Hydrogen abstraction, carotenoids

Woodall et al. suggested that the different reactivities of the carotenoids against free radicals can be partly attributed to the differences in electron distribution along the polyene chain of different chromophores that would alter the susceptibility of free radical addition to the conjugated double bond system. However, other factors must be also considered. Stearic hindrance, hydrogen abstraction from the allylic position to the polyene chain (C-4 of P-carotene and its derivatives, end of lycopene), would reduce radical scavenging activity. In addition, the stability of the polyene radical is important in determining the rate of the loss of carotenoids and hence it affects their antioxidant activity. 

Hydrogen atom abstraction (CAR- -R CAR -6 RH), where a hydrogen atom is taken from the carotenoid allylic to the polyene chain, leaving a resonance-stabiUzed carotenoid radical. 

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