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Aldoses, Ketoses, Fischer Projections and Epimers
Carbohydrates, also known as polysaccharides, are made up of carbon and water and have an empirical formula of (CH2O)n, where n is a positive integer. The simplest type of sugar is called a monosaccharide. The simplest monosaccharide has an n-value of three. Monosaccharides come in two types; they can either contain an aldehyde group, in which case they are called aldoses, or they can contain a ketone group, in which case they are called ketoses. The simplest aldose is dihydroxyacetone. The simplest ketose comes in two isomeric forms - the D-glyceraldehyde and the L-glyceraldehyde. These are mirror images of one another and so are called enantiomers. In fact, the D and L designations are used to differentiate between the two enantiomeric forms. If two monosaccharides have the same molecular formula, different arrangement of atoms and are not enantiomers, then they are called diastereoisomers. One important subcategory of diastereoisomers are epimers. Two monosaccharides are said to be epimers if they differ in stereochemistry at a single stereogenic (chiral) carbon atom. D-glucose and D-mannose are examples of diastereoisomers that are also epimers. In order to describe the three-dimensional arrangement of atoms of any open-chain sugar molecule, we commonly use the Fischer projection method. In this method, horizontal lines are made to come out of the board while the vertical lines are made to go into the board.
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