Sakakibara models

The 1970s and 1980s witnessed several additional attempts to adequately depict possible lignin structures. The most complex structure resulted from studies by Glasser and Glasser (13), using a computerized model called SIMREL. In hindsight, this model suffered from various critical limitations: For example, it was unable to predict the presence of various substructures, such as dibenzodioxicin (5-5′, 8-0-4′, 7-0-4′, substructure V in Figure 7.2D) moieties and it also contained a large number of hypothesized interunit linkages that have never been observed in lignins, e. g., C7-C8, C8-C7, C7-C6, and C7-0-C9. As for the Freudenberg and Forss/Fremer lignin models, hypothetical thioacidolytic cleavage of potential monomer/dimer releasable moieties in this representation would not provide quantitative data in agreement with later experimental observations, e. g., by thioacidolysis. This computer-generated model can, therefore, be eliminated from further consideration as adequately representing lignin structure.

At more or less the same time, a proposal for a representative lignin structure in the angiosperm, beech (Fagus silvatica), was made by Nimz (309). This model was based upon the analysis, identification, and quantification, of various monomeric, dimeric, trimeric, and tetrameric fragments released when beech woodmeal was treated with thioacetic acid and catalytic amounts of boron trifluoride at 20°C for 1 week, followed by hydrolysis of the thioacetates with NaOH (2 N, 60°C, 24 h) and Raney nickel treatment (8 h) (310). The proposed beech wood lignin structure (not shown) contained 27 inter-linked monomeric units of which, on a per monomer basis, there were 7 potentially cleavable G/S monomers, as well as 8-І7 (4), 8-5′ (2), 8-8′ (1.5), 7-8′ (1), 5-5′ (1), and 4- 0-5′ (0.4)/7-6/ (0.1) dimers. In hindsight, some of these fragments may have resulted from rearrangement, e. g., of sy — ringaresinol (substructure IIIC in Figure 7.2D) during the chemical degradative procedures employed, as observed for thioacidolysis cleavage/Raney Ni treatment where syringaresinol (70) can be converted into the tetrahydronaphthalene derivative 73 (Figure 7.17B) (217, 311) (Jourdes etal., unpublished results). Furthermore, the linkage frequencies in the Nimz model are not fully consistent with data obtained for other angiosperm lignins, such as with Arabidopsis and alfalfa, this perhaps being due to low chromatographic recoveries of various products, etc. Other linkages in the Nimz model, such as 7-8′, are also not known to be present, and may thus potentially represent artifacts as well. As before for the other lignin structures discussed, various other known subunit structures, such as 5-5′, 8- O-4′, 7-0-4′-dibenzodioxocin, were absent from this model.

Two other “representative” structures for gymnosperm lignins were reported by Adler (312) and by Sakakibara (313). The first (not shown) contained a proposed spruce lignin depiction of 16 aromatic monomer (H/G) units, with 5 potential thioacidolysis releasable monomers, including one again being the unlikely sinapyl alcohol (5). Other linkages in­cluded thioacidolysis cleavable 8-5′ (1), 5-5′ (1), and 8-6′ (1) dimeric entities, with the remaining five monomers envisaged to be linked together via 8-1′,4- O-5′, 8-8′, and 5-5′ in­terunit linkages. Sakakibara (313), by contrast, proposed another “representative” softwood (gymnosperm) lignin structure (also not shown), based upon hydrolysis and hydrogenolysis analyses. This contained 35 monomeric aromatic units which could be potentially cleaved (e. g., by thioacidolysis) to afford G/S monomers (7), 8-1′ dimers (3), 8-5′ dimers (3), 8-8′ dimers (1), trimers (3) consisting of 5-574-0-5′, 8-875-5′, and 4-0-178-5′ linkages, as well as a proposed five-unit (8-5′, 8-5′, 5-5′, 8-8′) linked moiety. None of these proposed structures, however, again apparently adequately account for gymnosperm lignins, in terms of interunit type and frequency. Nor did they contain the additional substructures more recently discovered, such as the 5-5′, 8-O-4′, 7-0-4′-dibenzodioxocin (V, Figure 7.2D). Thus, while recognizing all of these to be valuable studies, they fell short of being adequate representations of native lignin structure(s).