Amino acid side chains sulfhydryl-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). 

Photooxidations are not normally considered a protein deteriorative reaction because they usually go unseen or are found only when purposely contrived, such as in the chemical modification of proteins (Figure 17). However, the possibility of their occurrence in foods, particularly those containing added dyes, should not be overlooked. Several of the important amino acid side chains are readily modified, including the sulfhydryl, imidazole, phenoxy, indole, and th1o ether (Figure 18). More general and detailed coverage 1s provided in another article in this volume (34). 

The structures of the amino acids are given in Figure 21.2. A side chain is nonpolar if it is mostly hydrocarbon in nature (like alanine). Polar side chains may contain the hydroxyl group (—OH), the sulfhydryl group (—SH), or a second amino (—NH3) or carboxyl (—COOH) group. 

In summary, protein molecules may contain up to nine amino acids that are readily derivatizable at their side chains aspartic acid, glutamic acid, lysine, arginine, cysteine, histidine, tyrosine, methionine, and tryptophan. These nine residues contain eight principal functional groups with sufficient reactivity for modification reactions primary amines, carboxylates, sulfhydryls (or disulfides), thioethers, imidazolyls, gua-nidinyl groups, and phenolic and indolyl rings. All of these side chain functional groups in addition to the N-terminal a-amino and the C-terminal a-carboxylate form the full complement of polypeptide reactivity within proteins (Fig. 12). 

Hair is composed principally of keratin. An important difference between hair keratin and other proteins is its high content of the amino acid cysteine, which contains the —SH (sulfhydryl) group in its side chain. 

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