Probing metal-mediated O 2 activation in chemical and biological systems

Abstract

The use of oxygen ( 18O) isotope fractionation as a mechanistic probe of chemical and biological oxidation reactions, particularly those which involve metal–O 2 adducts, is currently being explored. Summarized here are reactions of enzymes and inorganic compounds for which competitive isotope effect measurements have been performed using natural abundance molecular oxygen and isotope ratio mass spectrometry. The derived 18O equilibrium isotope effects (EIEs) and kinetic isotope effects (KIEs) reflect the ground state and transition state structures, respectively, for reactions of 16O– 16O and 18O– 16O. Normal isotope effects (>1) characterize the binding of O 2 to transition metal centers. The magnitudes, which are primarily determined by the decrease in the O–O force constant accompanying formal electron transfer from the metal to O 2, suggest that metal superoxo complexes can be distinguished from metal peroxo complexes. Because 18O isotope effects can be measured during catalytic turnover, they complement existing approaches to elucidating the structures of activated oxygen intermediates based on low-temperature spectroscopy and crystallographic analysis of inorganic model compounds.