Comparative studies on radical cation formation from carotenoids and retinoids

A comparative study of radical cation formation from selected polyenes, namely carotenoids (C40) and retinoids (C20), has been carried out by treatment with the Lewis acids BF3 as its etherate or SbCl3. The reaction in chloroform was monitored by vis/NIR and EPR spectroscopy at variable temperature. β,β-Carotene, β,β-caroten-4-ol, retinol (vitamin A), retinyl acetate and anhydroretinol were used as substrates. It is concluded that whereas BF3–diethyl etherate or SbCl3 is capable of effecting one-electron transfers to produce radical cations from the longer polyenes (carotenoids), no radical cations were obtained from the retinoids. The results of SbCl3 treatment of the retinoids have a bearing on the current studies on the mechanism of the Carr–Price blue colour reaction previously used for quantitative analysis of vitamin A.