Retinyl chromophore

FIGURE 30. Plot of FOS (ppm) for vinyl-F labels at several locations on the chain of the retinyl chromophore in the ( ) 11-cA, dashed line, (A) 9-cis, solid line, and (o) 9,11-di-as configurations. Where appropriate, the averaged FOS values from mono- and dilabeled analogs were used for connecting lines. Solid symbols are for CF3. Reprinted with permission from Reference 48. Copyright (1996) American Chemical Society... 

The sequence of events involved in the vision process are listed in Fig. 3. The overall process triggered by the 11 -cis retinyl chromophore is very specific to the protein medium. Through cis-trans isomerization of the chromophore, light energy is transduced into chemical free energy, which in turn is utilized to cause conformational changes in the protein and ultimately activate the retinal G-protein. The important role of the protein... 

Retinol R C OH (ROH), Retinyl Acetate R CH20C0CH3 (RAc), and Retinyl-n-butylamine R Ch NHR" (RBA). These derivatives of the retinyl chromophore are distinguished by their relatively high room-temperature fluorescence yields (< >f = 0.025 for all three... 

Resonance Raman and NMR Studies. The major support to the protonation hypothesis is presently based on the recent application of resonance-Raman spectroscopy. (For recent reviews, see refs. 217-219.) The method uses an incident beam which is in resonance with the absorption of the retinyl chromophore. This results in the selective enhancement of the Raman cross sections coupled with the chromophore, relative to the very weak, non-resonant, modes of the opsin. Characteristic spectra are shown in Fig. 6. Early evidence for protonation came from the observation of a close similarity between the C=N vibrational frequency in rhodopsin and in a model protonated Schiff base (220). More conclusive arguments were provided by Oseroff and Callender, who carried out experiments at low temperatures in order to control sample photoability (221). It was observed that deuteration shifts the C=N vibration frequency from 1655 cm- to 1630 cm-- -, both in the pigment and in a model protonated Schiff base. 

Z-E isomerization via simple geometric inversion (one-bond flip, OBF, Fig. 2.3A) involves the torsional relaxation of the perpendicular excited state via an adiabatic mechanism which implies a non-volume-conserving process. This is not compatible with the ultrafast CTI in polyenes, in particular retinyl chromophores, and two other possible ways of photo-CTI have been proposed over the past 15 years [11]. 

Raman spectroscopy has been particularly useful in studies of rhodopsin and bacteriorhodopsin. As discussed in Chap. 4, excitatirai of rhodopsin or bacteriorho-dopsin by light causes isomerization of the retinyl chromophore. In rhodopsin, the chromophore changes from W-cis to all-trans in bacteriorhodopsin, it goes from dA -trans to 13-cA. Resonance Raman measurements showed that the isomerization is essentially complete in metastable intermediate states that form within a few ps [32-38]. The conformations of these states were ascertained by comparisons of the resonance Raman spectra with those of model compounds. 

Interpreting photoisomerization of the visual chromophore. Seltzer calculated the energy requirement of the isomerization process of the free retinyl chromophore by way of conventional torsional relaxation and the BP and the HT processes. BP was judged as an untenable, activated process while both the OBF and HT processes were considered energetically feasible. 

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