CHEMICAL FUNDAMENTALS AND PROPERTIES
General description
Polysaccharides (PSs) are produced biologically in plants, animals, and microorganisms from other carbon-containing compounds such as carbohydrates, lipids, and ultimately, carbon dioxide. In nature, PSs have numerous functions, such as structural elements and energy storage, and their chemical structures have evolved to fit the individual, specialized performance needs of each organism. Thus, PSs have a great diversity of structures and functional properties which we may exploit for commercial applications.
Chemical structures
Polysaccharides are characterized as polymeric
chains of cyclic sugar (monosaccharide) repeat units. The sugar units
are linked by an acetal
ether bond between a hydroxyl group of one sugar and the acetal
(carbonyl) carbon of
another sugar (see Figure 1). Thus, the simplest PSs are long chains of
5- or 6-membered rings with acetal ether linkages and pendant hydroxyl
groups. Chain length varies greatly depending on the polysaccharide and
its source, with the number of repeat units, n, ranging generally from 50 to
50,000 and molecular weights of 10,000 to 10,000,000 g/mol.
Fig 1. A general PS structure.
The great structural diversity of PSs, even greater than protein and
nucleic acid biopolymers, arises due to the following:
1. Diversity in repeat unit
structure. Each monosaccharide ring carbon is chiral - thus, for
example, a repeat unit consisting of a six-carbon, six-membered ring
with five chiral centers can actually be any one of 32 (25)
isomeric structures. In addition, substituents other than hydroxyl
groups can be present on the monosaccharide rings. In naturally
occurring PSs, the most common are carboxylic acids, methyl ethers,
acetate esters, hydrogen, acetamides, and other monosaccharides. Repeat
units also may consist of more than one monosaccharide,
i.e., with different structures.
2. Diversity in linkage structure.
The acetal bond between rings can be formed by any one of the available
hydroxyl groups on the monosaccharide with the carbonyl carbon of
another ring. Thus for a
6-carbon, 6-membered ring repeat unit, as in Fig 1, there are 4
possible hydroxyl groups to form the backbone linkages of the PS.
3. Branching. Branchs along
the PS backbone can be formed by attachment of chains of
monosaccharides to the pendant
hydroxyl groups. Branching accounts for the difference between the two
components of
starch, amylopectin (highly branched) and amylose (unbranched), which
are
otherwise made up of the same monosaccharide (glucose) and linkage.
Finally, polysaccharides often form intra- or intermolecular, noncovalently bonded structures such as helices or sheets which afford more rigidity and play an important role in solid phase and solution properties.
Overall, the key features of PSs which directly affect physical properties and applications are:
- polar functional groups
- high molecular weight
- relatively rigid backbone
Synthetic derivatives
Naturally occuring PSs have been synthetically modified to produce materials with new properties of great industrial value. The hydroxyl functional groups of PSs provide reactive sites for derivatization to form esters, ethers, etc. For example, cellulose which is water-insoluble can be converted to water-soluble derivatives by etherification (e.g., methylcellulose, carboxymethyl cellulose). Cellulose also does not melt upon heating without decomposition but can be converted to melt-stable thermoplastic derivatives by esterification (e.g., cellulose acetate and cellulose butyrate).
Optimization of performance through structural change can also be effected biologically. In the case of microbially produced PSs, this can be accomplished by changing the culture conditions in the fermentation production process. Genetic and related bioengineering techniques can also effect structural changes.
Properties
Chemical: Non-derivatized PSs are relatively stable at temperatures up to about 200 °C for short periods of time. Derivatization has been observed to increase thermal stability. Aqueous mixtures are generally stable under autoclaving conditions (120 °C, 1 h) and pH 4-10. At lower and higher pHs, hydrolysis and depolymerization of the PS backbone occurs. PSs are slowly oxidatively degraded in the presence of oxygen and light or radical sources and are sensitive to strong oxidizing agents such as bleach. Naturally occurring PSs are enzymatically biodegraded by microorganisms. Derivatization of the PS hydroxyl groups generally decreases the rate of biodegradation.
Physical state: PSs are semicrystalline or amorphous solids when dry.
Thermal behavior: Non-derivatized PSs have high interchain cohesive forces and thus high melting or softening temperatures (Tm and Tg, resp). Many PSs decompose before they melt at temperatures greater than about 200 °C. The thermal transitions are very sensitive to the presence of plasticizers such as water and other miscible low molecular weight compounds. Derivatization of the polar hydroxyl groups of PSs with less polar groups like alkyl ethers or alkyl esters also generally depresses Tm and Tg.
Solubilities: Some PSs which have high degrees of crystallinity, such as cellulose and chitin, are not soluble in water or common organic solvents. Recently, however, ionic liquids have been found to be powerful solvents for these PSs. Many other non-derivatized PSs are soluble in water and polar aprotic solvents, such as dimethylsulfoxide. PSs generally become more soluble in lower polarity organic solvents as they are derivatized with lower polarity functional groups. Some PSs which have a limited solubility in water become highly swollen and form semisolid hydrogels. In some cases, hydrogels may be formed by PSs with low degrees of crosslinking, either with noncovalent bonds (ionic, hydrogen bonds, hydrophobic associations) or by covalent bonds formed synthetically.
Mechanical properties: In the dry state, non-derivatized polysaccharide materials are generally brittle and stiff. This results from their relatively stiff polymer backbone and high interchain cohesive forces. When plasticized or derivatized, PSs can acquire more toughness and even become rubbery at ambient temperature.
PSs which form hydrogels in water show viscoelastic behavior, i.e., they behave as viscous liquids under relatively high shear forces and elastic solids under low shear. This behavior is a characteristic of "biofilms". In nature, microorganisms commonly exist in the form of highly hydrated biofilms which provide a favorable environment for cells to adhere together and onto surfaces. The biofilms are hydrogels which are composed of water, bacterial cells, and "extracellular polymeric substances", which are primarily polysaccharides.
Aqueous PS solutions, or hydrocolloids, show complex behavior: non-Newtonian flow, as well as pseudoplastic (shear thinning) and yield stress behaviors. For more detailed analysis of aqueous polysaccharide mixtures, see "Behavior of Polysaccharide Solutions, Dispersions, and Gels" in Carbohydrate Chemistry for Food Scientists, Whistler and Bemiller, 1997, Eagan Press, St Paul, MN, pp 91-114, and Professor Martin Chaplin's website: "Water Structure and Behavior."