25 research outputs found
Wettability of pristine and alkyl-functionalized graphane
Graphane is a hydrogenated form of graphene with high bandgap and planar structure insensitive to a broad range of chemical substitutions. We describe an atomistic simulation approach to predict wetting properties of this new material. We determine the contact angle to be 73°. The lower hydrophobicity compared to graphene is explained by the increased planar density of carbon atoms while we demonstrate that the presence of partial charges on carbonand hydrogen atoms plays only a minor role. We further examine the effects of graphane functionalization by alkyl groups of increasing chain lengths. The gradual increase in contact angle with chain length offers a precise control of surface wettability. A saturated contact angle of 114° is reached in butylated form. We find the saturation of contact angle with respect to the length of the functional groups to coincide with the loss of water\u27s ability to penetrate the n-alkyl molecular brush and interact with carbon atoms of the underlying lattice. Since no experimental data have yet become available, our modeling results provide the first estimate of the wettability of graphane. The results also show how its alkyl functionalization provides the basis for a variety of chemical modifications to tune hydrophilicity while preserving the planar geometry of the substrate
Thermodynamics of folding and association of lattice-model proteins
Closely related to the “protein folding problem” is the issue of protein misfolding and aggregation. Proteinaggregation has been associated with the pathologies of nearly 20 human diseases and presents serious difficulties during the manufacture of pharmaceutical proteins. Computational studies of multiprotein systems have recently emerged as a powerful complement to experimental efforts aimed at understanding the mechanisms of proteinaggregation. We describe the thermodynamics of systems containing two lattice-model 64-mers. A parallel tempering algorithm abates problems associated with glassy systems and the weighted histogram analysis method improves statistical quality. The presence of a second chain has a substantial effect on single-chain conformational preferences. The melting temperature is substantially reduced, and the increase in the population of unfolded states is correlated with an increase in interactions between chains. The transition from two native chains to a non-native aggregate is entropically favorable. Non-native aggregates receive ∼25% of their stabilizing energy from intraprotein contacts not found in the lowest-energy structure. Contact maps show that for non-native dimers, nearly 50% of the most probable interprotein contacts involve pairs of residues that form native contacts, suggesting that a domain-swapping mechanism is involved in self-association
Curvature dependence of the effect of ionic functionalization on the attraction among nanoparticles in dispersion
Solubilization of nanoparticles facilitates nanomaterial processing and enables new applications. An effective method to improve dispersibility in water is provided by ionic functionalization.We explore how the necessary extent of functionalization depends on the particle geometry. Using molecular dynamics/umbrella sampling simulations, we determine the effect of the solute curvature on solventaveraged interactions among ionizing graphitic nanoparticles in aqueous dispersion. We tune the hydrophilicity of molecular-brush coated fullerenes, carbon nanotubes, and graphane platelets by gradually replacing a fraction of the methyl end groups of the alkyl coating by the ionizing –COOK or –NH3Cl groups. To assess the change in nanoparticles’ dispersibility in water, we determine the potential-of-mean-force profiles at varied degrees of ionization. When the coating comprises only propyl groups, the attraction between the hydrophobic particles intensifies from spherical to cylindrical to planar geometry. This is explained by the increasing fraction of surface groups that can be brought into contact and the reduced access to water molecules, both following the above sequence. When ionic groups are added, however, the dispersibility increases in the opposite order, with the biggest effect in the planar geometry and the smallest in the spherical geometry. These results highlight the important role of geometry in nanoparticle solubilization by ionic functionalities, with about twice higher threshold surface charge necessary to stabilize a dispersion of spherical than planar particles. At 25%–50% ionization, the potential of mean force reaches a plateau because of the counterion condensation and saturated brush hydration. Moreover, the increase in the fraction of ionic groups can weaken the repulsion through counterion correlations between adjacent nanoparticles. High degrees of ionization and concomitant ionic screening gradually reduce the differences among surface interactions in distinct geometries until an essentially curvature-independent dispersion environment is created. Insights into tuning nanoparticle interactions can guide the synthesis of a broad class of nonpolar nanoparticles, where solubility is achieved by ionic functionalization
Field exposed water in a nanopore: liquid or vapour?
