Carbohydrates and Polysaccharides

Objectives: provide you with a basic understanding of the chemical nature of carbohydrates and how they are converted to polysaccharides.

Important Concepts
  1. Chirality (enantiomers versus diastereomers). Organic molecules that are similar in structure are referred to as isomers. There are different kinds of isomers, and it can get somewhat complicated. What is comes down to is that the specific attachement of four groups to a particular carbon atom makes a difference. A pair of compounds that are non-superimposable mirror images of one another are called enantiomers. For an example, go to the Chime site for Wood 3434 and access the two forms of lactic acid. Orientation in space is a very important part of biological chemistry.
  2. Fischer, Haworth and Chair Ring Forms of Carbohydrates. These are the ways that we view six-membered rings. The chair form is the most realistic. The Haworth structures for the major carbohydrates are shown in the figure at the bottom of this page. For Chime images, click here and access the carbohydrate page.
  3. Major Carbohydrates in Softwoods and Hardwoods. The major carbohydrate found in wood is glucose (glucose is the building block of cellulose). The other major carbohydrates found are listed in the Table below. Note the difference between the xylose and mannose contents for hardwoods and softwoods.
  4. Mutarotation. In aqueous solution, carbohydrates exist in several forms. The inter-convert between forms by the process of mutarotation. The anomeric position (position one) interconverts between acyclic (no ring) and cyclic (ring) forms. The handout on carbohydrates explains this in further detail.
  5. Glycoside Bond Formation (Polysaccharide Biosynthesis). Individual carbohydrates can form bonds between one another. This bond occurs between the anomeric position on one carbohydrate and an hydroxyl on another. This is a condensation reaction with the by-product being water. The handout on carbohydrates explains this in further detail.
  6. Degree of Polymerization. The degree of polymerization (DP) is how many carbohydrates are present in an individual polysaccharide. This can range from 150 units for a hemicellulose to 10,000 for cellulose. Note that polysaccharides are not biosynthesized to an exact length or DP. A range of DPs is produced.
  7. Reducing and Non-reducing Ends. The reducing end of a polysaccharide is the end where the anomeric position does not have a glycosidic bond. The non-reducing end is the end of the chain that does.
Non-Glucose Units of the Polysaccharides of Various Woods
 Species  Arabinose Xylose Galactose Mannose Uronics Acetyl
Balsam Fir
Abies balsamea
1.1  5.2 1.0 10.0 4.8 1.4
Tamarack
Larix laricina
1.3 6.0 2.4 12.3 2.8 1.6
White Spruce
Picea glauca
1.1 7.0 1.9 12.0 4.4 1.2
Black Spruce
Picea mariana
1.5 6.0 2.0 9.4 5.1 1.3
White Pine
Pinus strobus
1.7 7.0 3.8 8.1 5.2 1.2
Eastern Hemlock
Tsuga canadensis
1.0 3.3 1.8 10.6 4.7 1.4
Northern White Cedar
Tsuga occidentalis
1.7 3.8 1.5 7.4 5.8 0.9
Red Maple
Acer rubrum
1.0 18.1 1.0 3.3 4.9 3.6
Yellow Birch
Betula alleghaniensis
0.3 18.5 0.9 1.8 6.3 3.7
Paper Birch
Betula papyrifera
0.5 23.9 1.3 2.0 5.7 3.9
American Beech
Fagus grandifolia
0.9 21.7 0.8 1.8 5.9 4.3
Quaking Aspen
Populus tremuloides
0.9 21.2 1.1 3.5 3.7 3.9
Black Locust
Robinia pseudoacacia
0.4 16.7 0.8 2.2 4.7 2.7
American Elm
Ulmus americana
0.4 15.1 0.9 3.4 4.7 3.0
 

Fundamental Carbohydrate Chemistry.
Structure and Functions of Polysaccharides are Related to Composition
1. Chemical Nature of Monomers.
2. Degree of Polymerization (DP, molecular weight distribution).
3. Type of Linkage (alpha or beta).
4. Chain Conformation.
5. Chain Associations (hydrogen bonding).
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