Reactivity with p-nitrophenyl acetate and interaction between the amino and imidazole groups of histidine and related compounds

Abstract

The study of the reaction of p-nitrophenyl acetate (PNPA) with histidine and certain derivatives showed that the species in which the amino group is unprotonated (R(NH 2)Im) react with second-order rate constants ( k 2 am ) that are higher than predicted by a Brønsted relation for a series of neutral amino acids. The reason for this behavior was investigated through an analysis of the kinetics of the reaction of PNPA with these compounds in order to assess the reactivities of the amino and imidazole groups in the two species R(NH 3 +) Im and R(NH 2) Im . The rate constant for the reaction with the imidazole group ( k 2 im ) of N π-methyl histidine agrees with the value predicted by a Brønsted relation obtained from a series of model imidazole compounds. N τ-Methyl histidine, however, is unreactive, indicating that N τ is the reactive nitrogen in the imidazole ring of histidine. The k 2 im values found for histidine, histidine methyl ester, and N α-dimethyl histidine are lower than predicted by the Brønsted relation. This behavior was found to be due to low reactivity of the R(NH 3 +) Im species, in contrast with the normal reactivity of R(NH 2) Im . The evidence presented suggests that the lower reactivity of R(NH 3 +) Im is due to an ion-dipole interaction between the protonated amino group and the unprotonated imidazole ring, which displaces the tautomeric equilibrium toward the unreactive N τ-H form. The higher reactivity of the imidazole group in the species R(NH 2)Im, relative to that in R(NH 3 +) Im , is responsible for the observed high k 2 am values for histidine, for histidine methyl ester, for N τ-methyl histidine, and for N α-dimethyl histidine, in contrast with the normal k 2 am value found for N τ-methyl histidine. The conclusions from this study of histidine and its derivatives support the proposal of an interaction between the protonated N-terminal amino group and the imidazole ring of His 6 in the octapeptide hormone angiotensin.