If we replace the anomeric OH group, we obtain a group of compounds called glycosides. Here are two simple examples of their synthesis, which can be carried out under laboratory conditions:

Mother Nature, of course, knows how to make glycosides. Here are a couple of examples:

Linamarin is a glycoside found in manioc root and flax seeds. It can be quite toxic, because the glycosidic bond is cleaved in aqueous acid, and the resulting cyanohydrin evolves hydrogen cyanide.

Sinigrin is found in mustard seeds and horseradish root, and is in part responsible for their spicy or "hot" taste.

The most abundant biomolecule on the planet, cellulose, is also an O-glycoside. In this case (below), the alcohol used to replace the anomeric OH is the 4-OH of another glucose.

• The result of the 1,4- connections is a linear polymer composed of up to 40,000 glucose units, forming the long, strong fibers of cotton, wood, and other vegetable fibers.

• Note that as indicated on the drawing, the linkage between units is b.
  • Because the molecules are linear, they can pack closely together and form strong interunit hydrogen bonds, accounting for the strength of macroscopic cellulose fibers.

  • The linearity is evident in the 4-glucose piece of cellulose, derived from a molecular mechanics calculation, that is shown below:
  

A polyglycoside that all Mainers are familiar with is chitin, a polymer of N-acetylglucosamine:

Chitin is the principle constituent of lobster and crab shells. Glucosamine itself has a certain notoriety these days; the dietary supplement quacks are recommending it as a cure for arthritis.

N-acetylglucosamine and short oligomers of it are very common etherified to serine residues of proteins.

Another naturally occurring polymer, starch, is a glycoside of glucose in which the connection at the anomeric carbon is a- rather than b.

The a-connection twists the molecule into a helix, with six residues comprising a full turn. Below is a tetrameric piece of the starch amylose, derived from a molecular mechanics calculation:

The difference in molelcular shape from cellulose is evident in even this small fragment. Some starches also contain branches, created by making a second link to a given glucose molecule, using the 6-position. These are called amylopectins.

Recent research has uncovered the important role played by glycoproteins - glycosides that are coupled to protein chains - on cell surfaces.