Structural analysis of hardwood native lignins by quantitative 13C NMR spectroscopy

Milled wood lignins from alkaline pretreated wood with very low sugar content and a wide range of syringyl-to-guaiacyl (S/G) ratio between 1.2 and 3.0 were isolated from 12 industrially valuable hardwood (HW) species. The lignin preparations were investigated by means of a comprehensive 13C nuclear magnetic resonance (NMR) methodology to address the possibilities and limitations of this approach for HW native lignins and to estimate the structural variations within HW lignins. Good correlations were found for different independent methods for the quantification of major lignin moieties. The results were reliable at the C6 level and not only for relative comparison. The correlation was good between methoxyl group determinations by wet chemistry and those by 13C NMR spectroscopy. The limitations of the 13C NMR method were also pointed out. The differences in the S/G ratios can be large, but other structural deviations are less significant. Strong correlations between the S/G ratios and the amounts of other structural peculiarities could not be found by the 13C NMR approach. However, with increasing S/G ratios, the β-O-4 content showed increasing tendencies and the degree of condensation showed decreasing tendencies.