Oxidation of carotenoids by free radicals: relationship between structure and reactivity

The relationship between structure and reactivity is reported for a collection of carotenoids in solution reacted with oxidants generated by a modified Fenton process or with peroxyl radicals generated via the azo-initiators AMVN and AIBN. The initial rates of oxidation were in the order: lycopene > beta,beta-carotene, zeaxanthin > echinenone, isozeaxanthin > astaxanthin, canthaxanthin. The oxidative degradation caused rapid bleaching, due to disruption and breakdown of the polyene chromophore. A number of reaction mechanisms are likely to be involved. Isozeaxanthin, canthaxanthin and astaxanthin, in which the C-4 and C-4' positions are occupied by functional groups, react more slowly than beta,beta-carotene and zeaxanthin, in which this position is free. Products such as the 4-methoxy (or 4-ethoxy) and 4,4'-dimethoxy (or 4,4'-diethoxy) derivatives were isolated from reactions of beta,beta-carotene with peroxyl radicals in the presence of methanol or ethanol. Electron density calculations suggest that the different reactivities cannot be attributed solely to differences in electron distribution along the polyene chain of the different chromophores, which would alter the susceptibility to free-radical addition to the conjugated double-bond system. Other reactions must therefore be considered, including hydrogen abstraction from positions allylic to the polyene chain (C-4 of beta,beta-carotene and its derivatives, and of lycopene). Lycopene, lutein and zeaxanthin all reacted rapidly with oxidising agents, so these dietary carotenoids must also be considered as potential antioxidants.