PROPERTIES OF THE fAROTENOIDS 637 



has found that the effect of acids in producing isomerization can be pre- 

 ^'ented by the addition of such bases as pyridine, quinoUne, or dimethyl- 

 anihne. 



(e) cis-trnns Isomerizafion Produced by Photodiemical Action. All known 

 carotenoids are more or less sensitive to the effect of photochemical action. 

 However, marked variations in susceptibility to the effect of such light rays 

 occur between the different carotenoids. In general, the cis compounds 

 (particularly the poly-c/s derivatives) would appear to be more sensitive to 

 such treatment than are the all-/ran.s isomers. Thus, it was shown'*^^ that 

 when all-frans-/3-carotene was insolated for 45 minutes in a petroleum ether 

 solution, only 2% of stereoisomers resulted. When neo-^S-carotene U was 

 similarly treated, 55% was changed to the all-^rans form, 6% to the neo-/3 

 isomer, and 2.5% to the mixed C + D neo compounds. When neo-|8- 

 carotene B was used as the starting material, only 27% remained after 

 insolation for 45 minutes, while the following isomers were present: Neo V, 

 2.5%; all-/rans, 60%; neo B, 5%; and mixed C + D neo isomers, 5.5%. 

 The distribution of isomers after insolation of various members of the a- 

 carotene set is given in Table 27. The marked susceptibility of such poly- 

 cis compounds as prolycopene and pro-7-carotene to insolation was shown 

 by the fact that the almost colorless pigments turned intensely yellow after 

 exposure to sunlight for a few minutes. ^'*^ This change was accompanied 

 by a marked change in the absorption maxima, which were found at longer 

 wave lengths. 



Table 27 



Relative Colorimetric Values (per Cent of Pigment Recovered) of \.iAj-lrans- 



q-Carotene and of Some of Its Stereoisomers Formed by Insolation of Petroleum 



Ether Solutions for 45 Minutes" 



" L. Zeohmei.ster, Chem. Revs., 3/,, 267-344 (1944). 



The photochemical action is produced more efficiently by the visible 

 than by the ultraviolet spectiTim. The effect can be shown to be inde- 

 pendent of the thermal process, since it proceeds equally well when precau- 

 tions are taken to prevent heating. Considerable destruction ma.y also 

 accompany the photochemical process, particularly if some oxygen is 

 present, or catalysts are accidentally available. In practice, the effect of 



