THE VISUAL PIGMENTS 



or, in more convenient shorthand, 



-CHpH 



It is, therefore, a polyene alcohol, i.e. is characterized by a regular 

 alternation of double and single valency bonds in the molecule — a 

 feature common to all the carotenoids. In chloroform solution, 

 vitamin A^ has a single absorption band in the ultraviolet (Amax = 

 332 mju) and, when mixed with antimony trichloride reagent, gives a 

 transient blue colour (Amax = 620 mju). Retinenci has a single 

 absorption band in the near ultra-violet (Amax = 385 m/x in chloro- 

 form) and, like vitamin A^, also yields a blue colour (Amax = 664 m/Li) 

 with antimony trichloride. 



The probable structural formula for vitamin Ag (CigHgg.CHgOH), 

 the corresponding decomposition product of visual violet (porphy- 

 ropsin) is, 



i.e. the ring structure contains one conjugated double bond more 

 than that of vitamin A^. 



RELATIONSHIP BETWEEN ABSORPTION SPECTRA AND STRUCTURE 

 IN THE CONJUGATED POLYENES 



In the structural formulae of organic chemistry each single valency 

 bond is equivalent to two electrons, more or less equally shared 

 between the two atoms forming the bond. Similarly a double bond 

 is equivalent to four shared electrons. 



Where there is a conjugated chain of alternate single and double 

 bonds, however, a special situation arises. For example, the formula 

 for the conjugated fragment. 



H H 



I I 



1 I 



H H 



could equally well be written 



1 V 1 



100 



