Inherent Chirality Dominates the Visible/Near-Ultraviolet CD Spectrum of Rhodopsin

Abstract

The visible (alpha) and near-UV (beta) CD bands of rhodopsin have been studied extensively, but their source(s) have never been definitively established. Do they result from the intrinsic chirality of the polyene chromophore of the protonated Schiff base of retinal (retPSB) or from the coupling of the transitions of this chromophore with those of protein groups? We have calculated the contributions of these two mechanisms to the CD of rhodopsin. The intrinsic CD of the retPSB chromophore was calculated using time-dependent density functional theory (TDDFT) and, for comparison, the semiempirical ZINDO method. First-order perturbation theory was used to calculate the effects of coupling of the retPSB transitions with the pi pi* transitions of the aromatic chromophores and the pi pi* and n pi* transitions of the peptide groups in rhodopsin. Calculations were performed for eight structures based upon the two molecules in the asymmetric unit of four crystal structures. The most reliable results were obtained from TDDFT calculations on the structure of Okada et al. (J. Mol. Biol. 2004, 342, 571), PDB 1U19. Averaging over the two molecules in the asymmetric unit, the intrinsic rotational strengths are 0.62 +/- 0.00 DBM (Debye-Bohr magneton) and 0.90 +/- 0.03 DBM for the alpha- and beta-bands, respectively. The contributions from coupling with protein groups are, respectively, -0.32 +/- 0.05 and -0.01 +/- 0.03 DBM. Our results show that the visible/near-UV CD bands of rhodopsin are determined by the intrinsic chirality of the retPSB chromophore and that the contributions of coupling with the protein are significantly smaller for the alpha-band and negligible for the beta-band.