Rules for Determining D-,L-Configurationsin Haworth Structures Jerry L. Wilson California State University, Sacramento, CA 95819 Others have published in this Journnll detailed and rigorous rules for determining stereochemistrv of carbohvdrates. These rules are, however, too complex-for use in-a basic biochemistry course. I offer below a simple set of rules for determining the proper stereochemistry of carbohydrates when using Haworth projections. Using these rules, one can place the projection in any orientation on the paper. The validity of these rules can be readily demonstrated by the use of molecular models and the definition that a D-suear is one in which the penultimate hydroxyl points to the right when thecarbohvdrate is held verticallv in itsextended form with the most oiidized carbon a t the top.
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1. For five-carbonsugars in fursnase rings and six-carbon sugars in pyranose rings. This is the situation in which the^., L-reference hvdroxvl is oart of the rine. a. If. in going clockwise aruund the ring, starting from theorygen, the antrmeric carhon ir encountered firat and the hydroxymethylgroup on the last ring carlmn is in the up pwitim, the
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sugar is a D-isomer; if the hydroxymethyl group is down, the sugar is..I b. If you have to go counterclockwiseto encounter first the anomeric carbon,then an up hydroxymethyl means and adown hydroxymethyl means D. c. In each of the above cases the beta anomer is the one in which
the anomeric hydroxyl points in the same direction as the hydroxymethyl. The alpha anomer points, of course, in the opposite direction. 2. For five-carbonaldmes and six-carbon ketoses in pyranose rings. This is the situation where the D-, L-reference hydroxyl is not part of the ring. a. If moving clockwise from oxygen you encounter the anomeric carbon, and the last hydroxyl is down, the sugar is o;if the last hydroxyl is up, the sugar is L. b. If you have to go counterclockwiseto first reach the anomeric earhan, and the last hydroxyl is down, the sugar is L;if up, the sugar is D. c. In each of the above cases, the alpha anomer is that one in which the anomeric hydroxyl is pointing in the same direction as the last hydroxyl, and the beta anomer has the hydroxyl pointing in the opposite direction. 3. One can always identify the anomeric carbon as the only carbon in the ring that is attached to two oxygens. These rules work for both the Haworth projections and the chairboat type of projection.
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Shallenberger, R. S.: Wroldstad. R. E.; Kerschner. L. E. J. Chem. Educ. 1981,58.599-601.
Volume 65
Number 9
September 1988
783
