MWL fraction with a high concentration of lignin-carbohydrate linkages: Isolation and 2D NMR spectroscopic analysis

Abstract

Abstract A preparation enriched in lignin-carbohydrate fragments (LCC-AcOH) was isolated in the course of purification of loblolly pine crude milled wood lignin (MWL). The preparation contained approximately equal amounts of lignin and carbohydrates, with high amounts of arabinose and galactose compared to their levels in wood. LCC-AcOH was investigated by 2D 1H-13C (HMQC and HMBC) correlation NMR techniques and quantitative 13C NMR. The HMQC spectra allowed direct detection of phenyl glycoside, ester and benzyl ether lignin-carbohydrates linkages in high amounts. The assignment of these structures was supported by the HMBC technique. It is noteworthy that in the ester lignin-carbohydrate linkages, a uronic acid residue was attached not to the benzylic position of lignin, as commonly believed, but to the γ-position of the side chain.