A large kinetic isotope effect in the reaction of ascorbic acid with 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO˙) in aqueous buffer solutions.

Ikuo Nakanishi,Toshihiko Ozawa,Shunichi Fukuzumi,Kei Ohkubo,Ken-Ichiro Matsumoto,Yoshimi Shoji

doi:10.1039/d0cc05214k

Abstract

A large kinetic isotope effect (KIE, kH/kD) of 12.8 was observed for the hydrogen-transfer reaction from ascorbic acid to 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO˙) in a phosphate buffer solution (0.05 M, pH/pD 7.0) at 298 K. The isotopic difference in the activation energies (6.8 kJ mol-1) determined from the temperature dependence of the KIE suggests that quantum mechanical tunneling may partly play a role in the reaction, although the isotopic ratio of the Arrhenius prefactor (AH/AD = 0.86) is within the semiclassical limits.

Highlights

Ikuo Nakanishi, *a Yoshimi Shoji,a Kei Ohkubo, ab Toshihiko Ozawa,c Ken-ichiro Matsumoto a and Shunichi Fukuzumi *de
Ursic et al reported that a large kinetic isotope effect (KIE, kH/kD) of 24.2 was observed in water for the hydrogentransfer reaction from ascorbic acid (AscH2), one of the representative water-soluble antioxidants, to 2,2,6,6-tetramethylpiperidine1-oxyl (TEMPO) radicals.[5]
Li has recently reported a new and simple antioxidant assay in vitro using 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide radicals (PTIO), one of the nitronyl nitroxide radicals, where a hydrogen transfer occurred from antioxidants to PTIO.[6]

Summary

Introduction

Ikuo Nakanishi, *a Yoshimi Shoji,a Kei Ohkubo, ab Toshihiko Ozawa,c Ken-ichiro Matsumoto a and Shunichi Fukuzumi *de. A large kinetic isotope effect (KIE, kH/kD) of 12.8 was observed for the hydrogen-transfer reaction from ascorbic acid to 2-phenyl4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) in a phosphate buffer solution (0.05 M, pH/pD 7.0) at 298 K.

Results

Conclusion