Nomenclature:

a. monosaccahride - a simple sugar like glucose
b. disaccharide - two simple sugars linked together via a glycosidic bond
c. oligosaccharide - several simple sugars linked together
d. polysaccharide - many simple sugars linked

e. sizes (number of carbons) of sugars: tetroses (4), pentoses (5), hexoses (6), heptoses (7)

f. state of oxidation (of most highly oxidized carbon): if it is an aldehyde group, the sugar is called a aldose; if a ketone, the sugar is a ketose

g. stereochemical configuration: D or L (are not an indicator of optical activity) is based on the configuration of the lowest chiral center (i.e., the carbon away from the higher oxidation state groups).

For example: D (+) glucose is an aldohexose which rotates plane polarized light in a clockwise direction and has the same stereochemical configuration at C-5 as D-glyceraldehyde.

Ring configurations

A common chemical reaction of aldehydes and ketones in monosaccharides is the formation of hemiacetals and hemiketals. This reaction involves the intramolecular nucleophilic attack of the alcohol oxygen on the carbonyl carbon of the aldehyde or ketone group. Very common chemistry!

These two structures are also called anomers and the C-1 carbon is referred to as the anomeric carbon.

Hemiacetals and hemiketals are in equilibrium with the open-chain form (under certain conditions). Formation of the full acetal by (reaction with methanol for example) changes the sugar chemistry such that one no longer has a reducing sugar and can have no mutarotation.

Rule for configuration about C-1: alpha = below the plane of the ring in Haworth projection; beta = above the plane of the ring in Haworth projection

Chair configuration

The Haworth structure is relatively easy to draw and shows the disposition of the hydroxyl groups clearly. For these reasons it is most frequently used in texts, even though the chair (and boat) structure more accurately reflects the true configuration of sugars.

In the chair format, alpha hydroxyl groups are in the axial position, and the beta hydroxyl groups are in the equitorial position.

The stereochemistry of sugars explains why glucose is such a common sugar. The hydroxyl group (which is more bulky than a hydrogen) is more stable in the equitorial position; in glucose all the hydroxyl groups are in the equitorial position!

Ring sizes

When sugars undergo hemiacetal or hemiketal formation, the rings that form can be five or six-membered; this is reflected in their naming by the use of furan (5) and pyran (6).

Pyran and furan are the names of the five and six-membered cyclic ethers (for example, tetrahydrofuran, which has been used as a common organic solvent).

Memory devices:

For four carbon sugars: Erythrose --> E Threose --> T

For five carbon sugars: RAXL (ribose, arabinose, xylose, lyxose)

For six carbon sugars: All Alchemists Gladly Make Gin In Gallon Tanks

(Allose Altose Glucose Mannose Gulose Idose Galactose Talose)

Derivatives of Monosaccharides

a. phosphate esters ---> ribose 5'-phosphate
b. acids (oxidized aldehydes) and lactones ---> gluconic acid, and gluconolactone
c. alditols (reduced aldehydes) ---> mannitol (all hydroxyls)
d. amino sugars ---> glucosamine and galactosamine

Glycosidic linkages

Glycosidic linkages are formed by elimination of water between the anomeric hydroxyl of one sugar and a hydroxyl of another sugar molecule. Since the anomeric carbon is involved, glycosidic linkages can be either alpha or beta.

Continuation of this process of glycosidic bond formation with additional simple sugars yields polysaccharides. Examples are cellulose, starch, and glycogen.