Identification of Side-Chain Structures in a Poplar Lignin Using Three-Dimensional HMQC−HOHAHA NMR Spectroscopy

Abstract

The structure of an acetylated 13C-enriched poplar wood lignin preparation was studied using three-dimensional HMQC−HOHAHA NMR spectroscopy. This method takes advantage of the large dispersion of 13C chemical shifts to resolve individual 1H chemical shifts. The whole spin system of a 1H−13C correlation observed in an HMQC spectrum, even for minor components and unknown structural units, can be traced out. It is shown here that both trans- and cis-isomers of 6,7-dihydrodibenzo(e,g)(1,4)-dioxocin, a recently discovered prominent linkage in softwood lignins, occur also in hardwood lignin. Moreover, the poplar lignin preparation contains small amounts of noncyclic α-aryl ether linkages. Signals from a side chain of unknown structure are tentatively assigned to a spiro-cyclohexadienone. Keywords: NMR; 3D HMQC−HOHAHA; lignin