ISOMERISM AND THE VISUAL PIGMENTS 



little or no regeneration. They found further, that if a solution 

 bleached by yellow Hght was then exposed to blue, further changes 

 (bleaching) occurred — maximal at short wavelengths — and that the 

 solution would then exhibit some regeneration. They concluded 

 that the solutions contained a yellow (i.e. short-wave-absorbing) 

 substance the bleaching of which assisted in the regeneration of 

 rhodopsin. 



These observations were confirmed by hubbard and wald (1952). 

 Thus a solution of cattle rhodopsin which was bleached by orange 

 light, regenerated in darkness to the extent of 7 per cent; another 

 sample of the same preparation, when bleached with white light, 

 regenerated 26 per cent. When the regenerated pigments were 

 bleached a second time — but with the bleaching Hghts reversed — 

 there was a similar difference between the amounts of pigment 

 subsequently regenerated, viz. 8 per cent in one case (where orange 

 light was used for the second bleaching) and 26 per cent in the other. 



These results were explained by hubbard and wald in terms of 

 the isomerization of the retinene released as a result of bleaching. 

 The absorption spectra of rhodopsin solutions after bleaching were 

 slightly different depending on whether a non-isomerizing (long 

 wavelength) or isomerizing (short wavelength) light had been used 

 for bleaching. Thus when a solution of rhodopsin (pH 6-5) was 

 bleached by orange hght the product of bleaching had Amax = 

 386 m/i. On irradiating the bleached solution with white light the 

 maximum shifted to 380 m/i. When digitonin solutions of all-trans 

 neo-a and neo-b retinenesi were isomerized by exposure to white hght 

 a similar shift occurred only in the case of all-rra«5 retinenCi. hub- 

 bard and WALD thus concluded that when rhodopsin was bleached 

 by non-isomerizing (i.e. long- wavelength) hght the retinene was 

 released in the 2M-trans form. 



In the synthesis of cattle rhodopsin, however, the neo-b isomer of 

 retinene is required, a fact which strongly suggests that the retinene 

 moiety of this rhodopsin molecule is in the neo-b form. At some 

 stage in the bleaching process, therefore, conversion of the retinene 

 moiety from the neo-b to the 2i\\-trans structure occurs. If this con- 

 version is a consequence solely of the absorption of light, then it 

 cannot happen at the indicator yellow (retinene-protein) or retinene 

 stages of the bleaching process for neither of these appreciably absorb 

 orange light. It must occur in circumstances where orange hght is 

 fairly strongly absorbed, viz. at the transient orange stage or even 



147 



