The resonance Raman spectrum of the 11-cis retinal protonated Schiff base chromophore in rhodopsin exhibits low-frequency normal modes at 93, 131, 246, 260, 320, 446, and 568 cm -1 . Their relatively strong Raman activities reveal that the photoexcited chromophore undergoes rapid nuclear motion along torsional coordinates that may be involved in the 200-fs isomerization about the C 11 dC 12 bond. Resonance Raman spectra of rhodopsins regenerated with isotopically labeled retinal derivatives and demethyl retinal analogues were obtained in order to determine the vibrational character of these low-frequency modes and to assign the C 11 dC 12 torsional mode. 13 C substitutions of atoms in the C 12 -C 13 or C 13 dC 14 bond cause the 568-cm -1 mode to shift by ∼8 cm -1 , and deuteration of the C 11 dC 12 bond downshifts the 568-and 260-cm -1 modes by ∼35 and 5 cm -1 , respectively. The magnitudes of these shifts are consistent with those calculated for modes containing significant C 11 dC 12 torsional character. Thus, we assign the 568-cm -1 mode to a localized C 11 dC 12 torsion and the 260-cm -1 mode to a more delocalized torsional vibration involving coordinates from C 10 to C 13 . Consistent with these assignments, these two modes are not Raman active in 13-demethyl, 11-cis rhodopsin which has a planar C 10 ‚‚‚C 13 geometry. Furthermore, the relative Raman scattering strengths of the 260-and 568-cm -1 modes are ∼2-fold higher with preresonant excitation. These data quantitate the instantaneous torsional dynamics of the chromophore about its C 11 dC 12 bond on the S 1 surface and indicate that the isomerization process is facilitated by vibronic coupling of the S 1 and S 2 surfaces via C 11 dC 12 torsional distortion, which reduces the excited-state barrier along the reaction trajectory. We have also examined the low-frequency Raman spectrum of the trans primary photoproduct, bathorhodopsin, and discuss the relevance of its low-frequency torsional modes at ∼54, 92, 128, 151, 262, 276, 324, and 376 cm -1 to the observed femtosecond photochemical dynamics.

The role of intramolecular steric interactions in the isomerization of the 11-cis-retinal chromophore in the photoreceptor protein rhodopsin is examined with resonance Raman and CD spectroscopy combined with quantum yield experiments. The resonance Raman spectra and CD spectra of 13-demethylrhodopsin indicate that its chromophore, an analog in which the nonbonded interaction between the 10-H and the 13-CH3 groups is removed, is less distorted in the C10...C13 region than the native chromophore. The reduced torsional and hydrogen-out-of-plane resonance Raman intensities further indicate that the excited state potential energy surface has a much shallower slope along the isomerization coordinate. This is consistent with the decrease in quantum yield from 0.67 in rhodopsin to 0.47 in 13-demethylrhodopsin. The resonance Raman intensities show that the steric twist is reintroduced by addition of a methyl group at the C10 position. However, the quantum yield of 10-methyl-13-demethylrhodopsin is found to be only 0.35. This is attributed to nonisomorphous protein-analog interactions. The nonbonded interaction between the 10-hydrogen and the 13-methyl group in 11-cis-retinal makes this isomer particularly effective as the light-sensing chromophore in all visual pigments.

The spectroscopic properties of spheroidene and a series of spheroidene analogs with extents of π-electron conjugation ranging from 7 to 13 carbon−carbon double bonds were studied using steady-state absorption, fluorescence, fluorescence excitation, and time-resolved absorption spectroscopy. The spheroidene analogs studied here were 5‘,6‘-dihydro-7‘,8‘-didehydrospheroidene, 7‘,8‘-didehydrospheroidene, and 1‘,2‘-dihydro-3‘,4‘,7‘,8‘-tetradehydrospheroidene and taken together with data from 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene already published (DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lugtenburg, J.; Farhoosh, R.; Frank, H. A. Biochim. Biophys. Acta 1992, 1102, 107) provide a systematic series of molecules for understanding the molecular features that control energy transfer to bacteriochlorophyll in photosynthetic bacterial light-harvesting complexes. All of the molecules were purified by high-pressure liquid chromatographic techniques prior to the spectroscopic experiments. The absorption spectra of the molecules were observed to red-shift with increasing extent of π-electron conjugation. The room temperature fluorescence data show a systematic crossover from dominant S1 → S0 (2Ag → 11Ag) emission to dominant S2 → S0 (11Bu → 11Ag) with increasing extent of conjugation. The S2 fluorescence quantum yields of all the carotenoids in the series were measured here and indicate that 3,4-dihydrospheroidene with nine carbon−carbon double bonds has an S2 quantum yield of (2.7 ± 0.3) × 10-4 which is the highest value in the series. The lifetimes of the S1 states of the molecules were determined from time-resolved transient absorption spectroscopy and found to decrease as the conjugated chain length increases. The transient data are discussed in terms of the energy gap law for radiationless transitions which allows a prediction of the S1 energies of the molecules. The implications of these results for the process of light harvesting by carotenoids in photosynthesis are discussed.