(610g) Recovery and Characterization of Plant Cell Wall Biopolymers After An Alkaline Pre-Extraction of Hardwoods

Authors: 
Hodge, D., Michigan State University


In an alkaline environment hemicellulose and lignin compounds within plant cell walls have the ability to be extracted into an aqueous form.  This is partly due to the amorphous structure of the hemicellulose polysaccharide and its morphological placement in the cell wall.  Hardwood trees, a key feedstock for pulp production in North America and northern Europe, contain a hemicellulose polymer of β(1-4) linked xylose residues with side chain substitutions of acetyl and 4-O-methylglucuronic acid units.  During an alkaline extraction the hardwood xylan hemicellulose chain is fractured into shorter chain oligomers without significant degradation due to chain stabilization by the 4-O-methylglucuronic acid substituent.  After extraction the dissolved material can be removed from the aqueous form through the use of an organic liquid.    These steps significantly impact the hemicellulose by providing changes to the chemical and physical structure of the polymer.  To understand the changes taking place during extraction and precipitation the recovered material was analyzed for total composition and molecular weight.  The hardwood species of trees tested included Acer saccharum (sugar maple), Betula pendula (silver birch), Populus nigra X. P. maximowiczii cv. NM6 (a hybrid poplar), and Populus tremuloides X. P. tremula (a hybrid aspen).  A sample of switchgrass, Panicum virgatum cv. Cave-In-Rock, was used as a comparison feedstock to see differences in the chemical characteristics of HC.  Sugar maple and switchgrass performed the best in terms of biopolymer recovery exceeding 70% of the total amount of material dissolved after extraction.  Solvents, such as ethanol and methanol, were compared to see if the route of precipitation varies depending on the solvent used.  The performance of either solvent in recovery of the dissolved biopolymers will help to understand overall solubility characteristics of the extracted cell wall material. 

After precipitation the recovered material was then subjected to chemical and physical characterizations to understand the factors that influence hemicellulose solubilization during extraction and precipitation.  Composition studies show how the chemical make-up of the polymeric material can influence recoverability.  Physical studies on the material, such as size exclusion chromatography (SEC), allow for analysis on the polymers actual size once extracted and recovered.  Degree of polymerization (DP) of the xylan chain was also checked with a novel spectrophotometric method to determine the number of sugar reducing ends by using 2,2’-bicinchoninate acid (BCA).    These studies show that changes in the biopolymer influenced by extraction and precipitation affect the overall solubility.  The total amount of material recovered is not entirely polysaccharides due to non-covalently and covalently linked lignin fractions being extracted and recovered simultaneously with the hemicellulose forming lignin-carbohydrate complexes (LCC).  These compounds, due to the aromatic linkages, have been known to influence polymer aggregation that can inflate molecular weight analysis by SEC.  The BCA procedure is sensitive to these compounds as well.  A bleaching step with hydrogen peroxide was applied on the precipitated material to remove these interfering compounds for more accurate characterization.

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