Complexes with lignin

A thermo-analytical study of untreated and CCA treated wood samples (more or less cylindrically shaped with diameter less than 2 mm) was performed in order to examine the influence of the presence of CCA on the pyrolytic behaviour of wood samples (TG study), as well as the release of metals (Cu, Cr and As) during the pyrolysis process in the kinetically controlled regime. Since arsenic is the most problematic compound during pyrolysis of CCA treated wood and almost all arsenic is present in the treated wood as CrAs04 (precipitated on cellulose or complexed with lignin) [9], a... 

When chromium is reduced from the hexavalent to the trivalent state it reacts readily with arsenic to form CrAs04, which, in turn, has the ability to complex with the lignin and cellulose. In treated wood, approximately 85% of the arsenic reacts with chromium the remaining arsenic forms fairly soluble complexes with lignin and cellulose (5J). 

The above studies raise the question of the selectivity of the bleaching reaction toward other lignin chromophores and leads to the work on the bleaching chemistry of tert-butylamineborane complex with lignin model compounds (Figure 13.13) in aqueous solutions [73]. 

Rossberg A, Baraniak L, Reich T, Hennig C, Bernhard G, Nitsche H (2000) EXAFS structural analysis of aqueous uranium(VI) complexes with lignin degradation products. Radiochim Acta 88 593-597 Rothe J, Kneedler EM, Pecher K, Tonner BP, Nealson KH, Grundi T, Meyer-Ilse W, Warwick T (1999) Spectromicroscopy of Mn distributions in micronodules produced by biomineralization. J Synchrotron Rad 6 359-361... 

Nitrogen is present in peat in various forms not well understood. A considerable portion is known to be in the form of protein, but not in a free state. It is complexed with lignin, cellulose and hemicellulose. In such forms it is not readily released as ammonia, but does undergo slow release as the peat decomposes. It is very fortunate that it is not readily released, because in this age of abundant supplies of commercial synthetic ammonia, the nitrogen is far more valuable to agriculture when present as complexes than if present as ammonia. 

Since compound II has a Soret peak at 420 nm and the oxycomplex 416 nm, the equilibrated solution of compound II, H2O2, and compound III should exhibit an intermediate absorption peak. The existence of such an equilibrium has been established with HRP (22) and was also observed with lignin peroxidase (Cai, D., and Tien, M., unpublished results 23). Wariishi and Gold (25) recently proposed the existence of a new enzyme intermediate when compound in is formed in the presence of H2O2. They referred to this intermediate as compound III and suggested it to be a complex between compound III and H2O2. The existence of such an intermediate was based primarily on the Soret spectral properties where a 419 nm maximum was observed. In contrast, we see no evidence for existence of such a complex. The observed 419 nm absorbance maximun can be best explained by the existence of both compounds II and III in solution. 

Tannins are water-soluble phenolic compounds which are usually extracted from plant material by hot water. After lignins, they are the second most abundant group of plant phenolics. Their tanning property is due to their capacity to combine with proteins. However, they can also complex with other polymers such as alkaloids, cellulose, and pectins. 

Cross and Bevan Cellulose- An impure cellulose isolated from the complex it forms with lignin, in materials such as wood straw. The method of isolation is described by Doree (Ref 1) Ott (Ref 2). Cross Bevan cellulose contains both furfural-yielding substances (pentosans), and hexosans of the mannan type. It retains, in the case of coniferous woods, 40-60% of the total furfural-yielding substances in the original wood, and about 55 66% iq the case of hard woods. 

A reasonable model has been proposed to accommodate these results (2/y 23). The presence of quinoid functions in lignin would give rise to electron donor-acceptor complexes with existing phenolic groups. These complexes, like quinhydrone, would form stable radical anions (semiquinone anions) on basification, according to the scheme shown below. Both biological and chemical oxidation would create more quinone moieties, which in turn would increase the contribution of Reactions 1 and 2. Alternately, enzymatic (< ) and/or alkaline demethylation 16) would produce... 

Recent work using the atmospheric dioxane extraction of cork has proved extremely difficult, yielding only small amounts of a lignin- enriched material (7). This was explained by the presence of suberin, a complex structure of phenolic and aliphatic domains and the interaction with lignin. Further work using a saponified cork stressed these arguments (8). 

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