For a period now approximating well over 30 years, NMR spectroscopy has been applied to the study of various lignin isolates (71, 131, 132, 154-177). In this way, it has been possible, together with the study of various dimeric monolignol-derived products, to identify a number of the most abundant substructures in lignins. Many of these were previously shown in Figure 7.2D, for coupling of the H-, G-, and S-derived monolignols, respectively, with the so-called 8-O-4′ interunit linkages generally being acknowledged as the most prevalent. There are, however, still major limitations in current NMR spectroscopic analyses. One is in the inability to determine the sequences ofinterunit linkages within thebiopolymers, because it is not yet currently possible (using natural abundance 13C) to readily go beyond the ether interunit linkages (e. g., 8- O-4′,4- O-5′ linkages) to adjacent flanking substructure(s). A second major difficulty is that of the polymeric nature of the lignins: in general, the spectral band width lines for polymers can be very broad, due to molecular weight (size), polydispersity of samples, molecular aggregation and molecular heterogeneity, etc. For high molecular weight entities, such as with lignins, the molecules can thus experience slow tumbling which, in turn, results in very large transverse magnetization rates (efficient T2 relaxation) leading to broad spectral band width lines. Furthermore, given that all lignin isolation procedures result in preparations that are polydisperse, this — together with possible molecular heterogeneity (= different substructures) in the polymeric chains — can lead to

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EtSH/BFo-EtoO
20-40%
NBO	Thioacidolysis

very small changes in chemical shifts at the local bonding sites and hence signal broadening. A further complication is the ability of the lignins to aggregate/self-associate which further confounds the spectroscopic analyses.

On the other hand, it is also well known that the more mobile functionalities in polymeric backbones can readily be detected [e. g., acetate groups ofxanthan gum preparations (178)], whereas the polymeric backbones are more extensively line-broadened. A similar situation also presumably holds for different segments of the lignin polymer chains. Furthermore, even with observable resonances by various forms of NMR analyses, quantification of the signals relative to the entire polymeric entities can be quite problematic, again further il­lustrating issues as regards precise quantification. As a further caveat, chemical degradation protocols can be used to identify, detect, and quantify various lignin substructures which cannot readily be detected by NMR spectroscopic techniques. Thus, NMR spectroscopy, while a most powerful technique, currently only provides a very incomplete assessment of the nature of the lignin macromolecule(s). For these reasons, the question of lignin struc­tural analysis has thus been limited to date in attempts to identify both interunit linkages, and less precisely to estimations of probable amounts of the distinct substructures, in the various lignin-enriched isolates — at least, for those that can be distinguished/identified/ detected.