What Have Studies of Model Compounds Revealed about the Fungal Degradation of Lignin?

The author reviews the use of dimeric model compounds (two aromatic nuclei) in investigations of the degradation of lignin by white-rot fungi, and compares the results of such studies with those obtained using l i g n i n i t s e l f . Findings are summarized for the four types of compounds that have been studied: a) Phenolic β-O -4 and β-5 mode1 compounds are invariably oxidized in cultures by phenol oxidases (POs). The major reaction type is radical coupling/polmerization to produce complex products, although a variety of degradative reactions can follow disproportionation between radicals. Even so, POs probably do not play a crucial role in structural degrada t ion . b) Non-phenolic β-O -4 model compounds apparently are degraded via several pathways, but the specific reac t ions a re not comple te ly known. Cα-Oxidation, Cα-Cβ c l e a v a g e , a n d β-ether cleavage are indicated as important primary reactions. c) Non-phenolic β-5 model compounds are degraded to compounds oxidized in the coumaran ring. Ring cleavage of the 5-linked aromatic nucleus may be important in further degradation. d) Nonphenolic β-1 model compounds are degraded via an initial cleavage between Cα and Cβ forming a Cα-derived aldehyde and a phenylglycol. This reaction is well understood. It shows no stereoselectivity and involves incorporation of oxygen from molecular O2 into a hydroxyl group. Findings of research using each of the four types of model compounds agree closely with those from research using lignin; moreover, model compounds and lignin have each contributed unique information.