734 LIGHT AND LIFE 



The first result of absorbing a quantum of light is to isomerize 

 ll-a\y to 2i\\-trans retinene, still in protonated Schiff base linkage to 

 opsin. This is the transitory, still highly colored product, lumi- 

 rhodopsin. The next step appears to involve changes in the con- 

 figuration of opsin, including an opening of its structure, that ex- 

 poses 2 to 3 sulfhydryl groups per molecule, as also one proton-bind- 

 ing group with pK about 6.6. This is now metarhodopsin. There- 

 after, given a high enough temperature and access to water, the Schiff 

 base linkage hydrolyzes, and retinene and opsin result, the retinene 

 in part free, in part unspecifically attached in Schiff base linkage 

 to a variety of amino groups. It is this last reaction that constitutes 

 bleaching. 



In line with this explanation, it might be expected that any pro- 

 cedure that destroys the fit between retinene and opsin should bleach 

 a visual pigment. So, for example, the denaturation of opsin, by 

 changing its physical configuration, might have this effect, without 

 necessarily involving the isomerization of retinene. This has proved 

 to be true. Hubbard (22, 23) has shown that when rhodopsin and 

 iso-rhodopsin are bleached by heating, about 70 per cent of the reti- 

 nene emerges in the original 11- or 9-cis configuration. (The re- 

 mainder has apparently been isomerized by warming in the presence 

 of protein.) The process of thermal bleaching can be sinnmarized as 

 follows: 



heat 

 rhodopsin > W-cis retinene -\- denatured opsin 



heat 

 iso-rhodopsin > 9-cis retinene -|- denatured opsin 



The increased resonance associated with closeness of fit between 

 the retinene and opsin is, as already said, a principal source of the 

 stability of a visual pigment or iso-pigment. The iso-pigments, in 

 which this fit is less close, are correspondingly less stable. Thus iso- 

 rhodopsin is bleached by heat at a somewhat lower temperature, and 

 with a slightly lower Arrhenius energy of activation than rhodopsin 



(23) . 



Visual excitation must depend upon the initial steps in such a 

 sequence of changes as those shown in Fig. 4; that is, it must depend 

 upon the formation of the lumi- or at most the meta-pigments (54) . 

 The hydrolysis to retinene and opsin is much too slow a process to 

 accoimt for excitation. Indeed, in a number of invertebrate eyes 



