Hush theory in a copper mixed valence bioinorganic model generated photochemically

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In the present study a copper mixed valence (MV) compound has been generated photochemically from the (CH 3COO) 2Cu(II)Cu(II)(OOCCH 3) 2 dimeric species in 95/5% (vol./vol.) methanol/acetic acid mixture as solvent. The MV species shows two intervalence (IT) bands at 19.6 and 10.4 kK and seven complex lines in the EPR spectra at room temperature. Although the two bands are IT bands in character, only the second (960 nm) could be related through the Hush and Hopfield theories to a thermal electron transfer process between the copper centers with kinetic parameters Δ G #=21.6 kJ mol −1 and k th=1.2×10 9 s −1 (Hush) or 4.0×10 11 s −1 (Hopfield) at 298 K and related to the change from (d x 2− y 2 ) 2Cu(I)(d x 2− y 2 )Cu(II) to (d x 2− y 2 ) 1Cu(II)(d x 2− y 2 ) 2Cu(I) electronic configuration ( D 4 h symmetry for each copper center). The electronic assignation of the more energetic band is also discussed. The value obtained for k th is in the range measured by EPR spectroscopy for other Cu(II)Cu(I) dimeric systems and provides evidence for the validity of the essence of the theoretical approach in copper binuclear systems although the value for the electronic coupling parameter is relatively high. The complex studied in this work has some characteristics in common with those of the type III copper observed in copper oxidases in its MV state such as: dimeric structure with tetragonal environment for the coppers, with oxygen perhaps as bridging ligand, EPR signal related to one unpaired electron interacting with both coppers and with k th values which compare well with some biological systems; so we believe it is a good bioinorganic model for type III copper in multicopper oxidases.