[Show abstract][Hide abstract] ABSTRACT: The structure of the adsorbed layer of mixtures of a low molecular mass nonionic surfactant (alkyl heptaglycolether, C12EO7) with low molecular mass anionic (sodium dodecylsulfate, SDS, and sodium dodecylbenzenesulfonate, SDBS) and high molecular mass nonionic surfactants (EO37PO56EO37) was studied by atomic force microscopy in the combination with adsorption isotherms and adsorption enthalpies by isothermal titration calorimetry. The single surfactants C12EO7 and SDS show domains of parallel stripes on the graphite lattice in the atomic force microscopy (AFM) images. These parallel stripes can be related to hemicylindrical aggregates of a width of about 5 nm repeated in a periodic order which was proposed by different authors before. These structures are backed up by the results of the adsorption isotherms and the calorimetric measurements of the adsorption enthalpies. For SDBS and the high molecular mass nonionic surfactant, no such structures are observed in the AFM images despite an adsorption of surfactants on the surface. Mixtures of C12EO7 and the anionic surfactants give similar parallel stripes as the nonionic surfactant alone even in combination with SDBS, which indicates ideal mixing behavior on the surface. The concentration ratio of the surfactants on the surface differs from that in solution. For a mixed system of C12EO7 and the high molecular mass nonionic surfactant, a demixing of the surfactants on the surface can be assumed.
No preview · Article · Sep 2015 · Colloid and Polymer Science
[Show abstract][Hide abstract] ABSTRACT: Strong evidence exists for a central role of amyloid β-protein (Aβ) oligomers in the pathogenesis of Alzheimer's disease. We have developed a fast, reliable and robust in vitro assay, termed QIAD, to quantify the effect of any compound on the Aβ aggregate size distribution. Applying QIAD, we studied the effect of homotaurine, scyllo-inositol, EGCG, the benzofuran derivative KMS88009, ZAβ3W, the D-enantiomeric peptide D3 and its tandem version D3D3 on Aβ aggregation. The predictive power of the assay for in vivo efficacy is demonstrated by comparing the oligomer elimination efficiency of D3 and D3D3 with their treatment effects in animal models of Alzheimer´s disease.
[Show abstract][Hide abstract] ABSTRACT: Human islet amyloid polypeptide (IAPP) is the major component of pancreatic amyloid deposits in type 2 diabetes. The structural conversion of IAPP from a monomeric state into amyloid assemblies is the subject of intense research. Recombinant production of IAPP is, however, difficult due to its extreme aggregation propensity. Here we describe a novel strategy for expression of IAPP in Escherichia coli, based on an engineered protein tag, which sequesters IAPP monomers and prevents IAPP aggregation. The IAPP-binding protein HI18 was selected by phage display from a β-wrapin library. Fusion of HI18 to IAPP enabled the soluble expression of the construct. IAPP was cleaved from the fusion construct and purified to homogeneity with a yield of 3mg of isotopically labelled peptide per liter of culture. In the monomeric state, IAPP was largely disordered as evidenced by far-UV CD and liquid-state NMR spectroscopy but competent to form amyloid fibrils according to atomic force microscopy. These results demonstrate the ability of the engineered β-wrapin HI18 for shielding the hydrophobic sequence of IAPP during expression and purification. Fusion of aggregation-inhibiting β-wrapins is a suitable approach for the recombinant production of aggregation-prone proteins.
No preview · Article · Jun 2014 · Journal of Biotechnology
[Show abstract][Hide abstract] ABSTRACT: There is strong evidence that the amyloid-beta peptide (Aβ) plays a central role in the pathogenesis of Alzheimer's disease (AD). In this context, a detailed quantitative description of the interactions with different Aβ species is essential for characterization of physiological and artificial ligands. However, the high aggregation propensity of Aβ in concert with its susceptibility to structural changes due to even slight changes in solution conditions has impeded surface plasmon resonance (SPR) studies with homogeneous Aβ conformer species. Here, we have adapted the experimental procedures to state-of-the-art techniques and established novel approaches to reliably overcome the aforementioned challenges. We show that the application of density gradient centrifugation (DGC) for sample purification and the use of a single chain variable fragment (scFv) of a monoclonal antibody directed against the amino-terminus of Aβ allows reliable SPR measurements and quality control of the immobilized Aβ aggregate species at any step throughout the experiment.
