Thiols formation

Nitrosimines should be stored at lower temperature and protected from light. They are relatively stable when dissolved in aqueous buffers. They release both NO and N2O, the latter via a nitroxyl intermediate (HNO). In the presence of thiols, formation of N2O increases at the expense of NO production, which suggests that NO release involves an oxidative mechanism. However, nitrososydnonimines can release up to two moles of NO per molecule, one from the nitrosimine and another from the resulting sydnonimine. 

Mechanistic Studies of Organosulfur (Thiol) Formation in Coastal Marine Sediments... 

Further evidence for the addition of H2S to carbon-carbon double bonds very early in sediments, and further insights into reaction mechanisms, have been reported by Vairavamurthy and Mopper in 1987 and 1989 (109.110). They identified 3-mercaptopropionic acid (3-MPA) as a major thiol in anoxic intertidal marine sediment and demonstrated that the thiol formation could occur by the reaction of HS with acrylic acid in sediment water and seawater at ambient temperature The formation of 3-MPA was hypothesized to occur by a Michael addition mechanism whereby the nucleophile HS adds to the activated double bond in the a,/3-unsaturated carbonyl system ... 

The photolyses of diphenyl, di-o-octyl and di-/>-tolyl disulfides in organic solvents at 2540 A produce the corresponding thiols which are formed via H-abstraction of the thiyl radical the quantum yield of thiol formation is 0.046 for all three disulfides . 

A number of mechanisms for thiol formation using other sulfur species are plausible. Two nucleophilic displacement type mechanisms appear most promising. However, these mechanisms may not occur in each marsh because of differing physical and geochemical characteristics which are described later. 

Sulfoxidation. Sulfoxidation Is common in sulfur biochemistry (43,44), and has been observed with many of the transamination products listed in Table III, with mercapturic acids (45,46,47) and cysteine conjugates (12). Sulfoxidation is also common with the methylthio-containing metabolites produced from cysteine conjugates (see "Thiol Formation" below). Sulfoxidation increases the polarity of compounds and therefore affects their excretability. 

Thiol Formation. Thiol formation from cysteine conjugates has been known since 1951 (57), toxicity of the thiol formed durinq dichlorovinylcyst ne metabolism was reported in 1957 (15), and the enzyme involved was termed a "thionase" in 1965 (58). The enzyme(s) is now called a cysteine conJugate-8-lyase (C-S lyase) and has been purified from rat liver (59,60), partially purified from microorganisms (61,62) shown to function in the intestinal microflora (9,49), and in kidney (63). The general reaction catalyzed by C-S lyase is shown in Equation 11. 

The rate of thermal decomposition of ZDDPs can be greatly inhibited by the presence of overbased detergents [47]. Inhibition of volatile alkyl thiol formation due... 

Whether the enzyme action is a result of some type of nitroso thiol formation, the conversion of the N02 to a more reactive nitric oxide free radical (NO°) or other species is not yet established. Figure 10-23 summarizes these possible events. It should be understood that all the pieces in the puzzle are not yet in place. 

The contribution of the dissociatively adsorbed H2S in the reaction pathway was stated on the basis of the dependence of the thiols formation on the Me -FAU zeolite electronegativity. ... 

The Increase in the zeolite electronegativity resulted from the alkali cation exchange causes the decrease in the thiols formation. 

Above Steps (1) and (2) are peptide chain cleavage, Steps (3) and (4) are thiol formation by hydrolysis. Steps (5) and (6) are hemithioacetal regeneration via cyclization of the thiol. 

Vairavamurthy M. A. and Mopper K. (1989) Mechanistic studies of organo-sulphur (thiol) formation in coastal marine sediments. In Biogenic Sulfur in the Environment (eds. E. S. Saltzman and W. J. COOPER), pp. 231-242. ACS, Washington, DC. 

Triphenylmethanol treated with 0.55 eqs. of Lawesson s reagent in DME at room temp, under argon for 15 h (or in toluene under reflux for ca. 10 min) triphenyl-methanethiol. Y 100%. Reaction of tert. alcohols possessing an alkyl group at the a-position led to alkene formation (especially in refluxing toluene), but thiol formation predominated at lower temp. F.e. [of sec. and tert. benzylic or allylic mercaptans] s. T. Nishio, J. Chem. Soc. Chem. Commun. 1989, 205-6. 

In a commercial application of thiol formation by this nucleophilic substitution, the sodium salt of thioglycolic acid is prepared by the reaction of sodium hydrosuMde and sodium iodoacetate. 

Disulfides, such as the diallyl disulfide shown in Rgure 8.24, can be obtained by the reaction of sodium disulfide (NaiSi), itself prepared by the dissolution of an equivalent of sulfur in a concenctrated sodium sulfide (Na2S) solution, with the corresponding alkyl halide. Reduction of the disulfide (e.g., with zinc dust in acetic acid) results in cleavage of the sulfur-sulfur bond and thiol formation (Scheme 8.105). Alternatively, as might be expected, thiols are rather easily oxidized (e.g., by hypohalites [NaOCl]) and the products are disulfides (Equation 8.60). 

The thiol formates have CH vibrations which absorb at 2835-2825 cm" (CH stretch), 1345-1340 cm (CH deformation), and 2680-2660 cm (deformation overtone). The thiol acids in solution absorb at 2585-2565 cm (SH stretch) and at 837-828 cm" (SH in-plane deformation). ... 

---