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2.4: D and L Monosaccharides

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    After completing this section, you should be able to

    1. identify a specific enantioner of a monosaccharide as being D or L, given its Fischer projection.
    2. identify the limitations of the D, L system of nomenclature for carbohydrates.
    3. Draw the Fischer projection formula for a monosaccharide, given its systematic name, complete with the configuration of each chiral carbon atom.


    The D,L-Configuration of Monosaccharides


    Using D-glyceraldehyde as the starting compound, the complete series of D-monosaccharides can de be developed. Being an aldotriose, glyceraldehyde has only one chiral center. Starting with the isomer in which the OH is on the right, we can another -CH(OH)- group to build a tetrose. The OH group can now be on the right or on the left, generating D-erythrose and D-threose, respectively. By successively adding more carbons, we can build the Fisher structures for all D-monosaccharides, from aldotriose to aldohexose. 


    Tetrose formation.png

    To illustrate this concept, 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. The last chiral center in an aldose chain (farthest from the aldehyde group) was chosen by Fischer as the D / L designator site. If the hydroxyl group in the projection formula pointed to the right, it was defined as a member of the D-family. A left directed hydroxyl group (the mirror image) then represented the L-family.  The mirror images of these configurations were then designated the L-family of aldoses.  It is important to notice that monosaccharides found in nature belong to the D-series. 



    It is also important to recognize that the sign of a compound's specific rotation (an experimental number) does not correlate with its configuration (D or L). It is a simple matter to measure an optical rotation with a polarimeter. 

    What is the Fisher projection for an L-monosaccharide?

    For every D-aldose on the figure above, there is a L-aldose. These two compounds are enantiomers from each ether. For example, the structural formulas for D-Glucose and L-Glucose are shown below. Notice that every chiral center must be reversed,  not only the one farthest from the aldehyde group.


    Image by The original uploader was NicolasGrandjean at French Wikipedia., CC BY-SA 3.0, via Wikimedia Commons


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