Why Not Trans? Inhibited Radical Isomerization Cycles and Coupling Chains of Lipids and Alkenes with Alkane -thiols.

Abstract

Reversible addition of thiyl radicals to cis fatty acids converts them into trans fatty acids, L Z + S• ⇄ SL• ⇄ L E + S•, in a cycle that, uninterrupted, would rapidly isomerize lipids exposed to radicals and thiols. One reason this does not happen in foods and organisms is because the cycle is interrupted, by exothermic allylic abstraction, L + S• → L• + SH. Autoinhibition limits the cis-trans cycle length to around 400-500 (L E per S•) in a MUFA model (methyl oleate) and just ∼13-15 in a PUFA lipid model (methyl linoleate). The weak C-H bonds in bisallylic groups in PUFAs thereby act as the first line of defense against thiyl cis-trans cycles in biolipid solutions (±O2). With the intriguing exception of vitamin E in MUFA, thiyl-active antioxidants inhibit isomerization in much the same way as they protect against peroxidation. Applied to thiol-ene coupling (TEC), the allylic abstraction, degraded-chain paradigm resolved a raft of hitherto contradictory trends and findings in "click" TEC polymerization and organic synthesis methods.