The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Frank Trixler - One of the best experts on this subject based on the ideXlab platform.
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locally triggered Hydrophobic Collapse induces global interface self cleaning in van der waals heterostructures at room temperature
2D Materials, 2020Co-Authors: Stefan Wakolbinger, Frank Trixler, Fabian R Geisenhof, Felix Winterer, Samuel Palmer, Juri G Crimmann, Kenji Watanabe, Takashi Taniguchi, Thomas R WeitzAbstract:Mutual relative orientation and well defined, uncontaminated interfaces are the key to obtain van-der-Waals heterostacks with defined properties. Even though the van-der-Waals forces are known to promote the "self-cleaning" of interfaces, residue from the stamping process, which is often found to be trapped between the heterostructure constituents, can interrupt the interlayer interaction and therefore the coupling. Established interfacial cleaning methods usually involve high-temperature steps, which are in turn known to lead to uncontrolled rotations of layers within fragile heterostructures. Here, we present an alternative method feasible at room temperature. Using the tip of an atomic force microscope (AFM), we locally control the activation of interlayer attractive forces, resulting in the global removal of contaminants from the interface (i.e. the contaminants are also removed in regions several µm away from the line touched by the AFM tip). By testing combinations of various Hydrophobic van-der-Waals materials, mild temperature treatments, and by observing the temporal evolution of the contaminant removal process, we identify that the AFM tip triggers a dewetting-induced Hydrophobic Collapse and the van-der-Waals interaction is driving the cleaning process. We anticipate that this process is at the heart of the known "self-cleaning" mechanism. Our technique can be utilized to controllably establish interlayer close coupling between a stack of van-der-Waals layers, and additionally allows to pattern and manipulate heterostructures locally for example to confine material into nanoscopic pockets between two van-der-Waals materials.
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revealing the physico chemical basis of organic solid solid wetting deposition casimir like forces Hydrophobic Collapse and the role of the zeta potential
arXiv: Chemical Physics, 2019Co-Authors: Alexander Eberle, Thomas Markert, Frank TrixlerAbstract:Supramolecular self-assembly at the solid-solid interface enables the deposition and monolayer formation of insoluble organic semiconductors under ambient conditions. The underlying process, termed as the Organic Solid-Solid Wetting Deposition (OSWD), generates two-dimensional adsorbates directly from dispersed three-dimensional organic crystals. This straightforward process has important implications in various fields of research and technology, such as in the domains of low-dimensional crystal engineering, the chemical doping and band-gap engineering of graphene, and in the area of field-effect transistor fabrication. However, till date, lack of an in-depth understanding of the physico-chemical basis of the OSWD prevented the identification of important parameters, essential to achieve a better control of the growth of monolayers and supramolecular assemblies with defined structures, sizes, and coverage areas. Here we propose a detailed model for the OSWD, derived from experimental and theoretical results that have been acquired by using the organic semiconductor quinacridone as an example system. The model reveals the vital role of the zeta potential and includes Casimir-like fluctuation-induced forces and the effect of dewetting in Hydrophobic nano-confinements. Based on our results, the OSWD of insoluble organic molecules can hence be applied to environmental friendly and low-cost dispersing agents, such as water. In addition, the model substantially enhances the ability to control the OSWD in terms of adsorbate structure and substrate coverage.
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revealing the physicochemical basis of organic solid solid wetting deposition casimir like forces Hydrophobic Collapse and the role of the zeta potential
Journal of the American Chemical Society, 2018Co-Authors: Alexander Eberle, Thomas Markert, Frank TrixlerAbstract:Supramolecular self-assembly at the solid–solid interface enables the deposition and monolayer formation of insoluble organic semiconductors under ambient conditions. The underlying process, termed as the organic solid–solid wetting deposition (OSWD), generates two-dimensional adsorbates directly from dispersed three-dimensional organic crystals. This straightforward process has important implications in various fields of research and technology, such as in the domains of low-dimensional crystal engineering, the chemical doping and band gap engineering of graphene, and in the area of field-effect transistor fabrication. However, to date, lack of an in-depth understanding of the physicochemical basis of the OSWD prevented the identification of important parameters, essential to achieve a better control of the growth of monolayers and supramolecular assemblies with defined structures, sizes, and coverage areas. Here we propose a detailed model for the OSWD, derived from experimental and theoretical results ...
