ISOMERISM AND THE VISUAL PIGMENTS 



Melting points and absorption characteristics of retinene^ isomers 

 in various solvents 



(from Hubbard, Gregerman and Wald, 1953, Wald and Brown, 1953) 



* For 1 per cent solution in a cell of 1 cm path length. The molecular extinction 



M.W. 

 coefficient is obtained by multiplying by . i.e. by 28-4. 



3,780). This reaction thus affords a convenient means of estimating - 

 retinene without regard for its stereochemical form. 



ALLOCATION OF STRUCTURES TO THE RETINENEj ISOMERS 



In attempting to assign spatial formulae to the five retinenes^ we 

 are faced with an immediate difficulty : only four isomers are expected 

 according to the simple theory of cis-trans isomerism, outlined in the 

 earher part of this chapter. The fact that five isomers have been 

 described means, either that the simple theory is incorrect, or, that 

 some of the isomers depend on other than cis-trans differences. 

 Although, at present, it is not possible to reach an unequivocal 

 solution of this problem, it is interesting to examine the evidence. 



The arguments by which spatial formulae for the isomers can be 

 deduced depend very largely on comparisons between absorption 

 spectra (see Fig. 5.4). Since the retinenes are 'half-carotenoids' their 

 principal absorption bands he in the near ultraviolet, roughly in the 



139 



