title: The role of the beta-ionone ring in the photochemical reaction of rhodopsin.
abstract: Time-dependent density functional theory (TDDFT) calculations on the photoabsorption process of the 11-cis retinal protonated Schiff base (PSB) chromophore show that the Franck-Condon relaxation of the first excited state of the chromophore involves a torsional twist motion of the beta-ionone ring relative to the conjugated retinyl chain. For the ground state, the beta-ionone ring and the retinyl chain of the free retinal PSB chromophore form a -40 degrees dihedral angle as compared to -94 degrees for the first excited state. The double bonds of the retinal are shorter for the fully optimized structure of the excited state than for the ground state suggesting a higher cis-trans isomerization barrier for the excited state than for the ground state. According to the present TDDFT calculations, the excitation of the retinal PSB chromophore does not primarily lead to a reaction along the cis-trans torsional coordinate at the C11-C12 bond. The activation of the isomerization center seems to occur at a later stage of the photo reaction. The results obtained at the TDDFT level are supported by second-order Møller-Plesset (MP2) and approximate singles and doubles-coupled cluster (CC2) calculations on retinal chromophore models; the MP2 and CC2 calculations yield for them qualitatively the same ground state and excited-state structures as obtained in the density functional theory and TDDFT calculations.
authors.affiliation: Institut für Physikalische Chemie, Universität Karlsruhe, Kaiserstrasse 12, D-76128 Karlsruhe, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Send
authors.forename: Dage
authors.initials: D
authors.lastname: Sundholm
mesh_terms: D001436:Bacteriorhodopsins / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry; D002627:Chemistry, Physical / Q000379:methods; D018360:Crystallography, X-Ray; D004583:Electrons; D008958:Models, Molecular; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry; D045792:Norisoprenoids / Q000737:chemistry*; D010777:Photochemistry / Q000379:methods*; D012160:Retina / Q000378:metabolism; D012172:Retinaldehyde / Q000737:chemistry; D012243:Rhodopsin / Q000737:chemistry*; D012545:Schiff Bases / Q000737:chemistry; D012984:Software
chemical_list: D045792:Norisoprenoids; D012545:Schiff Bases; D001436:Bacteriorhodopsins; D002244:Carbon; D012243:Rhodopsin; C008157:beta-ionone; D009584:Nitrogen; D012172:Retinaldehyde