Quantitative Characterization of a Hardwood Milled Wood Lignin by Nuclear Magnetic Resonance Spectroscopy

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Abstract
The structure of Eucalyptus grandis milled wood lignin (MWL) was investigated by 2D 1H−13C HSQC, HMQC, and 1H−1H TOCSY correlation NMR techniques and by quantitative 13C NMR as well as by the permanganate oxidation degradation technique. The combination of 2D NMR and quantitative 13C NMR spectroscopy of nonacetylated and acetylated lignin preparations allowed reliable identification and calculation of the amount of different lignin structures. About 85% of side-chain moieties were estimated on the structural level. This information was substantiated by data on the quantity of various functional groups and interunit linkages as a whole. A modified method for calculation of the h:g:s ratio has been suggested and compared with previously suggested approaches. E. grandis MWL has been determined to have an h:g:s ratio of 2:36:62. The amounts of various phenolic/etherified noncondensed/condensed guaiacyl and syringyl moieties were approximately estimated. E. grandis MWL contained ∼0.60/Ar of β-O-4 moieties along with small amounts of other structural units such as pino/syringyresinol (0.03/Ar), phenylcoumaran (0.03/Ar), and spirodienone (0.05/Ar). The degree of condensation was estimated at ∼21%; the main condensed structures are 4-O-5 moieties (∼0.09/Ar). The structure of E. grandis MWL was compared with those of other lignin preparations isolated from various hardwoods. Keywords: Correlation 2D NMR; hardwood; lignin; milled wood lignin (MWL); quantitative 13C NMR