Progressive EPR power saturation for the radical intermediate trapped at 100 ms reaction of PGIS (150 μM) with PA (1.5 mM). EPR parameters were microwave power, 12.9 μW–25.8 mW; microwave frequency, 9.3 GHz; modulation amplitude, 2 G; and temperature, 110 K. Arrow indicates the direction of increasing microwave power. Lower left inset: microwave saturation curve of the g = 2 radical. The P1/2 value, fitted by eq 2 in which b was given as 1, is 0.73 mW. Upper right inset: Experimental (solid line) and simulated (dashed line; using the parameters in Table 2) X-band spectra of the freeze-quenched sample collected by reaction of 700 μM PGIS with 7 mM PA for 25 ms. Microwave power, 0.1 mW; microwave frequency, 9.3 GHz; modulation amplitude, 1 G; and temperature, 110 K.