Photo- and radiation chemical induced degradation of lignin model compounds

The basic mechanistic aspects of the photo- and radiation chemistry of lign in model compounds (LMCs) are discussed with respect to important processes related to lignin degradation. Several reactions occur after direct irradi ation, photosensitized or radiation chemically induced oxidation of LMCs. D irect irradiation studies on LMCs have provided supportive evidence for the involvement of hydrogen abstraction reactions from phenols, beta -cleavage of substituted alpha -aryloxyacetophenones and cleavage of ketyl radicals (formed by photoreduction of aromatic ketones or hydrogen abstraction from arylglycerol beta -aryl ethers) in the photoyellowing of lignin rich pulps. Photosensitized and radiation chemically induced generation of reactive ox ygen species and their reaction with LMCs are reviewed. The side-chain reac tivity of LMC radical cations, generated by radiation chemical means, is al so discussed in relation with the enzymatic degradation of lignin. (C) 2000 Elsevier Science S.A. All rights reserved.