A comparative study of the magnetic properties of six representative type 1 Cu(II) blue copper centers was carried out by using electron nuclear double resonance (ENDOR). Four centers are EPR single-site proteins: plastocyanins from bean (Phaseolus vulgaris) and poplar leaves (Populus nigra italica), azurin from Pseudomonas aeruginosa, and stellacyanin from Rhus vernicifera. Two are in laccases, fungal (Polyporous versicolor) and tree (R. vernicifera), that have had their type two centers reduced. In each case the low-frequency (v ≤ 35 MHz) region of the spectrum is dominated by the resonances from strongly coupled (AH > 20 MHz) methylene protons of a coordinated cysteinyl mercaptide. In all cases but one, experiments at two microwave frequencies (9.6 and 11.6 GHz) also permitted detection of resonances from two, inequivalent, nitrogenous ligands. The coordination environments in the type 1 Cu(II) sites of the six proteins are broadly similar, but detailed analysis suggests that the stereoelectronic structure of the single-site proteins, as a group, differs in subtle but significant ways from that of the type 1 Cu(II) center of the laccases. The reduction potentials of the single-site type 1 Cu(II) centers correlate well with the bonding within a center, as reflected in the ligand ENDOR parameters: Reduction potentials decrease with decreasing bonding to (or ligand field at) the type 1 Cu(II). This correlation does not hold for the laccase type 1 sites, which appear to have enhanced π-bonding to mercaptide sulfur. Analysis of these results suggests that fine tuning of the reduction potentials of a type 1 Cu center is primarily achieved by altering the properties of the reduced, Cu(I), state.
ASJC Scopus subject areas
- Colloid and Surface Chemistry