El artículo seleccionado no se encuentra disponible por ahora a texto completo por no haber sido facilitado todavía por el investigador a cargo del archivo del mismo.

ABSTRACT:The structure of the lignin in wheat straw has been investigated by a combination of analytical pyrolysis, 2D-NMR, and derivatization followed by reductive cleavage (DFRC). It is a p-hydroxyphenyl-guaiacyl-syringyl lignin (with an H:G:S ratio of 6:64:30) associated with p-coumarates and ferulates. 2D-NMR indicated that the main substructures present are β-O-4′-ethers (∼75%), followed by phenylcoumarans (∼11%), with lower amounts of other typical units. A major new finding is that the flavone tricin is apparently incorporated into the lignins. NMR and DFRC indicated that the lignin is partially acylated (∼10%) at the γ-carbon, predominantly with acetates that preferentially acylate guaiacyl (12%) rather than syringyl (1%) units; in dicots, acetylation is predominantly on syringyl units. p-Coumarate esters were barely detectable (<1%) on monomer conjugates released by selectively cleaving β-ethers in DFRC, indicating that they might be preferentially involved in condensed or terminal structures. KEYWORDS: wheat straw, Py-GC/MS, TMAH, HSQC, DFRC, milled wood lignin, p-coumarate, ferulate, coniferyl acetate, tricin ■ INTRODUCTIONEnergy consumption has increased gradually over the last decades as the world population has grown and more countries have become industrialized. Crude oil has been the major resource used to meet the increased energy demand. However, concerns about declining of energy resources and the need to mitigate green-house gas emissions and decrease our dependency on fossil fuel reserves have focused attention on the use of plant biomass as a source for the production of biofuels and/or bioproducts. 1The first generation of biofuel feedstocks included sugar cane and cereal grains. Bioconversion of such crops to biofuels, however, competes with food production for land and has a considerable effect on food and feed prices. A promising alternative for second generation biofuels will come from cultivated lignocellulosic crops or agricultural wastes, which are available in high amounts at relatively low cost and could be a widely available and relatively inexpensive source for biofuels and/or bioproducts. Therefore, increasing attention is being paid to the use of lignocellulosic biomass as a renewable feedstock for the above industrial uses. 2−4Common lignocellulosic feedstocks considered for second generation biofuel production include woods (e.g., poplar or eucalyptus), perennial energy crops (e.g., switchgrass or Miscanthus species), and agricultural wastes (e.g., corn stover or cereal straws). Among them, wheat straw has the greatest potential of all agricultural residues because of its wide availability and low cost. 4,5 Wheat straw is an abundant byproduct from wheat production in many countries. The average yield of wheat straw is 1.3−1.4 kg/kg of wheat grain, with a world production of wheat estimated to be around 680 million tons in 2011. Wheat straw contains 35−45% cellulose, 20−30% hemicelluloses, and around 15% lignin, which makes it an attractive feedstock to be converted to ethan...

Lignin changes during plant growth were investigated in a selected Eucalyptus globulus clone. The lignin composition and structure were studied in situ by a new procedure enabling the acquisition of two-dimensional nuclear magnetic resonance (2D-NMR) spectra on wood gels formed in the NMR tube as well as by analytical pyrolysis-gas chromatography-mass spectrometry. In addition, milled-wood lignins were isolated and analyzed by 2D-NMR, pyrolysis-gas chromatography-mass spectrometry, and thioacidolysis. The data indicated that p-hydroxyphenyl and guaiacyl units are deposited at the earlier stages, whereas the woods are enriched in syringyl (S) lignin during late lignification. Wood 2D-NMR showed that b-O-4# and resinol linkages were predominant in the eucalypt lignin, whereas other substructures were present in much lower amounts. Interestingly, open b-1# structures could be detected in the isolated lignins. Phenylcoumarans and cinnamyl end groups were depleted with age, spirodienone abundance increased, and the main substructures (b-O-4# and resinols) were scarcely modified. Thioacidolysis revealed a higher predominance of S units in the ether-linked lignin than in the total lignin and, in agreement with NMR, also indicated that resinols are the most important nonether linkages. Dimer analysis showed that most of the resinol-type structures comprised two S units (syringaresinol), the crossed guaiacyl-S resinol appearing as a minor substructure and pinoresinol being totally absent. Changes in hemicelluloses were also shown by the 2D-NMR spectra of the wood gels without polysaccharide isolation. These include decreases of methyl galacturonosyl, arabinosyl, and galactosyl (anomeric) signals, assigned to pectin and related neutral polysaccharides, and increases of xylosyl (which are approximately 50% acetylated) and 4-O-methylglucuronosyl signals.

SummaryLignin is the second most abundant constituent of the cell wall of vascular plants, where it protects cellulose towards hydrolytic attack by saprophytic and pathogenic microbes. Its removal represents a key step for carbon recycling in land ecosystems, as well as a central issue for industrial utilization of plant biomass. The lignin polymer is highly recalcitrant towards chemical and biological degradation due to its molecular architecture, where different non‐phenolic phenylpropanoid units form a complex three‐dimensional network linked by a variety of ether and carbon–carbon bonds. Ligninolytic microbes have developed a unique strategy to handle lignin degradation based on unspecific one‐electron oxidation of the benzenic rings in the different lignin substructures by extracellular haemperoxidases acting synergistically with peroxide‐generating oxidases. These peroxidases posses two outstanding characteristics: (i) they have unusually high redox potential due to haem pocket architecture that enables oxidation of non‐phenolic aromatic rings, and (ii) they are able to generate a protein oxidizer by electron transfer to the haem cofactor forming a catalytic tryptophanyl‐free radical at the protein surface, where it can interact with the bulky lignin polymer. The structure–function information currently available is being used to build tailor‐made peroxidases and other oxidoreductases as industrial biocatalysts.