The effect of the protein environment on the structure and charge distribution of the retinal Schiff base in bacteriorhodopsin

The effects of the amino acid side chains of the binding pocket of bacteriorhodopsin (bR) and of a water molecule on the structure of the retinal Schiff base have been studied using Becke3LYP/6-31G* level of density functional theory. A model protonated Schiff base structure including six conjugated double bonds and methyl substituents was optimized in the presence of several amino acid side chains and of a water molecule, separately. The Schiff base structure was also calculated in the form of a neutral species. At each optimized complex geometry the atomic charges of the model Schiff base were calculated using Mulliken population analysis. In agreement with previously proposed counterion(s) of the protonated retinal Schiff base in bR, the results show that Asp₈₅ and Asp₂₁₂, which are present in the form of negatively charged groups, have significantly large effects on the structure and electronic configuration of both unprotonated and protonated model Schiff bases. The presence of a water molecule in the vicinity of the Schiff base demonstrates significant effects which are comparable to those of aspartate groups. Other side chains studied did not show any significant effect in this direction. Apart from the aspartate groups and the water molecule, in none of the other complexes studied are the atomic charges and the bond alternation of the model Schiff base significantly influenced by the presence of the neighboring amino acids.