Methyl thiyl radical

At present the exact details of the mechanisms leading to each of the products are far from well understood (7.81. However, it seems likely that oxidation of the common naturally-occurring organo-sulfur compounds (dimethyl sulfide, methyl mercaptan and dimethyl disulfide) proceeds, at least in part, through formation of the methyl thiyl radical, CHjS (4.9-111. That is the main reason for carrying out direct studies on CH3S radical reactions where the individual reactions are isolated. 

Consideration of the relative concentrations of various oxidants in the marine boundary layer leads to the conclusion that reaction with O3, along with CH3SOO adduct formation, is likely to be the dominant fate of the methyl thiyl radical under typical atmospheric conditions. The CH3S+O3 reaction has been the subject of several kinetic and mechanistic studies (see Table 3) the rate coefficient for this reaction has been found to exhibit a slight negative activation energy (295-367 K) and no dependence on pressure (75-325 Torr He) [59]. The mechanism of the CH3S-1-O3 reaction remains uncertain. The thermodynamically viable reaction channels include reactions (6a) to (6f) [61J ... 

No direct studies of CH3S reactions with HO2 or organic peroxy radicals have been reported. However, because the atmospheric concentrations of peroxy radicals are much smaller than typical concentrations of O3, peroxy radicals will generally be unimportant as CH3S reaction partners. Indirect experimental evidence suggests that the methyl thiyl radical reacts with both HO2 and CH3O2 via O atom transfer [63, 64] (see Table 3). 

REACTIONS OF METHYL THIYL RADICALS WITH UNSATURATED HYDROCARBONS... 

The investigation into thiyl-catalyzed isomerization of olefins eventually resulted in the measurement of relative rate coefficients for several CH3S + olefin reactions. These early relative rate studies relied on the interpretation of complex chemical systems to arrive at rate coefficients for the elementary reactions involving CH3S attack on the olefin double bond. More recently, the characterization of the CH3S laser induced fluorescence (LIF) spectrum [90,91] has led to direct measurements of rate coefficients for methyl thiyl radical reactions with unsaturated hydrocarbons. Reported relative rate coefficients are tabulated along with the recent direct measurements of Balia et al. [92] (see Table 6). This has been done to allow for observation of any reactivity trends that may be present and to facilitate qualitative discussion of proposed CH3S+olefin reaction mechanisms. 

It has been demonstrated that the MCR enzyme is active only if the metal center of coenzyme F430 is in the Ni1 form.1857 The natural substrate Me-CoM or simple methyl thioethers, however, do not react with Ni1 F430, which has lead to the proposal of a catalytic mechanism in which the addition of a thiyl radical to the S atom of the thioether giving a sulfuranyl radical intermediate is... 

In a similar manner, many additions of heteroatom radicals to unsaturated positions have been studied. In many cases, addition reactions of heteroatom radicals to alkenes are reversible and thermodynamically disfavored, but their occurrence is apparent. For example, the rapid addition and elimination of thiyl radicals to unsaturated fatty acid methyl esters results in isomerization reactions from which kinetic parameters can be obtained. Additions of group 14 (IV A) metal-centered... 

Fang X, Jin F, Jin H, von Sonntag C (1998) Reaction of the superoxide radical with the A/-centered radical derived from N-acetyltryptophan methyl ester. J Chem Soc Perkin Trans 2 259-263 Ferreri C, Costantino C, Landi L, Mulazzani QG, Chatgilialoglu C (1999) The thiyl radical-mediated isomerisation of c/s-monounsaturated fatty acid residues in phospholipids a novel path of membrane damage Chem Commun 407-408... 

ItoO, Matsuda M (1979) Evaluation of addition rates of the thiyl radicals to vinyl monomers by flash photolysis. 2. Substituent effect on addition of substituted benzenethiyl radicals to methyl methacrylate or styrene. J Am Chem Soc 101 5732-5735 Ito O, Matsuda M (1981) Evaluation of addition rates of thiyl radicals to vinyl monomers by flash photolysis. 3. Polar effect in addition reactions of substituted benzenethiyl radicals. J Am Chem Soc 103 5871-5874... 

