. Can. J . Chem. 67, 1426Chem. 67, (1989. The rate constants for detritiation from the C(2) position of 1-meth~l[2-~~]histidine have been determined in a series of aqueous buffers at 8S0C. The resulting sigmoidal rate-pH profile was indicative of a mechanism involving hydroxide ion attack on the N(3)-protonated (4) and the amino-protonated (5) forms of 1-methylhistidine, and dissection of the kinetic data allowed the extraction of the second-order rate constants for the two pathways, k and k t . The unusually large value of k' for a species not protonated at N(3) of the imidazole ring suggested the involvement of a kinetically equivalent zwitterionic form of the substrate (7). Comparison of the rate constant k with values determined previously for closely related substrates, such as histidine, 1-methylimidazole, and imidazole, led to the use of FMO theory to explain the effect of the various structural changes, e.g., the effect of methylation and a positively charged side chain on k and k'. The addition of MeHgN03 resulted in a decrease in the pseudo-first-order rate constant for dehitiation. The rate retardation was discussed in terms of two mechanisms (Schemes 2 and 3). Analysis of the data in terms of the various metal-ion-coordinated species present under the experimental conditions showed that the reactivity of the protonated substrate greatly exceeds that of the metal-coordinated species. The difference in the catalytic ability of H+ vs. MeHg+ is discussed in terms of the extent of positive charge developed on the ligating heteroatom in the ylide (carbenoid) reaction intermediate.Key words: methylmercury, 1-methylhistidine, isotopic exchange, proton transfer, metal ion catalysis.ERWIN BUNCEL, HELEN A. JOLY et DIANE C. YEE. Can. J. Chem. 67, 1426Chem. 67, (1989.On a dktermink les constantes de vitesse de la dktritiation h partir de la position C(2) de la rn~th~l-l[~~-2]-histidine dans une skrie de solutions tampons ?i 85°C. Le profil sigmoidal de la courbe de vitesse en fonction du pH obtenue, tkmoigne I'existence d'un mkcanisme faisant intervenir l'attaque de l'ion hydroxyle sur les formes protonkes en N(3) (4) et amino-protonke (5) L'addition du MeHgN03 conduit h une diminution de la constante de vitesse de pseudo ordre un de la dktritiation. On discute de la diminution de la vitesse en fonction de deux mkcanismes (schkmas 2 et 3). L'analyse des donnkes en fonction des diverses espi?ces ion mktallique-coordonnk prksentes dans les conditions exp6rimentales dkcrites, montre que la rkactivitk du substrat proton6 exckde de beaucoup celle des espkces mktalliques coordonnkes. On discute de la diffkrence de pouvoir catalytique de H+ versus MeHg+ en fonction du taux de charge positive dkveloppke au niveau de l'hktkroatome jouant le r6le de ligand dans l'intermkdiaire (carbknoide) de rkaction de l'ylide.Mots clks : mercure-mkthyle, mtthyl-1-histidine, tchange isotopique, transfert de proton, catalyse d'ion mttallique.[Traduit par la revue]Histidine (1) is one of the most important amino acids found in biologi...