ISOMERISM AND THE VISUAL PIGMENTS 



the formation of a different isomer if, between the interchange and 

 the reinstatement, rotations about single bonds take place. For 

 example : 



all-transform /X/X.,.^%v/ > .^^^^.^^^^^^^"^^^ 



a cw-isomer 



i 



Any process which facilitates a reversible interchange of single and 

 double bonds in a conjugated chain will cause isomerization to occur. 

 Such changes might be initiated by the accession of energy to the 

 molecule, either from collision with other molecules (resulting in 

 thermal isomerization) or by absorption of light. 



The effect of iodine in facihtating isomerization might result from 

 photosensitization of the carotenoid to visible Hght, which iodine 

 strongly absorbs, or from an ephemeral addition of iodine across a 

 double bond, thereby converting it temporarily to a single bond. 



CIS-TRANS ISOMERS OF VITAMIN Ai IN THE 

 SYNTHESIS OF VISUAL PURPLE 



The present chapter was opened with wald and Hubbard's 

 observation that rhodopsin can be synthesized in solution from liver 

 oil vitamin A^ but not from crystaUine vitamin A^. 



The results of the experiment are shown in Fig. 5.2. The synthesis 

 was followed by measuring the rising extinction at 500 mju with time 

 (Fig. 5.2, left). In the case of the solution containing the liver oil 

 concentrate (empty circles) the rise was very marked and was still 

 proceeding after 3 hr. With the solution containing crystaUine all- 

 trans vitamin A^ (filled circles), however, there was only a moderate 

 rise in extinction which ceased after about 1 hr. After 3 hr hydroxyl- 

 amine was added to both solutions to break up 'fortuitous combina- 

 tions of retinene with other complexes.' The changes in absorption 

 caused by bleaching (difference spectra) were then measured and are 

 shown on the right of Fig. 5.2. Crystalline vitamin Aj had yielded 

 only about one tenth as much rhodopsin as had that from the liver 

 oil concentrate. Another isomer of vitamin Aj, namely crystaUine 

 «eo-vitamin Aj,a (robeson and Baxter, 1947) was found to be 



135 



