Lignin isolation procedures

In spite of their intractable nature, lignins can be fully removed, but only under quite drastic chemical conditions using either strong acid [e. g., sulphite pulping processes (147-150)] or alkaline [e. g., kraft (8)] conditions at elevated temperatures and pressures. They can also be partially removed using milder procedures, such as by extensive ball milling for 4-5 days followed by extraction in dioxane:H2O (9:1) (71). Indeed, the “milled wood lignin” or “Bjorkman” lignin (151,152) preparations are often considered as representing the mildest forms of treatment necessary to solubilize lignin-derived components with these frequently being described as structurally closest to native lignins. The overall yields resulting from such ball-milling manipulations are though generally low (<20%), but can be somewhat improved by pretreatment with cellulase(s) and other hemicellulose degrading enzymes. There is also one report of attempting to solubilize entire plant tissue using a mixture of DMSO/tetrabutylammonium fluoride (TBAF) on very finely ground plant cell wall residue (CWR) (153). In our hands, this approach was unsuccessful, in large part because total solubilization was not achieved, with the suspension also becoming very black, viscous, gelatinous, and unworkable.

In terms ofthe acidic sulphite treatment ofwoody plant tissue, the effects on delignification have been somewhat studied. For example, using a continuous flow (pulping) apparatus, it was possible to essentially delignify the tissue completely when treating black spruce (Picea mariana) stem wood. Characterization of the components in the resulting eluant established that initially various paucidisperse low molecular weight moieties, such as mono-, di-, and tri-sulphonated monomers, e. g., 38-41 (Figure 7.10), were released and these were facilely identified (147-150). These were then followed by solubilization of higher molecular weight sulphonated polydisperse lignin chains of increasing molecular weights ranging from 3.3 to 120 kDa (147); however, these polymeric entities have not yet been fully characterized in terms of their chemical structures thus far.

Alkaline (kraft) pulping delignification has also long been investigated, together with the nature of the accompanying chemical delignification reactions. Perhaps most interesting, Sarkanen etal. (8) demonstrated that solubilized “kraft” lignins, assumed to contain reflec­tions of the original lignin primary structures, displayed a capability to self-associate. This was interpreted then, and now, as due to extensive noncovalent dynamic electronic corre­lations between the associating (primary) chains resulting in aggregation of the polymeric lignin chains (e. g., up to several millions in molecular weight). This property is yet an­other complicating and potentially confounding feature in lignin analyses; such properties limit further the ability to facilely study lignin from a structural perspective (e. g., by NMR spectroscopy) as discussed below.

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