Lignin-polyphenol interaction in azobe (Lophira alata) heartwood. A study on milled wood lignin (MWL) and Klason residues

Findings about anomalous high Klason residues of azobe (Lophira alata) hear twood prompted a search for methods of determination of non lignin-type pol yphenols in lignin preparations. A four step procedure was performed: (1) e thylation of milled wood lignins (MWLs) from azobe and beech heartwood, (2) permanganate oxidation, (3) methylation, and (4) GC analysis of the oxidat ion products. Besides the well known degradation products of hemipinic, iso -hemipinic, and meta-hemipinic acid type, methyl-3,4-diethoxy-benzoate (com pound 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 rem arkably high amount of 10.7 mol% of I. As lignins do not contain two adjace nt 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 t o MWLs. Ethylation followed by KMnO4 oxidation is an effective tool to dete ct polyflavonoids in lignins.