(methionine, tyrosine). The pH was chosen to match experimental conditions under which second-order reaction rate constants of the probe compounds with 1 O 2 were determined. Oxygen isotope analysis O/ O ratios of dissolved O 2 were measured by gas chromatography coupled to isotope ratio mass spectrometry (GC/IRMS) as described previously. The magnitude of O isotope fractionation associated with O 2 consumption is reported in terms of ϵ value (in , ± 95% confidence intervals), which was calculated as the slope of a linear regression according to eq 2.