Pulse Radiolysis Study of Imidazole and Histidine in Water

Authors: P. S. Rao, M. Simic, E. Hayon

The reaction rate constants of e-/sub aq/ and OH radicals with imidazole (ImH), N-methylimidazole (MeIm), histidine, and N-methylhistidine were determined. The k(e-/sub aq/ + S) values were strongly dependent on the state of protonation of the ”pyridine” nitrogen in these molecules, with rates of approximately 5 x 109 M-1 sec-1 at pH approximately 6.0 and (approximately 1-2) x 107 M-1 sec-1 at pH approximately 10.0. The k(OH + S) values show only small but significant changes with pH with rates of approximately 5 x 109 M-1 sec-1 at pH 4 to 5 and approximately 9.0 x 109 M-1 sec-1 at pH 9 to 10. The transient absorption spectra of the e-/sub aq/ and OH radical adducts to these molecules were observed. For e-/sub aq/ addition to imidazole at pH approximately 5.0, the ImH2. radical suggested to be formed has maxima at 300 and 360 nm and epsilon300 5.6 x 103 M-1 cm-1; the MeImH. radical has maxima at 310 and 360 nm with epsilon360 6.1 x 103 M-1 cm-1. The OH radical adducts to ImH2+ and ImH have maxima at 295 and 400 nm and at 310 and 390 nm, respectively. A pK/sub a/(radical) = 6.1 is observed for the equilibrium OH.ImH2+ reversible OH.ImH + H+, which is close to pK/sub a/ = 7.1 for loss of a proton from the ”pyridine” nitrogen of imidazole. The results from the OH radical adduct to N-methylimidazole support this assignment. Similar results wereobtained for histidine and N-methylhistidine. The decay kinetics of the free-radical intermediates were determined.

https://doi.org/10.1021/j100580a006