Gilbert C Walker - One of the best experts on this subject based on the ideXlab platform.
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How osmolytes influence Hydrophobic polymer conformations: A unified view from experiment and theory.
Proceedings of the National Academy of Sciences of the United States of America, 2015Co-Authors: Jagannath Mondal, Gilbert C Walker, Duncan Halverson, Guillaume Stirnemann, Bruce J BerneAbstract:It is currently the consensus belief that protective osmolytes such as trimethylamine N-oxide (TMAO) favor protein folding by being excluded from the vicinity of a protein, whereas denaturing osmolytes such as urea lead to protein unfolding by strongly binding to the surface. Despite there being consensus on how TMAO and urea affect proteins as a whole, very little is known as to their effects on the individual mechanisms responsible for protein structure formation, especially Hydrophobic association. In the present study, we use single-molecule atomic force microscopy and molecular dynamics simulations to investigate the effects of TMAO and urea on the unfolding of the Hydrophobic homopolymer polystyrene. Incorporated with interfacial energy measurements, our results show that TMAO and urea act on polystyrene as a protectant and a denaturant, respectively, while complying with Tanford-Wyman preferential binding theory. We provide a molecular explanation suggesting that TMAO molecules have a greater thermodynamic binding affinity with the Collapsed conformation of polystyrene than with the extended conformation, while the reverse is true for urea molecules. Results presented here from both experiment and simulation are in line with earlier predictions on a model Lennard-Jones polymer while also demonstrating the distinction in the mechanism of osmolyte action between protein and Hydrophobic polymer. This marks, to our knowledge, the first experimental observation of TMAO-induced Hydrophobic Collapse in a ternary aqueous system.
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interfacial free energy governs single polystyrene chain Collapse in water and aqueous solutions
Journal of the American Chemical Society, 2010Co-Authors: Isaac T S Li, Gilbert C WalkerAbstract:The Hydrophobic interaction is significantly responsible for driving protein folding and self-assembly. To understand it, the thermodynamics, the role of water structure, the dewetting process surrounding hydrophobes, and related aspects have undergone extensive investigations. Here, we examine the hypothesis that polymer−solvent interfacial free energy is adequate to describe the energetics of the Collapse of a Hydrophobic homopolymer chain at fixed temperature, which serves as a much simplified model for studying the Hydrophobic Collapse of a protein. This implies that changes in polymer−solvent interfacial free energy should be directly proportional to the force to extend a Collapsed polymer into a bad solvent. To test this hypothesis, we undertook single-molecule force spectroscopy on a Collapsed, single, polystyrene chain in water−ethanol and water−salt mixtures where we measured the monomer solvation free energy from an ensemble average conformations. Different proportions within the binary mixture ...
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solvent effect on the unfolding force of a single Hydrophobic polymer
Biophysical Journal, 2010Co-Authors: Matthew F Paige, Gilbert C WalkerAbstract:The Hydrophobic Collapse of a homopolymer is a much simplified model for studying the Hydrophobic Collapse of a protein. It is widely believed and theorized that the driving force for the Hydrophobic Collapse is the interfacial free energy between the polymer and the solvent. Therefore, changes in interfacial free energy should be directly proportional to the force to unfold the polymer in bad solvents. To test this hypothesis, we used single molecule force spectroscopy to unfold a single polystyrene chain in water-ethanol mixtures. Different percentage of binary mixture is used to create solvents with different interfacial energy with polystyrene. However, we do not see a linear correlation between the interfacial tensions with the unfolding forces. This is an indication that macroscopic properties such as the interfacial free energy cannot be directly applied to study certain systems in microscopic scale. In this study we also hypothesized a mechanism for the cause of this inconsistency.
