Alkaline nitrobenzene oxidation (NBO), thioacidolysis degradation, and permanganate ox­idation procedures are also currently routinely applied to the analysis of plant materials, due to their abilities to cleave various linkages in lignins, as well as that of related phenolics. For example, NBO oxidation of lignin “model” compounds 42-44 results in formation of the various lignin-derived products (45-50) (Figure 7.10B) (189, 190), and is thus employed to probe both lignin compositions and contents. With lignins, this results in homolytic oxidative fission of their 7-8 linkages, and ultimately cleavage of the 8- O-4′ bonds (Figure 7.10C) (189, 190). Moreover, if there are significant amounts of other non-lignin cell wall bound p-hydroxycinnamic acids/aldehydes present, this method gives overestimations of lignin contents/compositions (77). Thioacidolysis is also now perhaps even more routinely employed, again to probe both lignin contents and composition; with this method, the over­all main monomeric, monolignol-derived, cleavable degradation products, obtained from lignins proper are compounds 54-56 (Figure 7.10D). These conversions have also been well studied with model compounds 51-53 that contain 8- O-4′ interunit linkages. Other work is currently under way to fully identify dimeric and other oligomeric entities released by this method. Both NBO and thioacidolysis procedures are generally also employed to estimate H:G:S ratios, as well as amounts of releasable products relative to lignin contents and/or cell wall residues (CWR). Overall, the recoveries of monomeric and dimeric components, rela­tive to original lignin contents, are very low. For example, alkaline NBO and thioacidolysis generally only account for circa 15-25% and 20-40% by weight, respectively, of the lignin presumed present (71, 132) (Figure 7.10E). Thus the bulk of the lignin polymer(s) remains unaccounted for using either procedure. Lastly, permanganate oxidation has also been quite extensively employed with various carboxylic acid methyl esters ultimately being formed (191). The yields of these products are also low (192) (~ 10-30% or so, Figure 7.10E), with the bulk of the lignin polymer again being unaccounted for.

In spite of these serious shortcomings, as discussed below, little has been done until now to attempt to establish overall trends in interunit linkage placement (as a function of lignin deposition); in accurately determining response factors, etc., for calibration and quantifica­tion purposes; in developing new/improved methodologies to study both lignins and their degradation products. For these reasons, and before discussing the results so obtained, it was thus first necessary to consider the advantages and limitations of the procedures currently employed.