80 



VITAMINS A AND CAROTENES 



this be called isorhodopsin; there is as yet no evidence that it occurs in the 

 living retina. Isoretinene b itself appears to be inactive; but on long incuba- 

 tion, or on very short exposure to light, it isomerizes to isoretinene a, which 

 yields isorhodopsin (Fig. 24). 



It will be noted that we have mentioned five crystalline isomers of 

 retinene, though present theory limits the number of cis-trans isomers to 



400 



600 



450 500 550 

 wavelength — mju 



Fig. 24. The synthesis of rhodopsin and isorhodopsin from isomers of retinene. 

 Difference spectra of the products formed when single stereoisomers of retinene are 

 incubated in the dark with cattle opsin. A.\\-trans retinene and neoretinene a yield 

 no light-sensitive pigment; neoretinene h yields rhodopsin (Xmax 500 m/i) indistinguish- 

 able from that extracted from the dark-adapted retina; and isoretinene a yields iso- 

 rhodopsin (Xmax 487 m^u). Isoretinene h itself appears to be inactive l)ut isomerizes 

 preferentially to isoretinene a, which yields isorhodopsin. (From R. Hubbard and 

 G. Wald."^ 



four. We have examined the properties of the retinene isomers with particu- 

 lar care, with the thought that one or more of them might involve some type 

 of structural modification other than cis-trans isomerism. It has turned out 

 that all five isomers are interconvertible on gentle treatment. All of them, 

 on simple exposure to light, isomerize to what appears to be the same 

 e(iuilibrium mixture of isomers. All of them also yield an identical product 

 in the reaction with antimony chloride, and in the same (molar) amount. 