Sprinz H, Adhikari S, Brede O (2001) Transformation of phospholipid membranes by thiyl radicals via ds-tmns isomerization in fatty acid residues. Adv Colloid Interface Sci 89/90 313-325 Stefanic I, Bonifacic M, Asmus K-D, Armstrong DA (2001) Absolute rate constants and yields of transients from hydroxyl radical and H atom attack on glycine and methyl-substituted glycine anions. J Phys Chem A 105 8681-8690... 

Figure 5 Proposed catalytic cycle of MCR. Methyl CoM binds first to the active site pocket, followed by CoB, which plugs the channel. The methyl group is transferred from CoM to nickel (step 1). The donor for the hydron that is taken up is suggested here to be a tyrosyl residue in the active site. The Ni(III)-methyl group is a strong oxidizing species and oxidizes the CoM-SH to a thiyl radical CoM-S" (step 2). Release of the methyl group takes place by hydronation of the methyl-Ni(II), while the CoM forms a disulfide with CoB, with electron transfer to the Ni(I) these two processes are combined in step 3. Repulsion of the sulfonate group by the Ni(I) would favor the release of the product disulfide from the active site (step 4). (Adapted from Ref. 119.)...

Scheme 13. The catalytic reaction was proposed to go through a sulfuranyl radical by addition of a thiyl radical to the sulfur atom of the thioether MeSPh. Interestingly, these studies are consistent with the proposed mechanism of the enzyme the thiyl radical of cofactor HS-CoB resulting from one-electron reduction of F430 by S-CoB couples with Me-CoM to afford the sulfuranyl radical, CoB-S-S (Me)-CoM. The sulfuranyl radical transfers a methyl radical to Ni -p43o to generate Me-Ni -F43o and the heterodisulfide CoM-S-S-HTP...

Schwinn J, Sprinz H, Drossier K, Leistner S, Brede O. (1998) Thiyl radical-induced cis/tr r-isomerization of methyl linoleate in methanol and of linoleic acid residues in liposomes. Int JRadiat Biol 74 359-365. 

The gas phase photolysis of methyl disulfide at 2537 or 2288 A in the presence of NO results in the formation of methylthionitrite without added NO, the only major product is CH3SH. The results are consistent with initial S-S cleavage into excited thiyl radicals... 

Chemically activated methyl disulfide, formed by the recombination of thiyl radicals, suffers collisional deactivation in competition with S-S cleavage. Although the molecular elimination... 

CTI reactions of methyl linoleate [35], y-linolenate [35], and arachidonate [36] catalyzed by thiyl radicals have been studied in some detail. Each isolated double bond in PUFA behaves independently as discussed above. Indeed, the time profiles of methyl linoleate disappearance and formation of mono-trans and di-trans isomers in these experiments indicated that the CTI occurs stepwise (Scheme 6.5). The number of possible geometrical isomers increases according to 2n, where n is the number of double bonds, and the complete analysis may be difficult and incomplete for high unsaturation as in the case of methyl arachidonate. 

The role of sulphur-containing compounds in photopolymerization appears to have attracted some interest. Bis(j -methylpyridazinyl)-3,3 -disulphide has been found to initiate the photopolymerization of styrene but inhibits the thermal polymerization. The role of thiyl radicals (PhS-) in photoinitiated polymerization of vinyl monomers by aromatic thio-compounds has been postulated by several workers. In one study, flash photolysis was used to identify the nature of the radical. Sulphur-containing monomers such as 4-methyl-2-(vinylthio)thiazole and thiocyclanes have been photopolymerized and copolymerized with other vinyl monomers. Luca et al. have devised a mathematical model for the photopolymerization of 2,3-dimethylbutadiene and thiourea. 

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