THE VISUAL PIGMENTS 



mainly attached to the protein since carotenoids are not easily 

 adsorbed in the presence of high concentrations of acetone' (hecht, 

 1942). From these facts dartnall inferred that after bleaching, 

 the altered visual purple chromophore was still attached to the 

 parent protein, this arrangement constituting the indicator yellow 

 molecule. 



In reply, ball, collins and morton (1948) stated that *if under 

 the action of Hght retinene is split off, the conditions are such that it 

 is available for recombination with any protein to form new com- 

 pounds.' Spectroscopic evidence showed that there were three 

 materials in bleached visual purple solutions, namely, retinene (or 

 retinene *loosely bound to protein') and the acid and alkaline forms 

 of indicator yellow. In their opinion the same protein was concerned 

 in all three compounds, the chemical evidence suggesting that, in 

 indicator yellow, the retinene was firmly bound but in neutral bleached 

 visual purple solutions only weakly bound. 



BLISS (1948) suggested that the reason why lythgoe had been 

 unable to extract retinene from bleached visual purple solution was 

 *because he did not shake his extract with enough vigour to compen- 

 sate for the slow diffusion of the liberated retinene through the 

 aqueous phase to the petroleum ether.' However, in support of the 

 view that retinene is not spUt from the visual purple molecule by 

 light, he pointed out that even in the presence of an active agent such 

 as alcohol, retinene is not extracted by petroleum ether for a period 

 of 2-10 min after the solution has been bleached (bliss, 1948). 

 BLISS also recalled lythgoe's (1937) observation of the two 

 forms of acid indicator yellow : a stable form (Amax = 450 mfi) 

 at pH's less than 4 and an unstable form (Amax = 445 mju) at 

 pH's between 4 and 6. The unstable form of acid indicator yellow 

 readily releases retinene when its solutions are shaken with petro- 

 leum ether and bliss supposed that this form corresponded to 

 the *loosely bound retinene' mentioned by ball, collins and 



MORTON. 



The above discussion has been reproduced in some detail because 

 it illustrates the different views held then, and to some extent even 

 now. At the time (1948) it seemed that the question at issue was: 

 indicator yellow or retinene? When it should have been: which 

 comes first ? — a situation only imperfectly grasped (except, perhaps 

 by bliss). 



However, collins and morton (1950b) soon devised tests to 



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