of lignin structure

With the incremental advances made in the study of lignins, such as by application of NMR and mass spectroscopic analyses, together with thioacidolytic degradation, further refinement of possible lignin representations were attempted for these polymers in spruce (15) and wheat straw (314). The Brunow et al. (15) model speculated the existence of random-linked structures, such as the 28-unit structure shown (Figure 7.3C), and also that dibenzodioxocin (substructure V, Figure 7.2D) and 5-5′ linkages (substructure VII) served as important branching points. The envisaged structure (Figure 7.3C), while acknowledging herein its hypothetical basis as emphasized by the researchers themselves, would nevertheless contain on a per monomer basis 10 potentially cleavable thioacidolysis monomers (35%), with the remaining eighteen units (65%) being releasable as 8-5′ (3), 5-5′ (2), 8-1′ (2), 4-0-5′ (1), and 8-8′ (1) linkedmoieties — where one of the latter has a presumed C-5 linkage to an adjacent lignin subunit, respectively. As gleaned from inspection of this proposed “hypothetical” structure, much of it would presumably be readily susceptible to thioacidolysis degradation. However, release of such fragments/substructures in the rel­ative amounts experimentally determined has not been observed. Furthermore, nor has it been established that either the dibenzodioxocin or 5,5′ substructures serve as branching points. Thus, this proposed structure again does not meet experimental scrutiny.

A linear (8-5′) linked lignin macromolecule has also been proposed (314), based on mass spectrometric analysis (APCI-MS, MS/MS, and MALDI-TOFMS) of the extracted lignin from wheat straw using the AVIDEL (315) procedure. Such proposed structures, however, need to be verified, quantified and placed in context with the existing chemistry of lignins, in order to assess what, if any, their relative merits and contributions are to lignin macromolec­ular configuration. Whatever the limitations of this approach, these researchers sought to obtain needed primary sequence data, unlike many of the previous highly speculative lignin structural models which did not.