates were doped with this pair of dyes. Despite the presence of the shell, which was intended to increment the average separation between the probes, interaction of the dyes was observed, although with lower efficiencies. A similar study was performed with C153 and 4-(dicyanomethylene)-2-methyl-6-p-(dimethylamino)styryl-4H-pyran (DCM). FRET studies indicated that the probes were placed in proximity to each other. We confirmed these observations by means of fluorescence lifetime measurements, which showed a decrease in the lifetime of the donor upon addition of the acceptor.
    Nanotechnology 12/2009; 21(5):055703. · 3.84 Impact Factor
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    ABSTRACT: Bee venom phospholipase A(2) (bvPLA(2)) is part of the secretory phospholipase A(2) (sPLA(2)) family whose members are active in biological processes such as signal transduction and lipid metabolism. While controlling sPLA(2) activity is of pharmaceutical interest, the relationship between their mechanistic actions and physiological functions is not well understood. Therefore, we investigated the interfacial binding process of bvPLA(2) to characterize its biophysical properties and gain insight into how membrane binding affects interfacial activation. Attention was focused on the role of membrane electrostatics in the binding process. Although dynamic light scattering experiments indicated that bvPLA(2) does not lyse lipid vesicles, a novel, nonhydrolytic activity was discovered. We employed a supported lipid bilayer platform on the quartz crystal microbalance with dissipation sensor to characterize this bilayer-disrupting behavior and determined that membrane electrostatics influence this activity. The data suggest that (1) adsorption of bvPLA(2) to model membranes is not primarily driven by electrostatic interactions; (2) lipid desorption can follow bvPLA(2) adsorption, resulting in nonhydrolytic bilayer-disruption; and (3) this desorption is driven by electrostatic interactions. Taken together, these findings provide evidence that interfacial binding of bvPLA(2) is a dynamic process, shedding light on how membrane electrostatics can modulate interfacial activation.
    Langmuir 12/2009; 26(6):4103-12. · 4.38 Impact Factor
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    ABSTRACT: Second-, third-, and fourth-generation hyperbranched aliphatic polyols namely Boltorn® H20, Boltorn H30, and Boltorn H40 were endcapped with azido and activated acetylenic groups in good to excellent yields (75–95%) following an acid catalyzed procedure. The resultant terminally functionalized dendritic azido and acetylenic groups undergo 1,3-dipolar cycloaddition using methyl (or ethyl) propiolate and benzyl azide, respectively, under catalytic or noncatalytic conditions below 40 °C to yield 1,2,3-triazole dendrimeric polymers in 82–95% yield, under extremely mild conditions that could be applied for compounds sensitive to acid, base, or heat. The dendritic azido and activated acetylenic derivatives may act as novel scaffolds to tune the mechanical properties of different polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3748–3756, 2009
    Journal of Polymer Science Part A Polymer Chemistry 06/2009; 47(15):3748 - 3756. · 3.54 Impact Factor
  • Biophysical Journal 01/2009; 96(3). · 3.67 Impact Factor
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    ABSTRACT: Water-dispersible organosilica nanoparticles were synthesized using microemulsions and micellar solutions. Octadecyltrimethoxysilane (OTMS) was used as the silica source resulting in particles having a hydrophobic interior with the ability to host oil. The diameters of the formed particles could be varied between 60 and 200 nm, depending on the amount of added oil and OTMS. The size of the particles was determined using dynamic light scattering and transmission electron microscopy. Exchange-coupled diffusion nuclear magnetic resonance experiments were performed to study the exchange rates between the particles and the surrounding media. Triethylamine and tributylamine were used as probe molecules, and it was shown that they had longer mean residence times in the particles compared with in the bulk. Moreover, it was found that the mean residence time of the probe molecules increased significantly when the particles contained oil. The results also showed that the mean residence time of tributylamine was longer than that of triethylamine. Furthermore, by the use of UV−vis spectrophotometry, it was shown that the particles were able to take up benzophenone from water solutions.
    Journal of Physical Chemistry C - J PHYS CHEM C. 10/2008; 112(44).
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    ABSTRACT: S-layer proteins are commonly found in bacteria and archaea as two-dimensional monomolecular crystalline arrays as the outermost cell membrane component. These proteins have the unique property that following disruption by chemical agents, monomers of the protein can re-assemble to their original lattice structure. This unique property makes S-layers interesting for utilization in bio-nanotechnological applications. Here, we show that the addition of S-layer proteins to bilayer lipid membranes increases the lifetime and the stability of the bilayer. M2delta ion channels were functionally incorporated into these S-layer stabilized membranes and we were able to record their activity for up to 20 h. Transmission electron microscopy (TEM) was used to visualize the 2D crystalline pattern of the S-layer and the M2delta ion channel characteristics in bilayer lipid membrane's were compared in the presence and absence of S-layers.
