QM/MM study of the structure, energy storage, and origin of the bathochromic shift in vertebrate and invertebrate bathorhodopsins.

By comparing the results from a hybrid quantum mechanics/molecular mechanics method (SORCI+Q//B3LYP/6-31G*:Amber) between vertebrate (bovine) and invertebrate (squid) visual pigments, the mechanism of molecular rearrangements, energy storage, and origin of the bathochromic shift accompanying the transformation of rhodopsin to bathorhodopsin have been evaluated. The analysis reveals that, in the presence of an unrelaxed binding site, bathorhodopsin was found to carry almost 27 kcal/mol energy in both visual pigments and absorb (λ(max)) at 528 nm in bovine and 554 nm in squid. However, when the residues within 4.0 Å radius of the retinal are relaxed during the isomerization event, almost ∼16 kcal/mol energy is lost in squid compared to only ∼8 kcal/mol in bovine. Loss of a larger amount of energy in squid is attributed to the presence of a flexible binding site compared to a rigid binding site in bovine. Structure of the squid bathorhodopsin is characterized by formation of a direct H-bond between the Schiff base and Asn87.