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Isolation and characterization of lignin from populus Stewart, Jaclyn Jeanette

Abstract

Wood chemistry, and in particular lignin chemistry, impacts chemical pulp yield and quality. In this research, a thorough lignin chemical structure was elucidated and related to pulp properties of natural aspen clones and genetically modified hybrid poplar. The chemical differences among phenotypically distinct naturally grown aspen clones indicated that considerable variability exists in the concentration of carbohydrates, lignin, and extractive. Lignin monomer ratio was determined using thioacidolysis and nitrobenzene oxidation, and varied from 74.4 to 80.3% syringyl units. The ease of chemical pulping was very strongly correlated with the mol % S derived from nitrobenzene oxidation. Two clones (10-1 and 16-2) that revealed the greatest differences in Kraft pulping efficiencies (difference of yield and Kappa) were subject to extensive lignin analyses. The peak molecular weight of sample 10-1 lignin was shown to be significantly higher than sample 16-2 lignin. These two samples were also employed for in-depth NMR analysis in order to determine side chain linkages, functional group differences, and aromatic carbon nature. It was shown that the structure of the residual lignin generated by ball milling was consistent with Kraft pulp efficacy; the phenolic hydroxyl content and methoxyl content were both higher for sample 16-2 than for sample 10-1. A similar study, employing highly selective and controlled genetically modified lignin, involved in-depth lignin analysis. Hybrid poplar expressing the Arabidopsis ferulate 5-hydroxylase gene under the control of a cinnamate 4-hydroxylase promoter was the model sample. As expected, the modified lignin had an increase in syringyl content as measured by thioacidolysis and methoxyl analysis. In addition to this monomer ratio shift, the molecular weight of the lignin was reduced in the modified lignin, which may further enhanced chemical pulping efficiency. This is the first report of a dramatic molecular weight decrease due to up-regulation of a gene in the lignin biosynthetic pathway.

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