Sulfhydryl groups, amino acid

The amino acid residues in neurotoxins which are important for neurotoxic action are still not entirely clarified. Some neurotoxins contain one free SH group, while others do not. From this fact, it would be logical to assume the sulfhydryl group is not essential. This was actually proven to be the case. 

Biosorption is a rather complex process affected by several factors that include different binding mechanisms (Figure 10.4). Most of the functional groups responsible for metal binding are found in cell walls and include carboxyl, hydroxyl, sulfate, sulfhydryl, phosphate, amino, amide, imine, and imidazol moieties.4 90 The cell wall of plant biomass has proteins, lipids, carbohydrate polymers (cellulose, xylane, mannan, etc.), and inorganic ions of Ca(II), Mg(II), and so on. The carboxylic and phosphate groups in the cell wall are the main acidic functional groups that affect directly the adsorption capacity of the biomass.101... 

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). 

The iodoacetyl group of both isomers reacts with sulfhydryls under slightly alkaline conditions to yield stable thioether linkages (Figure 9.7). They do not react with unreduced disulfides in cystine residues or with oxidized glutathione (Gorman et al., 1987). The thioether bonds will be hydrolyzed under conditions necessary for complete protein hydrolysis prior to amino acid analysis. 

FIGURE 3.22 Protection (A) of carboxyl groups of amino acids as ferf-butyl esters,58 (B) of carboxyl groups of /V-substituted amino acids as fert-butyl esters,61 and (C) of phenolic and sulfhydryl groups as ethers. The amino acid esters are isolated as the hydrochlorides. 

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