Vibrational Assignment of Torsional Normal Modes of Rhodopsin:  Probing Excited-State Isomerization Dynamics along the Reactive C11C12 Torsion Coordinate

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 C11C12 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 C11C12 torsional mode. 13C substitutions of atoms in the C12−C13 or C13C14 bond cause the 568-cm-1 mode to shift by ∼8 cm-1, and deuteration of the C11C12 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 C11C12 torsional character. Thus, we assign the 568-cm-1 mode to a localized C11C12 torsion and the 260-cm-1 mode to a more delocalized torsional vibration involving coordinates from C10 to C13. Consistent with these assignments, these two modes are not Raman active in 13-demethyl, 11-cis rhodopsin which has a planar C10···C13 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 C11C12 bond on the S1 surface and indicate that the isomerization process is facilitated by vibronic coupling of the S1 and S2 surfaces via C11C12 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.