Using an Oval to Represent Carbohydrates with Hexopyranose Rings John E. Fulkrod University of Minnesota Technical College, Waseca, Waseca, MN 56093 Haworth projection formulas are widely used to represent carbohydrate structures with hexopyranose rings. These structures do not give an accurate representation of the molecular conformation in the pyranose ring, and, although they are easy enough to draw, they are very difficult for students to learn because there is no guiding principle for placing OH'S and H's up or down. Several modern textbooks stress the geometry of the hexopyranose ring. These hooks stress principles students have encountered hy emphasizing that the rather large hydroxymethyl group of aldohexoses (C-6) is attached to the ring carbon (C-5) in the preferred equatorial position to reduce steric interference. To stress conformational effects and help the student understand why 8-D-(+)-glucose is the most common naturally occurring compound, it is necessary to use the chair conformation. However, I have found that students get so hogged down when drawine chair conformation structures that thev do not we relationships between compounds and important structural differences. To overcome this I have develooed a way to represent hexopyranose rings that I have foundquite ~~
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successful. This "shorthand" method uses an oval with a large dot for the oxygen atom position in the hexopyranose ring. The reason for the use of the oval is that this gives an approximate size perception of the six-membered ring. The oval pictured in this way also gives a perspective view of a ring in a plane perpendicular to the paper. This is illustrated in structures 1and 2.
In 8-n-(+)-glucopyranose the conformation is approximately achair with the bulky -CH20H group (C-6)in the equatorial position. In this conformation all the other -OH groups are located in the equatorial positions. In our "shorthand" we designate D-(+)-glycopyranose with an oval and a large dot for the position of the ring oxygen atom. We always show the -OH group of the anomeric carbon atom (C-1). None of the other groups are shown, and it is understood that this "shorthand" designates the pyranose form of D-(+) glucose. This is illustrated hv structure 4. The numbers for the locationof all the ring carbons as well as C-6 are also shown. The students are required to memorize these positions. It is also noted that C - f i s positioned across from the ring oxygen atom, while C-6 is connected to C-5 in the equatorial position and goes back behind the oval. The equatorial position of C-2 and C-3 would both come out in front of the oval.
Structure 6 shows how the student would represent n-D-(+)glucopyranose and shows C-6 for emphasis.
Flgure 1. Disaccharide srmctures: (from top to bottom) a-milose, @-maltose. a-lactose. and a-iso-o-maltose.
The method used to represent the equatorial and axial positions on C-1 and C-5 are also emohasized at this point. Once the student knows the "shorthkd" for the glu>opyranose structure and what it represents, other related structures Volume 66
Number 6 June 1989
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the configuration about C-4 and C-2, respectively. The "shorthand" for galactopyranose and mannopyranose is shown in 7 and 8.
Figure 2. Polyracchsride snumres: (top) repeating use 01 arnylose (linear t o m of starch) (u-o-glucopyranme units nkec 1.4) an0 (oonorn)a section of a w l p e n i n (branched chain form of starch) (oi-o-glucopyranose unik linked 1.4 with some 1.6 branching).
can quickly he drawn if the difference between them is explained or illustrated. Two other naturally occurring aldohexoses are galactose and mannose. These also are found in the pyranose ring structure. They differ from glucose only by
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I stress the alternating up and down p i t i o n of the axial groups in the chair conformation of the six-membered ring. From this point we can also explain glycoside formation and illustrate disaccharide structures containing the glycosidic acetal link. Some examples are shown in Figure 1. Many interesting topics can be discussed and related readily to the structural differences. I have found that these "shorthand" structures focus student attention on the real structural differences while still using approximate geometric relatiom ships of atoms. The chair conformation oft he hexopyranose ring structure can he emphasized without a lot-of extra structural features to distract the student from key structural differences. Another key advantages is that structures may also be represented in considerably less time. The polysaccharide structures for amylose and amylpectin using the "shorthand" designation are illustrated in Figure 2. One last suggestion that I have given to students (although many do not take it) is to cut out of rigid cardboard an oval to speed up the drawing of the structures. In the last four years that I have used this "shorthand", our students have found it easier to relate carbohydrate structures to chemical properties.
