X-band EPR spectra of Met121Cys azurin at different pH values. The EPR spectra are in black while the simulated spectra are in red. X-band experimental conditions: microwave frequency, 9.04 GHz; power, 0.2 mW; modulation amplitude 5 G, time constant, 32 ms; sweep time, 60 s; number of scans, 10; gain, 8000; temperature, 29 K. The spike at g=2.0 signal is an artifact of the EPR tubes used.