High-frequency (A) and low-frequency (B) RR spectra of RFQ samples of [metMb + 14NO] at 6, 20, 75, and 280 ms, and of metMb and metMb(NO). Low-frequency RR spectra of metMb(14NO) and metMb(15NO) (C) (experimental conditions as in Figure 4). Fraction of metMb(NO) in individual RFQ samples on the basis of the intensity of the Fe-NO stretching mode (open black square), high-frequency porphyrin skeletal modes (open grey circle), and a single-exponential fit (black line) (D).