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Configurations of Monosaccharides

Monosaccharides have chiral centers and thus exhibit optical activity. Let’s explore the structure of glyceraldehyde. In the three-dimensional representation, the dotted wedges indicate the bonds that extent backwards from the chiral carbon (away from you or into the plane of the page); and the solid wedges indicate the bonds that are projected toward you (out of the plane of the page). In stereochemistry, Fischer projection is an important way to represent the spatial orientation of molecules. In Fischer projection representation, the bonds that are pointed backward (away from you or into the page) are indicated by vertical lines, and the bonds that extend toward you (out of the page) are represented by horizontal lines. 

 

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By convention, the carbonyl group is always drawn on top. With that as the standard, we can describe the structure of D-glyceraldehyde as follows:
D-glyceraldehyde has a hydroxyl group represented on the right side of the chiral carbon. L-Glyceraldehyde has a hydroxyl group represented on the left side of the chiral carbon. To determine whether a sugar structure shown is L or D, we have to locate the chiral carbon that is farthest from the carbonyl group, and see whether the hydroxyl group is on the right or on the left of the chiral carbon.
 





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