Lignin phenolic subunits

Earlier work on the Co(salen)-catalyzed 02 oxidation of p-substituted phenols to p-benzoquinones has been extended to include substrates that serve as models for lignin phenolic subunits. Lignin is a renewable source of carbon and its oxidation to p-benzoquinone derivatives would allow conversion to useful intermediates. Certain p-substituted phenolics can be oxidized to p-benzoquinones with dioxygen using the Co(salen) complexes A and B in Figure 4 as catalysts ". ... 

An example of the use of the nitrobenzene oxidation to elucidate differences in lignin subunit composition between wild-type and mutant Arabidopsis plants can be found in Chappie et al. (1992). They dried and ground stem tissue of Arabidopsis and extracted 50 mg with methanol (three times 1 mL) and deionized water (twice, 1 mL) at 60°C. Esterified phenolics... 

VM, percentage matter volatilized in pyrolysis CHYDR, carbohydrates with pentose and hexose subunits PHLM, phenols and lignin monomers LDIM, lignin dimers LIPID, lipids, alkanes, alkenes, bound fatty acids, and alkylmonoesters ALKY, alkylaromatics NCOMP, mainly heterocyclic N-containing compounds STEROL, sterols PEPTI, peptides SUBER, suberin FATTY, free fatty acids in % of total ion intensity. 

The network structure of lignin, which is made of phenol units, coagulates the cell wall in wood tissue, which is composed of cellulose and hemicellulose. Lignin is currently a waste product because of its complicated structure [1-4], It is produced by an oxidative polymerization of coniferyl alcohol, sinapil alcohol, and cumarol alcohol (Figure 1) catalyzed by metalloenzymes such as laccase and peroxidases. Laccase is a protein whose active center contains four coppers per one subunit [5-20],... 

The observed properties of DOM are reasonably expected to be some kind of weighted average of the properties of the individual compounds of which DOM is comprised. Ideally, DOM could be studied at this level— the level of individual compounds. Practically, the mixture is too complex for this approach, so DOM is studied at the level of elemental composition and at the level of functional groups and structural subunits, the latter of which include the amino acids, sugars, and lignin-derived phenols that can be liberated from DOM by hydrolysis or oxidative degradation. 

This section of the review of DOM in freshwaters will examine its isolation by XAD resins and by membranes, the molecular weight distribution of DOM, its elemental composition, its acidic functional groups, its distribution of carbon among structural subunits, and the low-molecular-weight molecules (amino acids, sugars, and lignin-derived phenols) that are liberated from DOM by hydrolysis or oxidative degradation. For each of these subtopics, a statistical summary of published data will be presented. 

---