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.

Path integral Monte Carlo computations have been done to study the local structure of water molecules around an isolated lithium ion (Li-7(+)) at T=298K. The solute was treated as a quantal particle and the water solvent was treated classically. The water-water interaction was modelled by the MCY pair-potential and the solute-water interaction by the Kistenmacher et al. pair-potential. Purely classical simulations, at the same conditions and using the same model potentials, where also performed for comparison. Significant changes are observed on the results of the quantal simulations when compared with the results of the classical simulation. The major difference is the coordination number that increases from 5, the result of the classical simulation, to 6, for the quantal simulation. In addition, structural analysis of the generated configurations showed that the local structure of the water molecules surrounding the ion is also clearly different in the two simulations. The reliability of the results is discussed.

A new ab initio effective two-body potential that aims at mimicking the average copper-water interaction energy of the first solvation shell was developed. This new potential, together with the MCY water-water potential and a three-body ion-water-water induction potential, is tested in simulations of gas-phase clusters [Cu2+-(H2O)20] and diluted solutions [Cu2+-(H2O)200] at T = 298 K. The results of simulations with conventional ab initio pair potentials, with and without three-body induction corrections, are also presented. The different types of copper-water interaction potentials are evaluated comparatively and the efficiency of the newly proposed effective pair potential is discussed.

A new ab initio pair potential is developed to describe the nickel-water interactions in Ni(II) aqueous solutions. Results of Monte Carlo simulations for the Ni(II)-(H2O)200 system are presented for this pair potential with and without three-body classical polarization terms (the water-water interaction is described by the ab initio MCY potential). The structure of the solution around Ni(II) is discussed in terms of radial distribution functions, coordination numbers and thermal ellipsoids. The results show that the three-body terms have a non-negligible effect on the simulated solution. In fact, the experimental coordination number of six is reproduced with the full potential while a higher value is predicted when the simple pairwise-additive potential is used. The equilibrium NiO distance for the first hydration shell is also dependent on the use of the three-body terms. Comparison of our distribution functions with those obtained by neutron-diffraction experiments shows a reasonable quantitative agreement. Statistical pattern recognition analysis has also been applied to our simulations in order to better understand the local thermal motion of the water molecules around the metal ion. In this way, thermal ellipsoids have been computed (and graphically displayed) for each atom of the water molecules belonging to the Ni(II) first hydration shell. This analysis revealed that the twisting and bending motions are greater than the radial motion, and that the hydrogens have a higher mobility than the oxygens. In addition, a thermodynamic perturbation method has been incorporated in our Monte Carlo procedure in order to compute the free energy of hydration for the Ni(II) ion. Agreement between these results and the experimental ones is also sufficiently reasonable to demonstrate the feasibility of this new potential for the nickel-water interactions.

Monosubstituted benzenes, in which the substituents participate in the π‐electron system, are studied following a classification in two classes according to the π‐electronic structure of the substituent. For this type of molecule, a relation is established between the nature of the substituent and, on the one hand, the energies of the two highest occupied molecular orbitals and, on the other hand, their respective differences. The two orbitals referred to above have π‐character and belong to the a2 and b1 species if a C2v point group is assumed. Simple symmetry arguments lead to the conclusion that the a2 orbitals have, essentially, an intraring character, whereas the π‐orbitals of the substituents do give an important contribution to the b1 orbitals. Therefore, an a2 electron must have a larger interaction with the benzene ring and a smaller kinetic energy, whereas a b1 electron must have a larger interaction with the substituent and a larger kinetic energy. It is also expected that the changes in the π‐electronic structure of the substituent must much more influence the variations on the b1 energies and on the components of orbital energies associated with the substituent than the variations on the a2 energies and on the intraring components of the orbital energies. A modified version of the MOPAC program was prepared to perform the decomposition of the orbital energies in their kinetic and potential energy components and these in their monocentric and bicentric terms. MNDO calculations on nine monosubstituted benzenes, using the modified MOPAC program, give good confirmation of the symmetry predictions and prove the consistency of the classification of the substituents that is introduced. © 1993 John Wiley & Sons, Inc. Copyright © 1993 John Wiley & Sons, Inc.

The interaction energy DELTA-E of the systems Cu+-H2O and Cl--H2O has been computed over a wide range of distances and orientations with the MINI-1 basis set in the SCF approximation. The interaction energy has been decomposed according to the Kitaura-Morokuma scheme, with and without counterpoise (CP) corrections to the basis set superposition error. The importance of this correction is analysed by its effect upon Monte Carlo calculations of the Cu+-water and Cl--water systems, using two-body potentials without and with CP corrections. The effect of CP corrections on the DELTA-E analysis is similar to that found in other systems of analogous composition (of the general type ion plus neutral ligands), but with significant differences in the details. The effect of the CP corrections to the interaction potential, and then on the results of the Monte Carlo simulations, is small for the Cu+ ion, but remarkable for the Cl- ion.