An Investigation of the Milled Wood Lignin Isolation Procedure by Solution- and Solid-State NMR Spectroscopy.

Abstract

Milled wood lignin (MWL) is a fraction of the total lignin in wood but is considered the isolated lignin most representative of lignin in its native state. It is useful for lignin studies because it is isolated relatively free of carbohydrates, however structural changes occur during its isolation. This study presents attempts to evaluate the whole lignin structure with the goal of analysis its structure in intact wood. Solution-state and solid-state NMR experiments and degradative techniques were performed on soluble and insoluble lignins to make an estimation of the interunit linkages.

MWL and cellulolytic enzyme lignin (CEL) exhibit only minor structural differences. MWL is lower in b-aryl ether content, has a higher degree of condensation, and a slightly higher amount of oxidized end group moieties. MWL may derive from the middle lamella to a larger extent than CEL.

MWL and lignins isolated from milled wood and REL (Residual Enzyme Lignin) were dissolved using dimethyl sulfoxide (DMSO)/N-methylimidazole (NMI) for NMR analysis. HMQC showed that these lignins are similar from the standpoint of structural moieties present. Quantitative 13C NMR indicates that MWL has a b-O-4' content lower than that of the wood. Solid-state NMR suggests that MWL had a higher degree of condensation. The milled wood and REL contain a significant portion of high MW material (~55,000 g/mol) which contributes to their insolubility in aqueous dioxane.

Solution-state NMR indicates that prolonged rotary ball milling results in more structural changes than the standard milling technique. There are few differences between samples milled in toluene or under N2 in a vibratory ball mill. Milling under N2 results in a higher MWL yield and the isolated lignin contains higher aliphatic and phenolic hydroxyl contents.

The DFRC method is inefficient because it does not completely cleave all b-O-4' linkages. Inefficiency increases with molecular weight indicating that the cause is either accessibility or molecular mobility in the reductive cleavage step. Thioacidolysis completely degrades the b-aryl ether linkages in lignin and is therefore preferrable for analysis of these structures.