Singlet oxygen initiates lipid peroxidation via a non- free radical mechanism by reacting directly with unsaturated lipids to form lipid hydroperoxides

L-H represents a lipid, generally a polyunsaturated fatty acid moiety (PUFA). (See Figure 1 legend for an explanation of the abbreviations used to denote the various chemical species associated with lipid peroxidation). The rate of the propagation event is governed by the various carbon-hydrogen bond dissociation energies along the lipid chain (Cosgrove et al., 1987). The weakest carbonhydrogen bonds are those of the bis-allylic methylene positions, which have carbon-hydrogen bond energies of approximately 75 kcal/mol compared to 101 kcal/mol for typical alkyl C-H bonds (Figure 2). In homogeneous solution the oxidizability of unsaturated lipids increases linearly with the number of bis-allylic positions in the lipid chain (Cosgrove et al., 1987). However, the rate of cellular lipid peroxidation has been shown to increase exponentially with the number of bisallylic positions contained in cellular lipids (Wagner et al., 1994).