Published: December 13, 2011 r2011 American Chemical Society 2249 dx.doi.org/10.1021/jp205918m | J. Phys. Chem. B 2012, 116, 2249–2258 ARTICLE pubs.acs.org/JPCB The Effect of Protein Environment on Photoexcitation Properties of Retinal Ville R. I. Kaila,* ,† Robert Send,* ,‡ and Dage Sundholm* ,§ † Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, United States ‡ Institut f€ ur Physikalische Chemie, Karlsruher Institut f€ ur Technologie, Kaiserstraße 12, 76131 Karlsruhe, Germany § Department of Chemistry, University of Helsinki, P.O. Box 55 (A. I. Virtanens plats 1), FIN-00014 Helsinki, Finland b S Supporting Information ’ INTRODUCTION Rhodopsin (Rh) is a G protein-coupled receptor (GPCR), responsible for light absorption in vertebrate visual pigments. 1,2 The light-absorbing chromophore of Rh is 11-cis retinal (Figure 1), which is a conjugated polyene linked to residue Lys-296, forming a Schi ff base. Absorption of a photon leads to a cisÀtrans isomerization of the retinal chain, 3À7 activating the G protein transducin by conformational changes in the photopigment. 1,2 The activated G protein influences a phosphodiestrase en- zyme, which lowers the intracellular cyclic guanosine mono- phosphate (cGMP) concentration and blocks the influx of cations into the cell. The cell is eventually hyperpolarized as a consequence of the signaling cascade, leading to the vision process. 1,2 In addition to Rh (λ max = 498 nm), which is the light pigment employed for dim-vision in the rod cells of the retina, color vision is realized in humans by the three cone pigments sensitive for blue (λ max = 414 nm), green (λ max = 530 nm), and red (λ max = 560 nm) light. 8 Photoisomerization of light pigment-bound retinal is one of the fastest processes in nature, taking place on a femtosecond time scale. After the initial absorption process, 11-cis retinal is trans- formed within 200 fs into photorhodopsin (PHOTO-RH), 7,9,10 followed by 800 fs relaxation to bathorhodopsin (BATHO), which is the first thermodynamically stable all-trans retinal state. 4,11,12 The reaction proceeds with the blueshifted intermediate (BS I) within 120 ns, 13 lumirhodopsin (LUMI) within 150 ns, and metarhodop- sin I (META I) is formed after 10 μs. 14,15 In the BATHO to LUMI step, retinal relaxes to a more planar all-trans structure. 16 Later states in the photocycle are characterized by a deprotonated Schiff base, many of which have recently been crystallized. 12,17,18 META II, which forms within 1 ms of photoabsorption, is the first inter- mediate with the ability to initiate signal transduction. 16,19,20 Large- scale protein conformational changes during the LUMI-to-META Received: June 23, 2011 Revised: November 30, 2011 ABSTRACT: Retinal is the photon absorbing chromophore of rhodopsin and other visual pigments, enabling the vertebrate vision process. The effects of the protein environment on the primary photoexcitation process of retinal were studied by time- dependent density functional theory (TDDFT) and the alge- braic diagrammatic construction through second order (ADC (2)) combined with our recently introduced reduction of virtual space (RVS) approximation method. The calculations were performed on large full quantum chemical cluster models of the bluecone (BC) and rhodopsin (Rh) pigments with 165À171 atoms. Absorption wavelengths of 441 and 491 nm were obtained at the B3LYP level of theory for the respective models, which agree well with the experimental values of 414 and 498 nm. Electrostatic rather than structural strain effects were shown to dominate the spectral tuning properties of the surrounding protein. The Schiff base retinal and a neighboring Glu-113 residue were found to have comparable proton affinities in the ground state of the BC model, whereas in the excited state, the proton affinity of the Schiff base is 5.9 kcal/mol (0.26 eV) higher. For the ground and excited states of the Rh model, the proton affinity of the Schiff base is 3.2 kcal/mol (0.14 eV) and 7.9 kcal/mol (0.34 eV) higher than for Glu-113, respectively. The protein environment was found to enhance the bond length alternation (BLA) of the retinyl chain and blueshift the first absorption maxima of the protonated Schiff base in the BC and Rh models relative to the chromophore in the gas phase. The protein environment was also found to decrease the intensity of the second excited state, thus improving the quantum yield of the photoexcitation process. Relaxation of the BC model on the excited state potential energy surface led to a vanishing BLA around the isomerization center of the conjugated retinyl chain, rendering the retinal accessible for cisÀtrans isomerization. The energy of the relaxed excited state was found to be 30 kcal/mol (1.3 eV) above the minimum ground state energy, and might be related to the transition state of the thermal activation process.