    Colloids and surfaces B: Biointerfaces 10/2008; 65(2):178-85. · 4.28 Impact Factor
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    ABSTRACT: Using a water-in-oil microemulsion system, silica nanoparticles containing superparamagnetic iron oxide (SPIO) crystals have been prepared and characterized. With this method, the loading of iron oxide crystals, the thickness of the silica shells, and the overall particle sizes are tunable. Moving from low to high water concentration, within the microemulsion region, resulted in a gradual shift from larger particles, ca. 100 nm and fully loaded with SPIOs, to smaller particles, ca. 30 nm containing only one or a few SPIOs. By varying the amount of silica precursor, the thickness of the silica shell was altered. Field dependent magnetization measurements showed the magnetic properties of the SPIOs were preserved after the synthesis.
    Langmuir 05/2008; 24(7):3532-6. · 4.38 Impact Factor
  • Jorge L Chavez, Jeffrey L Wong, Randolph S Duran
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    ABSTRACT: Core-shell nanocapsules intended to be used as drug scavengers were prepared using a surfactant mixture containing octadecyltrimethoxysilane (OTMS) as a reactive amphiphile, to form spherical templates. A siloxane shell was grown on the surface of the templates by reacting tetramethoxysilane (TMOS) with the silanol groups obtained after the hydrolysis and condensation of OTMS. Dynamic light scattering (DLS) showed that particles with diameters in the range of 100-200 nm were obtained, with core and shell sizes controlled by varying component compositions. Atomic force microscopy (AFM) was used to study the effect of the silica coating of the templates on their robustness after deposition on a substrate. Subsequently, we present studies on the encapsulation of two hydrophobic fluorescent dyes, which are sensors of polarity and rigidity. Steady-state fluorescence spectroscopy was used to examine the fluorescence response of the dyes before and after shell growth. Changes in the emission of the encapsulated dyes were related to changes in the polarity and rigidity of the microenvironment where the dyes were located and correlated to the AFM results. Finally, dye-free core-shell particles were used to sequester the dyes from aqueous suspensions. Fluorescence of the sequestered species was compared to the dye-loaded particles to determine the final fate of the fluorophores in the nanoparticles.
    Langmuir 04/2008; 24(5):2064-71. · 4.38 Impact Factor
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    ABSTRACT: The mechanosensitive (MS) ion channel is gated by changes in bilayer deformation. It is functional without the presence of any other proteins and gating of the channel has been successfully achieved using conventional patch clamping techniques where a voltage has been applied together with a pressure over the membrane. Here, we have for the first time analyzed the large conducting (MscL) channel in a supported membrane using only an external electrical field. This was made possible using a newly developed technique utilizing a tethered lipid bilayer membrane (tBLM), which is part of an engineered microelectronic array chip. Single ion channel activity characteristic for MscL was obtained, albeit with lower conductivity. The ion channel was gated using solely a transmembrane potential of 300 mV. Computations demonstrate that this amount of membrane potential induces a membrane tension of 12 dyn/cm, equivalent to that calculated to gate the channel in patch clamp from pressure-induced stretching of the bilayer. These results strengthen the supposition that the MscL ion channel gates in response to stress in the lipid membrane rather than pressure across it. Furthermore, these findings illustrate the possibility of using the MscL as a release valve for engineered membrane devices; one step closer to mimicking the true function of the living cell.
    Biosensors & Bioelectronics 02/2008; 23(6):919-23. · 6.45 Impact Factor
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    ABSTRACT: Fourteen commercial polyols have been characterized by GPC, NMR spectroscopy and elemental analysis. From these, eight corresponding tosylates, six nitrate esters, seven mesylates, thirteen alkynes and fourteen azides have been prepared and all these derivatives have been fully characterized. Five alkyne monomers and eight azide monomers were also prepared. Twelve alkynes and thirteen azides (functionality 2−4) were combined in 1,3-dipolar cycloaddition reactions under neat conditions to prepare triazole-cured polymers, avoiding any heavy metal catalyst. Characterization by NMR spectroscopy, elemental analysis and gel permeation chromatography, indicated triazole polymers 14, 22, 23, 28 and 30 with degrees of polymerization of 17−28 to be the best candidates for future work.
