Histidine alkyl halides

Hannig et al. used C- and N-protected histidine to synthesise a chiral imidazoUum salt by quarternisation of the two ring nitrogen atoms with alkyl halides [63] (see Figure 6.20). 

Alkyl halides are even less reactive than acyl halides, as indicated by the compilation of reaction rates of thiolate anions with various types of alkyl halides (282). Nevertheless, potentially useful affinity labels have been synthesized with alkyl halide substituents and have been shown to specifically inactivate several enzymes, albeit slowly the low reactivity of the alkyl halides may minimize nonspecific reaction. Adenosine 5 -(2-bromoethyl)phosphate has been characterized and reported to inactivate NAD -dependent isocitrate dehydrogenase (283). The 2 - and 3 -(2-bromoethyl)-AMP labels have also been synthesized, and model reactions of the bromoethyl-AMPs with cysteine, lysine, histidine, and tyrosine have been studied (284). More recently, esters of adenosine 5 -monophosphate have been prepared with ethyl, propyl, or hexyl moieties and bromo or chloro substituents at the w position (285). Yeast alcohol dehydrogenase exhibited enhanced inactivation by the hexyl derivative, but inactivation rates of other dehydrogenases were unremarkable. Two iodopropyl derivatives of cAMP have been described, namely, 1, A -(3-iodopropyleno)adenosine 3, 5 -cyclic monophosphate and 3 -0-(2-iodo-3-hydroxypropyl)adenosine 3, 5 -cyclic monophosphate the latter inactivates cAMP phosphodiesterase from human platelets, with a pseudo-first-order rate constant of 0.147 hr" (286). 

Alkyl halide, sodium hydride in DMSO 2.5.1 General, substances insoluble in common organic solvents, polysaccharides, lipopolysaccharides, peptides, sterically hindered functions, amides, amino acids Esters, and as stated for procedure 2.4 peptides containing sulfur-amino acids, histidine and arginine 132] O-Acyl groups are replaced with O-alkyl groups. For N-isopropyl amino acids, isopropyl ester formation, (see Chapter 2)... 

Another potent group of alkylators are the a-haloketones. The classic example is the alkylator TPCK (tosyl-L-phenylalanylchloromethyl ketone) which interacts specifically with one imidazole ring (histidine residue) in the enzyme a-chymotrypsin (see Section 7.2.3). These compounds are significantly more reactive in Sn2 displacements than alkyl halides. For example, nucleophilic attack by iodide will proceed 33,000 times as quickly on chlo-roacetone as on n-propyl chloride (acetone solvent, 50°C). Inductive pull of the carbonyl function would be expected to both increase the electrophilicity of the methylene function, and stabilize the approaching anionic nucleophile. a-Haloacids and amides will exhibit a similar effect, and both iodoacetic acid and iodoacetamide have found use as biochemical probes for the alkylation of purified enzymes. 

All three elimination reactions—E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol rather than an alkyl halide, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the -OH group is assisted by simultaneous protonation. 

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. ... 

Trimethylsilyl chloride also gives the N -product alone in simple models, but this is probably the consequence of thermodynamic control in any event N -trialkylsilyl derivatives are much too labile to be useful as stepping stones to N"-alkylhistidines. The only thoroughly studied example of exclusive mono-N -derivatization is the reaction of acetic anhydride with acylhistidine esters, when thermodynamic control appears to operate. Thus, N -(benzyl-oxycarbonyl)histidine methyl ester gives only the M-acetyl derivative, providing a third kind of intermediate which is useful for N"-alkylation with reactive halides (see Section 2.6.2.3).1 1... 

Oxidation is one of the major causes of protein degradation and has been widely studied. The side-chains of histidine (His), methionine (Met), cysteine (Cys), tryptophan (Trp) and tyrosine (Tyr) residues in proteins are potential oxidation sites. The reactive oxygen species include singlet oxygen O2, superoxide radical O2, alkyl peroxide ROOH, hydrogen peroxide H2O2, hydroxy radicals (HO or HOO ), and halide complexes (CLO )." The reactivity of these oxidants is ... 

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