Oxidation of Histidine by Pyridinium Bromochromate in Aquo-Acetic Acid Medium: A Kinetic Investigation

Abstract

Amino acids do not react with CrO₃ and K₂Cr₂O₇ but react with pyridinium bromochromate. Oxidation of histidine with pyridinium bromochromate (PBC) has been studied in 30% acetic acid-70% water (v/v) mixture containing perchloric acid and leads to 2-imidazole aldehyde, carbon dioxide and ammonia as products. The reaction rate is first order with respect to each in [histidine] and [PBC] while inverse second order with respect to [H⁺]. Michaelis-Menten type kinetics is observed with respect to histidine. The rate of reaction increases with decrease in polarity of solvent indicating ion-dipole interaction. Absence of primary kinetic isotope effect (k_H/k_D) = 1.12 at 303 K) rules out the α-C-H bond fission in the rate determining step. Activation parameters have been evaluated. A rate limiting C-C bond fission of the chromic-ester formed by zwitter ion of histidine and unprotonated PBC is consistent with the observed results. The rate law proposed and derived is in agreement with experimental results.