EPR spectra of Fe(II)-HppE·HPP nitrosyl complexes. Blue line: Fe(II)-HppE (250μM) mixed with 2-S (2.5 mM) and exposed to NO; Black line: Fe(II)-HppE (250 μM) mixed with 2-R (2.5 mM) and exposed to NO. Instrumental parameters: temperature 2 K, microwave frequency 9.6 GHz, microwave power 0.6 mW, modulation amplitude 5 G, time constant 0.02 s, and a sweep rate of 50 G/s.