Beginning with the studies of Freudenberg and others, it was initially assumed that lignins were three-dimensional cross-linked biopolymers, albeit also in the absence of any experi­mental data in support of this assumption. Such assumptions, however, did not survive fur­ther experimental scrutiny either. In a study by Dolk etal. (316), using kraft lignin derivatives, it was estimated that at best there was less than one cross-link per 19 monomeric units. Later studies by Mlynar and Sarkanen (317) and by Smith etal. (318), using ultracentrifugation/ size exclusion chromatography and modeling of delignification with a computer program (SIMREL), respectively, also concluded that there were no detectable cross-links. Moreover, other studies probing lignin biophysical properties in situ have indicated that lignins were more akin to linear-like polymeric entities (319). Interestingly, Forss and Fremer (308) had underscored many of the experimental shortcomings in the proposed Freudenberg three­dimensional cross-linked structure which went unanswered. It is thus quite bewildering why many researchers have arbitrarily dismissed the concept of regular structure in lignins without the appropriate experiments having even been either designed and/or conducted. Yet this incorrect notion of three-dimensional cross-linked polymer for lignins, however, exists even today in many literature contributions.

Lignins also display a profound tendency to associate, as demonstrated, for example, in the study of kraft lignins (8). Specifically, it has been reported that kraft lignins — while chemically modified but which nevertheless are presumed to retain important vestiges of native lignin structure — contain discrete molecular entities. These can associate with one another to form multimodal distributions of interconvertible supramolecular complexes estimated to be comprised of 103-104 individual species. Such selectivity is considered to result from association of well-defined (regular) structures in the participating polymer chains (320). This presumably would not be expected if random (1066/100-mer) and/or combinatorial chemistry was occurring.

In presumed agreement with a concept of defined template polymerization, one-electron oxidation of coniferyl alcohol (3) moieties in open solution in the presence of a lignin template apparently engendered preferential formation of high molecular weight species in preliminary studies. In this regard, Sarkanen etal. (50, 51) reported the effects of polymer­ization of coniferyl alcohol (3) in the presence of peroxidase/H2O2 and a methylated “kraft” lignin preparation (Mw ~15 400,2.7 x 10-8 M initial concentration). In the presence of the putative template, higher molecular weight entities were reportedly preferentially formed as shown by the elution profiles (Figure 7.18A), and to a much lower extent in absence of the template (Figure 7.18B). These researchers interpreted these findings as due to template poly­merization effects, where the preformed (methylated kraft) lignin macromolecule assumed the role of a progenitorial template in vitro. That is, monolignol radicals were considered to be positioned on adjacent loci of the template with new interunit linkages determined by either the corresponding substructure in the lignin template chain and/or the chemical nature of the monolignol (radical) species aligned on the template. Work has, however, not yet been described as to the chemical (subunit) nature of these interunit linkages and how/if the fidelity of the replication process is maintained relative to that of the macromolecular template itself. Nevertheless, the presumed ability to polymerize monolignol radicals on a preexisting lignin template needs to be considered as regards possible relevance to cell wall assembly mechanisms.