Structure of Monosaccharide’s
Although a large number of monosaccharide’s are found in nature, we will confine our discussion here to four of them only viz. D-glucose, D-fructose, D-ribose and 2-deoxy-D-ribose. D-Glucose (an aldohexose) is the monomer for many other carbohydrates. Alone or in combination, glucose is probably the most abundant organic compound on the earth. D-Fructose (a ketohexose) is a sugar that is found withglucose in honey and fruit juices. D-Ribose (an aldopentose) is found in ribonucleic acids (RNA) while. 2-Deoxy-D-ribose is an important constituent of the deoxyribonucleic acids (DNA). Here, the prefix 2-Deoxy indicates that it lacks oxygen at carbon no. 2
These monosaccharides generally exist as cyclic compounds in nature. A ring is formed by a reaction between the carbonyl group and one of the hydroxyl groups present in the molecule. Glucose preferentially forms the six member rings which can be in two different isomeric forms called α- and ß-forms (shown below as I & II). The two forms differ only in the arrangement of the hydroxyl group at carbon No.1. Such isomers are called anomers. Formation of these cyclic structures (I and II) from the open chain structure can be shown as follows.
The α- and ß-forms of other sugars also exist in the cyclic form. D-Ribose forms a five member ring structure as shown below
D-before the name of above example indicates the configuration of particular stereoisomer. Stereoisomers are assigned relative configurations as D– or L –. This system of assigning the relative configuration refers to their relation with glyceraldehydes. Glyceraldehydes contain one asymmetric carbon atom so exists in two enantiomeric forms as shown below.
All those compounds which can be correlated to (+) - glyceraldehyde are said to have D-configuration and those can be correlated to (–) -glyceraldehyde are said to have L–configuration. In monosaccharides it is the lowest asymmetric carbon atom (shown in the box) by which the correlation is made. As in (+) glucose the lowest asymmetric carbon atom has –OH group on the right side which matches with (+) glyceraldehyde hence it is assigned D-configuration.
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