62 



VITAMINS A AND CAROTENES 



discussion which follows will concentrate primarily upon the rhodopsin 

 system, which has led the way in all retinal chemistry. The other visual 

 systems will be considered only as minor variations upon this central theme. 



B. VITAMIN A AND RETINENE 



As already noted, the bleaching of rhodopsin is a complex process. It 

 begins with a typical photochemical reaction, which converts rhodopsin 

 to a highly unstable, orange-red product called lumi-rhodopsin. In the dark, 



CH3 



CH3 



C 



CH, 



CH3 



HoC 



C— C=C- 

 H H 



-C=C— C=C— C=C— CHoOH 

 H H H H 



H2 C C — CHj 



c 

 H2 



CH3 CH 



\ / 



c 



/ 



H2C 



Vitamin Ai, C20H29OH 



CH3 



CH3 



H2C 



C— C=C— C=C— C=C— C=C— C=0 

 H H H H H H H 



C— CH3 



C 

 H2 



Retinenei , C20H28O 

 Fig. 14. Structural formulas of vitamin Ai and rotineiiei. 



lumi-rhodopsin continues to react, going over with little change in color to 

 meta-rhodopsin. Then, given access to water, meta-rhodopsin bleaches in the 

 dark to a mixture of retinene and opsin. ^ 



We owe the identification of retinene to Morton and his coworkers. Vita- 

 min A is the primary alcohol, C19H27CH2OH. Morton has shown that 

 retinene is vitamin A aldehyde, CigHorCKO.^ The structures of these sub- 

 stances are shown in Fig. 14; their properties are listed in Table XVI. 



Morton showed also how to prepare retinene by the mild oxidation of 

 vitamin A. His simplest procedure was to add a pinch of manganese dioxide 



« G. Wald, J. Durell, and R. C. C. St. George, Science 111, 179 (1950). 

 ' S. Ball, T. W. Goodwin, and R. A. Morton, Biochem. J. 42, 516 (1948). 



