Lignin-Polyphenol Interaction in Azobe (Lophira alata) Heartwood. A Study on Milled Wood Lignin (MWL) and Klason Residues

Summary

Findings about anomalous high Klason residues of azobe (Lophira alata) heartwood prompted a search for methods of determination of non lignin-type polyphenols in lignin preparations. A four step procedure was performed: (1) ethylation of milled wood lignins (MWLs) from azobe and beech heartwood, (2) permanganate oxidation, (3) methylation, and (4) GC analysis of the oxidation products. Besides the well known degradation products of hemipinic, iso-hemipinic, and meta-hemipinic acid type, methyl-3,4-diethoxy-benzoate (compound I) has also been detected among the oxidation products. The oxidation of beech MWL gave rise only to 0.4 mol % of I but azobe MWL yielded the remarkably high amount of 10.7 mol % of I. As lignins do not contain two adjacent aromatic hydroxyl groups, we concluded that I is of polyflavonoid origin. Hence the extraordinarily high Klason residues of azobe heartwood can be rationalized with large amounts of acid insoluble polyphenols which are an integral part of the azobe cell wall and which is also closely associated to MWLs. Ethylation followed by KMnO₄ oxidation is an effective tool to detect polyflavonoids in lignins.