By far the most important polysaccharides are polymers of D-glucose. These materials are generally divided into two categories depending upon 1. the nature of the glycosidic bond that connects one monomeric unit to the next 2. whether the "backbone" of the polymer is branched or unbranched. In this topic we will focus on the most common polysaccharides, cellulose and starch. We will also look briefly at an interesting class of polysaccharides known as cyclodextrins.
Cellulose is a "straight chain" polymer of D-glucose in which the monomeric units are connected together by b-1,4 linkages. This means that the oxygen at C-4 of one D-glucose is connected to C-1 of another D-glucose, and that the C1-OC4 bond, the glycosidic linkage, occupies an equatorial position in the D-glucopyranose ring. This bond is highlighted in red in Figure 1.
A b 1,4 Linkage
Cellulose is not a single compound, but rather a mixture of polymers of D-glucose in which the chain length varies from one molecule to the next. A typical polymer chain may contain 10,000-15,000 D-glucose units. Because each monomer unit contains several OH groups, inter-chain hydrogen bonding is extensive. Figure 2 illustrates the secondary bonding interactions between polymer chains.
Between the Sheets
The figure shows fragments of four polymer chains, one pair of which lie in the roughly same plane, the second pair lying in a parallel plane. The dashed red lines indicate inter-chain H bonds for those molecules that lie in the same plane. The dashed blue lines suggest inter-chain H bonds between chains in different planes.
Starch is a mixed polysaccharide consisting of two main components, a-amylose and amylopectin. The primary difference between cellulose and a-amylose is the nature of the glycosidic linkage. In cellulose it is a b-1,4 linkage, while in a-amylose the D-glucose units are joined together by a-1,4 linkages. While this is a seemingly small difference, it has a major impact on the shapes and functions of these two polymers. Unlike cellulose, the polymer strands of a-amylose do not assemble in planar sheets. Rather, as Figure 3 suggests, they adopt a helical structure similar to that found in nucleic acids. Typical chain lengths for a-amylose are approximately 1000 monomer units.
Around the Bend
The second component of starch is amylopectin. Like a-amylose, the D-glucose units in amylopectin are connected by a-1,4 linkages. The major difference between a-amylose and amylopectin is that amylopectin is a branched polymer; at irregular intervals there are branch points where a secondary polysaccharide chain is connected to the main chain by a-1,6 linkages. In amylopectin the branches occur, on average, every 24-30 D-glucose units along the main chain. Starch is the form in which plants store excess D-glucose. In animals it is stored as glycogen, which is similar in structure to amylopectin, except that it is more highly branched. Typically there is an a-1,6 linkage to a side chain every 8-12 D-glucose units along the main chain. Figure 4 offers a generic structure for amylopectin and glycogen. Two side chains are shown in color.
Energy Storage in Plants and Animals
The highest concentrations of glycogen are found in muscle and liver cells.
Exercise 1 Classify the glycosidic linkage highlighted in red an blue in the following polysaccharide fragment:
The bond highlighted in red isa-1,2 b-1,2 a-1,3 b-1,3 a-1,4 b-1,4 a-1,6 b-1,6
The bond highlighted in blue isa-1,2 b-1,2 a-1,3 b-1,3 a-1,4 b-1,4 a-1,6 b-1,6
Linking D-gluocse units together with a-1,4 linkages means that the growth of the polysaccharide follows a helical path. Occassionally, this coiling brings the D-glucose at the end of the growing polymer chain close enough to the one at the beginning that a glycosidic bond can form between them, thereby creating a cyclic polysaccharide. These structures are known as cyclodextrins. Figure 5 presents the structure of one such compound which contains a ring comprised of eight D-glucose units. This compound is known as g-cyclodextrin.
A Cyclodextrin Molecule
Cyclodextrins are natural products formed by the action of enzymes called cycloglucosyltransferases, CGTases, on starch. These enzymes are found in a microorganism called Bacillus macerans. Cyclodextrins participate in host-guest interactions, serving as hosts for a variety of small molecules. The number of monomer units in the macrocyclic ring determines the size of the cavity the host makes available to the guest. The ability of cyclodextrins to "encapsulate" small molecules has led to the development of a number of interesting applications. They have been used to
- enhance the chromatographic separation of chiral drugs
- protect the active ingredients in perfumes
- increase the solubility of antineoplastic drugs for use in chemotherapy
- separate cholesterol from dairy products
In Figure 5 you are looking down on the cavity from above. Figure 6 presents a perspective drawing of the 3-dimensional structure of g-cyclodextrin. Note that the polar OH groups project to the exterior of the structure while the hydrogens attached to the glucose units point into the cavity. Thus the interior is comparatively non-polar. These structural features make the polymer water soluble while still able to transport non-polar materials such as cholesterol.