Kinetic Magnetic-Field Effect Involving the Small Biologically Relevant Inorganic Radicals NO and O2

Abstract

Oxidation of dihydrorhodamine 123 (DHR) to rhodamine 123 (RH) by oxoperoxonitrite (ONOO−), formed through recombination of NO and O2.− radicals resulting from thermal decomposition of 3-morpholinosydnonimine (SIN-1) in buffered aerated aqueous solution at pH 7.6, represents a kinetic model system of the reactivity of NO and O2.− in biochemical systems. A magnetic-field effect (MFE) on the yield of RH detected in this system is explored in the full range of fields between 0 and 18 T. It is found to increase in a nearly linear fashion up to a value of 5.5±1.6 % at 18 T and 23 °C (3.1±0.7 % at 40 °C). A theoretical framework to analyze the MFE in terms of the magnetic-field-enhanced recombination rate constant krec of NO and O2.− due to magnetic mixing of T0 and S spin states of the radical pair by the Δg mechanism is developed, including estimation of magnetic properties (g tensor and spin relaxation times) of NO and O2.− in aqueous solution, and calculation of the MFE on krec using the theoretical formalism of Gorelik at al. The factor with which the MFE on krec is translated to the MFE on the yield of ONOO− and RH is derived for various kinetic scenarios representing possible sink channels for NO and O2.−. With reasonable assumptions for the values of some unknown kinetic parameters, the theoretical predictions account well for the observed MFE.