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Multi-frequency e.p.r. studies of a derivative of laccase containing one mercury atom and three of copper were carried out at -150 degrees C. The e.p.r. signal of the mercury derivative and fluoride-binding studies establish that a type-2-like copper centre is present. The signal suffers broadening, owing to g-strain, but at low frequencies (S-band) ligand hyperfine splitting can be resolved, and it can be explained in terms of coupling to three nitrogen atoms. The g values and the effect of solvent deuteration on the line width suggest that the fourth ligand in the equatorial plane is a water molecule. Simulations of the e.p.r. spectrum reveal that the site is slightly rhombic at -150 degrees C, a finding in accord with the proposed N3O donor set. Finally, it is emphasized that a structural reorganization of the type-2 copper site occurs with the binding of fluoride at low temperature. The reorganization may be linked to a conformational change which has previously been claimed to occur on cooling; however, this transition is not necessarily relevant to the temperature-dependence of fluoride binding.

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