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

Abstract

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.

Mechanism and bottlenecks in strand photodissociation of split green fluorescent proteins (GFPs)

“…In contrast to fluorescent proteins, PYP and rhodopsin have exceedingly low fluorescence quantum yields [2 × 10 −3 (64) and 1 × 10 −5 (51), respectively], while exhibiting excellent cis-trans isomerization efficiencies [quantum yields 35% (65) and 65% (66), respectively]. Early studies suggest their similarities (52,67,68): they have steep Franck-Condon regions (i.e., strong vibronic coupling along the reaction coordinate) and either small or no excited-state barriers, allowing the excited population to rush down to the conical intersection without much impediment, and their conical intersections are relatively more peaked than sloped to increase isomerization quantum yields while suppressing competing internal conversion (45,46,53,69). Although fluorescent proteins and photosensory proteins have drastically different fluorescence and isomerization quantum efficiencies from one another, they can all be described with the same type of PES topology, suggesting that we can learn from photosensory proteins to improve the photodissociation of split GFPs.…”

Computational Studies of the Primary Phototransduction Event in Visual Rhodopsin

“…[33][34][35] In contrast, however, resonance Raman data 25,36,37 and recent double-quantum solid-state NMR experiments 32 converge toward single-bond character for C14-C15, suggesting that the polaronic charge defect might hop over the vicinity of the Schiff base, settling in the middle of the polyene chain next to C12. 32 An objective of recent DFT QM/MM studies has been to resolve such a contradictory picture by providing a detailed first principles description of the electronic density of the chromophore as influenced by the protein environment.…”

1 H and 13 C MAS NMR evidence for pronounced ligand–protein interactions involving the ionone ring of the retinylidene chromophore in rhodopsin

“…In the protein, however, there is a nonplanar 12-s-trans conformation ( Fig. 1 A) (4,10,43). Also the most recent refined x-ray model for rhodopsin includes an out-of-plane distortion in the C 10 -C 13 segment with a dihedral angle for C 10 -C 11 -C 12 -C 13 of ϭ 7.9°(4).…”