Haworth structure

Draw Haworth structures for the two possible isomers of D-altrose (Figure 7.2) and D-psicose (Figure 7.3). 

Figure 2. Structural properties of alginate are shown, with the linear array of Haworth structures given at the top, the conformational structure given next, and the effect of calcium on the formation of complexes between two polymeric strands of alginate given at the bottom. The epimerase catalyzed conversion of / ( —4) linked D-mannuronate to a(l—4) linked L-guluronate residues of poly(ManA) to the catenated structure of poly(GulA) and the formation of the eggbox structure upon the complexing of two polymer strands with Ca. (Reproduced with permission from reference 7. Copyright 1988 Elsevier.)...

Haworth structures are easy to draw and unambiguous in depicting configurations,14 but they also do not show the spatial relationships of groups attached to other rings correctly. For this reason conformational formulas of the type described in Section 2 and shown in Fig. 4-4 are used most often in this book. 

Haworth structures are unambiguous in depicting configurations, but even they do not show the true spatial relationship of groups attached to rings. The normal angle between the bonds formed by the saturated carbon atoms (109°) causes the pyranose molecule to pucker into either a chairlike or boatlike conformation. For glucose and most other pyranoses the chair form (fig. 12.6c) predominates. However, we usually display pyranoses by the Haworth projection because it is easier to draw. 

Following are chair and Haworth structures for this repeating disaccharide. 

Figure 9.7 Structures of some sugars. (A) L-Fucose (pyranose form). Note that the -CH3 group points downward this indicates l series in the Haworth structural convention. (B) Maltose (a-D-glucopyranosyl-l,4-D-glucopyranose) (C) cellobiose (/3-D-glucopyranosyl-1,4-D-glycopyranose) and (D) sucrose (a-D-glucopyranosyl-l,2-/3-D-fructofuranoside). (Reproduced by permission from Diem K, Lentner C. Scientific Tables. Basel Ciba-Geigy, 1971.)...

The simplest way to draw Haworth structures for these two sugars is to draw the chair conformations and then draw the flat rings with the same substituents in the up and down positions. For practice, however, let s lay down the Fischer projection for galactose. You should follow along with your molecular models. 

Fructose forms a five-membered cyclic hemiacetal. Five-membered rings are usually represented as flat Haworth structures. 

It was stressed in the previous section that the Haworth structures for the anomers of D-glucopyranose do not represent the true shape of the rings. The carbon atoms of glucose are all saturated, and the most stable form of a ring will be one that is strain free, i.e., where the angles formed by the bonds at each carbon atom are 109°, the tetrahedral angle. 

A Haworth structure for a monosaccharide is translated readily into a structure showing the true shape of the molecule. 

The Haworth structure tells us that a substituent that is above the general plane of the monosaccharide ring must also appear above the general plane of the chair whether this is axial or equatorial will depend on the carbon atom being considered. For example, looking at each carbon of 0-D-glucopyranose in turn, we find... 

Substituent Haworth Structure— Position Relative to Ring Chair Conformers ... 

D-Mannose and D-galactose are readily translated into six-membered Haworth structures. The a anomers only are shown. 

Question Translate D-ribose into five- and six-membered Haworth structures. Give the ft anomers only. 

The substituents of /3-L-glucopyranose are all on the opposite side of the Haworth structure from those of /3-D-glucopyranose (i.e., they are in the positions occupied by the H s in /3-D-glucopyranose). Therefore, from Example 2.13, it may be seen that in the 1C chair conformer, all the substituents of /3-L-glucopyranose would be equatorial. 

Haworth structures of linked segments of cellulose, chitin, chitosan, murein, xylo-/ -glucan, and... 

Haworth structures more closely depict proper bond angles and lengths than do Fischer representations. To convert from the traditional Fischer formula of a D-pen-tose or D-hexose to a Haworth formula, the following steps should be followed ... 

Convert the following Fischer structures into cyclic Haworth structures QUESTION 7.3... 

Compare the information provided by (a) the space-filling model and (b) the Haworth structure. Carbon atoms are green, oxygen atoms are red, and hydrogen atoms are white. 

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