Lignin dimers

Table I. Degradation Mechanism of Lignin Dimer Models...

Hie GOG-Dimer formed from the lignin dimer compound (GOG) by laccase III was also degraded by laccase HI, resulting in the formation of compounds (I), (VI), and biphenyl dimer of 2-methoxy-l,4-benzoquinone (DC). This indicates that GOG-Dimer also suffers the cleavage of -0-4 ether linkage via the alkyl-phenyl cleavage. 

Figure 14.1. Temperature-resolved intensities of precise masses in a high-resolution Py-FIMS measurement of wheat straw. The thermograms of palmitic acid (m/z 256.240) and a thermal fragment (m/z 256.108) of the lignin dimer C16Hi404 (m/z 270.089) are shown with a mass window of +30mmu.

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. 

Microbial transformations of DOM were also studied by incubation experiments. The DOM extracted from maize straw and forest floors was incubated for 90 days and samples taken before and after incubation were analyzed by Py-FIMS and complementary UV absorbance, fluorescence emission spectroscopy, FTIR-spectroscopy, 1H NMR spectroscopy, and 13C natural abundance (Kalbitz et al., 2003). The Py-FI mass spectra showed increases in the proportions of phenols and lignin monomers at the expense of lignin dimers and alkylaromatics during... 

Figure 14.14. Thermograms for volatilization of lignin dimers and lignin monomers of wheat straw (a) before incubation, without incubation (control), and with inoculation with Tricho-derma satumisporum (+T. sat.), (b) after 4 weeks, and (c) after 13 weeks of incubation. Reprinted from Wiedow, D., Baum, C., and Leinweber, P. (2007). Inoculation with Tricho-derma satumisporum accelerates wheat straw decomposition on soil. Archives of Agronomy and Soil Science 53,1-12, with permission from Taylor Francis.

Filley, T.R., Minard, R.D., and Hatcher, P.G. (1999) Tetramethylammonium hydroxide (TMAH) thermochemolysis proposed mechanisms based upon the application pf 13C-labeled TMAH to a synthetic model lignin dimer. Org. Geochem. 30, 607-621. 

Goni, M.A., and Hedges, J.I. (1992) Lignin dimers Structures, distribution, and potential geochemical apphcations. Geochim. Cosmochim. Acta 56, 4025 1043. 

Goni, M. A., and J. I. Hedges. 1992. "lignin dimers Structures, distributions and potential geochemical applications." Geochimica Cosmochimica Acta 56 4025—4043. 

It is likely that the benzenecarboxylic acids are derived from lignin units where the a-carbon of the side-chain has been oxidized to a carboxyl group (52), and different studies have interpreted the structures and distributions of the methyl esters of benzenecarboxylic acids as indicating that these structural units exist as either free or ester-bound structures in the HA 10,16,17,24)- However, it has recently become clear that certain benzenecarboxylic acids are produced from unoxidized lignin moieties by the TM AH reagent in the course of the reaction at elevated temperatures 41), Examination of the products from the TMAH thermochemolysis of a model lignin dimer, free of any carboxylic functionality, showed the production of relatively large amounts of methylated benzenecarboxylic acid derivatives. These authors... 

FIGURE 10.22 Reactions of a substituted lignin dimer model. (Dimmel, D.R. and L.R, Schuller, J Wood Chem Technol, 6, 535, 1986.)... 

To better understand the mechanisms for the oxidation of lignin by a laccase mediator system, a laccase from Polyporous sp, kindly provided by Novozymes, was used in combination with 1-HBT. The redox mediator was found to be partly regenerated during the oxidation of lignin dimer 1 in the presence of laccase. A free radical of 1-HBT generated by laccase was probably responsible for the oxidation of I [146]. The free radical of 1-HBT was, however, transformed to benzotriazole, which could not mediate the oxidation of I. A proposed mechanism for the laccase mediator oxidation of nonphenolic lignins is given in Scheme 14.1. 

To investigate the importance, not only of laccase mediators, but also of lacca-ses per se, several laccases were studied for the oxidation of the nonphenolic lignin dimer I. In the presence of the redox mediators 1-HBT or violuric acid, it was found that the oxidation rates of dimer I by the laccases differed considerably. In oxidation of dimer I, both 1-HBT and violuric acid were to some extent, consumed. The consumption rate followed the same order of laccases as the oxidation rates of dimer I. The oxidation rate of dimer I was found to be dependent on both k, jt and the stability of the laccase in question. Both 1-HBT and violuric acid inactivated the laccases— violuric acid to a greater extent then 1-HBT. The presence of dimer I in the reaction mixture slowed down this inactivation. Inactivation seems to be mainly due to the reaction of the redox mediator free-radical with the laccases. No relationship between the carbohydrate content of the laccases and their inactivation was found. When the redox potential of the laccases is in the range of 750-800 mV, i.e., about that of the redox mediator, a further increase in redox potential does not affect k(,jt and the oxidation rate of dimer I [147]. 

Figure 6 Degradation of a p-O-4 lignin dimer via a one-electron oxidation initiated by electron bombardment in a mass spectrometer, ligninase and 5 or 6 using t-butylhydroperoxide...

Oak wood chips Dichloromethane 150 C/20 MPa Lignin dimer derivatives Vichietal. 2007 [55]... 

Kawamoto H, Ryoritani M, Saka S. Different pyrolytic cleavage mechanisms of (3-ether bond depending on the side-chain structure of lignin dimers. J Anal Appl Pyrol. 2008 81 88-94. 

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