Fischer projections, monosaccharide structures

Monosaccharide structures may be depicted in open-chain forms showing their carbonyl character, or in cyclic hemiacetal or hemiketal forms. Alongside the Fischer projections of glucose, ribose, and fructose shown earlier, we included an alternative... 

The Fischer projection is a convenient way of showing the configurations of the linear forms of monosaccharides. This convention depicts the concepts of stereochemistry established by Jacobus Henricus van t Hoff and Joseph Achille Le Bel in a simplified form. While these abbreviated structural formulas are simple to write and easy to visualize, there are some guidelines that should be taken into account when converting a three-dimensional structure into a Fischer projection and in its manipulation (Fig. 1.2) ... 

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 ... 

Drawing Monosaccharide Structures The Fischer Projection. From the stereochemical point of view, monosaccharides are considered to be derived from the two trioses, D- and L-glyceraldehyde (see Fig. III-l). These two parent compounds differ only in the steric arrangement of the atoms about the central asymmetric carbon they are mirror images (enantiomorphs, optical antipodes) of one another. 

Do not rotate a Fischer projection formula in the plane of the page, because you might inadvertently convert a compound into its enantiomer. When using Fischer projections it is usually best to convert them to structures with wedges and dashes, and then manipulate them. Although a Fischer projection formula can be used for the stereogenic center in any compound, it is most commonly used for monosaccharides. 

Although Fischer projections are commonly used to depict monosaccharides with many stereogenic centers, care must be exercised in using them since they do not give a true picture of the three-dimensional structures they represent. Because each stereogenic center is drawn in the less stable eclipsed conformation, the Fischer projection of glucose really represents the molecule in a cylindrical conformation, as shown in Figure 27.2. 

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. 

Figure 1.40 Structural summary of all main monosaccharides found in natural carbohydrates. ALL monosaccharides are displayed as their Fischer Projections to shown differences in absolute configuration.

Monosaccharides have chiral centers and thus exhibit optical activity. Let s explore the structure of glyceraldehyde. In the three-dimensional representation, the dotted wedges indicate the bonds that extent backwards from the chiral carbon (away from you or into the plane of the page) and the solid wedges indicate the bonds that are projected toward you (out of the plane of the page). In stereochemistry, Fischer projection is an important way to represent the spatial orientation of molecules. In Fischer projection representation, the bonds that are pointed backward (away from you or into the page) are indicated by vertical lines, and the bonds that extend toward you (out of the page) are represented by horizontal lines. For a more detailed approach, refer to Chapter 19 (Stereochemistry). 

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. 

Carbohydrates are polyhydroxy aldehydes and ketones that are commonly known as sugars. A monosaccharide is a carbohydrate composed of one sugar unit and a disaccharide is a carbohydrate composed of two monosaccharide units. An oligosaccharide is composed of 2-10 monosaccharide units a polysaccharide is composed of 10 or more monosaccharide units. An aldose is a carbohydrate that contains an aldehyde unit and a ketose is a carbohydrate that contains a ketone unit. Monosaccharides are categorized by the total number of carbons in the structure triose, tetrose, pentose, hexose, etc. The d and 1 configurations of a monosaccharide are based on the Fischer projection of d-glyceraldehyde. A Fischer projection is an older representation of sugars. 

A Fischer projection of a monosaccharide is used to show its structure and thus keep track of stereochemistry. 

In a Fischer projection, the monosaccharide is drawn in the open-chain form and the carbonyl carbon atom is placed at the top of the structure. Horizontal lines represent groups projecting above the plane of the paper, and vertical lines represent groups projecting below the plane of the paper. 

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

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]. 

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