The Formation and Structure of the Sulfoxyl Radicals RSO·, RSOO·, RSO₂·, and RSO₂OO· from the Reaction of Cysteine, Glutathione and Penicillamine Thiyl Radicals with Molecular Oxygen

Abstract

This work reports an electron spin resonance study of the reactions of cysteine, glutathione and penicillamine thiyl radicals with molecular oxygen in frozen aqueous solutions at low temperatures. For all three thiols, the thiyl radical, RS., is found to react with oxygen to form the thiol peroxyl radical, RSOO(.). On the absorption of visible light, RSOO(.) photoisomerizes to the sulfonyl radical, RSO2(.), which subsequently reacts with molecular oxygen to form RSO2OO(.), the sulfonyl peroxyl radical. The identities of the sulfonyl and sulfonyl peroxyl radicals were confirmed by their production by a different route, from sulfinic acid. Sulfinyl radicals, RSO(.), are found as the final radical species in the reactions of thiyl radicals and oxygen. Parallel 17O hyperfine couplings (A parallel) are reported for each sulfoxyl radical and a correlation between the spin density on oxygen and the reactivity of the radical is suggested. As a result of this correlation sulfonyl peroxyl radicals are predicted to be far more reactive than thiol peroxyl radicals. We also report molecular orbital calculations on the nature of the spin density distribution and the molecular geometry of the model radicals CH3SO2(.) and CH3SO2OO(.).

Intra-molecular reactions between cysteine sulfinyl radical and a disulfide bond within peptide ions

“…The reactivity of peptide cysteine sulfinyl radical ions with organic disulfides, thiols, and molecular oxygen in a linear ion trap mass spectrometer was investigated via ion/molecule reactions recently [18]. The sulfinyl radical appeared to be the least reactive relative to the thiyl and perthiyl radical, which agreed with studies in the condensed phase [19][20][21]. The low reactivity was in-part due to the delocalization of the spin between the sulfur and oxygen atom within SO Cys as shown by theoretical calculations [22].…”

S-OO bond dissociation energies and enthalpies of formation of the thiomethyl peroxyl radicals CH3S(O)nOO (n=0,1,2)

“…Under atmospheric conditions equilibrium between CH 3 S and CH 3 SOO is rapidly established with approximately 20-80 % of CH 3 S in the form of CH 3 SOO at 298 K. The computational investigations of the equilibrium CH 3 S+O 2 CH 3 SOO [5][6][7][8][9] have produced a binding energy for S-OO ranging from 8.8 to 14.8 kcal mol -1 , depending on the method, in agreement with the experimental estimate of ~11 kcal mol -1 [4]. Photo-isomerization to the methylsulfonyl radical, CH 3 S(O) 2 , [10][11][12][13] and further coupling with O 2 give rise to the possible formation of a series of moderately bound peroxy radicals [14][15] (The notation by Yin et. al [16] is followed, where sulfonyl or sulfinyl oxygen atoms are put in parentheses).…”

The Interaction of Hypotaurine and Other Sulfinates with Reactive Oxygen and Nitrogen Species: A Survey of Reaction Mechanisms

“…One and two-electron pathway for the oxidative reactions of sulfinates To briefly overview the one-electron mechanism: the reaction between sulfinates (RSO 2 -) and one-electron oxidants, such as ONOO -, CO 3 •-and • NO 2 , is accompanied by the generation of intermediate sulfonyl radicals (RSO 2 • ). Acting as a strong oxidizing species, RSO 2 • can produce the sul-fonyl-peroxyl radical (RSO 2 OO • ) when oxygen is present ( in Scheme 7) (Sevilla et al 1990;Flyunt et al 2001). RSO 2 OO…”

“…It may initiate an oxygen-dependent radical chain propagation with a possible amplification of the oxidative mechanism ( in scheme 7) (Sevilla et al 1990;Flyunt et al 2001) or it can dimerize to form disulfone derivative (RSO 2 -SO 2 R), that subsequently hydrolyzes to yield RSO 3 -( in Scheme 7) (Ricci et al 1978;Fellman et al 1987;Green and Fellman 1994). The dimerization route does not require oxygen and can be operative in systems when low oxygen uptake is observed or in anaerobiosis (Ricci et al 1978;Pecci et al 1999;Baseggio Conrado et al 2015).…”

“…• is an intermediate in this mechanism and also a highly reactive species that can react with excess RSO 2 -and produce peroxysulfonate (RSO 2 OO -) (Scheme 4) (Sevilla et al 1990). …”