Lignin degradation by a non-enzymatic system supposed to be active in white rot fungi
P Lamaipis, W Gindl, T Watanabe, K Messner
Electron microscopic investigations have shown that ligninolytic enzymes of white-rot fungi are only able to penetrate the wood cell wall in late stages of degradation. Thus, the selective degradation of lignin of certain white-rot fungi can only be explained on the basis of a low molecular weight, highly diffusible system. A system, consisting of copper, a coordination compound and either H2O2 or organic peroxide was found to be highly ligninolytic and has been proven on different types of lignin model compounds. In this paper, it is shown that this system is also capable of delignifying soft and hardwood lignin. Delignification was indicated on wood sections by differential staining with astra-blue and safranine and was quantified by UV-microscopy in cell walls. It was demonstrated that especially when organic peroxide was used in the copper system, even the recalcitrant softwood lignin was depolymerized while with H2O2 only hardwood lignin was oxidized efficiently. Due to the selectivity of lignin degradation it is assumed that rather peroxyl, alkoxyl or carbon centered radicals than hydroxyl radicals are the active compounds. In principle the copper system is comparable to the diffusible system active in brown-rot, but with the latter one leading to cellulose degradation. A deeper insight into the mechanism of white-rot decay, including also the likelihood of non-enzymatic reactions could alter the generally accepted picture of purely enzymatic reactions and could probably offer a new approach to chemical wood preservation.