Monosaccharides (from
Greek monos single,
sacchar sugar) are the most basic unit of
carbohydrates. They consist of one sugar and are usually
colorless,
water-
soluble,
crystalline solids. Some monosaccharides have a
sweet taste. Examples of monosaccharides include
glucose (dextrose),
fructose,
galactose,
xylose and
ribose. Monosaccharides are the building blocks of
disaccharides such as
sucrose (common sugar) and
polysaccharides (such as
cellulose and
starch). Further, each carbon atom that supports a
hydroxyl group (except for the first and last) is
chiral, giving rise to a number of
isomeric forms all with the same chemical formula. For instance, galactose and glucose are both
aldohexoses, but have different chemical and physical properties.
With few exceptions (e.g., deoxyribose), monosaccharides have the chemical formula Cx(H2O)y with the chemical structure H(CHOH)nC=O(CHOH)mH. If n or m is zero, it is an aldehyde and is termed an aldose, otherwise it is a ketone and is termed a ketose. Monosaccharides contain either a ketone or aldehyde functional group, and hydroxyl groups on most or all of the non-carbonyl carbon atoms.
Most monosaccharides form cyclic structures, which predominate in aqueous solution, by forming hemiacetals or hemiketals (depending on whether they are aldoses or ketoses) between an alcohol and the carbonyl group of the same sugar. Glucose, for example, readily forms a hemiacetal linkage between its carbon-1 and the hydroxyl group of its carbon-5. Since such a reaction introduces an additional stereogenic center, two anomers are formed (a-isomer and ß-isomer) from each distinct straight-chain monosaccharide. The interconversion between these two forms is called mutarotation.[1]
A common way of representing the cyclic structure of monosaccharides is the Haworth projection. In Haworth projection, the a-isomer has the OH- of the anomeric carbon under the ring structure, and the ß-isomer, has the OH- of the anomeric carbon on top of the ring structure. In chair conformation, the a-isomer has the OH- of the anomeric carbon in an axial position, whereas the ß-isomer has the OH- of the anomeric carbon in equatorial position.
