Hyperfine coupling of 17O to Mo(V) in the rapid epr signal of xanthine oxidase: Evidence for an oxygen ligand of the metal

The spectrum of the Rapid Mo(V) electron paramagnetic resonance signal from xanthine oxidase dissolved in ¹⁷O-enriched water is presented. Difference technqiues have been used to eliminate the ¹⁶O contribution. Clearly observed structure in the spectrum is attributed to moderately strong hyperfine coupling of one oxygen atom to molybdenum. Though complete interpretation of the spectrum has not been attempted, one component of A(¹⁷O) is about 1.6 mT. The possibility that the oxygen is present in a MoOH group, whose proton is the strongly-coupled proton of the Rapid signal, is discussed.