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

Abstract

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 + S ⇄ SL → SL') that are slowly reduced by thiol (SL' + SH → SL'H + S). The first-order grow-in rate of the L signal (k) 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 [O]. Implications of this new paradigm for the thiol-ene reactivity fall in an interdisciplinary research area spanning from synthetic applications to metabolomics.

Prooxidative chain transfer activity by thiol groups in biological systems

“…2 D). Notably, cis-trans -isomerization of monounsaturated fatty acid (MUFAs) has been described before as specific footprint of thiyl radical formation in biological systems [ 21 , 22 ]. Consistently, dodecanol at identical concentrations did not induce fatty acid changes in human cells ( Fig.…”

[Cu(TPMA)(Phen)](ClO4)2: Metallodrug Nanocontainer Delivery and Membrane Lipidomics of a Neuroblastoma Cell Line Coupled with a Liposome Biomimetic Model Focusing on Fatty Acid Reactivity

“… 11 , 12 The reaction of metal complexes with thiols, which are active biomolecules, can lead to the generation of thiyl radicals that either catalyze a cis–trans isomerization of double bonds and/or initiate a lipid peroxidation. 64 The biomimetic model of liposomes—made of phospholipids—was designed and used to follow the fatty acid fate. 11 In this section, we applied this biomimetic model to study the reaction of Cu-TPMA-Phen with thiols in vesicle suspensions made of l -α-phosphatidylcholine derivatives ( Figure 9 ).…”

Cis/Trans Isomerization of Unsaturated Fatty Acids in Polysorbate 80 During Light Exposure of a Monoclonal Antibody–Containing Formulation

“…In 1959, Walling and Helmreich 41 reported on the isomerization of olefins by thiyl radicals. About 40 years later, Chatgilialoglu et al initiated a series of mechanistic studies on thiyl radicaleinduced isomerization of double bonds in monounsaturated and polyunsaturated FAs, [14][15][16][17][18][19] proceeding according to equilibria 5 and 6, where in tert-butanol at room temperature k a Z ¼ 1.6 Â 10 5 M À1 s À1 , k a E ¼ 2.9 Â 10 À1 s À1 , k f Z ¼ 1.7 Â 10 7 M À1 s À1 , and 42 This cis/trans isomerization proceeds via a chain reaction, which is subject to retardation by several pathways. 30 First, retardation may proceed via hydrogen abstraction from allylic or bisallylic methylene groups such as present in monounsaturated and polyunsaturated FAs.…”

“…Here, we report that the light exposure of polysorbate 80 (PS80)-containing formulations can induce the cis/trans isomerization of unsaturated FAs in polysorbate esters when proteins are present. These isomerization reactions likely result from reactions of thiyl radicals, [14][15][16][17][18][19] which can be generated during the light exposure of proteins. [20][21][22] Evidence for thiyl radical-mediated cis/ trans isomerization of monounsaturated and polyunsaturated FAs comes from a series of mechanistic studies with isolated FAs and their methyl esters, phospholipids, micelles, and vesicles.…”

“…[20][21][22] Evidence for thiyl radical-mediated cis/ trans isomerization of monounsaturated and polyunsaturated FAs comes from a series of mechanistic studies with isolated FAs and their methyl esters, phospholipids, micelles, and vesicles. [14][15][16][17][18][19] Light exposure of a monoclonal antibody (mAb) can generate thiyl radicals (RS ) via different pathways. First, the photoionization of a protein tryptophan (Trp) residue 23 can lead to a hydrated electron, which efficiently reduces a protein disulfide bond to yield a protein thiyl radical and thiol (reactions 1 and 2).…”