25.9 Polysaccharides and Their Synthesis

Polysaccharide Structure and Composition

Polysaccharides are polymers built from monosaccharide units connected by glycosidic bonds. Understanding how these bonds differ in stereochemistry and position explains why cellulose, starch, and glycogen have such different physical properties and biological roles. This section also covers glycal assembly, a synthetic strategy for building polysaccharides in the lab with precise stereochemical control.

Structure of Cellulose and Starch

A glycosidic bond forms when the anomeric carbon (C1) of one sugar undergoes a condensation reaction with a hydroxyl group on another sugar, releasing water. The stereochemistry at C1 (α vs. β) and which hydroxyl is involved (C4, C6, etc.) determine the polymer's overall shape.

Cellulose is a linear polymer of glucose linked by β(1→4) glycosidic bonds. Because the β configuration places each successive glucose unit flipped 180° relative to its neighbor, the hydroxyl groups are oriented equatorially on both sides of the chain. This allows extensive interchain hydrogen bonding, producing rigid, insoluble fibers. Cotton and wood are largely cellulose.

Starch is a mixture of two components:

• Amylose: a linear polymer with α(1→4) glycosidic bonds. The axial orientation of the α linkage causes the chain to coil into a helix rather than lying flat.
• Amylopectin: a branched polymer with α(1→4) linkages along the main chain and α(1→6) branch points every 24–30 glucose units. This branching makes amylopectin more soluble than amylose. Potatoes and rice are rich in starch.

Glycogen as Energy Storage

Glycogen is the primary energy-storage polysaccharide in animals, concentrated in the liver and skeletal muscle. Structurally, it resembles amylopectin: the backbone uses α(1→4) bonds with α(1→6) branch points. The difference is that glycogen is much more highly branched, with branch points every 8–12 glucose units compared to amylopectin's 24–30.

This extensive branching creates a compact, roughly spherical particle. The large number of non-reducing ends (chain termini) is functionally important: glycogen phosphorylase cleaves glucose units from these ends simultaneously, allowing rapid glucose mobilization. A debranching enzyme handles the α(1→6) linkages at branch points. The released glucose-1-phosphate is converted to glucose-6-phosphate, which enters glycolysis or, in the liver, is dephosphorylated and exported to the blood.

Glycal Assembly for Polysaccharides

Glycal assembly is a synthetic strategy that uses glycals (unsaturated sugars with a double bond between C1 and C2) as building blocks for constructing polysaccharides with controlled stereochemistry. Glycals are typically prepared from glycosyl halides or hemiacetals by reductive elimination.

The synthesis follows these steps:

1. Protection: Hydroxyl groups not involved in the desired glycosidic bond are masked with protecting groups. Esters (e.g., acetates) are removed under mild basic conditions; ethers (e.g., benzyl ethers) are removed by hydrogenolysis. The choice depends on the stability needed during subsequent reactions.
2. Epoxidation: The C1–C2 double bond of the glycal is oxidized with dimethyldioxirane (DMDO) to form a 1,2-anhydrosugar (an epoxide at the anomeric center). The face selectivity of this epoxidation controls the stereochemistry of the eventual glycosidic bond.
3. Coupling: A free hydroxyl group on the acceptor sugar attacks the anomeric carbon (C1) of the 1,2-anhydrosugar in an $S_N2$-like ring opening, forming the new glycosidic bond with predictable stereochemistry.
4. Deprotection: After all coupling steps are complete, protecting groups are removed under appropriate conditions (base hydrolysis for esters, $H_2$/Pd for benzyl ethers) to reveal the free polysaccharide.

This approach is modular: the same glycal building blocks and coupling chemistry can be combined in different sequences to produce both linear and branched polysaccharides. The stereoselectivity of the epoxidation and ring-opening steps gives glycal assembly an advantage over less controlled glycosylation methods.

In biological systems, glycosyltransferases catalyze glycosidic bond formation using activated sugar donors (such as UDP-glucose), achieving the same stereochemical precision that synthetic chemists work hard to replicate.

Carbohydrate Diversity and Glycobiology

Carbohydrates exist across a range of complexity. Monosaccharides (glucose, galactose, mannose) are the simplest units. Oligosaccharides contain 2–10 sugar units and are often found attached to proteins and lipids on cell surfaces, where they play roles in cell recognition and signaling. Polysaccharides contain hundreds to thousands of units and serve structural or energy-storage functions.

Glycobiology is the field that studies how these saccharide structures are built, recognized, and used in living systems. Much of the motivation for synthetic methods like glycal assembly comes from glycobiology: researchers need pure, well-defined oligosaccharides and polysaccharides to study their biological roles.