Vibrational Assignment of Torsional Normal Modes of Rhodopsin:  Probing Excited-State Isomerization Dynamics along the Reactive C11C12 Torsion Coordinate

• Published in: The journal of physical chemistry. B 1998-04, Vol.102 (15), p.2787-2806
• Main Authors: Lin, Steven W, Groesbeek, Michel, van der Hoef, Ineke, Verdegem, Peter, Lugtenburg, Johan, Mathies, Richard A
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp972752u

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 C11C12 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 C11C12 torsional mode. 13C substitutions of atoms in the C12−C13 or C13C14 bond cause the 568-cm-1 mode to shift by ∼8 cm-1, and deuteration of the C11C12 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 C11C12 torsional character. Thus, we assign the 568-cm-1 mode to a localized C11C12 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 C11C12 bond on the S1 surface and indicate that the isomerization process is facilitated by vibronic coupling of the S1 and S2 surfaces via C11C12 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.