Halogenated retinals

Brominated and fluorinated retinals 26 [183], 27 [183], 28 [184] and 29 [184] also reacted with cattle opsin but 13-desmethyl-14-bromoretinal and the 1-cis isomer of 

Chromophores in which the allenic bond fixes the ring at right angles with the polyene chain were useful not only to study the role of the twist about the 6-s-bond in normal retinals (see p. 307), but also to appreciate the steric and chiroptical requirements of opsin towards the chromophore since they contain two chiral centers, one at C-5 and the other at C-6. 

Both the 9-cw and 9,13-dim isomers of 30 formed visual pigment analogs [165], with Amax at 460 nm and 455 nm (mixed diastereomers), respectively. Moreover, upon incubation of a ca. 1 1 diastereomeric mixture of 30 (the 9-cis isomers) followed by extraction of the bound chromophore by the CH2C12 procedure and 

HPLC and CD measurements, it was found that opsin binds equally well with both of the antipodes. 

In view of the high specificity usually encountered in binding sites, the leniency of opsin in accepting all four diastereomers and enantiomers was surprising. Therefore, adamantyl allenic retinals, 31, were synthesized [87]. The structure of these com- 

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Adhikari, S., Kapoor, S., Chattopadhyay, S., and Mukheijee, T. 2000. Pulse radiolytic oxidation of P-carotene with halogenated alkylperoxyl radicals in a quaternary microemulsion Formation of retinal. Biophys. Chem. 88 111-117. 

Fig. 3.5 Triplet-sensitized isomerization of retinal starting from the all-trans (filled circles), 7-cis (open circles), 9-cis (open triangles), 11-c/s (stars) and 13-c/s (open squares) isomers. The sensitizer, Zn-tetraphenylporphyrin, was irradiated with a 250 W halogen lamp after passing through water and a Toshiba V-Y52 filters [8].

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