Blue copper proteins: Synthesis, spectra, and structures of CuN(3)(SR) and CuN(3)(SR) active site analogues

The reaction of Cu(SR) or [Cu(SR)][ClO₄] derivatives (SR = p-nitrobenzenethiolate or O-ethylcysteinate) with potassium hydrotris(3,5-dimethyl-1-pyrazolyl)borate produces redox pairs of the stoichiometry Cu^IN₃(SR) and Cu^IIN₃(SR). These complexes are well-defined synthetic approximations to the proposed N₃S binding sites of blue (type 1) copper electron transfer proteins. The compounds were investigated by a variety of chemical and spectral (optical, resonance Raman, and electron paramagnetic resonance) techniques; the complex K[Cu(hydrotris(3,5-dimethyl-1-pyrazolyl)borate)(p- NO₂C₆H₄S]-2 acetone was also studied by single-crystal x-ray diffraction methods. The spectrochemical characteristics of the Cu^IIN₃(SR) species are in large part similar to the native system and thus provide some perspective regarding the origin of the unique type 1 spectral parameters and electron transfer properties.