title: Color tuning in short wavelength-sensitive human and mouse visual pigments: ab initio quantum mechanics/molecular mechanics studies.
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 beta-ionone terminals of 11-cis-retinal in the ground and first excited states.
authors.affiliation: Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA. aaltun@emory.edu
authors.forename: Ahmet
authors.initials: A
authors.lastname: Altun
authors.forename: Shozo
authors.initials: S
authors.lastname: Yokoyama
authors.forename: Keiji
authors.initials: K
authors.lastname: Morokuma
mesh_terms: D000818:Animals; D002417:Cattle; D003116:Color; D004583:Electrons; D006801:Humans; D051379:Mice; D008958:Models, Molecular; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry / Q000378:metabolism; D011522:Protons; D011789:Quantum Theory; D012168:Retinal Pigments / Q000737:chemistry* / Q000378:metabolism*; D012172:Retinaldehyde / Q000737:chemistry / Q000378:metabolism; D012545:Schiff Bases / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D011522:Protons; D012168:Retinal Pigments; D012545:Schiff Bases; D009584:Nitrogen; D012172:Retinaldehyde