Ab initio calculations of proton potential functions of some rhodopsin modelling systems

Abstract

Proton potential functions of three hydrogen bonded systems (allylmethylimine—formic acid, allylmethylimine—water—formic acid and allylimine—phenol) were computed with the ab initio MO scheme using the 4-31G basis set. The effect of a polarizable solvent cavity was mimicked by the method of Tomasi—Miertus̆ [1,2]. The in vacuo calculations yielded double minimum type functions with the deeper minimum nearer to the acid moiety, i.e., neutral hydrogen bonded complexes are more stable. Inclusion of the solvent effect reversed the depth of the minima, thus favouring the ionic forms of the complexes. The calculations demonstrate that changes in the medium may assist proton translocations that are essential components of the proton pumping by bacteriorhodopsin and photon detection by visual rhodopsin.