Initial Excited-State Relaxation of the Isolated 11-cis Protonated Schiff Base of Retinal: Evidence for in-Plane Motion from ab Initio Quantum Chemical Simulation of the Resonance Raman Spectrum

The intensity distribution in the resonance Raman (RR) spectrum of the 11-cis protonated Schiff base of retinal (PSB11) is modeled for the first time on the basis of ab initio quantum chemical calculations. To adequately represent the structure of PSB 11, 4-cis-y,η-dimethyl-C9H9 NH2+ is chosen as a model. The RR spectra of the model PSB11 and of several isotopomers are compared with the experimental spectra of PSB11 in solution. An excellent agreement is obtained in the structurally sensitive fingerprint region of the spectra (1100-1300 cm-1), where most of the observed details are quantitatively reproduced by the simulations. The 900-1100-cm-1 region of the RR spectrum of PSB11, which contains the signatures of the S0,S1 potential energy changes due to the protein environment, is also well reproduced. On the basis of the simulations, it is concluded that the activity observed at ca. 970 cm-1 in the spectrum of PSB11 in solution is due to in-plane modes, while a superposition of in-plane and out, of-plane motions is responsible for the increased RR activity in rhodopsin. The present analysis of RR activities along with the computed relaxation path structure provides support for the interpretation of the initial relaxation of photoexcited PSB11 in solution in terms of initial in-plane motion out of the Franck-Condon region followed by slow out-of-plane (i.e., cis → trans torsional) evolution along a flat energy plateau. Furthermore, the quality of the simulated spectra suggests that the quantum chemical method used in this work can be employed quantitatively to assist in the characterization of photoreaction intermediates in the visual cycle.