we noticed that the signal changed almost completely from a protium-to a deuteriumcontaining glycyl radical upon changing the solvent from H2O to D2O, but appeared to revert only partially when the buffer was changed back to H2O, resulting in a mixed signal (Fig. ). Aromatoleum toluolicum was used, either as prepared (blue), after exchanging the solvent to D2O (green), and after exchanging it back to H2O (black). The red curve approximates the black spectrum by adding the blue and green spectra at a ratio of 0.44:0.21, which fits well to a 40% loss of radical content in this sample as calculated by spin quantitation. Note that the amplitude-based 68:32 ratio shown here is converted to an 80:20 ratio when analyzed based on the spectrum integrals.