GEORGE WALD 729 



neo-b before its configuration was established by synthesis. This re- 

 lationshi]) has been demonstrated in both rod and cone pigments, 

 constructed with vitamin A or A.„ in vertebrate and invertebrate eyes 

 (48). 



It is significant that one of the unhindered cis isomers of retinene 

 also combines with opsin to form photosensitive pigments (28) . This 

 is the unhindered 9-cis isomer, Avhich most resembles the W-cis isomer 

 in shape. 9-cis retinene, and retinenco combine with the opsins to 

 form photosensitive pigments that greatly resemble those found 

 naturally, but have never yet been extracted from any retina, and 

 must as yet be regarded as artefacts. We call them the iso pigments 

 — iso-rhodopsin, iso-porphyropsin, iso-iodopsin, and iso-cyanopsin. 



The hindered W-cis configuration of vitamin A or retinene is sur- 

 prisingly stable, once formed. I think it is clear now that though 

 Pauling's strictines were valid thermodynamically, the molecule es- 

 capes them through special kinetic circumstances. The hindered 11- 

 cis isomer represents a higher energy state than the unhindered iso- 

 mers, and under true equilibrium conditions little of it should occur. 

 It seems to be stabilized, however, by lying in a potential energy 

 valley walled off by high potential barriers. It takes a high activation 

 energy either to form the W-cis isomer or to re-isomerize it, once 

 formed. In our experience the molecule goes in either direction 

 conveniently only as the result of absorbing light. 



Under certain conditions the W-cis isomer is indeed favored among 

 the possible configurations of retinene. If a solution of 2M-trans 

 retinene in ethyl alcohol is exposed to the light of an ordinary tung- 

 sten lamp, after about 10 minutes it has isomerized to a steady-state 

 mixture of all possible isomers. About 25 per cent of this mixture 

 is the hindered W-cis species, neo-b (6). 



This being needed to make a visual pigment, what configuration 

 of retinene emerges when a visual pigment is bleached? It turns out 

 that the retinene produced by bleaching a visual pigment is inactive 

 in its resynthesis. It is indeed the 2L\\-trans isomer. It must be re- 

 isomerized to the hindered W-cis configuration before resynthesis can 

 occur (28). 



Fig. 3 shows one passage through such a cycle (49) . On the left, 

 a mixture of W-cis retinene and cattle opsin combines to form rho- 

 dopsin. On the right, this rhodopsin is bleached back to retinene and 

 opsin. To begin the experiment, a solution of cattle opsin in an 

 absorption cell was placed in position in a Gary recording spectro- 

 photometer. A solution of W-cis retinene in digitonin was added. 



