Haworth projection, monosaccharides

Two sugars can link to each other by losing water from OHs to form disaccharides. Figure 4.6 shows the Haworth projection formulas of four important disaccharides sucrose, lactose, maltose, and cellobiose, which all have the same molecular formulas, C12H22011. Sucrose and lactose are the most abundant and most important disaccharides of natural origin. Maltose and cellobiose are repeating units of polymeric starch and cellulose, respectively. Disaccharides may hydrolyze to form two monosaccharide molecules. 

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

A major drawback of cyclic Fischer projections is the unrealistic manner in which the structures are depicted. In 1929, Haworth designed a representation to address this deficiency. Haworth projections provide a simple way to represent cyclic monosaccharides with a three-dimensional perspective. The following process allows the conversion of a Fischer projection into a Haworth representation ... 

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]. 

Draw the following monosaccharides, using chair conformations for the pyranoses and Haworth projections for the furanoses. 

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. 

The Haworth projection formulas are neater ways of writing the ring forms shown in the equilibria above and yet preserving the configuration shown at each chiral carbon. It is not difficult to translate the open-chain structure for a monosaccharide into the Haworth ring structure. 

To convert an acyclic monosaccharide to a Haworth projection, follow a stepwise procedure. 

Sample Problem 27.3 shows how to convert a Haworth projection back to the acyclic form of a monosaccharide. 

Haworth projection (Section 27.6A) A representation of the cyclic form of a monosaccharide in which the ring is drawn flat. 

Open chain monosaccharides (like the Fischer projection above) have n-2 chiral carbons, so 2"-2 isomers (assuming no internal symmetry). Note that sugars can also react internally to produce a variety of rings forms this introduces an extra chiral centre, as you can see in the Haworth projection above. 

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... 

The depictions of glucopyranose and fructofuranose shown in Figures 11.4 and 11.5 are Haworth projections. In such projections, the carbon atoms in the ring are not explicitly shown. The approximate plane of the ring is perpendicular to the plane of the paper, with the heavy line on the ring projecting toward the reader. Like Fischer projections, Haworth projections allow easy depiction of the stereochemistry of sugars. We will return to a more structurally realistic view of the conformations of cyclic monosaccharides shortly. 

D-Glucose is the most abundant of the monosaccharides in human metabolism, and the open- or straight-chain stmcture only occurs when glucose is in solution even then it represents only 1% of the glucose molecules. The other forms are known as a- and P-anomers (Chap. 3). The two forms are represented in Haworth projections, as shown in Chap. 3, and we use these in the diagrams in this chapter. 

Haworth projection A conventional planar representation of a cyclized monosaccharide molecule. The hydroxyls that are represented to the right of the chain in a Fischer projection are shown below the plane in a Haworth projection. 

For a D-monosaccharide, the terminal —CH2OH points up in the Haworth projection. 

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. 

A monosaccharide existing as a five-membered ring is a furanose one existing as a six-membered ring is a pyranose. A pyranose is most commonly drawn as a Haworth projection or a chair conformation ... 

Draw a-D-glucopyranose (a-D-glucose) as a Haworth projection. Now, using only the following information, draw Haworth projections for these monosaccharides (See Example 17.2)... 

What term would you use to describe the stereochemical relationship between D-xylose and D-lyxose Monosaccharides Fischer and Haworth Projections... 

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