Extended Data Fig. 9: Generation of EPR-active STEMPO• under open circuit potential (OCP) in the absence and presence of MBA at pH 7.3.
The potential was held at +0.6 V such that only the EPR-active species (STEMPO•) is present, and then stepped to +1.1 V such that EPR-silent (STEMPO + ) is generated exclusively. The current is monitored at OCP over time and the experiment was performed with and without substrate. EPR spectra were recorded throughout the chronoamperometry-OCP measurement. a Double integrals of EPR spectra showing regeneration of STEMPO• in the absence of substrate (black dots), and in the presence of 1 mM (red dots) and 6.8 mM (blue dots) MBA after potential steps at + 0.6 V and + 1.1 V vs SHE, for 100 s each. b STEMPO• regeneration rates as a function of substrate concentration (0, 1, 2.4, 6.8, 10 and 20 mM MBA; the colours of the data points match the data in a and in Supplementary Figure 16), deduced from a 1st order relaxation fit of the variation of STEMPO• concentration over time, as quantified from the EPR data. The solid line shows the fit using the Michaelis-Menten model with KM = 0.96 mM and kcat = 0.09 s−1.
