GEORCE WALD 735 



(squill, lobster) , true bleaching hardly occurs under j)hysiological 

 conditions, the action of light on the visual pigments ending with the 

 production of nieta-rhodopsin (18) . 



It has already been said that simple exposure to light isomerizes 

 free retinene, eventually to a steady-state mixture of all possible 

 isomers; and that in a polar solvent about 25 per cent of this steady 

 state mixtiue is ll-r/^, about 25 per cent other cis isomers, and about 

 half M-tya)is. Similar isomerizations can occur with retinene attached 

 as chromophore to opsin (25, 26, 30) . If rhodopsin, for example, 

 is irradiated at about — 60°C, in a glycerol-water mixture, the first 

 quantum absorbed by the molecule isomerizes W-cis retinene to all- 

 tra72s, yielding lumirhodopsin; but the absorption of further quanta 

 at this temperature continues to isomerize retinene, ending in such 

 a steady-state mixture of isomers as just described. Since, however, 

 in this case the retinene remains attached to unaltered opsin at the 

 specific chromophoric site, the fraction of this mixture which is ll-cis 

 is now again rhodopsin — rhodopsin made from lumirhodopsin 

 through re-isomerization of the retinene by light. Another fraction, 

 which is 9-cis, is now isorhodopsin, also made so by light. The re- 

 mainder of the retinene is in configurations which do not serve as 

 the chromophores of photopigments: a.ll-tra7TS, \S-cis, etc.; and this 

 is the fraction — about half the total — that bleaches to retinene and 

 opsin on warming in the dark. A mixtuie of rhodopsin and iso- 

 rhodopsin constitutes the remaining half of the product. 



A similar steady state is instituted when metarhodopsin, stabilized 

 by holding below — 20 °C, is irradiated with visible light. The form 

 of this steady state is shown in the following scheme, together with 

 estimates of the quantum efficiencies (y) of the component photo- 

 reactions (25, 26, 30) . In this scheme the quantum yield of the 

 isomerization from the W-cis to the a\\-trans chromophore is set arbi- 

 trarily at 1, and the others are rated accordingly. 



y := 1 y = 0.09 



Rhodopsin ^ Metarhodopsin "-" Isorhodopsin 



y = 0.5 y = 0.3 



(ll-cis) (all-trans) (9-cis) 



When it was first found that about half the product of bleaching 

 rhodopsin after irradiation in the cold is still in the form of photo- 

 sensitive pigment, it was thought that the latter must result from 

 some "dark" reversion process. We now realize that, on the con- 

 trary, this residue of rhodopsin and isorhodopsin is formed as de- 



