Haworth Structures of Monosaccharides

STEP D Turn the Fischer projection clockwise by 90 . The —H and —OH groups on the right of the vertical carbon chain are now below the horizontal carbon chain. Those on the left of the open chain are now above the horizontal carbon chain. 

Fold the horizontal carbon chain into a hexagon and bond the O on carbon 5 to the carbonyl group. 

Draw the new—OH group on carbon 1 below the ring to give the a isomer or above the ring to give the j8 isomer. 

D-Mannose, a carbohydrate found in immunoglobulins, has the following Fischer projection. Draw the Haworth structure for /3-o-mannose. 

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In drawing cyclic Haworth structures of monosaccharides, certain rules should be followed so that all our structures will be consistent. First we must draw the ring with its oxygen to the back ... 

Carbohydrates Chiral Molecules Fischer Projections of Monosaccharides Haworth Structures of Monosaccharides Chemical Properties of Monosaccharides Disaccharides Polysaccharides... 

Haworth Structures of Monosaccharides LEARNING GOAL Draw and identify the Haworth structures for monosaccharides. 

Haworth projection common way of representing the cyclic structure of monosaccharides using a three-dimensional perspective. 

William Mills described a similar convention to depict the structures of monosaccharides. While the ring atoms of the Haworth projections are oriented perpendicular to the paper, Mills chose to depict the carbon skeleton in the plane of the paper (Fig. 1.5). Although Fischer, Haworth, and Mills projections are useful tools for depicting the structures of carbohydrates, the planar nature of these representations does not provide an accurate picture of the actual geometry of the molecules. In order to understand carbohydrate function and reactivity, recognition of each distinct conformation and the properties associated with it is required [15]. 

Haworth formulas are named after the English chemist W. N. Haworth (University of Birmingham), who, in 1926, along with E. L. Hirst, demonstrated that the cyclic form of glucose acetals consists of a six-membered ring. Haworth received the Nobel Prize for his work in carbohydrate chemistry in 1937. For an excellent discussion of Haworth formulas and their relation to open-chain forms, see The Conversion of Open Chain Structures of Monosaccharides into the Corresponding Haworth Formulas, Wheeler, D. M. S., Wheeler, M. M., and Wheeler, T. S.,J. Chem. Educ. 1982, 59, 969-970. 

Using methods similar to Fischer s, the straight-chain form of any monosaccharide can be worked out. As we have seen, however, monosaccharides exist mostly as cyclic pyra-nose or furanose hemiacetals. These hemiacetals are in equilibrium with the open-chain forms, so sugars can react like hemiacetals or like ketones and aldehydes. How can we freeze this equilibrium and determine the optimum ring size for any given sugar Sir Walter Haworth (inventor of the Haworth projection) used some simple chemistry to determine the pyranose structure of glucose in 1926. 

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... 

The configurations of monosaccharides are described by several types of formulas. As an example, the following formulas are shown for a-D-glucopyranose structural, Haworth, modified Fischer, and Fischer. In Fischer formulas the -H or -OH groups are above or below the plane formed by the hemiacetal bonds and the carbon chain (-H bonds are shown shorter). 

CONFORMATIONAL STRUCTURES Although Haworth projection formulas are often used to represent carbohydrate structure, they are oversimplifications. Bond angle analysis and X-ray analysis demonstrate that conformational formulas are more accurate representations of monosaccharide structure (Figure 7.10). Conformational structures are more accurate because they illustrate the puckered nature of sugar rings. 

Given the linear structure of a monosaccharide, draw the Haworth projection of its a- and p-cyclic forms and vice versa. 

Drawing the Haworth Projection of a Monosaccharide from the Structural Formula... 

Haworth receives the Nobel prize in chemistry for his work in carbohydrates, including describing the pyranose ring structure of the monosaccharides N-sulfation of heparin identified... 

You would do well to remember the configuration of groups on the Haworth projection of both a-D-glucopyranose and jS-D-glucopyranose as reference structures. Knowing how the Fischer projection of any other monosaccharide differs from that of D-glucose, you can then construct the Haworth projection of that other monosaccharide by reference to the Haworth projection of D-glucose. 

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