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experimental evidence of polymer Hydrophobic Collapse due to water density fluctuation
Biophysical Journal, 2009Co-Authors: Nikhil Gunari, Gilbert C WalkerAbstract:Hydrophobic Collapse of protein chain is a major driving force for its folding in water. However, the detailed mechanism of Hydrophobic Collapse is not clear. One compelling theory explains the Hydrophobic Collapse due to the density fluctuation of water creating local voids in the vicinity of a Hydrophobic polymer chain that drives its Collapse. This density fluctuation happens rapidly in the time scale of picoseconds and is therefore difficult to detect directly. However, a result of the fluctuation is the much longer lasting Collapsed structure of Hydrophobic polymer chains, which can be detected by single molecule force spectroscopy (SMFS). We performed experiments to investigate the effect of water density fluctuation in the Collapse of single polystyrene (PS) chains using the atomic force microscope (AFM). In particular, we applied a constant tension to single PS chain and detected end-to-end distance fluctuation, which directly probes the Collapsed state of the PS chain and provides us insight to the Hydrophobic Collapse of the molecule.
Alexander Eberle - One of the best experts on this subject based on the ideXlab platform.
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revealing the physico chemical basis of organic solid solid wetting deposition casimir like forces Hydrophobic Collapse and the role of the zeta potential
arXiv: Chemical Physics, 2019Co-Authors: Alexander Eberle, Thomas Markert, Frank TrixlerAbstract:Supramolecular self-assembly at the solid-solid interface enables the deposition and monolayer formation of insoluble organic semiconductors under ambient conditions. The underlying process, termed as the Organic Solid-Solid Wetting Deposition (OSWD), generates two-dimensional adsorbates directly from dispersed three-dimensional organic crystals. This straightforward process has important implications in various fields of research and technology, such as in the domains of low-dimensional crystal engineering, the chemical doping and band-gap engineering of graphene, and in the area of field-effect transistor fabrication. However, till date, lack of an in-depth understanding of the physico-chemical basis of the OSWD prevented the identification of important parameters, essential to achieve a better control of the growth of monolayers and supramolecular assemblies with defined structures, sizes, and coverage areas. Here we propose a detailed model for the OSWD, derived from experimental and theoretical results that have been acquired by using the organic semiconductor quinacridone as an example system. The model reveals the vital role of the zeta potential and includes Casimir-like fluctuation-induced forces and the effect of dewetting in Hydrophobic nano-confinements. Based on our results, the OSWD of insoluble organic molecules can hence be applied to environmental friendly and low-cost dispersing agents, such as water. In addition, the model substantially enhances the ability to control the OSWD in terms of adsorbate structure and substrate coverage.
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revealing the physicochemical basis of organic solid solid wetting deposition casimir like forces Hydrophobic Collapse and the role of the zeta potential
Journal of the American Chemical Society, 2018Co-Authors: Alexander Eberle, Thomas Markert, Frank TrixlerAbstract:Supramolecular self-assembly at the solid–solid interface enables the deposition and monolayer formation of insoluble organic semiconductors under ambient conditions. The underlying process, termed as the organic solid–solid wetting deposition (OSWD), generates two-dimensional adsorbates directly from dispersed three-dimensional organic crystals. This straightforward process has important implications in various fields of research and technology, such as in the domains of low-dimensional crystal engineering, the chemical doping and band gap engineering of graphene, and in the area of field-effect transistor fabrication. However, to date, lack of an in-depth understanding of the physicochemical basis of the OSWD prevented the identification of important parameters, essential to achieve a better control of the growth of monolayers and supramolecular assemblies with defined structures, sizes, and coverage areas. Here we propose a detailed model for the OSWD, derived from experimental and theoretical results ...
Nico F. A. Van Der Vegt - One of the best experts on this subject based on the ideXlab platform.
