OXIDATION OF CAROTENOIDS BY FREE-RADICALS - RELATIONSHIP BETWEEN STRUCTURE AND REACTIVITY

The relationship between structure and reactivity is reported for a co llection of carotenoids in solution reacted with oxidants generated by a modified Fenton process or with peroxyl radicals generated via the ate-initiators AMVN and AIBN. The initial rates of oxidation were in t he order: lycopene > beta,beta-carotene, zeaxanthin > echinenone, isoz eaxanthin > astaxanthin, canthaxanthin. The oxidative degradation caus ed rapid bleaching, due to disruption and breakdown of the polyene chr omophore. A number of reaction mechanisms are likely to be involved. I sozeaxanthin, canthaxanthin and astaxanthin, in which the C-4 and C-4' positions are occupied by functional groups, react more slowly than b eta,beta-carotene and zeaxanthin, in which this position is free. Prod ucts such as the 4-methoxy (or 4-ethoxy) and 4,4'-dimethoxy (or 4,4'-d iethoxy) derivatives were isolated from reactions of beta,beta-caroten e with peroxyl radicals in the presence of methanol or ethanol. Electr on density calculations suggest that the different reactivities cannot be attributed solely to differences in electron distribution along th e polyene chain of the different chromophores, which would alter the s usceptibility to free-radical addition to the conjugated double-bond s ystem. Other reactions must therefore be considered, including hydroge n abstraction from positions allylic to the polyene chain (C-4 of beta , beta-carotene and its derivatives, and of lycopene). Lycopene, lutei n and zeaxanthin all reacted rapidly with oxidising agents, so these d ietary carotenoids must also be considered as potential antioxidants. (C) 1997 Elsevier Science B.V.