S-Alkyl-cysteine

All these enzymes have similar specificity and require the L-cysteine sulfoxide portion of the molecule. S-alkyl cysteines are not substrates 18, 24, 27, 39) neither are sulfoxides of N-substituted L-cysteine, / -di-methyl-L-cysteine, y8-thiopropionic acid 18), D-cysteine 19), and dl-methionine 18, 24, 39), nor do the enzymes act on L-cysteine itself 24, 27, 39), cysteic acid, cysteinesulfinic acid 24, 27), or cycloalliin 24, 39). There may be some action on S-ethylcysteine sulfone (42). 

New metab. of propylbrotnide (I) in rat is n-propylmercapturic acid sulfoxide ilk%), trace of S contg. metab. suggest that S-alkyl. cystein residues formed only with (I) (1). Haloalkanes gen. metab. to mercap-turic ac. and similar S-linked conj. Addl. metab. reaction is hydroxy-lation (2). 

S-alkyl cysteines, such as S-allyl cysteine (SAC), S-methyl cysteine (SMC), and S-propyl cysteines are slowly formed during sprouting from the action of y-glutamyl transpeptidase on y-glutamyl cysteines. S-allyl cysteine is also rapidly formed in the body by hydrolysis of y-glutamyl cysteine particularly in the kidney which contains an abundant amount of Y-glutamyl transpeptidase activity [2]. 

Froelich, J.M. Reid, G.E. Mechanisms for the proton mobility dependent gas-phase fragmentation reactions of S-alkyl cysteine sulfoxide-containing peptide ions. J. Am. Soc. Mass Spectrom. 2007,18, 1690-1705. 

Activated derivatives of S-alkyl-cysteine suffer base catalyzed racemization even when their amino group is blocked by the benzyloxycarbonyl or other urethane-type protecting group. A simple, but not uncontested, explanation is reversible -elimination... 

Racemization during the synthesis of peptides is a complex problem. The diversity of possible courses followed in the process is compounded by the individuality of the amino acids. This was already shown on the example of S-alkyl-cysteine residues which lose chiral purity by a special mechanism even if protected by a urethane-type protecting group that prevents racemization in other acylamino acids. The opposite end of the scale is represented by proline... 

The aroma of many vegetables is due to volatile sulfur compounds obtained by a variety of enzymatic reactions. Examples are the vegetables of the plant families Brassicacea and Liliaceae their aroma is formed by decomposition of glucosinolates or S-alkyl-cysteine-sulfoxides (cf. 17.1.2.6.7). 

Its threshold values are 0.27-0.53 ppm (in water) or 12.5-25 ppm (in edible oil) It is derived biosynthetically from an S-alkyl cysteine sulfoxide, lentinic acid. Truffles, edible potato-shaped fungi, contain approx. 50ng/g 5a-androst-16-ene-3 a-ol, which has a musky odor that contributes to the typical aroma (cf. 3.8.2.2.1). 

Figure 7.7 Some of the S-alkyl cysteine sulphoxides found in Allium species.

Triola G, Brunsveld L, Waldmann H (2008) Racemization-free synthesis of S-alkylated cysteines via thiol-ene reaction. J Org Chem 73 3646-3649... 

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