Panels a and b, two-pulse ESEEM spectra (cosine FT) of wt hpH cSO obtained at gZ (Bo = 1055 mT) and gX (Bo = 1071.5 mT), respectively. Traces 1 and 2 in each panel, spectra of samples prepared with H2 17O with long and short incubation time, respectively. Trace 3 in each panel, spectra of samples prepared with H2 16O. Experimental conditions: mw frequency, 29.34 GHz; mw pulses, 2 × 15 ns; temperature, 21 K. Asterisks, diamonds and triangles indicate the ΔmI = 1 fundamental lines of axial oxo-17O, equatorial 17O (from the OH ligand), and unidentified weakly coupled 17O, respectively. The vertical dashed line in each panel shows the Zeeman frequency of 17O, νO.