Lignin radicals

Barsberg, S., P. Matusek, M. Towrie, H. Jprgensen, and C. Felby. 2006. Lignin radicals in the plant cell wall prohed hy Kerr-gated resonance raman spectroscopy. Biophys. J. 90 2978-2986. 

Chemical Properties. Lignin is subject to oxidation, reduction, discoloration, hydrolysis, and other chemical and enzymatic reactions. Many ate briefly described elsewhere (51). Key to these reactions is the ability of the phenolic hydroxyl groups of lignin to participate in the formation of reactive intermediates, eg, phenoxy radical (4), quinonemethide (5), and phenoxy anion (6) ... 

A polymeric composition for reducing fluid loss in drilling muds and well cement compositions is obtained by the free radical-initiated polymerization of a water-soluble vinyl monomer in an aqueous suspension of lignin, modified lignins, lignite, brown coal, and modified brown coal [705,1847]. The vinyl monomers can be methacrylic acid, methacrylamide, hydroxyethyl acrylate, hydroxypropyl acrylate, vinylacetate, methyl vinyl ether, ethyl vinyl ether, N-methylmethacrylamide, N,N-dimethylmethacrylamide, vinyl sulfonate, and additional AMPS. In this process a grafting process to the coals by chain transfer may occur. 

Russell, W. R. Forrester, A. R. Chesson, A. Burkitt, M. J. Oxidative coupling during lignin polymerization is determined by unpaired electron delocalization within parent phenylpropanoid radicals. Arch. Biochem. Biophys. 1996, 332, 357-366. 

Hatfield, R. D. Ralph, J. Grabber, J. H. A potential role of sinapyl p-coumarate as a radical transfer mechanism in grass lignin formation. Planta 2008, 228, 919-928. 

Recent work by Atalla(H) supports the idea that lignin is at least a semi-ordered substance in wood with the plane of the aromatic ring parallel to the cell wall surface. Woody plants synthesize lignin from trans-coniferyl alcohol (pines), trans-sinapyl alcohol 2 (deciduous), and trans-4-coumaryl alcohol 3 by free radical crosslinking initiated by enzymatic dehydrogenation(l2). Structures of these alcohols are given in Figure 1. 

LiP catalyze several oxidations in the side chains of lignin and related compounds [26] by one-electron abstraction to form reactive radicals [27]. Also the cleavage of aromatic ring structures has been reported [28]. The role of LiP in ligninolysis could be the further transformation of lignin fragments, which are initially released by MnP. 

For delignified boards, a constant or at higher temperatures an increasing rate of heat release with time is interpreted as a radical initiated oxidation, maybe catalysed by some of the intermediate oxidation products - but retarded by radical scavengers formed in the degradation of lignin. 

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