Resonance Raman multicomponent spectra of bovine rhodopsin, isorhodopsin, and bathorhodopsin have been obtained at low temperature. Application of the double beam "pump-probe" technique allows us to extract a complete bathorhodopsin spectrum from the mixture in both protonated and deuterated media. Our results show that the Schiff base of bathorhodopsin is fully protonated and that the extent of protonation is unaffected by its photochemical formation from either rhodopsin or isorhodopsin. The Raman spectrum of bathorhodopsin is significantly different than that of either parent pigment, thus supporting the notion that a geometric change in the chromophore is an important component of the primary photochemical event in vision. A normal mode analysis is carried out with particular attention devoted to the factors that determine the frequency of the C=N stretching vibration. We find that the increased frequency of this mode in protonated relative to unprotonated Schiff bases is due to coupling between C=N stretching and C=N-H bending motions, and the shift observed upon deuteration of the Schiff base can also be understood in these terms. Various models for the primary event are discussed in light of our experimental and theoretical results.

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The nature of the first n π* excited state of the pyrazine molecule has been investigated by studying the vibrational structure of the spectrum in the region of 3200 Å. The spectrum was observed in the vapor, in the pure crystal, and in crystalline solid solutions in cyclohexane and benzene. The phosphorescence and fluorescence spectra were used to determine ground state frequencies in the various media and to give the Franck-Condon factors of the various modes. Assignments of ν1, ν6a, ν9a, and ν10a in the excited state have been made. It was found that ν10a has a strongly quartic component in its potential function and that there is a large negative anharmonic coupling term between 6a and 10a. These results are interpreted qualitatively in terms of the expected electron redistribution in the n π* state.

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