Ion-specific thermodynamics of multicomponent electrolytes: a hybrid HNC/MD approach.

Using effective infinite dilution ion-ion interaction potentials derived from explicit-water molecular dynamics (MD) computer simulations in the hypernetted-chain (HNC) integral equation theory we calculate the liquid structure and thermodynamic properties, namely, the activity and osmotic coefficients of various multicomponent aqueous electrolyte mixtures. The electrolyte structure expressed by the ion-ion radial distribution functions is for most ions in excellent agreement with MD and implicit solvent Monte Carlo (MC) simulation results. Calculated thermodynamic properties are also represented consistently among these three methods.

Ionic force field optimization based on single-ion and ion-pair solvation properties.

Molecular dynamics simulations of ionic solutions depend sensitively on the force fields employed for the ions. To resolve the fine differences between ions of the same valence and roughly similar size and in particular to correctly describe ion-specific effects, it is clear that accurate force fields are necessary. In the past, optimization strategies for ionic force fields either considered single-ion properties (such as the solvation free energy at infinite dilution or the ion-water structure) or ion-pair properties (in the form of ion-ion distribution functions).

Ion specific effects of sodium and potassium on the catalytic activity of HIV-1 protease.

The behavior of HIV-1 protease in aqueous NaCl and KCl solutions is investigated by kinetic measurements and molecular dynamics simulations. Experiments show cation-specific effects on the enzymatic activity. The initial velocity of peptide substrate hydrolysis increases with salt concentration more dramatically in potassium than in sodium chloride solutions.

Specificity of ion-protein interactions: complementary and competitive effects of tetrapropylammonium, guanidinium, sulfate, and chloride ions.

The interactions of ions with a model peptide (a single melittin alpha-helix) in solutions of tetrapropylammonium sulfate or guanidinium chloride were examined by molecular dynamics simulations. The tetrapropylammonium cation shares the geometrical property of essentially flat faces with the previously examined guanidinium cation, and it was found that that this geometry leads to a strong preference for tetrapropylammonium to interact in a similar stacking-type fashion with flat nonpolar groups such as the indole side chain of tryptophan. In contrast to guanidinium, however, tetrapropylammonium does not exhibit strong ion pairing or clustering with sulfate counterions in the solution.

Specific ion binding to nonpolar surface patches of proteins.

Employing detailed atomistic modeling we study the mechanisms behind ion binding to proteins and other biomolecules and conclude that (1) small, hard ions bind via direct ion pairing to charged surface groups and (2) large, soft ions bind to nonpolar groups via a solvent assisted attraction. Our predictions are in qualitative agreement with bulk solution data and may provide an important clue for the basic understanding of ion-specific effects in biological systems.

Quantification and rationalization of the higher affinity of sodium over potassium to protein surfaces.

For a series of different proteins, including a structural protein, enzyme, inhibitor, protein marker, and a charge-transfer system, we have quantified the higher affinity of Na+ over K+ to the protein surface by means of molecular dynamics simulations and conductivity measurements. Both approaches show that sodium binds at least twice as strongly to the protein surface than potassium does with this effect being present in all proteins under study. Different parts of the protein exterior are responsible to a varying degree for the higher surface affinity of sodium, with the charged carboxylic groups of aspartate and glutamate playing the most important role.

Homogeneous freezing of water starts in the subsurface.

Molecular dynamics simulations of homogeneous ice nucleation in extended aqueous slabs show that freezing preferentially starts in the subsurface. The top surface layer remains disordered during the freezing process. The subsurface accommodates better than the bulk the increase of volume connected with freezing.

Interior and interfacial aqueous solvation of benzene dicarboxylate dianions and their methylated analogues: A combined molecular dynamics and photoelectron spectroscopy study.

Aqueous solvation of benzene dicarboxylate dianions (BCD(2-)) was studied by means of photoelectron spectroscopy and molecular dynamics simulations. Photoelectron spectra of hydrated o- and p-BCD(2-) with up to 25 water molecules were obtained. An even-odd effect was observed for the p-BCD(2-) system as a result of the alternate solvation of the two negative charges.

Selected biologically relevant ions at the air/water interface: a comparative molecular dynamics study.

Interfacial behavior of selected biologically and technologically relevant ions is studied using molecular dynamics simulations employing polarizable potentials. Propensities of choline, tetraalkylammonium (TAA), and sodium cations, and sulfate and chloride anions for the air/water interface are analyzed by means of density profiles. Affinity of TAA ions for the interface increases with their increasing hydrophobicity.

Specific ion effects at protein surfaces: a molecular dynamics study of bovine pancreatic trypsin inhibitor and horseradish peroxidase in selected salt solutions.

The distribution of sodium, choline, sulfate, and chloride ions around two proteins, horseradish peroxidase (HRP) and bovine pancreatic trypsin inhibitor (BPTI), is investigated by means of molecular dynamics simulations with the aim to elucidate ion adsorption at the protein surface. Although the two proteins under investigation are very different from each other, the ion distributions around them are remarkably similar. Sulfate is always strongly attached to the proteins, choline shows a significant, but unspecific, propensity for the protein surfaces, and sodium ions have a weak surface affinity, while chloride has virtually no preference for the protein surface.

Aqueous ionic and complementary zwitterionic soluble surfactants: molecular dynamics simulations and sum frequency generation spectroscopy of the surfaces.

An aqueous ionic surfactant, 1-dodecyl-4-(dimethylamino)pyridinium (DMP) bromide, and the corresponding zwitterion 2-[4-(dimethylamino)pyridinio]dodecanoate (DPN) were explored by means of molecular dynamics (MD) simulations and, for the ionic system, by infrared-visible sum frequency generation (IR-vis SFG). The molecular structure of the interfacial layer was investigated for the ionic and zwitterionic systems as a function of surfactant concentration, both in water and in salt (KF or KBr) solutions, by MD simulations in a slab geometry. The buildup of the surface monolayer and a sublayer was monitored, and density and orientational profiles of the surfactants were evaluated.

Brine rejection from freezing salt solutions: a molecular dynamics study.

The atmospherically and technologically very important process of brine rejection from freezing salt solutions is investigated with atomic resolution using molecular dynamics simulations. The present calculations allow us to follow the motion of each water molecule and salt ion and to propose a microscopic mechanism of brine rejection, in which a fluctuation (reduction) of the ion density in the vicinity of the ice front is followed by the growth of a new ice layer. The presence of salt slows down the freezing process, which leads to the formation of an almost neat ice next to a disordered brine layer.