Polysaccharides

Viscoelastic polymer; excipient; food additive

Sodium salt of a polysaccharide obtained from the brown seaweeds (e.g. Laminaria hyperborea, Fucus vesiculosus, Ascophyllum nodosum). The chemical structure consists of blocks of (1,4) linked-β-D-polymannuronic acid (poly M), (1,4) linked-α-L-polyguluronic acid (poly G) and alternating blocks of the two uronic acids (poly MG). Alginates form strong gels with divalent metal cations and the egg box model has been used to describe this form of gelation. The main use for alginate is in textile printing as a thickener in the printing of cottons with reactive dyes. In the food industry it is used as a thickener and gelling agent.  Recently, it has been shown that ternary mixtures of Konjac glucomannan, Xanthan gum and Sodium alginate can form a non-covalently linked complex which exhibits enhanced rheological properties of value in, for example, functional foods. The image was kindly provided by Prof Mike Guiry from Cork who runs ‘The Seaweed Site’.

Tara gum is a galactomannan that is obtained from the seeds of the tara shrub Caesalpinia spinosa which is a native of the northern regions of Africa and South America. The gum is described as having a backbone of (1,4)-linked β-D-mannopyranosyl units one third of which possess a single unit side chain of (1,6)-linked α-D-galactopyranose. The gum forms gels with carrageenan and xanthan in a similar manner to guar and locust bean gum. Currently, only small quantities of tara gum are sold for use as food thickeners and stabilisers.

A linear β-(1,4)-galactan from Acacia gum. Other linear β-(1,4)-galactans have been isolated from lemon peel, potato tubers and Norwegian spruce compresssion wood.

The disaccharide repeating unit of chondroitin sulphate consists of N-acetyl galactosamine sulphate linked β1,4 to glucuronic acid. Each monosaccharide may be left unsulphated, sulphated once, or sulphated twice. The most common pattern is that the hydroxyl groups of the 4 and 6 positions of the N-acetyl-galactosamine are sulphated, with some chains having the position 2 of the glucuronic acid sulphated.

Hydroxypropyl methylcellulose (HPMC or hypromellose) is a semisynthetic, inert and viscoelastic polymer that is used as eye drops and as semi-synthetic substitute for tear-films. When applied, a hypromellose solution acts to swell and absorb water, by increasing the thickness of the tear-film, resulting in decreased eye irritation.  In addition to its use in ophthalmic liquids, hypromellose has been used as an excipient in oral tablet and capsule formulations, where, depending on the grade, it functions as controlled-release agent. It is also used as a binder and as a component of tablet coatings.  Hypromellose in aqueous solution, unlike methylcellulose, exhibits thermal gelation properties. HPMC is approved as a food additive, emulsifier, thickening and suspending agent, and as an alternative to animal gelatin (Codex Alimentarius code (E number) is E464).

A fucan sulphate found in brown marine algae (Phaeophyta-typically Fucus vesiculotus, Sargassum (illustrated), Alaria and Cladosiphon) and has been shown to have anticoagulant activity. The main constituents are α-1,4 and α-1,2 linked L-fucose sulphates although galactose also occurs and there are many variations of the basic structure found in different species of Phaeophyta. The image was kindly provided by Prof Mike Guiry from Cork who runs ‘The Seaweed Site’.

Heparin is a glycosaminoglycan which occurs in many mammalian tissues and has important anticoagulant and thrombolytic properties. The chemical structure is composed mainly of two disaccharide repeating units A and B. A is L-iduronic acid 2-suplhate linked α-(1,4) to 2-deoxy-2-sulfamido-D-galactose 6-sulphate while B is D-glucuronic acid β-(1,4) linked to 2-deoxy-2-sulfamido-D-glucose 6-sulphate.

In water, hydroxypropyl cellulose forms liquid crystals with many mesophases depending on concentration. These mesophases include isotropic, anisotropic, nematic and cholesteric, the latter resulting in many colors such as violet, green and red. Pharmaceutical applications include treatments for medical conditions such as dry eye syndrome (keratoconjunctivitis sicca), recurrent corneal erosions, decreased corneal sensitivity, exposure and neuroparalytic keratitis. It is also used as a binder in tablets. Hydroxypropylcellulose is also used as a thickener, a binder and emulsion stabiliser in foods with E number E463.  HPC is used as a support matrix for DNA separations by capillary and microchip electrophoresis.

Nigeran is a polysaccharide found in the cell wall of lower fungi. In certain Aspergillus and Penicillium spp., nigeran was first isolated from Penicillium expansum and Aspergillus niger (illustrated). The polysaccharide contains unbranched α-D- glucopyranose residues linked 1,3 and 1,4. Nigeran is part of the hyphal cell wall, where it can contribute up to 40 % of the cell dry weight. The polysaccharide occupies several domains or location on the hyphal wall and is highly crystalline in vivo. Deposition of nigeran is primarily at the outer surface of the hyphal wall.

A linear β-(1,4)-galactan from potato tubers. Other linear β-(1,4)-galactans have been isolated from lemon peel, potato tubers and Norwegian acacia gum.