THE STRUCTURE OF VISUAL PURPLE (RHODOPSIN) 



ascertain the answer, ball, collins, dalvi and morton (1949) 

 had shown that retinene would readily react with /7-aminobenzoic 

 acid to give a substance (not an indicator yellow analogue) which 

 had an absorption maximum at 535 m^w. collins and morion 

 therefore carried out the following four experiments. /7-amino- 

 benzoic acid in excess was added (1) to a freshly bleached, slightly 

 acid visual purple solution and the mixture was made first alkaline 

 and then acid; (2) to a slightly acid unbleached visual purple solu- 

 tion, which was then bleached, made alkaUne and finally acid; (3) 

 to a freshly bleached neutral visual purple solution, which was then 

 acidified; (4) to a neutral unbleached visual purple solution, which 

 was then bleached and made acid. In all cases results were negative, 

 i.e. no band at 535 m« appeared. This indicated that there was no 

 free retinene in the solutions immediately after bleaching. Again, 

 retinene will combine with proteins or aliphatic amines (to form 

 indicator yellow analogues) only if the solutions are alkaline. If 

 retinene were formed before indicator yellow then, on bleaching an 

 acid solution of visual purple, one would expect to obtain only free 

 retinene (Amax = 385 m/0; not acid indicator yellow (Amax = 

 440 m^) as is, in fact, produced. From these results collins and 

 MORTON deduced that indicator yellow is formed first and may then 

 be either wholly or partly converted to retinene. They concluded 

 that *the structure of indicator yellow is of vital importance to the 

 structure of rhodopsin. It is clear that although transient orange is 

 the primary product of irradiation, indicator yellow is the first 

 reasonably stable product. The original attachment between carrier 

 protein and chromophoric groups is clearly unbroken and the C — N 

 link of the indicator yellow must also occur in rhodopsin.' (collins 

 and MORTON (1950).) 



Strong confirmatory evidence for this conclusion has recently been 

 advanced by collins (1953). His argument runs as follows. 

 Although, when acid solutions of visual purple are bleached, indica- 

 tor yellow is formed, it might, perhaps, be argued (taking the 

 opposite view that retinene is the first stable product of bleaching) 

 that freshly-formed retinene is in an activated state and in this 

 condition might then react with protein to form indicator yellow, 

 even though the conditions were acid. 



If retinene is formed first, then it ought to be possible — by 'block- 

 ing' all amino groups in the unbleached visual purple molecules — to 

 prevent the subsequent formation of indicator yellow. One way of 



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