MODELING THE BLUE PROTEIN ACTIVE-SITES - SYNTHESIS AND CHARACTERIZATION OF CUN2S2 COMPLEXES SHOWING RHOMBIC EPR-SPECTRA AND HIGH CU-II CU-IPOTENTIAL/

Two new tetradentate ligands have been synthesized by Schiff base cond ensation of diisobutyraldehyde disulphide with 2-mercaptoethylamine (L (1)) and 2-aminothiophenol (L(2)) respectively and then reducing the i mine linkages with NaBH4 in refluxing methanol. In the free ligands th e thiolate sulphur is protected with tertiary butyl groups which are c leaved 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 ar e consistent with a pseudotetrahedral coordination around the Cu-II io n. The EPR spectrum of the aliphatic thiolate in MeCN glass shows sign ificant rhombic splitting (g(x)-g(y) = 0.09 and A(x)-A(y) = 60 x 10(-4 ) cm(-1)) attributable to d(z)2 mixing into the ground state wavefunct ion. For the aromatic thiolate complex, however, the EPR spectrum was not well resolved although the rhombic nature of the spectrum could ea sily be observed. Both the complexes exhibit well-defined cyclic respo nses in their cyclic voltammograms at RT and in acetonitrile for the C u-II/Cu-I couple with E(1/2) = 0.5 V vs SCE. This high positive value for the redox couple is also consistent with a coordination geometry m uch distorted from planarity. The active sites of the blue proteins wh ich contain copper in distorted geometries exhibit Cu-II/Cu-I potentia l in the range 300-800 mV vs NHE at pH = 7.0.