[Show abstract][Hide abstract] ABSTRACT: Abstract Compatible solutes are small, uncharged, zwitter ionic, osmotically active molecules produced and accumulated by microorganisms inside their cell to counteract different kinds of environmental stress. They enhance protein stability without interfering with the metabolic pathways even at molar concentrations. In this paper, we report the stabilizing effects of compatible solutes, ectoine, betaine and taurine on membrane protein Bacteriorhodopsin at different concentrations. Using Atomic Force Microscopy-Single Molecule Force Spectroscopy the impact of the osmolytes was quantified by measuring the forces required to pull the protein out of the membrane and the change in the persistence lengths of the unfolded polypeptide chain. Increasing unfolding forces were observed indicating the strengthening of intra-molecular interactions, which are vital for protein stability. The decrease in persistence lengths was recorded showing increasing tendencies of the polypeptide strand to coil up. Interestingly, it was revealed that these molecules have different stabilizing effects on protein unfolding at different concentrations. The results show that the unfolding of single proteins provides insight to the structure-dynamic relationship between the protein and compatible solute molecules at sub-nanometer scale. This also helps to understand the molecular mechanism involved in protein stabilization by organic osmolytes.
No preview · Article · Sep 2013 · Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The adaptive immunity of bacteria against foreign nucleic acids, mediated by CRISPR (clustered regularly interspaced short
palindromic repeats), relies on the specific incorporation of short pieces of the invading foreign DNA into a special genomic
locus, termed CRISPR array. The stored sequences (spacers) are subsequently used in the form of small RNAs (crRNAs) to interfere
with the target nucleic acid. We explored the DNA-binding mechanism of the immunization protein Csn2 from the human pathogen
Streptococcus agalactiae using different biochemical techniques, atomic force microscopic imaging and molecular dynamics simulations. The results
demonstrate that the ring-shaped Csn2 tetramer binds DNA ends through its central hole and slides inward, likely by a screw
motion along the helical path of the enclosed DNA. The presented data indicate an accessory function of Csn2 during integration
of exogenous DNA by end-joining.
Full-text · Article · Jul 2013 · Nucleic Acids Research
[Show abstract][Hide abstract] ABSTRACT: Mechanical single molecule techniques offer exciting possibilities for investigating protein folding and stability in native environments at sub-nanometer resolutions. Compatible solutes show osmotic activity which even at molar concentrations do not interfere with cell metabolism. They are known to protect proteins against external stress like temperature, high salt concentrations and dehydrating conditions. We studied the impact of the compatible solute ectoine (1M) on membrane proteins by analyzing the mechanical properties of Bacteriorhodopsin (BR) in its presence and absence by single molecule force spectroscopy. The unfolding experiments on BR revealed that ectoine decreases the persistence length of its polypeptide chain thereby increasing its tendency to coil up. In addition, we found higher unfolding forces indicating strengthening of those intra molecular interactions which are crucial for stability. This shows that force spectroscopy is well suited to study the effect of compatible solutes to stabilize membrane proteins against unfolding. In addition, it may lead to a better understanding of their detailed mechanism of action.
No preview · Article · Apr 2012 · Protein and Peptide Letters
[Show abstract][Hide abstract] ABSTRACT: Interaction forces of membrane protein subunits are of importance in their structure, assembly, membrane insertion, and function. In biological membranes, and in the photosynthetic apparatus as a paradigm, membrane proteins fulfill their function by ensemble actions integrating a tight assembly of several proteins. In the bacterial photosynthetic apparatus light-harvesting complexes 2 (LH2) transfer light energy to neighboring tightly associated core complexes, constituted of light-harvesting complexes 1 (LH1) and reaction centers (RC). While the architecture of the photosynthetic unit has been described, the forces and energies assuring the structural and functional integrity of LH2, the assembly of LH2 complexes, and how LH2 interact with the other proteins in the supramolecular architecture are still unknown. Here we investigate the molecular forces of the bacterial LH2 within the native photosynthetic membrane using atomic force microscopy single-molecule imaging and force measurement in combination. The binding between LH2 subunits is fairly weak, of the order of k(B)T, indicating the importance of LH2 ring architecture. In contrast LH2 subunits are solid with a free energy difference of 90 k(B)T between folded and unfolded states. Subunit α-helices unfold either in one-step, α- and β-polypeptides unfold together, or sequentially. The unfolding force of transmembrane helices is approximately 150 pN. In the two-step unfolding process, the β-polypeptide is stabilized by the molecular environment in the membrane. Hence, intermolecular forces influence the structural and functional integrity of LH2.
