The Reaction of Thiyl Radical with Methyl Linoleate: Completing the Picture

Cis lipids can be converted by thiols and free radicals into trans lipids, which are therefore a valuable tell-tale for free radical activity in the cell's lipidome. Our previous studies have shown that polyunsaturated lipids are isomerized by alkanethiyl radicals (S^•) in a cycle propagated by reversible double-bond addition and terminated by radical H-abstraction from the lipid. A critical flaw in this picture has long been that the reported lipid abstraction rate from radiolysis studies is faster than addition-isomerization, implying that the "cycle" must be terminating faster than it is propagating! Herein, we resolved this longstanding puzzle by combining a detailed product analysis, with reinvestigation of the time-resolved kinetics, DFT calculations of the indicated pathways, and reformulation of the radical-stasis equations. We have determined thiol-coupled products in dilute solutions arise mainly from addition to the inside position of the bisallylic group, followed by rapid intramolecular H^• transfer, yielding allylic radicals (L_ZZ + S^• ⇄ SL^• → SL'^•) that are slowly reduced by thiol (SL'^• + SH → SL'H + S^•). The first-order grow-in rate of the L_-H^• signal (k_exp^280nm) may therefore be dominated by the addition-H-translocation rather than slower direct H^•-abstraction. Steady-state kinetic analysis of the new mechanism is consistent with products and the rates and trends for polyunsaturated fatty acids (PUFAs), monounsaturated fatty acids (MUFAs), and mixtures, with and without physiological [O₂]. Implications of this new paradigm for the thiol-ene reactivity fall in an interdisciplinary research area spanning from synthetic applications to metabolomics.