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Cosolvent Effects on Polymer Hydration Drive Hydrophobic Collapse
The journal of physical chemistry. B, 2018Co-Authors: Divya Nayar, Nico F. A. Van Der VegtAbstract:Water-mediated Hydrophobic interactions play an important role in self-assembly processes, aqueous polymer solubility, and protein folding, to name a few. Cosolvents affect these interactions; however, the implications for Hydrophobic polymer Collapse and protein folding equilibria are not well-understood. This study examines cosolvent effects on the Hydrophobic Collapse equilibrium of a generic 32-mer Hydrophobic polymer in urea, trimethylamine-N-oxide (TMAO), and acetone aqueous solutions using molecular dynamics simulations. Our results unveil a remarkable cosolvent-concentration-dependent behavior. Urea, TMAO, and acetone all shift the equilibrium toward Collapsed structures below 2 M cosolvent concentration and, in turn, to unfolded structures at higher cosolvent concentrations, irrespective of the differences in cosolvent chemistry and the nature of cosolvent–water interactions. We find that weakly attractive polymer–water van der Waals interactions oppose polymer Collapse in pure water, corroborati...
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Molecular origin of urea driven Hydrophobic polymer Collapse and unfolding depending on side chain chemistry.
Physical chemistry chemical physics : PCCP, 2017Co-Authors: Divya Nayar, Angelina Folberth, Nico F. A. Van Der VegtAbstract:Osmolytes affect Hydrophobic Collapse and protein folding equilibria. The underlying mechanisms are, however, not well understood. We report large-scale conformational sampling of two Hydrophobic polymers with secondary and tertiary amide side chains using extensive molecular dynamics simulations. The calculated free energy of unfolding increases with urea for the secondary amide, yet decreases for the tertiary amide, in agreement with experiment. The underlying mechanism is rooted in opposing entropic driving forces: while urea screens the Hydrophobic macromolecular interface and drives unfolding of the tertiary amide, urea's concomitant loss in configurational entropy drives Collapse of the secondary amide. Only at sufficiently high urea concentrations bivalent urea hydrogen bonding interactions with the secondary amide lead to further stabilisation of its Collapsed state. The observations provide a new angle on the interplay between side chain chemistry, urea hydrogen bonding, and the role of urea in attenuating or strengthening the Hydrophobic effect.
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on the urea induced Hydrophobic Collapse of a water soluble polymer
Physical Chemistry Chemical Physics, 2015Co-Authors: Francisco Rodriguezropero, Nico F. A. Van Der VegtAbstract:Stabilization of macromolecular folded states in solution by protective osmolytes has been traditionally explained on the basis of preferential osmolyte depletion from the macromolecule's first solvation shell. However recent theoretical and experimental studies suggest that protective osmolytes may directly interact with the macromolecule. An example is the stabilization of the Collapsed globular state of poly(N-isopropylacrylamide) (PNiPAM) by urea in aqueous solution. Based on Molecular Dynamics simulations we have characterized the mechanism through which urea stabilizes the Collapsed state of PNiPAM in water. Analysis and comparison of the different components of the excess chemical potentials of folded and unfolded PNiPAM chains in aqueous urea solutions indicates that enthalpic interactions play no role in stabilizing the Collapsed state. We instead find that with increasing urea, solvation of the unfolded state is entropically penalized over solvation of the folded state, thereby shifting the folding equilibrium in favour of the folded state. The unfavourable entropy contribution to the excess chemical potential of unfolded PNiPAM chains results from two urea effects: (1) an increasing cost of cavity formation with increasing urea, (2) larger fluctuations in the energy component corresponding to PNiPAM–(co)solvent attractive interactions. These energy fluctuations are particularly relevant at low urea concentrations (<3 M) and result from attractive polymer–urea van der Waals interactions that drive the formation of “urea clouds” but bias the spatial distribution of urea and water molecules with a corresponding reduction of the entropy. We further find indications that urea increases the entropy of the globular state.
Victor Munoz - One of the best experts on this subject based on the ideXlab platform.
