THE VISUAL PIGMENTS 



*cis peak' region for *whole' carotenoids (e.g. /5-carotene, Fig. 5.1): 

 likewise, the 'cis peak' region of the retinenes is located in the far 

 ultraviolet; in roughly the same spectral region as the quarter 

 chromophore bands of 'whole' carotenoids. Apart from these 

 differences from true carotenoids — differences which, of course, 

 arise from the shorter conjugated chain — the retinenes have absorp- 

 tion spectra which are analogous to those of the carotenoids and the 

 same rules apply. 



Considering first the principal bands of the retinenes we see 

 (Fig. 5.4) that the aW-trans isomer has its maximum (in alcohol) at 

 383 mju, while three other isomers have their maxima about 6 m/^ 

 lower, viz. iso-a at 376 m/*, neo-a at 377 mjbt and neo-b at 377-5 m/n. 

 Such a shift of maximum corresponds — in carotenoids — to a single 

 cis linkage in the conjugated chain. The remaining isomer, iso- 

 retinene b, has a principal-band maximum at 369 m/<, 14 m^ lower 

 than that for the a\\-trans isomer, and thus indicating the presence of 

 two cis links. From a comparison between the principal bands there- 

 fore, it would seem that the five isomers include an edl-trans form, 

 three mono-d5 forms, and one di-cis form. 



How does this conclusion tally with the absorption characteristics 

 in the cis peak region? The all-trans isomer has low and poorly 

 defined absorption here (Fig. 5.4), as would be expected, but so also 

 have wo-retinenesi a and b, the presumptive mono- and di-cis isomers. 

 Only the «eo-retineneSi a and b, show the well defined cis peaks 

 expected of mono-cis isomers. In any case, if there are three mono- 

 cis isomers (iso-a, neo-a and neo-b) one of them must be a sterically 

 hindered form, viz. the 2- or 4-cis isomer. 



2-cw-retinenei 4-c/5-retinenei 



Now recent work with the polyenes (oroshnik et al, 1952, car- 

 eers et al, 1952) has indeed shown that compounds containing 



140 



