Environmental effects on H-bond potentials: A SCRF MO CNDO/2 study of some model systems

Abstract

The self-consistent reaction field (SCRF) theory of solvent effects upon dipolar species has been applied to the study of H-bonded systems. A model to represent the enzyme core medium is discussed. It is shown, albeit heuristically, that this type of environment may be represented with the combined effect of a reaction field and an inhomogeneous external electric field acting over the site system, this latter being provided by the main chain dipolar peptide residues. The generalization of the reaction field concept attained within the SCRF theory allowed for this extention to be operationally implemented. The model systems considered are: a formaldehyde-water, acetone-water, a water dimer and a tetrahedrally arranged water trimer. The interest has been focuzed on the study of the dependance upon the site-surrounding coupling strength of the intermolecular potentials and interactions related to the H-bond. The orientational and intermolecular distance dependance of the pair potential and the proton potentials have been considered. The perturbing effect of the enzyme core medium is manifested through the drastic variation of many (but not all) of the H-bond potentials. The changes showed by the orientational component of the intermolecular potential together with that of the proton potential shape illustrate this point. As far as the proton and charge relay mechanisms are concerned, the results reported, while confirming our previous ones, shed a new light upon these matters. The sensitization of the H-bridge towards an external electric field is made possible by the presence of the reaction field, in absence of this latter, the external field effect is not significant.