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folding and aggregation kinetics of a β hairpin
Biochemistry, 2006Co-Authors: Victor Munoz, James Hofrichter, Rodolfo Ghirlando, Francisco J Blanco, Gouri S Jas, William A. EatonAbstract:We have investigated the solution structure, equilibrium properties, and folding kinetics of a 17-residue beta-hairpin-forming peptide derived from the protein ubiquitin. NMR experiments show that at 4 degrees C the peptide has a highly populated beta-hairpin conformation. At protein concentrations higher than 0.35 mM, the peptide aggregates. Sedimentation equilibrium measurements show that the aggregate is a trimer, while NMR indicates that the beta-hairpin conformation is maintained in the trimer. The relaxation kinetics in nanosecond laser temperature-jump experiments reveal a concentration-independent microsecond phase, corresponding to beta-hairpin unfolding-refolding, and a concentration-dependent millisecond phase due to oligomerization. Kinetic modeling of the relaxation rates and amplitudes yields the folding and unfolding rates for the monomeric beta-hairpin, as well as assembly and disassembly rates for trimer formation consistent with the equilibrium constant determined by sedimentation equilibrium. When the net charge on the peptides and ionic strength were taken into account, the rate of trimer assembly approaches the Debye-Smoluchowski diffusion limit. At 300 K, the rate of formation of the monomeric hairpin is (17 micros)(-1), compared to rates of (0.8 micros)(-1) to (52 micros)(-1) found for other peptides. After using Kramers theory to correct for the temperature dependence of the pre-exponential factor, the activation energy for hairpin formation is near zero, indicating that the barrier to folding is purely entropic. Comparisons with previously measured rates for a series of hairpins are made to distinguish between zipper and Hydrophobic Collapse mechanisms. Overall, the experimental data are most consistent with the zipper mechanism in which structure formation is initiated at the turn, the mechanism predicted by the Ising-like statistical mechanical model that was developed to explain the equilibrium and kinetic data for the beta-hairpin from protein GB1. In contrast, the majority of simulation studies favor a Hydrophobic Collapse mechanism. However, with few exceptions, there is little or no quantitative comparison of the simulation results with experimental data.
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how fast is protein Hydrophobic Collapse
Proceedings of the National Academy of Sciences of the United States of America, 2003Co-Authors: Mourad Sadqi, Lisa J Lapidus, Victor MunozAbstract:One of the most recurring questions in protein folding refers to the interplay between formation of secondary structure and Hydrophobic Collapse. In contrast with secondary structure, it is hard to isolate Hydrophobic Collapse from other folding events. We have directly measured the dynamics of protein Hydrophobic Collapse in the absence of competing processes. Collapse was triggered with laser-induced temperature jumps in the acid-denatured form of a simple protein and monitored by fluorescence resonance energy transfer between probes placed at the protein ends. The relaxation time for Hydrophobic Collapse is only ≈60 ns at 305 K, even faster than secondary structure formation. At higher temperatures, as the protein becomes increasingly compact by a stronger Hydrophobic force, we observe a slowdown of the dynamics of Collapse. This dynamic Hydrophobic effect is a high-temperature analogue of the dynamic glass transition predicted by theory. Our results indicate that in physiological conditions many proteins will initiate folding by collapsing to an unstructured globule. Local motions will presumably drive the following search for native structure in the Collapsed globule.
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Submillisecond kinetics of protein folding.
Current opinion in structural biology, 1997Co-Authors: William A. Eaton, Victor Munoz, Peggy A. Thompson, Chi-kin Chan, James HofrichterAbstract:Abstract New experimental methods permit observation of protein folding and unfolding on the previously inaccessible nanosecond—microsecond timescale. These studies are beginning to establish times for the elementary motions in protein folding — secondary structure and loop formation, local Hydrophobic Collapse, and global Collapse to the compact denatured state. They permit an estimate of about one microsecond for the shortest time in which a protein can possibly fold.
