![]() The (+) or D– designations indicate that the enantiomer rotates plane-polarized light in a clockwise fashion (dextrorotatory), while the (-) or L designations are applied to the enantiomer that rotates plane-polarized light in a counterclockwise (levorotatory) sense. One of the smallest compounds considered to be a carbohydrate is glyceraldehyde, which has a chiral center and therefore can exist as a pair of enantiomers, as shown. Glucose is an aldohexose, while fructose is a ketohexose the suffix – ose indicates that the molecule is a sugar. Glucose is an example of aldose, while fructose is an example of ketose.Īldoses and pentoses can be further classified on the basis of their number of carbons, using the typical prefixes tetr- (four carbons), pent– (five carbons), hex- (six carbons), and so forth. Simple monosaccharides can be classified as aldoses, when they contain an aldehyde group, or ketoses, when they contain a ketone group. five pairs of D/L enantiomers called allose, altrose, glucose, mannose, gulose, idose, galactose, and talose.īelow are the Fischer projects 3-6 carbon aldoses.In this article, I provide a brief introduction to the classification and representation of simple aldoses, carbohydrates with an aldehyde group and no more than five or six carbons. Aldohexoses have sixteen (2 4) possible stereoisomers. Four pairs of D/L enantiomers called ribose, arabinose, xylose, and lyxose.Īldohexoses: 6 carbon sugars with four chiral centers. Aldopentoses have eight (2 3) possible stereoisomers. Two pairs of D/L enantiomers called erythrose and threose.Īldopentoses: 5 carbon sugars with three chiral centers. Aldotetroses have four (2 2) possible stereoisomers. A pair of enantiomers called D-gylceraldehyde and L-glyceraldehyde.Īldotetroses: 4 carbon sugars with two chiral centers. Aldotrioses have two (2 1) possible stereoisomers. To illustrate using present day knowledge, Fischer projection formulas and names for the D-aldose family (three to six-carbon atoms) are shown below, with the asymmetric carbon atoms (chiral centers) colored red.Īldotrioses: 3 carbon sugars with one chiral center. The mirror images of these configurations were then designated the L-family of aldoses. ![]() Consequently, Fischer made an arbitrary choice for (D)-glucose and established a network of related aldose configurations that he called the D-family. At the time Fischer undertook the glucose project it was not possible to establish the absolute configuration of an enantiomer. His successful negotiation of the stereochemical maze presented by the aldohexoses was a logical tour de force, and it is fitting that he received the 1902 Nobel Prize for chemistry for this accomplishment. This challenge was accepted and met in 1891 by the German chemist Emil Fischer. These would exist as eight diastereomeric pairs of enantiomers, and the initial challenge was to determine which of the eight corresponded to glucose. The four chiral centers in glucose indicate there may be as many as sixteen (2 4) stereoisomers having this constitution.
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