Fig. 1.

The EPR spectra of the DNB label $R_1^{•}SS{R}_1^{•}$ (a) and its isotopically substituted analogues, $R_2^{•}SS{R}_2^{•}$ (b) and $R_3^{•}SS{R}_3^{•}$ (c). The spectra were obtained for 0.1 mM solution of the biradicals alone (red) and after the reaction with different concentrations of GSH added at pH 11.5 (0.1 mM, blue, and 1 mM, black). The spectrometer settings were as following: modulation amplitude, 0.5 G; sweep width, 50 G; microwave power, 10 mW. The maximal increase of the intensity of the “monoradical” component after complete splitting of the disulfide bond, $I_m^{max}/I_m^{t=0}$, was equal to (a) 4.6, (b) 3.0 and (c) 3.3. Note that double integration of the EPR spectra shows conservation of the integral spectral intensity during the observed spectral changes.