The Photolysis and Radiolysis of Simple Mercaptans in Dilute Glassy Matrices

Abstract

The e.p.r. spectrum of thiyl radicals is obtained when dilute solutions of aliphatic mercaptans are photolyzed in a xenon matrix at 77 OK. No proton hyperfine structure is observed at low concentrations of thiol but it appears at higher concentrations due to the reaction of hot hydrogen atoms with solute mercaptan molecules. When small amounts of the thiols are dissolved in glassy hydrocarbon matrices such as 3-methylpentane, there is produced after photolysis hyperfine structure characteristic of the solvent as well as the thiyl resonance. The e.s.r. absorption due to the solvent species disappears when the matrix is warmed a few degrees above 77 "K but thiyl radicals are stable up to the melting point of the glass. The corresponding y-irradiated glasses do not give thiyl absorption, but two other species are produced along with solvent radicals. No thiyl radicals are observed following radiolysis, or upon photobleaching the new species. The spatial distribution of the neutral radicals in the glasses is discussed in view of these results.Le spectre de r.p.e. de radicaux thiyl est obtenu lors de la photolyse de solutions dilukes de mercaptans aliphatiques dans une matrice de xenon a 77 OK. On n'observe pas la structure hyperfine du proton en utilisant de faibles concentrations de thiol, mais elle apparait lorsqu'on utilise des concentrations plus ClevCes a cause de la reaction des atomes d'hydrogkne chauds avec les molCcules de mercaptan en solution. Si on dissout de petites quantites de thiol dans des matrices d'hydrocarbure vitreuses, tel le methylpentane-3, il se produit apres photolyse, une structure hyperfine qui est caracteristique du solvant aussi bien que de la resonance du thiyl. L'absorption r.p.e. due a l'espece solvant disparait quand la matrice est chauffee a quelques degris au-dessus de 77 OK, mais les radicaux thiyl demeurent stables jusqu'au point de fusion du verre. Les verres irradits aux rayons-y ne donnent pas d'absorption thiyl, mais deux autres especes sont produites en meme temps que les radicaux de solvant. On n'observe pas de radicaux thiyl apres radiolyse, ou pendant la photodCcoloration des nouvelles espkces. On discute de la distribution spatiale des radicaux neutres dans les verres pour arriver aces rksultats.

Intramolecular Hydrogen Transfer Reactions of Thiyl Radicals from Glutathione: Formation of Carbon-Centered Radical at Glu, Cys, and Gly

“…20 Additional ESR experiments obtained with small model thiols also suggest the possibility for a formal 1,2-hydrogen transfer in thiyl radicals, that is, the observation of • CH 2 SH radicals after the photolysis of CH 3 SH, 22 and the detection of CH 3 C • H-SH radicals after photolysis of CH 3 CH 2 SH. 23 Theoretical calculations have examined activation energies for intramolecular hydrogen transfer reactions in GS • . 24 Here, the hydrogen transfer between the thiyl radical and the α C−H bond of γ-Glu displays a rather low activation energy of ΔG ⧧ = 37−42 kJ mol −1 , whereas the 1,3-hydrogen shift within the Cys residue shows ΔG ⧧ = 110 kJ mol radicals from Cys (CysS • ) exists, both in the gas phase 25 and in solution.…”

“…the observation of • CH 2 SH radicals after the photolysis of CH 3 SH, 22 and the detection of CH 3 C • H-SH radicals after photolysis of CH 3 CH 2 SH. 23 …”

“…The latter observation would suggest the possibility for a 1,3-hydrogen transfer in GS • , supported also by experimental observations on the oxidation of penicillamine . Additional ESR experiments obtained with small model thiols also suggest the possibility for a formal 1,2-hydrogen transfer in thiyl radicals, that is, the observation of • CH 2 SH radicals after the photolysis of CH 3 SH, and the detection of CH 3 C • H-SH radicals after photolysis of CH 3 CH 2 SH …”

Photochemistry of XCH2CN (X = −Cl, −SH) in Argon Matrices

“…Photolysis of thiol species of the form R–SH results mainly in scission of C–S and S–H bonds − with the main products being radicals and, depending on photolysis conditions, various radical recombination products. In 195 nm flash photolysis experiments on gaseous CH 3 SH, Callear et al observed formation of CH 3 S, CH 3 , and other radicals using UV absorption spectroscopy while 254 nm photolysis of C 2 H 5 SH in a xenon matrix was shown to produce the thiyl radical C 2 H 5 S* . Photolysis of CH 3 SH vapor at 254 and 214 nm has been shown to produce H 2 , H 2 S, C 2 H 4 , C 2 H 6 , and CH 3 SSCH 3 ; C 2 H 5 SH vapor photolyzed under the same conditions produced H 2 , H 2 S, C 2 H 4 , C 2 H 6 , and C 2 H 5 SSC 2 H 5 …”

“…In 195 nm flash photolysis experiments on gaseous CH 3 SH, Callear et al 52 observed formation of CH 3 S, CH 3 , and other radicals using UV absorption spectroscopy while 254 nm photolysis of C 2 H 5 SH in a xenon matrix was shown to produce the thiyl radical C 2 H 5 S*. 55 56 and C 2 H 5 SSC 2 H 5 . 57 Hydrogen atom production and escape of H 2 from the matrix cage has been reported as well, producing even more complex, dehydrogenated species, where photolysis of methanethiol at 121 nm in a nitrogen matrix at 14 K leads to formation of CH 4 , CS, CS 2 , CH 3 , and H 2 CS as the main products.…”