Fixation of copper-protein formulation in wood: Part 2. Molecular mechanism of fixation of copper(II) in cellulose, lignin and wood studied by EPR

Abstract

Abstract Electron paramagnetic resonance (EPR) spectra were recorded for copper(II) ions in cellulose, lignin and pine sapwood impregnated with a water based formulation containing CuSO4·5 H2O, tannic acid and animal protein SOLUTEIN. EPR parameters were derived from computer-simulated spectra. The parameters were interpreted in terms of molecular orbital theory giving coefficients of spin delocalization onto ligands. Five different spectra of chemically fixed complexes were identified. The complexes were octahedral with a different degree of elongation. For a copper concentration higher than approximately 0.5%, the additional EPR signal from the precipitated CuSO4·5 H2O salt appears. Two possible coordination sites are proposed for fixed copper complexes in cellulose structure and a single site is proposed for copper in a lignin. These complexes are different from the copper complex in wood, where Cu(H2O)4O2 octahedral complex exists with apical coordination to two oxygen atoms of deprotonated hydroxyl groups of polymeric units. When tannic acid is added to the impregnating solution, copper ions are strongly coordinated in a square-planar geometry of CuO4 and are easily leached. The copper-SOLUTEIN complex yields a distinguished EPR spectrum only in the impregnating solution, but not in the wood. Except for the copper(II) EPR spectra, where the signals from free radicals were detected in wood and lignin with a concentration in the order of magnitude of 1017 g-1 dependent on wood treatment.