Vibrational Assignment of Torsional Normal Modes of Rhodopsin:  Probing Excited-State Isomerization Dynamics along the Reactive C₁₁C₁₂Torsion 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 C-11=C-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=C-12 torsional mode. C-13 substitutions of atoms in the C-12-C-13 or C-13=C-14 bond cause the 568-cm(-1) mode to shift by similar to 8 cm(-1), and deuteration of the C-11=C-12 bond downshifts the 568- and 260-cm(-1) modes by similar to 35 and 5 cm(-1), respectively. The magnitudes of these shifts are consistent with those calculated for modes containing significant C-11=C-12 torsional character. Thus, we assign the 568-cm(-1) mode to a localized C-11=C-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 similar to 2-fold higher with preresonant excitation. These data quantitate the instantaneous torsional dynamics of the chromophore about its C-11=C-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=C-12 torsional distortion, which reduces the er,cited-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 similar to 54, 92, 128, 151, 262; 276, 324, and 376 cm(-1) to the observed femtosecond photochemical dynamics.