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 laceases, 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 laceases. 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 lacease type 1 sites, which appear to have enhanced tr-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.The mononuclear Cu(II) site of the blue copper (type 1) proteins has unusual optical and magnetic properties.6 The electronic spectrum is characterized by an intense absorption near 600 nm (e ~103-104), in contrast to the weak absorptions (< <102) exhibited by mononuclear copper complexes of low molecular weight. Typically, the EPR spectrum of blue copper also is quite different from that observed for "normal" copper; the values of AzCu, and ^iso0*, the isotropic coupling, all show unusually small values.7Near infrared, visible absorption, circular dichroism, and pulsed EPR spectroscopy7 are consistent with a distorted tetrahedral coordination geometry for blue copper site. It also has been suggested, in part from model compound studies,8 that the intense blue color is due to RS -* Cu charge-transfer excitation6,9 and that the unpaired electron is substantially delocalized onto a sulfur