Basic carbohydrate chemistry

Carbohydrates are classed into two groups: simple and complex. Simple sugars or monosaccharides are carbohydrates like glucose or fructose. They can't be hydrolysed into smaller units. Complex carbohydrates are made from two or more simple sugars linked together. Sucrose for example is a disaccharide, made up of one glucose molecule linked to a fructose molecule. Fructan for instance is a polysaccharide made up of a glucose molecule linked to many fructose molecules. When these molecules are hydrolysed they break up into their simple carbohydrate units.
Monosaccharides can further be classified into aldoses or ketoses. The -ose suffix tells you it's a carbohydrate, and the aldo- or keto- prefixes designate the nature of the carbonyl group (aldehyde or ketone). A carbonyl group is specified as a carbon atom with a double bound oxygen atom attached to it. Is this carbonyl group is at the end of a chain it is called an aldehyde, if it's in the middle of a chain it's called a ketone. The number of carbon atoms in the monosaccharide is given by the Greek numbers (tri-, tert-,pent- etc.) as the parent name. For example a glucose is an aldohexose, fructose is a ketohexose.
To graphically draw sugars Fisher and Haworth projections are used. A Fisher projection is basically the projection of a tetrahedral carbon atom onto a flat surface. A tetrahedral carbon atom is represented in Fisher projection by two crossed lines. By convention horizontal lines represent bonds going out of the page, and vertical lines represent bonds going into the page.


Only two kinds of motion with the flat molecules are allowed. 1 A Fisher projection may be rotated 180 degrees, nothing else. 2 A Fisher projection can have one group held steady, while the other three rotate in either a clockwise or counterclockwise direction. By convention, the carbonyl carbon is always placed either at the top or near the top when showing the Fisher projection of a carbohydrate. The atoms are numbered from the top downwards. These numbers are for example used to indicate which atom from one monosaccharide is linked to which atom froms another ( like 1 -> 2 linkage).
Nearly all monosaccharides that are synthesized in nature have the same stereochemical configuration at the chiral carbon atom farthest from the carbonylgroup. A chiral carbon atom is a carbon atom that has no plain of symmetry (for example it has four different groups attached). Historically these narural monosaccharides were called D-sugars (D from dextrorotatory). The non-natural form is called the L-sugar (L from levorotatory). D is also called right turning, and L left turning. In Fisher projections most naturally occurring sugars have the hydroxylgroup at the lowest chiral atom pointing to the right. Such monosaccharides are referred to as the D-sugars.
In reality only a very small percentage of the monosaccharides exist in the open-chain form with a free aldehyde- or keto-group. In solution the open-chain forms spontaniously form cyclic structures, in which the carbonylgroup is involved. This is called the hemiacetal formation. Hemiacetal formation is an addition reaction between a carbonyl and a hydroxyl group. The reaction is catalyzed by base or acid. In sugars the carbonyl and hydroxyl groups are both present, and can therefor form a cyclic structure. The addition of the hydroxyl group to the carbonyl group can lead to two mirror stereo configurations. These are called alpha and beta. In Fisher projections the molecule is called alpha if the newly formed hydroxyl group on the carbonyl group is on the same side as the lowest chiral atom. Elsewise it's called beta.

The picture below shows how one can convert a Fisher projection into a Haworth projection. Haworth projections are mostly used for depicting pyranose and furanose rings (six or five atom rings with an oxygen in it). They give a better representation of the true form of a cyclic carbohydrate. Although convenient, this view is not really accurate because pyranose rings are actually chair shaped rather than flat. When converting from a Fisher- to a Haworth-projection, keep the following in mind. A hydroxyl on the right in a Fisher projection is down in a Haworth projection and vice versa. For D-sugars the terminal -CH2OH group is always up in Haworth projections, whereas for L-sugars it is down.

Last updated on 19/08/96


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