Decay of WT PmoD CuA at 4 °C observed in parallel by (A) optical and (B) CW X-band EPR spectroscopies. The inset in B depicts scans measured in the gz region for the WT PmoD t = 144 hr sample, where brackets define the hyperfine splitting Az of the two type 2 Cu2+ centers (the fourth hyperfine line is outside of the range shown). Conditions: (B inset) 9.364-9.365 GHz microwave frequency, 40 s scan rate, 320 ms time constant, 12.5 G modulation amplitude, temperature 20 K; (B bottom) 9.364-9.366 GHz microwave frequency, 90 s scan rate, 320 ms time constant, 12.5 G modulation amplitude, temperature 20 K. Spectra intensities were normalized to account for different gain settings. The protein concentration was 150 μM for all samples.