Histidine alkylating agent

The quaternization of 1-substituted imidazoles is usually easy unless steric factors intervene, or strongly electron-attracting groups are present, for example, 1-acylimidazoles can only be alkylated at N(3) with powerful alkylating agents such as methyl fluorosulfonate or trialkyloxonium fluoroborates. Trimethyloxonium fluoroborate does not methylate 1-dimethylaminosulfonylimidazole. Regiospecific synthesis of 3-substituted L-histidines can be achieved by alkylation of Ar-/-butoxycarbonyl-l-phenacyl-L-histidinc methyl ester at N(3), followed by reductive removal of the phenacyl group (Scheme 15). 

C-Alkylation of the sodio derivative is accomplished by a technique similar to the alkylation of malonic ester. Primary halogen compounds, quaternary ammonium salts,and an alkene oxide have been used as alkylating agents. Alkylation by secondary halides has been less successful. Hydrolysis of the substituted esters to acetylated amino acids is described for leucine (64%) and phenylalanine (83%). Hydrolysis with deacylation has been used to prepare histidine (45%) and phenylalanine (67%). Glutamic acid (75%) is obtained from substituted acylaminomalonates prepared by the Michael condensation of methyl acrylate and the acylated amino esters. ... 

In the past ten years, there has been developed a series of enzyme inhibitors that combine the features of an alkylating agent with specificity for the active site of an enzyme, thus permitting alkylation and identification of a group at or near the active center of an enzyme, or a particular enzyme to be specifically inactivated. Thus a l-chloro-4-phenyl-3-p-toluenesulfonamido-2-butanone ( W-p-tolylsulfonylphenylalanine chloro-methyl ketone ) inactivates chymotrypsin (which cleaves a peptide bond adjacent to an aromatic residue), and 7-amino-l-chloro-3-p-toluene-sulfonamido-2-heptanone ( a-iV-p-tolylsulfonyllysine chloromethyl ketone ) inhibits trypsin (which cleaves a peptide bond adjacent to lysine. In both cases, a histidine residue at the active site is alkylated, and neither inhibitor will inhibit the other enzyme at low concentrations. 

Other alkylating agents that react with cysteine residue side-chains are methyl iodide, Wethylmaleimide, and acrylonitrile. Alkylating agents react most rapidly with cysteine residues but more slowly with lysine, histidine, methionine, serine, threonine, tyrosine, aspartic acid, and glutamic acid residue side-chains. 

Identification of the Site of Reaction. The alkylation of the active site histidine residue by a substrate related haloketone has been used often to locate a specific histidine in the primary sequency of a serine protease. However, haloketones are general alkylating agents, and if the reagent is too far removed in structure from a substrate, it is likely to alkylate nucleophilic sites other than the catalytic histidine. In one... 

A number of alkylating agents yield derivatives which are stable under the conditions for acidic hydrolysis of proteins. The reaction with ethylene imine giving an S-aminoethyl derivative and, hence, an additional linkage position in the protein for hydrolysis by trypsin, was mentioned in Section 1.4.1.3. lodoacetic acid, depending on the pH, can react with cysteine, methionine, lysine and histidine residues ... 

Oxiranes are highly toxic alkylating agents and the hydrolysis product of ethylene oxide ethylene glycol and reaction products of oxiranes with chloride ions is likewise toxic. The latter reaction yields 2-chloroethanol, which arises from oxirane, and isomeric vicinal chloropropanols (chlorohydrins) resulting from methyloxirane (Figure 11.3). Oxiranes react with a number of other food components, such as vitamins (riboflavin, pyridoxine, niacin and folic acid) or amino acids (methionine and histidine) to form biologically inactive products. 

Sulfur mustard has been found to be genotoxic and mutagenic in several microbial assays. The agent caused alkylation of DNA in the yeast Saccharo-myces cerevisiae (Kircher and Brendel 1983) and interstrand DNA cross-links (Venitt 1968) and inhibition of DNA synthesis (Lawley and Brookes 1965) in Escherichia coli. Using the histidine reversion assay, Stewart et al. (1989a) found that sulfur mustard induced point mutations in Salmonella typhimurium strain TA102 and frameshift mutations in TA 97 but neither type of mutation in strains TA98 and TAIOO. 

---