Mannose Ring Structure
The cyclic structure of mannose is a pyranose, a six-membered ring. Mannose is a type of carbohydrate that has the formula C6H12O6. It is a monosaccharide, therefore it is unable to hydrolyze to a simpler carbohydrate. It exists as a pair of enantiomers because of its chiral center on the penultimate carbon. Mannose forms a six-membered cyclic structure called a pyranose. The cyclic structure of mannose is commonly depicted in a Haworth projection, but is best represented in its chair conformation representation. Does this Spark an idea?
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Monosaccharides
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Mannose is a monosaccharide with six carbon atoms: a hexose. All monosaccharides contain a carbonyl group of a ketone or an aldehyde and are the components of carbohydrates. Monosaccharides cannot be hydrolyzed to a simpler compound. They are commonly classified by the number of carbon atoms. Mannose has six carbon atoms and is, therefore, classified as a hexose. Glucose, galactose, allose, altrose, gulose, idose and talose are other examples of hexoses.
D & L Naming System for Monosaccharides
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Mannose and other monosaccharides have a chiral center, therefore, they exist as a pair of enantiomers. Emil Fischer, the father of the Fischer projection, deduced the D & L naming system for carbohydrates. The hydroxyl group of the penultimate carbon serves as the point of reference for this naming system. It is the second to last carbon from the most highly oxidized carbon of a Fischer projection. A D-Mannose and other D-monosaccharides have its hydroxyl group attached to the penultimate carbon to the right. A L-Mannose has its hydroxyl group to the left in its Fischer projection.
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Pyranose
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Mannose forms a cyclic six-membered structure called a pyranose. Pyranose is a term used to describe any cyclic form of a monosaccharide with six carbons. The cyclic structure of mannose forms from an intermolecular hemiacetal reaction between the hydroxyl oxygen on the penultimate carbon and the carbonyl carbon of the aldehyde. The new chiral center formed from this reaction is called the anomeric carbon.
Haworth Projection
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In a Haworth projection mannose is shown as a planar hexagon. Groups attached to the carbon atoms of the ring are written directly above or below the carbon atoms. Intermolecular hemiacetal reaction of a pentose forms a planar pentagon structure. If the hydroxyl group attached to the anomeric carbon is on the side of the -CH2OH group, it is given the prefix beta. However, if it is opposite to the -CH2OH group then it is given the prefix alpha.
Conformational Projection
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The Haworth projection for a pentose more accurately reflects its structure than that for a pyranose. Therefore, it is more appropriate to draw the structure of mannose and other hexoses in a chair conformation. A chair conformation is more realistic due to the steric hindrance of the groups attached to the carbon atoms of the ring.
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References
- Pentoses and Hexoses Cyclize to Form Furanose and Pyranose Rings
- "Organic Chemistry: Fourth Edition"; William H. Brown, Christoper S. Foote, Brent L. Iverson; 2005