We study the behavior of ambient temperature water under the combined effects
of nanoscale confinement and applied electric field. Using molecular
simulations we analyze the thermodynamic causes of field-induced expansion at
some, and contraction at other conditions. Repulsion among parallel water
dipoles and mild weakening of interactions between partially aligned water
molecules prove sufficient to destabilize the aqueous liquid phase in isobaric
systems in which all water molecules are permanently exposed to a uniform
electric field. At the same time, simulations reveal comparatively weak
field-induced perturbations of water structure upheld by flexible hydrogen
bonding. In open systems with fixed chemical potential, these perturbations do
not suffice to offset attraction of water into the field; additional water is
typically driven from unperturbed bulk phase to the field-exposed region. In
contrast to recent theoretical predictions in the literature, our analysis and
simulations confirm that classical electrostriction characterizes usual
electrowetting behavior in nanoscale channels and nanoporous materials.Comment: 20 pages, 6 figures + T.O.C. figure, in press in PCC
Recommended from our members
Electrostatic interactions between peptides and the molecular chaperone DnaK
Recommended from our members
Protein-Folding Landscapes in Multi-Chain Systems
Computational studies of proteins have significantly improved our understanding of protein folding. These studies are normally carried out using chains in isolation. However, in many systems of practical interest, proteins fold in the presence of other molecules. To obtain insight into folding in such situations, we compare the thermodynamics of folding for a Miyazawa-Jernigan model 64-mer in isolation to results obtained in the presence of additional chains. The melting temperature falls as the chain concentration increases. In multi-chain systems, free-energy landscapes for folding show an increased preference for misfolded states. Misfolding is accompanied by an increase in inter-protein interactions; however, near the folding temperature, the transition from folded chains to misfolded and associated chains is entropically driven. A majority of the most probable inter-protein contacts are also native contacts, suggesting that native topology plays a role in early stages of aggregation
Tunable Wetting of Surfaces with Ionic Functionalities
Surface charges can dramatically improve the wettability
of solid
surfaces by water and aqueous solutions. This effect is common in
nature, as in ion channel proteins, and can be used to facilitate
fluid transport in microfluidic applications. While it is often possible
to make reliable predictions of contact angle reduction due to a uniform
electrode charge, we address the effect of discrete charge distribution
on a surface carrying a pattern of ionic functionalities. We perform
atomistic molecular dynamics simulations to investigate the wetting
regimes at the nanoscale on molecular-brush-coated graphane surfaces.
We tune hydrophilicity by covering the surface with a mixture of covalently
bonded butyl and potassium butyrate (or propylammonium chloride) chains
at different densities of ionizing groups. We use thermodynamic integration
to determine the relation between wetting free energy and the amount
of discrete charges on the substrate. We show that nanopores with
oppositely charged walls in neat water feature a Lippmann-like quadratic
dependence of the cosine of contact angle on the surface charge density
while discretely charged surfaces surrounded by neutralizing counterions
exhibit a linear dependence reminiscent of the Cassie–Baxter
relation. Nonuniform surfaces show a strong dependence on the distribution
pattern, with charge effects on maximally segregated surfaces about
4 times weaker than for the uniform distribution. The findings provide
guidance for the design of nanopatterned materials with tailored wettability
Recommended from our members
Effect of single-point sequence alterations on the aggregation propensity of a model protein
Sequences of contemporary proteins are believed to have evolved through process that optimized their overall fitness including their resistance to deleterious aggregation. Biotechnological processing may expose therapeutic proteins to conditions that are much more conducive to aggregation than those encountered in a cellular environment. An important task of protein engineering is to identify alternative sequences that would protect proteins when processed at high concentrations without altering their native structure associated with specific biological function. Our computational studies exploit parallel tempering simulations of coarse-grained model proteins to demonstrate that isolated amino-acid residue substitutions can result in significant changes in the aggregation resistance of the protein in a crowded environment while retaining protein structure in isolation. A thermodynamic analysis of protein clusters subject to competing processes of folding and association shows that moderate mutations can produce effects similar to those caused by changes in system conditions, including temperature, concentration, and solvent composition that affect the aggregation propensity. The range of conditions where a protein can resist aggregation can therefore be tuned by sequence alterations although the protein generally may retain its generic ability for aggregation