Subproject 2 – Lignin

Participants: Paulo Mazzafera (Institute of Biology – Unicamp). Collaboration with Dr. X. S. Xie (Harvard University).

Lignin is a phenolic polymer highly complex, and it is a component of the plant cell walls, conferring strenght to the whole structure. Lignin is the second most abundant organic compound on the Earth after cellulose. Plants stands in part because lignin in the vascular tissues, mainly in the xylem, responsible for the transport of water. Besides this function in vascular plants lignin has a role to control plant pathogens because the toxic nature of the polymerized phenolic compounds. Despite good for plants, this characteristic has been one of the major problem to use sugarcane bagasse to produce ethanol. In fact the creation of a new industry to produce ethanol on large scale and based on this sugarcane residue, named lignocellulose biofuel, will demand a huge knowledge on basic and applied areas. Among them, cellulose is a recalcitrant substrate for bioconversion and molecular engineered microrganism will be necessary as none known yeast can convert lignocellulose to simple sugars and then to ethanol. Additionally, the yeast lineages currently used to produce ethanol are not efficient in fermenting sugars other than
Glucose, and low concentrations of this alcohol kill the microorganism. Regarding lignin, it inhibits enzymatic hydrolysis of the the carbohydrates present in the cell wall and so far it can be removed only using corrosive chemical pretreatments. Lignin is also a problem in the cellulose industry for paper making. Two methods are used to obtain cellulose from wood, mechanical and chemical. Mechanical pulping is used when low quality paper is produced. The chemical method is used for high quality paper and employs hydrolyses and lignin solubilization, under high temperature and extreme pHs. The most used chemical pulping method is the Kraft method, where wood pieces are subjected to high temperature (150-170ºC), sodium hydroxyde and sodium sulfide treatments, when most of the lignin is hydrolyzed and solubilized. However, this may not be enough to remove al the lignin and bleaching compounds such as chloride, hypochloride and dioxidechloride may be used to reduce lignin to very low levels. From all these treatments results several compounds which are released in the atmosphere as gases or toxic liquid disposals which require expensive treatments for inactivation before being discarded in the environment. Due to its rigid structure very little is known about the whole organizational structure of lignin mainly regarding the chemical reactions occurring among its components. Lignin is made from three compounds: 4-hydroxy-cinammyl-alcohol, coniferyl alcohol and sinapyl alcohol, which are referred aldo as hydroxyphenyl unit, guaiacyl unit and siringyl unit, respectively. They differ mainly in the hydroxylation and methylation degree in the benzene ring, which allows a series of covalent reactions among molecules, making the final structure very complex. Although these compounds prevail in plants lignin, depending on the physiology of the plant as influenced by biotic and abiotic stresses, other phenolic compounds may also be polymerized. We believe that integrate CARS, SHG/THG and FLIM analysis of autofluorescence might contribute to a better understanding of lignin distribution and organization, and that Raman microspectroscopy could discriminate different components of lignin.