Fully reduced ferrous 50 µM iNOSox containing equimolar BH4 and 500 µM L-Arg (top spectrum), with BH4 alone (middle spectrum), or in the absence of BH4 (bottom spectrum) was reacted with air saturated 50 mM HEPES buffer (pH 7.5), containing 10 % glycerol, 0.1M NaCl, pH 7.4 at a 1:1 ratio at 22 °C, and the reaction mixture was freeze-trapped at 80 ms reaction time in the prechilled isopentane. EPR conditions were: 4 mW microwave power, 10 G modulation and 11K. The g values of both the high-spin (7.88 and 3.80) and low-spin (2.41 and 2.27) heme species are labeled by solid lines.