RETINAL PHOTOPIGMENTS 127 



The porphyropsin cycle was thus formulated by Wald as 

 follows : 



Porphyropsin 

 (522 m/i) 



X. 



Light 



Vitamin A2 + protein -^ Retinene2+ protein 



I \ 

 (355 m/i- in chloroform) (405 m/c in chloroform) 

 (SbCl 3 ^ 696 mju ) (SbCl 3 »-- 706 mju ) 



It is thus seen that the performance of the two systems 

 is identical in detail, but a constant difference in spectrum 

 divides all components in one cycle from their analogues in 

 the other. 



The difference between the two systems apparently lies 

 in the fact that throughout the porphyropsin system there 

 has been added one double bond in the carotenoid chain. 

 This is known from spectrophotometric studies on a series 

 of synthetic carotenoids to shift the spectrum 20-30 m/z 

 towards the red. The separation between the vitamin Ai 

 and A2 maxima is 27 m/x; between rhodopsin and porphyrop- 

 sin, 22 m/x; and between retinenei and retinene2, 18 m/x. The 

 differences are in satisfactory agreement with the proposed 

 structure. It appears, therefore, that the only chemical 

 difference between the two visual systems is the possession 

 by the porphyropsin system of one added double bond. 

 Wald found that all the fresh water fishes which he examined 

 possess only porphyropsin systems, but he also discovered 

 that those fishes which spawn in fresh water and live nor- 

 mally in the sea (anadromous) possess primarily the por- 

 phyropsin systems, whereas those which live mostly in fresh 

 water but spawn in the sea (catadromous) possess primarily 

 the rhodopsin systems. These so-called euryhaline fishes, 

 which are capable of existence in both environments, thus 

 possess either predominately or exclusively that photopig- 



