The [Cu(I)–Cu(II)] half-met form of the dinuclear copper site of tyrosinase has been probed by continuous wave electron paramagnetic resonance (EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopy in the presence and absence of inhibitors. In all cases the EPR spectrum is indicative of a dx 2−y 2 ground state for the unpaired electron. From the cross-peaks observed in the HYSCORE spectra, proton hyperfine coupling constants were obtained that are compatible with a hydroxide ion in an equatorial coordination position of the paramagnetic copper. After changing the water solvent to D2O or after addition of the inhibitors p-nitrophenol or L-mimosine, the proton signals disappear. The relevance of these findings for understanding the catalytic cycle is discussed.
