Opsin Effect on the Electronic Structure of the Retinylidene Chromophore in Rhodopsin.

Abstract

Direct examination of experimental NMR parameters combined with electronic structure analysis was used to provide a first-principle interpretation of NMR experiments and give a precise evaluation of how the electronic perturbation of the protein environment affects the electronic properties of the retinylidene chromophere in rhodopsin. To this end, we pursued a theoretical analysis using a combination of tools including quantum mechanics/molecular mechanics (QM/MM) at the Density Functional Theory (DFT) level, in conjunction with gauge independent atomic orbital (GIAO) calculations of (13)C NMR chemical shieldings and (1)J(CC) spin-spin coupling constants obtained with the Coupled Perturbed DFT (CPDFT) method. The opsin effect on the retinylidene chromophere is interpreted as an inductive effect of Glu-113 which readjusts the weighting factors of resonance substructures of the conjugated chain of the chromophere. These changes give a rationalization to the alternating effect of the (13)C chemical shifts magnitudes when comparing the retinylidene chromophere in the presence and absence of the protein environment. Conversely, perturbation of π orbitals has little to no effect over (1)J (13)C-(13)C spin-spin coupling constants, as they are mainly dominated by the Fermi contact term, and hence the counteraion effect is restricted to the vicinity of the perturbation. Thus, the apparent contradiction between experimental findings based on chemical shifts (deep penetration) and one-bond J-couplings (localized effects of the protonated Schiff base at the chain terminus) is in fact a consequence of different properties responding differently to the same external perturbation.