THE VISUAL PIGMENTS 



In a complex molecule like this, the effects of steric hindrance can 

 be more readily appreciated with the aid of models in which the 

 spatial orientation of valency bonds and the sizes and distances apart 

 of the constituent atoms are all properly represented. 



According to pauling (1939) a cis conformation in the carotenoids 

 and related substances can be assumed only by those double bonds 

 which carry methyl side chains (i.e. bonds 3 and 5) and not by those 

 which are adjacent to a C — CH3 group (bonds 2 and 4) or which are 

 located in a ring (bond 1). 



3' 



H 



'>H) (hT 





(a) (b) 



Cw-conformations for (a) =CH— C(CH3)=CH— CH= and 

 (b) =C(CH3)— CH=CH— CH== 



In (a) which is representative of bonds 3 and 5 there is Httle over- 

 lapping between the spheres of influence of the hydrogen atoms and 

 the strain thus caused can in any case be alleviated by small rotations 

 (about the single carbon-carbon bonds) out of coplanarity. In (b), 

 representing bonds 2 and 4 the overlap between the hydrogen and the 

 bulkier methyl group is much greater and cannot be reheved by 

 rotation about single bonds. The cis form in this example is a 

 strained and hence unstable configuration. 



Because of these hmitations only four (2^), instead of sixteen (2*) 

 isomers are likely for vitamin A^. Conventional formulae for these 

 are given below: 



all trans 3-cis 5-cis 3,5 di-cis 



Unhindered cis-trans isomers of vitamin Aj 



130 