    Journal of Polymer Science Part A Polymer Chemistry 11/2007; 46(1):238 - 256. · 3.54 Impact Factor
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    ChemBioChem 08/2007; 8(11):1246-50. · 3.74 Impact Factor
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    ABSTRACT: A novel approach to two-dimensionally crosslink polydienes at the air/water interface is proposed. The acid-catalyzed condensation of the triethoxysilane pendant groups of triethoxysilane-functionalized polybutadiene chains at the air/water interface successfully led to the formation of an insoluble crosslinked material which could be directly removed from the water surface. The efficiency of the cross-linking reaction was demonstrated through surface pressure measurements such as surface pressure-mean molecular area isotherms recorded at different reaction times and isobar experiments for different subphase pH values. The evolution of the monolayer topography during cross-linking was studied by atomic force microscopy imaging of the Langmuir-Blodgett films.
    Journal of Colloid and Interface Science 08/2007; 311(1):315-21. · 3.55 Impact Factor
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    ABSTRACT: Recently, tethered bilayer lipid membranes (tBLMs) have shown high potential as biomimetic systems due to their high stability and electrical properties, and have been used in applications ranging from membrane protein incorporation to biosensors. However, the kinetics of their formation remains largely uninvestigated. By using quartz crystal microbalance with impedance analysis (QCM-Z), we were able to monitor both the kinetics and viscoelastic properties of tether adsorption and vesicle fusion. Formation of the tether monolayer was shown to follow pseudo-first-order Langmuir kinetics with association and dissociation rate constants of 21.7 M-1 s(-1) and 7.43 x 10-6 s(-1), respectively. Moreover, the QCM-Z results indicate a rigid layer at the height of deposition, which then undergoes swelling as indicated by AFM. The deposition of vesicles to the tether layer also followed pseudo-first-order Langmuir kinetics with observed rate constants of 5.58 x 10(-2) and 2.41 x 10-2 s(-1) in water and buffer, respectively. Differential analysis of the QCM-Z data indicated deposition to be the fast kinetic step, with the rate-limiting steps being water release and fusion. Atomic force microscopy pictures taken complement the QCM-Z data, showing the major stages of tether adsorption and vesicle fusion, while providing a road map to successful tBLM formation.
    Langmuir 07/2007; 23(13):7344-55. · 4.38 Impact Factor
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    ABSTRACT: Membrane-bound ion channels are promising biological receptors since they allow for the stochastic detection of analytes at high sensitivity. For stochastic sensing, it is necessary to measure the ion currents associated with single ion channel opening and closing events. However, this calls for stability, high reproducibility, and long lifetimes. A critical issue to overcome is the low stability of the ion channel environment, that is, the bilayer membrane. A promising technique to surmount this is to connect the lower part of the membrane to a surface forming a tethered bilayer membrane. By reconstituting the synthetic ion channel, gramicidin A, into a tethered bilayer as part of a microchip design, we have been able to record the activity of single ion channels. The observed activity was compared with that obtained by a conventional electrophysiology method, tip dipping, to confirm its authenticity. These findings allow for the construction of stable biosensors based on ion channels and provide a novel technique for the characterization of ion channel activity.
    Langmuir 04/2007; 23(6):2924-7. · 4.38 Impact Factor
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    ABSTRACT: The interfacial properties of amphiphilic linear diblock copolymers based on poly(ethylene oxide) and poly(epsilon-caprolactone) (PEO-b-PCL) were studied at the air-water (A/W) interface by surface pressure measurements (isotherms and hysteresis experiments). The resulting Langmuir monolayers were transferred onto mica substrates and the Langmuir-Blodgett (LB) film morphologies were investigated by atomic force microscopy (AFM). All block copolymers had the same PEO segment (Mn = 2670 g/mol) and different PCL chain lengths (Mn = 1270; 2110; 3110 and 4010 g/mol). Isothermal characterization of the block copolymer samples indicated the presence of three distinct phase transitions around 6.5, 10.5, and 13.5 mN/m. The phase transitions at 6.5 and 13.5 mN/m correspond to the dissolution of the PEO segments in the water subphase and crystallization of the PCL blocks above the interface similarly as for the corresponding homopolymers, respectively. The phase transition at 10.5 mN/m was not observed for the homopolymers alone or for their blends and arises from a brush formation of the PEO segments anchored underneath the adsorbed hydrophobic PCL segments. AFM analysis confirmed the presence of PCL crystals in the LB films with unusual hairlike/needlelike architectures significantly different from those obtained for PCL homopolymers.
    Langmuir 03/2007; 23(5):2423-9. · 4.38 Impact Factor