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Recognizing D- from L- Monosaccharides

    The first step in identifying a sugar is to ascertain whether or not it is D- or L-. (Please note that being D- does not necessarily indicate that it will rotate light to the right. This D/L designation merely indicates the configuration of the highest-numbered carbon in the ring of the Haworth structure.

  1. Getting your bearings

        First you need to know how the carbons are numbered in a monosaccharide. The lowest numbered carbon IN THE RING is the only one bonded to TWO oxygen atoms - one oxygen is in the ring itself, and the other is an oxygen of a hydroxyl group. This lowest-numbered-carbon-in-the-ring is the epimeric carbon. If it is not bonded to any carbon not in the ring, it will be carbon #1, if it is bonded to a carbon not in the ring - as in fructose - it will be carbon #2, and the carbon not in the ring is carbon #1.

        Think of the Haworth structure of a monosaccharide as being like a rowboat with a number of oars (hydroxyl groups) both in and out of the water, and one "flag" (the -CH2OH group that includes the highest-numbered carbon in the total monosaccharide).

     

  2. Is it D? or L?

        Looking down onto the sugar's ring, start counting of the carbons starting with the epimeric carbon

    • if you go clockwise and the "flag" is up, then the monosaccharide is D
    • If you go counterclockwise and the "flag" is down, you have a D
    • Any other combination and you have an L-monosaccharide

  3. Now which pentose or hexose is it?

        Always denote the carbon-in-the-ring that carries the flag as being "1". Moving around the ring, designate the hydroxyls as being "1" if they are trans to the flag, or "0" if cis. Glucose will always be, for example, 1101 whether or not it is D or L. Click " for a full display of the binary mnemonic for remembering all of the monosaccharide structures.


    In 1962, two graduate-student teaching assistants in biochemistry at the Univ. of Illinois - Susan Schwartz and this author - developed this mnemonic model to help their undergraduate students in introductory biochemistry. Since that time, the mnemonic has appeared in a number of textbooks in both full and abbreviated form.