Color Tuning in Short Wavelength-Sensitive Human and Mouse Visual Pigments: Ab initio Quantum Mechanics/Molecular Mechanics Studies

• Authors: Ahmet Altun, Emory University; Shozo Yokoyama, Emory University; Keiji Morokuma, Emory University
• Journal Title: Journal of Physical Chemistry A
• Volume: Volume 113, Number 43
• Publisher: American Chemical Society | 2009-10-29, Pages 11685-11692
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1021/jp902754p
• Subjects: Chemistry, General

Abstract

We have investigated the protonation state and photoabsorption spectrum of Schiff-base (SB) nitrogen bound 11-cis-retinal in human blue and mouse UV cone visual pigments as well as in bovine rhodopsin by hybrid quantum mechanical/molecular mechanical (QM/MM) calculations. We have employed both multireference (MRCISD+Q, MR-SORCI+Q, and MR-DDCI2+Q) and single reference (TD-B3LYP and RI-CC2) QM methods. The calculated ground-state and vertical excitation energies show that UV-sensitive pigments have deprotonated SB nitrogen, while violet-sensitive pigments have protonated SB nitrogen, in agreement with some indirect experimental evidence. A significant blue shift of the absorption maxima of violet-sensitive pigments relative to rhodopsins arises from the increase in bond length alternation of the polyene chain of 11-cis-retinal induced by polarizing fields of these pigments. The main counterion is Glu113 in both violet-sensitive vertebrate pigments and bovine rhodopsin. Neither Glu113 nor the remaining pigment has a significant influence on the first excitation energy of 11-cis-retinal in the UV-sensitive pigments that have deprotonated SB nitrogen. There is no charge transfer between the SB and β-ionone terminals of 11-cis-retinal in the ground and first excited states.