Modelling the blue protein active sites: synthesis and characterization of CuN₂S₂ complexes showing rhombic EPR spectra and high Cu^II/Cu^I potential

Abstract

Two new tetradentate ligands have been synthesized by Schiff base condensation oof diisobutyraldehyde disulphide with 2-mercaptoethylamine (L¹) and 2-aminothiophenol (L²) respectively and then reducing the imine linkages with NaBH4 in refluxing methanol. In the free ligands the thiolate sulphur is protected with tertiary butyl groups which are cleaved in the presence of Cu^II-salts to give neutral CuN₂S₂ complexes. The copper complexes show ligand field transitions at 815 and 760 nm at room temperature which are independent of the solvents used and are consistent with a pseudotetrahedral coordination around the CuII ion. The EPR spectrum of the aliphatic thiolate in MeCN glass shows significant rhombic splitting (g_x−g_x = 0.09 and A_x−A_y = 60 × 10⁻⁴ cm⁻¹) attributable to δ _z² mixing into the ground state wavefunction. For the aromatic thiolate complex, however, the EPR spectrum was not well resolved although the rhombic nature of the spectrum could easily be observed. Both the complexes exhibit well-defined cyclic responses in their cyclic voltammograms at RT and in acetonitrile for the Cu^II/Cu^I couple with E_½ =0.5V vs SCE. This high positive value for the redox couple is also consistent with a coordination geomttry much distorted from planarity. The active sites of the blue protein which contain copper in distorted geomtries exhibit Cu^II/Cu^I potential in the range 300-800 mV vs NHE at pH = 7.0.