Opsin shift

In a crystal structure47111-ds-retinal has the 12-s-cis conformation shown at the top in Eq. 23-36 rather than the 12-s-frans conformation at the center and in which there is severe steric hindrance between the 10-H and 13-CH3. Nevertheless, H-and 13C-NMR spectroscopy suggest that the retinal in rhodopsin is in a twisted 12-s-frans conformation.472 4723 The Schiff base of 11-ds-retinal with N-butylamine has an absorption maximum at -360 nm but N-protonation, as in the structure in Eq. 23-36, shifts the maximum to 440 nm with emax = 40,600 M 1 cm 1 (Fig. 23-42). This large shift in the wavelength of the absorption maximum (the opsin shift) indicates that binding to opsin stabiliz-... 

OPSIN SHIFT Xmox(in cm- ) S8H — Xmox(in cm- ) Pigment 2700 cur1... 

A large number of both theoretical and experimental models were put forward to account for the opsin shifts encountered in visual pigments ... 

A series of hydroretinals were synthesized and were bound to bovine opsin to form visual pigment analogs, the hydrorhodopsins [92]. Model protonated Schiff bases were also prepared from each of these retinals, and the magnitudes of the opsin shifts were determined. Table 1 summarizes the data obtained and shows the opsin shift for bovine rhodopsin for comparative purposes. Binding studies were also carried out to ascertain that the hydroretinals occupy the same binding site as 11-cK-retinal in rhodopsin. This is required to show the relevance of the data to the natural system. 

Absorption maxima of hydroretinals, protonated Schiff bases, hydrorhodopsins and opsin shifts (from Nakanishi et al. [92])... 

TABLE 28. NMR chemical shifts of retinyUdene PSBs, (protonated schiff bases), rhodopsin pigments and corresponding F NMR opsin shifts... 

We recently [36] provide evidence that our CASPT2//CASSCF/AMBER QM/MM strategy can be correctly applied to the investigation of the excited state of Rh with a computational error <5 kcal mol Thus, our simulation allows for a semiquantitative analysis of the factor determining the properties of the protein environment. Comparing the computed vertical excitation energy with the experimental values (see Scheme 12.6), we found that for Rh the absorption maximum is reproduced with only 3 kcal mol (476 nm vi. 498 nm) while for the solution the error is only 1 kcal mol (433 nm vj . 442 nm). These results confirm the quality of our approach and allow to reproduce the so-called opsin-shift (the 445 nm A ax observed for PSB 11 in methanol is red shifted to 498 nm in Rh) with a 2 kcal mol error. 

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