How many stereoisomers of an aldotetrose can exist

When numbering stereoisomers that have more than three carbon atoms we look at the position of the OH group on the penultimate or next to last carbon atom because this determines whether it is an L or D stereoisomer. In this example we will look at the numbering of D-Glucose.

First we must find the reactive end of the molecule and assign its carbon the number one. We then number the remaining carbons in order through the end of the chain.

In theory, in glucose, the position of the OH group on each of the asymmetric carbon atoms, numbers two, three, four, and five could be flipped, producing a distinct stereoisomer each time, for a total of 16 or 2 4 stereoisomers. However, not all of these actually exist in nature. For fructose, there are only three asymmetric carbons, so only 8 or 2 3 stereoisomers can be produced.

Only a few of the monosaccharides exist free in nature. Most of them are usually found as sugar units in polysaccharides or in more complex molecules. Monosaccharides are often called simple sugars, and are sub-divided according to the number of C-atoms. These compounds are important metabolic intermediates in the oxidation of glucose to produce energy. Pentoses C 5 H 10 O 5 Three important pentoses are:.

D-ribose — a component of RNA, ribonucleic acid, vitamins riboflavin , and coenzymes. In its reduced form, deoxyribose, it is a component of DNA. L-arabinose — occurs in conifer heartwood and is a component of hemicelluloses where it occurs with xylose. It is also a component of pectin and can be a major component of gums gum Arabic.

Bacterial action in making silage can yield free arabinose. Arabans are polymers of arabinose. D-xylose — there are small amounts of D-xylose free in fruits, but it occurs mainly in hemicellulose, as xylans and hetero-xylans. Hemicellulose is a polysaccharide of xylose and arabinose a heteroxylan. The ratio of xylose to arabinose seems to affect digestibility as digestibility is reduced as the proportion of xylose increases.

Hemicelluloses constitute a considerable portion of the cell walls of plants so herbivores eat large amounts of them. These sugars are all aldopentoses. D-glucose — an aldohexose with various common names, including grape sugar, dextrose, corn sugar made from cornstarch.

Occurs free in plants, fruits, honey, body fluids, including CSF, blood, lymph. It is the major end-product of CHO digestion by non-ruminants and is therefore a primary energy form for non-ruminants. It is a major component of many oligosaccharides with galactose forms lactose and polysaccharides such as starch and cellulose.

In solution D-glucose exists as an equilibrium mixture of the straight chain form with two pyranose ring forms. Effectively, carbon atom number one reacts with carbon atom number five forming a ring. In fact two forms of the structure exist, called anomers. If the hydrogen atom is above carbon atom one then it is called an alpha anomer but if the hydrogen atom is below the carbon atom it is called a beta anomer. This structural information is very important because it governs how molecules of glucose join together to form larger molecules.

Starch is a polymer of the a- form and is water soluble and digestible by animal enzymes. Cellulose is a polymer of the b- form, it is not soluble and is not digestible by animal enzymes. Changing from a to b via an open chain structure is called mutarotation, and it requires the O-C bond to be broken to allow the C to swivel the H and OH upside down.

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