The interface between two immiscible liquids is a region with unique discontinuous properties. Knowledge of the structure and dynamics of this region plays a fundamental role in understanding, from a molecular point of view, many interfacial processes like, for example, charge transfer between the two phases. This paper reports the results of a molecular dynamics simulation of the interface between water and 1,2-dichloroethane (DCE). It is shown that this interface is very sharp at the molecular level, without a mixed region, but broadened by interpenetrating waves of one liquid into the other. In addition, an estimate of the frequency of these interpenetrating waves and a study of the dynamics of the global interface are presented. It is concluded that this dynamics is somewhat regular and has a long correlation time.

Molecular dynamics simulations were performed to study the structural and dynamic properties of the water/2-heptanone (HPT2) liquid/liquid interface. It was found that HPT? forms a bilayer structure at the interface, pointing its polar heads into the aqueous phase. Water molecules penetrate the hydrophilic headgroup region but not the hydrophobic core. At the hydrophilic region water molecules establish hydrogen bonds with the ketone oxygen of the HPT2 molecule. Behind that zone, the water molecules show a preference in keeping their dipoles in the interfacial plane and these orientations remain in two or three molecular layers. The water dipole distribution is slightly asymmetric, having an average excess in the resulting component normal to the interfacial plane. The water dipoles point toward the aqueous phase for waters in the aqueous side of the interface and into the organic phase for water molecules in the organic side of the interface. The water structure remains almost unchanged at the Gibbs dividing surface. The HPT2, structure is not so robust, and near the interface it is distorted by the presence of the aqueous phase. Self diffusion exhibits long range anisotropy, diffusion toward the interface being slower than diffusion in the interfacial plane. The water orientational dynamics is slowed down near the interface. The HPT2 reorientation becomes anisotropic at the interface as reorientations perpendicular to the interface an slower than those in the interfacial plane. The interface was found to be sharp, highly corrugated, and broadened by capillary waves.

This work focuses on the study of the properties of two liquid/liquid interfaces, the H2O/2-heptanone and the H2O/iso-octane interfaces, and on the transfer of the iodide ion across them. A detailed study of the properties of the first interface was already reported (J. Phys. Chem. B, 1999, in press). The iso-octane liquid is a hydrophobic analog of the very hydrophilic 2-heptanone, and the properties of the N2O/iso-octane interface are analyzed here and compared with the ones obtained for the H2O/2-heptanone system. It is shown that the basic features characterizing the interface structure (such as the non-existence of a mixed solvent region or the broadening of the sharp interface by capillary waves) are almost unaffected by the change of the hydrophilic nature of the organic solvent. A new method is proposed to calculate more accurately properties which depend on the distance to the interface. In the case of density profiles, the application of this method reveals that both liquids are packed in layers against the interface. This structural pattern, not detectable using classical methods, allows us to understand the reason for the oscillations in the density profiles calculated perpendicularly to the interfacial plane, an unsolved problem for more than one decade. The free energy profiles for the transfer of iodide across the two interfaces are computed and compared. In both cases they show a monotonous decrease in the free energy as the ion moves from the organic solvent into water. The value obtained for the Gibbs free energy of transfer is in good agreement with the available experimental data. In addition, the mechanism of the ion transfer is investigated. The process of transfer from the water phase to the organic one and the reverse process involve, in both cases, the formation of a water cone that connects the hydration sphere of the ion to the water phase. This water cone is a chain of molecules that can be as long as 10 Angstrom. After the disruption and retraction of the water cone, the ion in the organic solvent retains part of its first hydration shell. The mechanism of the transfer through both interfaces is, in qualitative terms, very similar, although the ion transfer free energies are very different, as expected considering the relative hydrophilicity of the present solvents.

This paper reports on studies of the interactions of halide ions with water. The standard Hartree-Fock (HF) method was used to calculate the interaction between each of the four halide ions and the water monomer. The structural properties of the X--H2O systems (X=F, Cl, Br, I) are presented with a detailed comparison with experimental energies. A new ion-water parameterised potential, derived from quantum calculations, is proposed for the description of the X--H2O interactions in simulations. This potential was used in Monte Carlo (MC) studies of the gas-phase formation of X-(H2O)(n) clusters (n = 1,..., 10) and of the solvation of the ions in dilute aqueous solutions. Thermodynamic properties, such as enthalpies, Delta H-n-1,H-n, Gibbs free energies, Delta G(n-1,n), and entropies, Delta S-n-1,S-n, are presented for the gas-phase reactions: X-(H2O)(n) + H2O reversible arrow X-(H2O)(n). The results follow the general experimental trends, but overestimate their absolute values for the smaller clusters. The structural properties of the small clusters were found to be in good agreement with the results of quantum calculations. For small n, the so-called surface (S) structure was found to be predominant, while for larger n the interior (I) structure is preferred. The transition from an (S) structure to an (I) structure was found to occur for n between 4 and 6, depending on the ion. In solution, the energy of solvation and the structural properties of each ion are reported and compared with the experimental data available. The energy values were found to be in good agreement with estimates reported for the three larger ions, while for fluoride they are slightly overestimated.

The importance of the guanidinium-carboxylate interactions has sprung from the observed salt bridges often present in biological systems involving the arginine-glutamate or arginine-aspartate side chains. The strength of these interactions has been explained on the basis of a great coulombic energy gain, due to the closeness of two charges of opposite sign and the occurrence of H-bond interactions. However, in some environments proton transfer, from guanidinium to carboxylate, can occur with the consequent annihilation of charge. In this work, both ab-initio (6-31G** and MP2/6-31G**) and semi-empirical (AM1) calculations were performed in vacuo on appropriate models, methylguanidinium-acetate and methylguanidine-acetic acid to simulate the zwitterionic and the neutral forms, respectively. The results obtained indicate that, in solvent-free hydrophobic environments, the neutral form should be more stable than the zwitterionic one.

The main goal of this work is the detailed study of the binding interactions in the trypsin-pancreatic trypsin inhibitor (PTI) complex and, here, we present how meaningful the Tyr(39)-Ile(19) interaction is to the stability of that particular complex using free energy methods. This knowledge should be very important in the design of new inhibitors for trypsin and enzymes homologous to it. In particular, it could help to decide whether it is possible to produce selective inhibitors for these enzymes by appropriate mutations of residues in the contact region of PTI. (C) Munksgaard 1997.

A Monte Carlo simulation is performed to investigate the role of solvent in the Cu2+/Cu+ electron exchange process in water. This study is based on the classical Marcus model using the energy gap between the reactant and product states as the reaction coordinate. Since the process has a rather high activation free energy, conventional simulations are not best suited to its complete description, and thus special sampling techniques are required. In this case, a mapping potential is used to drive the system from the reactants to the transition state, and free energies are computed by thermodynamic perturbation and thermodynamic integration methods. A comparison is made between the results obtained by both methods and conclusions are drawn concerning the applicability of the Marcus model to this kind of process.

Arginine-carboxylate interactions in proteins have aroused much interest due to the important role they play in the stability of biological systems. These interactions have usually been interpreted as being associated with a zwitterionic state as opposed to a neutral one. In this work, ab initio (6-31G** basis set) calculations were carried out in vacuo on appropriate models, methylguanidinium-acetate and methylguanidine-acetic acid, to simulate the zwitterionic and neutral forms, respectively. The results obtained reveal that the neutral form is more stable than the zwitterion, i.e, proton transfer should occur with the consequent annihilation of charge in some environments, possibly hydrophobic ones.