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Wood Chemistry PSE 406/Chem E 470
Lecture 8 Hemicellulose II PSE Lecture 8
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Class Agenda Arabinogalactans Minor Species Starch Chitin Glucans
Pectins Starch Chitin PSE Lecture 8
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Arabinogalactan Minor hemicellulose except in Larch 10-20% of Larch
0.5-3% of other woods DP ~ 220 Backbone 13 -D-Galactopyranose units Branches: Nearly every galactose in backbone attached 16 to: -D-Galactopyranose -L-Arabinofuranose, -L-Arabinopyranose -D-Glucopyranosyluronic acid Arabinogalactans are present as minor constituents in many different softwood species. Only in larch (larix) is this hemicellulose a major component (35% of wood). As you can see from the information presented above and from the structure on the next slide, arabinogalactans are very highly branched molecules. Because of this branching, these compounds can be extracted from wood with water. There are different combinations of sugars which make up the side chains. The combinations depend upon the wood species. Arabinoglactans in larch are mainly a heartwood component. The content increases from the pith to the sapwood and from the lower part of the tree to the top. In other species, arabinogalactans are found in elevated levels (still low percentages) in reaction woods. Text PSE Lecture 8
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Arabinogalactan 3--D-Gal-1 3--D-Gal-1 3--D-Gal-1 3--D-Gal-1 3--D-Gal-1 6 -D-Gal 1 R R = galactopyranose or L-Arabinofuranose or D-glucopyranosyluronic acid -L-Araf -L-Ara 3 PSE Lecture 8
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Pectins Often described as water soluble extractives, not as hemicelluloses. Found in middle lamella and primary cell wall. Pectins is a class of several compounds Galactans (discussed on next slide) Galacturonans: Rhamnogalacturonan: a 1®4 galacturonic acid backbone with rhamnose 1/8 units, sidechain of galcturonic units Arabinans 90% arabinose linked a 1®5 Pectins are very minor wood components but are found in large amounts in certain fruits. They comprise about one third of the mass of the primary cell wall. They are laid down initially when the cell divides. We will discuss this in a later lecture. Remember that the primary cell wall in wood cells is quite thin thus accounting for the low percentage of these compounds. Pectins are highly involved in cell wall properties such as cell porosity, etc. There are a number of different hemicelluloses which fit under this umbrella such as galacturonans, rhamnogalacturonans, arabinans, galactans and arabinogalactans. In many cases, pectins are not included in the hemicellulose category although we will. Pectins are used commercially in many applications. These pectins are not obtained from wood but from fruits. There are several companies which market these compounds into a large number of end uses. Searching the web you will find several sites which talk about the health benefits of pectins. These sites generally describe pectins as a single compound. Text PSE Lecture 8
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Galactans Very minor component in normal and tension wood. Can be major (10%) component of compression wood. Galactan from Tamarack b 1®4 galactopyranose backbone, 1/20 1®6 galacturonic acid Rhamnoarabinogalactan Sugar Maple Gal:Ara:Rha (1.7:1:0.2) Many other varieties PSE Lecture 8
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Glucans Glucans are very minor components in wood.
The major representatives of this group are starch and callose. Starch Amylose:14 -D-Glucopyranose Amylopectin: 14 -D-Glucopyranose + 16 D Glucopyranose Callose 13 -D-Glucopyranose These are all polysaccharides which like cellulose are made up of glucose. The difference from cellulose is in the bonding patterns. Typically these compounds are found in very limited amounts in wood. Text PSE Lecture 8
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Callose Callose is found in very minor amounts in wood.
Small granular hemicelluloses found in a few isolated locations. Polymer of 13 b-D-Glucopyranose PSE Lecture 8
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Starch in Plants Starch serves as an energy reserve in plants.
High concentrations of starch are found in seeds, bulbs, and tubers. Starch can be as high as 70-80% of certain tubers and seeds. Wood contains minor amounts of starch in the form of granules in living parenchyma cells. Typical amounts: % of total wood Sapwood >3% PSE Lecture 8
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Chemical Composition of Starch
Plants contain two types of starch, linear (helix) amyloses and branched amylopectins. The amounts of each of these starch types present is plant dependent. Typical amounts are 25% amylose, 75% amylopectin Mutant species can have from 50-90% amylose PSE Lecture 8
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Amylose 14 -D-Glucopyranose
Because of the bonding, this molecule forms a helix It takes 6 gluopyranose units for each turn Addition of iodide to amylose results in a deep blue color. It is believed to be due to a complex of I5- within the polyglucoside spiral. Amylose is not water soluble PSE Lecture 8
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Amylopectin 14 aD-Glucopyranose + 16 D Glucopyranose
Amylopectin is a branched polymer Branching inhibits helix formation This starch is therefore somewhat water (hot) soluble. PSE Lecture 8
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Isolation of Starch From tubers (potato, tapioca, etc.) From corn
Wet tuber is ground fine. Hot water dissolves soluble fraction of tuber. Insoluble starch is separated from the liquid phase. Starch is dried. From corn The skin and the germ (oil containing portion) are mechanically removed from the kernel after soaking in water. Isolation of the starch is then similar to tubers. PSE Lecture 8
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Chitin Cellulose type polymer found in insects and crustaceans; used in making the hard exoskeletons (~30%). Second or third most abundant biopolymer. Second most abundant if you study this kind of thing for a living. Third most if you are a lignin chemist. PSE Lecture 8
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Chitin Structure Chitin is an amino polysaccharide
It is a linear polymer of 14 b-D-Glucopyranose units just like cellulose The difference is that the hydroxyl group of C2 has been replaced by an amide group PSE Lecture 8
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