Full-text · Article · Jun 2011 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Scanning probe microscopy-based techniques can address and manipulate individual molecules. This makes it possible to use them for building nanostructures by assembling single molecules. Recently the formation of surface structures by positioning single molecules with the Atomic Force Microscope (AFM) was demonstrated on an irreversible delivery process. This inherits the drawback, that the transfer has to occur between differently functionalized surfaces and allows no proofreading of the built structures. Here we demonstrate a procedure for directed deposition of single DNA molecules, which intrinsically allows a reversible positioning. This method uses specific interactions between complementary DNA oligonucleotides for symmetric coupling of the transport molecules to the support and AFM tip, respectively. Thus, it allows for a simple "drag-and-drop" procedure, which relies on the statistical breakage of the molecular interaction under a force load. In addition, the delivery of the transport molecules was observed in real-time by single-molecule fluorescence microscopy.
Full-text · Article · Aug 2010 · Journal of Nanoscience and Nanotechnology
[Show abstract][Hide abstract] ABSTRACT: Force spectroscopy allows testing the free energy landscapes of molecular interactions. Usually, the dependency of the most probable rupture force on the force rate or the shape of the rupture force histogram is fitted with different models that contain approximations and basic assumptions. We present a simple and model free approach to extract the force-dependent dissociation rates directly from the force curve data. Simulations show that the dissociation rates at any force are given directly by the ratio of the number of detected rupture events to the time this force was acting on the bond. To calculate these total times of acting forces, all force curve data points of all curves measured are taken into account, which significantly increases the amount of information which is considered for data analysis compared to other methods. Moreover, by providing force-dependent dissociation rates this method allows direct testing and validating of any energy landscape model.
Full-text · Article · Nov 2009 · Biophysical Journal
[Show abstract][Hide abstract] ABSTRACT: Microbial rhodopsins are a family of seven-helical transmembrane proteins containing retinal as chromophore. Sensory rhodopsin II (SRII) triggers two very different responses upon light excitation, depending on the presence or the absence of its cognate transducer HtrII: Whereas light activation of the NpSRII/NpHtrII complex activates a signalling cascade that initiates the photophobic response, NpSRII alone acts as a proton pump. Using single-molecule force spectroscopy, we analysed the stability of NpSRII and its complex with the transducer in the dark and under illumination. By improving force spectroscopic data analysis, we were able to reveal the localisation of occurring forces within the protein chain with a resolution of about six amino acids. Distinct regions in helices G and F were affected differently, depending on the experimental conditions. The results are generally in line with previous data on the molecular stability of NpSRII. Interestingly, new interaction sites were identified upon light activation, whose functional importance is discussed in detail.
Full-text · Article · Aug 2009 · Journal of Molecular Biology
[Show abstract][Hide abstract] ABSTRACT: To measure forces acting on a chainlike molecule in liquid, we introduce a dynamic approach based on the frequency-modulation technique with constant-excitation. In difference to the classical approach where the force is recorded as a conventional force versus distance curve in a static measurement, we are able to detect simultaneously the conservative force as well as the energy dissipation during the elongation of a chainlike molecule. We apply this technique to dextran monomers and demonstrate the agreement of the experimental force curves with a ``single-click'' model.
[Show abstract][Hide abstract] ABSTRACT: Covalent chemisorption of biomolecules to surfaces with high density and low unspecific background is prerequisite for most optical and mechanical single molecule experiments and accordingly, many recipes have been developed. However, new establishment of the surface functionalization process in the lab usually is still difficult and time consuming due to the complex procedures containing many pitfalls. Therefore, based on the known recipes, we developed and optimized a simple straight forward protocol. We demonstrated it resulting in a high density of the coupled biomolecules, homogeneous surfaces and a low unspecific background when binding nucleic acids, peptides and proteins. The protocol was optimized for borosilicate cover glasses and silicon nitride atomic force microscope cantilevers commonly used in single molecule experiments and takes advantage of commonly used chemicals. It consists of only four steps, silanol group generation, amination, grafting of poly(ethylene glycol) to the surface and biomolecule coupling. All individual steps were optimized comparing different variations partially described in the literature. Finally, a detailed description is provided which allows avoiding most sources of contamination, often being a main hurdle on the way to single molecule experiments.
Full-text · Article · Mar 2009 · Colloids and surfaces B: Biointerfaces