Amino acid side chains thiol containing

Aryl halide compounds such as fluorobenzene derivatives can be used to form covalent bonds with amine-containing molecules like proteins. The reactivity of aryl halides, however, is not totally specific for amines. Other nucleophiles such as thiol, imidazolyl, and phenolate groups of amino acid side chains also can react (Zahn and Meinhoffer, 1958). Conjugates formed with sulfhydryl groups are reversible by cleaving with an excess of thiol (Shaltiel, 1967). 

Reagents containing thiol groups, usually mercaptoethanol or thioglycolic acid, are used to reduce the disulfide cross-links in wool [13,242]. Tributyl phosphine and sulfites are also used. Normally, reduction of wool affects only cystine residues no other amino acid side-chains react. 

Thiols also undergo oxidation by the loss of hydrogen atoms from each of two —SH groups. The oxidized product contains a disulfide bond —S—S—. Much of the protein in hair is cross-linked by disulfide bonds, which occur mostly between the side chains of the amino acids of cysteine, which contain thiol groups. 

It should be appreciated that amino acids such as serine, threonine, tyrosine, and cysteine all contain side-chain alcohol or thiol groups that may participate in hydrogen bonding and stabilize a particular protein conformation. 

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