ENERGETICS OF PROTONATION-DEPROTONATION OF THE CHROMOPHORE IN RETINAL PROTEINS

Abstract— We investigate the energetics of protonation and deprotonation of retinylidene Schiff-base (SB) which is realized in the functioning of retinal proteins. We first calculate the energy difference ΔE between the protonated and unprotonated states of the SB by the ab initio molecular orbital method, using two kinds of molecular model; a counter-ion model where a carboxyl group of Glu or Asp is directly hydrogen-bonded to the SB, and a water-bridge model where a water molecule bridges the carboxyl group and the SB. The calculated results indicate that the protonated SB state is unstable compared with the unprotonated SB state in either model. In addition, we find that coordination of some water molecules to the carboxyl group reduces ΔE significantly. The value of AE for the counterion model with two coordinated water molecules is 0.003 eV. Next, we calculate the electrostatic interaction energy between a tryptophan residue and the SB. We find that the protonated state is more stabilized than the unprotonated state by about 0.1 eV with one tryptophan residue. This fact indicates that if some aromatic amino acid residues work cooperatively, they can contribute to significantly reducing ΔE. We also discuss the possible role of amino acid residues which make hydrogen-bond with the carboxyl group of interest.