Monosaccharides can be classified by the number x of carbon atoms they contain: triose 3 , tetrose 4 , pentose 5 , hexose 6 , heptose 7 , and so on. The most important monosaccharide, glucose, is a hexose. Examples of heptoses include the ketoses , mannoheptulose and sedoheptulose. Monosaccharides with eight or more carbons are rarely observed as they are quite unstable.
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Monosaccharides can be classified by the number x of carbon atoms they contain: triose 3 , tetrose 4 , pentose 5 , hexose 6 , heptose 7 , and so on. The most important monosaccharide, glucose, is a hexose. Examples of heptoses include the ketoses , mannoheptulose and sedoheptulose.
Monosaccharides with eight or more carbons are rarely observed as they are quite unstable. In aqueous solutions monosaccharides exist as rings if they have more than four carbons. Monosaccharides are the simplest units of carbohydrates and the simplest form of sugar. In that case, the compound is termed an aldose. Ketoses of biological interest usually have the carbonyl at position 2. The various classifications above can be combined, resulting in names such as "aldohexose" and "ketotriose".
A more general nomenclature for open-chain monosaccharides combines a Greek prefix to indicate the number of carbons tri-, tetr-, pent-, hex-, etc. Open-chain stereoisomers[ edit ] Two monosaccharides with equivalent molecular graphs same chain length and same carbonyl position may still be distinct stereoisomers , whose molecules differ in spatial orientation. This happens only if the molecule contains a stereogenic center , specifically a carbon atom that is chiral connected to four distinct molecular sub-structures.
Those four bonds can have any of two configurations in space distinguished by their handedness. In a simple open-chain monosaccharide, every carbon is chiral except the first and the last atoms of the chain, and in ketoses the carbon with the keto group.
Therefore, it exists as two stereoisomers whose molecules are mirror images of each other like a left and a right glove. Monosaccharides with four or more carbons may contain multiple chiral carbons, so they typically have more than two stereoisomers. The number of distinct stereoisomers with the same diagram is bounded by 2c, where c is the total number of chiral carbons. The Fischer projection is a systematic way of drawing the skeletal formula of an acyclic monosaccharide so that the handedness of each chiral carbon is well specified.
Each stereoisomer of a simple open-chain monosaccharide can be identified by the positions right or left in the Fischer diagram of the chiral hydroxyls the hydroxyls attached to the chiral carbons. Most stereoisomers are themselves chiral distinct from their mirror images. In the Fischer projection, two mirror-image isomers differ by having the positions of all chiral hydroxyls reversed right-to-left.
Mirror-image isomers are chemically identical in non-chiral environments, but usually have very different biochemical properties and occurrences in nature. While most stereoisomers can be arranged in pairs of mirror-image forms, there are some non-chiral stereoisomers that are identical to their mirror images, in spite of having chiral centers.
In that case, mirroring is equivalent to a half-turn rotation. For this reason, there are only three distinct 3-ketopentose stereoisomers, even though the molecule has two chiral carbons. Distinct stereoisomers that are not mirror-images of each other usually have different chemical properties, even in non-chiral environments. Therefore, each mirror pair and each non-chiral stereoisomer may be given a specific monosaccharide name. For example, there are 16 distinct aldohexose stereoisomers, but the name "glucose" means a specific pair of mirror-image aldohexoses.
In the Fischer projection, one of the two glucose isomers has the hydroxyl at left on C3, and at right on C4 and C5; while the other isomer has the reversed pattern. These specific monosaccharide names have conventional three-letter abbreviations, like "Glu" for glucose and "Thr" for threose. Generally, a monosaccharide with n asymmetrical carbons has 2n stereoisomers. The number of open chain stereoisomers for an aldose monosaccharide is larger by one than that of a ketose monosaccharide of the same length.
Configuration of monosaccharides[ edit ] Like many chiral molecules, the two stereoisomers of glyceraldehyde will gradually rotate the polarization direction of linearly polarized light as it passes through it, even in solution. The two stereoisomers are identified with the prefixes D- and L-, according to the sense of rotation: D-glyceraldehyde is dextrorotatory rotates the polarization axis clockwise , while L-glyceraldehyde is levorotatory rotates it counterclockwise.
D- and L-glucose The D- and L- prefixes are also used with other monosaccharides, to distinguish two particular stereoisomers that are mirror-images of each other. Otherwise, it receives the L- prefix. In the Fischer projection, the D- and L- prefixes specifies the configuration at the carbon atom that is second from bottom: D- if the hydroxyl is on the right side, and L- if it is on the left side.
Note that the D- and L- prefixes do not indicate the direction of rotation of polarized light, which is a combined effect of the arrangement at all chiral centers. However, the two enantiomers will always rotate the light in opposite directions, by the same amount. Cyclisation of monosaccharides[ edit ] A monosaccharide often switches from the acyclic open-chain form to a cyclic form, through a nucleophilic addition reaction between the carbonyl group and one of the hydroxyls of the same molecule.
The reaction creates a ring of carbon atoms closed by one bridging oxygen atom. The resulting molecule has a hemiacetal or hemiketal group, depending on whether the linear form was an aldose or a ketose. The reaction is easily reversed, yielding the original open-chain form.
In these cyclic forms, the ring usually has five or six atoms. For example, the aldohexose glucose may form a hemiacetal linkage between the hydroxyl on carbon 1 and the oxygen on carbon 4, yielding a molecule with a 5-membered ring, called glucofuranose. The same reaction can take place between carbons 1 and 5 to form a molecule with a 6-membered ring, called glucopyranose.
Cyclic forms with a seven-atom ring the same of oxepane , rarely encountered, are called heptoses. Conversion between the furanose, acyclic, and pyranose forms of D-glucose Pyranose forms of some pentose sugars Pyranose forms of some hexose sugars For many monosaccharides including glucose , the cyclic forms predominate, in the solid state and in solutions, and therefore the same name commonly is used for the open- and closed-chain isomers. Thus, for example, the term "glucose" may signify glucofuranose, glucopyranose, the open-chain form, or a mixture of the three.
Cyclization creates a new stereogenic center at the carbonyl-bearing carbon. The molecule can change between these two forms by a process called mutarotation , that consists in a reversal of the ring-forming reaction followed by another ring formation.
Pyranoses typically adopt a chair conformation, similar to that of cyclohexane.