Hemicelluloses
- Objectives: use the knowledge you have gained on the
structural components of carbohydrates and polysaccharides to
understand the wood hemicelluloses.
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- Important Concepts
- Glycosidic Bond. This is how carbohydrate (ie., monosaccharides)
attach to one another. A glycosidic bond is a bond between the
anomeric position and an hydroxyl position on another carbohydrate.
This is an acetal. The condensation reaction releases water.
If formation of a glycosidic bond releases water, then the hydrolysis
of a glycosidic bond consumes water.
- Homo- and Heteropolysaccharides. Polysaccharides composed
of only one monosaccharide is termed a homopolysaccharides. Cellulose
and starch are examples of homopolysaccharides. Heteropolysaccharides
have more than one monosaccharide in their overall structure.
Hemicelluloses are heteropolysaccharides.
- Degree of Polymerization. How many monosaccharides
are present in one hemicellulose molecule. There is a distribution
of lengths.
- Reducing and Non-reducing Ends. The reducing end has
the anomer position without a glycosidic bond. The non-reducing
end is the end where there is a glycosidic bond, but no additional
carbohydrates attached at the hydroxyls.
- General Aspects of Wood Hemicelluloses
- 1. 20-30% of the weight of wood.
- 2. Composed of several carbohydrate monomers (heteropolysaccharides).
- 3. Degree of polymerization about 150-200.
- 4. Usually branched.
- 5. Acetate and methyl groups can be present.
- 6. Amorphous (not crystalline).
- 7. Can generally be solubilized by base, but this removes
the acetate groups (de-esterification).
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- Polysaccharide/Hemicellulose
Nomenclature Key
- Hardwood
Hemicelluloses.
- Softwood
Hemicelluloses.
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Wood 3434 Information Page