OXIDATION OF ARGININE BY MANGANESE(III) IN PYROPHOSPHATE AND ACETATE MEDIA - A KINETIC-STUDY

Kinetics of manganese(III) oxidation of L-arginine has been studied in the presence of pyrophosphate and acetate ions in acidic media at 328 K and 323 K, respectively. The nature of the oxidizing species formed in manganese(III) solutions was determined by spectrophotometric and redox potential measurements. The reaction shows a first-order dependence on [manganese(III) pyrophosphate] in the pyrophosphate medium, pH 2-3, and a half-order on [manganese(III) acetate] in HOAc-acetate medium. In both media, the kinetic order is one with respect to [arginine]. The dependencies of the rate on the reduction product, manganese(II), concentration are zero- and inverse first-orders in acetate and pyrophosphate media, respectively. Effects of varying dielectric constant of the medium and of added anions such as acetate, pyrophosphate, fluoride, chloride, and perchlorate have been investigated, in both media. There is evidence for the existence of free radicals as transient species. Activation parameters have been evaluated using the Arrhenius and Eyring plots. Mechanisms consistent with the observed kinetic data have been proposed and discussed. Kinetic data for the oxidations of some alpha-amino acids by manganese(III) species of different forms are summarized and compared. (C) 1994 John Wiley & Sons, Inc.