Retro-retinal

Proton translocation to the Schiff base nitrogen was proposed to occur by concerted double proton transfer (as shown in Fig. 15) leading to a retro retinal structure in the batho intermediate [127, 196], However, this model can be eliminated as it is inconsistent with the formation of batho intermediates from pigment analogs based on 5-desmethylretinal [145] and y-retroretinal [146,147], It also disagrees with the resonance Raman results. 

The process entails shifting of double bonds along the polyene chain, with the formation of a "retro-retinal" structure. Peters et al. (301) interpreted their observations by identifying PBAT with an excited state of rhodopsin, where single proton transfer toward the Schiff base nitrogen leads to the formation of bathorhodopsin. This approach has been supported by the theoretical interpretation of the spectrum of rhodopsin in terms of a nonprotonated Schiff base (214-216). A mechanism involving deprotonation of the Schiff base has also been suggested (310). All these models do not require cis-trans isomerization as a primary event in the chromo-phore. 

Reduction of bacteriorhodopsin during illumination in the presence of borohydride produces a fluorescent species which absorbs at 360 nm [195,196]. The fine structure of the absorption band of this form is like that of retro-retinal, but the participation of such an isomer in the photocycle of bacteriorhodopsin [215,216] is unlikely. Rather, planarization of the chromophore by constraints of the retinal binding site might be responsible for the fine structure [110]. 

Because of the presence of an extended polyene chain, the chemical and physical properties of the retinoids and carotenoids are dominated by this feature. Vitamin A and related substances are yellow compounds which are unstable in the presence of oxygen and light. This decay can be accelerated by heat and trace metals. Retinol is stable to base but is subject to acid-cataly2ed dehydration in the presence of dilute acids to yield anhydrovitamin A [1224-18-8] (16). Retro-vitamin A [16729-22-9] (17) is obtained by treatment of retinol in the presence of concentrated hydrobromic acid. In the case of retinoic acid and retinal, reisomerization is possible after conversion to appropriate derivatives such as the acid chloride or the hydroquinone adduct. Table 1 Hsts the physical properties of -carotene [7235-40-7] and vitamin A. 

In another study, Bienayme [65] obtained retinal in three steps from P-ionone, involving a Pd-catalyzed rearrangement of a mixed carbonate, derived from ethynyl-retro-ionol. 

Thus, the (3-ionone was smoothly deconjugated and ethynylated to give ethynyl-retro-ionol as a mixture of E/Z stereoisomers. Formation of the carbonate and its Pd-catalyzed rearrangement produced straightforward a mixture of aldehydes and a allene compound. After silica-gel chromatography, the allenic-aldehyde was conjugated with a catalytic amount of HBr in acetone. Retinal was obtained as a mixture of E and Z isomers (75/25), which could be converted into the all E isomer by simple equilibration, Fig. (33). 

A similar route was patented by Ancel and Meilland [66], The ethynyl-retro-ionol was acetylated (Ac20-DMAP-Et3N) and this propargylic acetate was reacted with methyl butadiene acetate in the presence of BF3-Et20. The allenic-retinal, obtained in 61% yield was isomerised in retinal by HBr in acetone (yield 50%), Fig. (34). 

The photoisomerization of retinal has been studied using super-short light pulses, and the neutral and protonated forms of the retinal analogue (47) have been examined by time-resolved methods.The irradiation of 3-ionone as a complex with P-CD in water leads only to the formation of retro-y-ionone." ... 

Retinal pigment epithelium (RPE), 3925, 3927-3929, 3937, 3939 3941, 3943-3947 Retinoids, 2675, 2695 Retinol, 3369 Retinopathy, 1450 Retroaldol-aldol mechanism, 2700 Retrochalcones, 1869, 1874, 1875 Retrograde amnesia, 292 Retronecine, 1058, 1064 Retro-Prince reaction, 3075, 3076 Retrorsine, 1061, 1063, 1385 Reversed phase... 

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