Glucose is a monosaccharide with formula C6H12O6 .It contains OH and CHO functional groups. The five hydroxyl (OH) groups are arranged in a specific way along its six-carbon backbone. The glucose structure can exist in an open-chain (acyclic) and ring (cyclic) form.

Open chain glucose : Linear glucose has four chiral carbons (C2 to C5). The number of stereoisomers is 2 n, where n is the number of asymmetric centers.Since glucose have 4 asymmetric centers, and thus 16 stereoisomers (8 D-sugars and 8 L-sugars). D-glucose (dextrose-the usual form of glucose found in nature is dextrarotatory) and the other way round for L-glucose ( levorotatory) .D and L designations are based on the configuration about the single asymmetric carbon in glyceraldehyde. For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group..D & L sugars are mirror images of one another as shown below. 


Cyclic glucose:

In the open chain form an intramolecular reaction between the aldehyde C1 atom and the C-5 hydroxyl group forms an intramolecular hemiacetal. An equilibrium state is found in both the forms in water solution. Cyclic form of glucose contains 5 carbon and one oxygen atoms, which resembles to form the structure of pyran, it is also referred to as glucopyranose. The ring-closing reaction makes carbon C-1 chiral, too, since its four bonds lead to -H, to -OH, to carbon C-2, and to the ring oxygen. These four parts of the molecule may be arranged around C-1 (the anomeric carbon) in two distinct ways, designated by the prefixes "α-" and "β as shown below.


Figure 6 (graphics6.jpg)

So D-glucopyranose and L-glucopyranose are cyclic form of glucose generate from D-glucose and L-glucose. They are optical isomer and mirror image of each other.one of the example is shown below.

Figure 6. Glucose enantiomers

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