Monosaccharides cyclic structure

The aldehyde or ketone group of monosaccharides can undergo an intramolecular reaction with one of its own hydroxyl groups to form a cyclic, hemiacetal, or hemiketal structure, respectively (Figure 1.26). In aqueous solutions, this cyclic structure actually predominates. The open-chain aldehyde or ketone form of monosaccharides is in equilibrium with the cyclic form, but the open structure exists less than 0.5 percent of the time in aqueous environments. It is the... 

Monosaccharides commonly form internal hemiacetals or hemiketals, in which the aldehyde or ketone group joins with a hydroxyl group of the same molecule, creating a cyclic structure this can be represented as a Haworth perspective formula. The carbon atom originally found in the aldehyde or ketone group (the anomeric carbon) can assume either of two configurations, a and /3, which are interconvertible by mutarotation. In the linear form, which is in equilibrium with the cyclized forms, the anomeric carbon is easily oxidized. 

Given the acyclic formula for a monosaccharide, draw its cyclic structure in either the pyranose or furanose form and either a or p configuration. 

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

This chapter begins with a discussion of the structure and stereochemistry of monosaccharides. Then the formation of cyclic structures front monosaccharides is discussed. This is followed by the presentation of a small number of reactions of these compounds. The classic series of experiments that was used to establish the structure of glucose is presented next. Finally, the structures of disaccharides, polysaccharides, and a few other types of carbohydrate-containing compounds are introduced. 

The building blocks for polysaccharides are monosaccharides (sugars) such as glucose or fructose (QH O ). Two cyclic structures of D-glucose are reproduced below (a third, open-chain structure is not shown). 

Drawing Cyclic Monosaccharides Cyclic hemiacetal structures may seem complicated at first glance, but they can be drawn and recognized by following the process illustrated in Figure 23-6. 

Because poiysaccharides are natural macromolecules occurring in all living organisms, the structure of some polysaccharides can be much more complex, as they are not made only from simple monosaccharides. In the composition of natural polymeric carbohydrates, a wide variety of sugars are found. Among these, the most common are pentoses and hexoses. The structural formulas of three common pentoses are shown below. They frequently form cyclic structures with five-member rings (furanoses). 

When monosaccharides are in aqueous solution, they exist in both open-chain and cyclic structures. The cyclic structures are more stable and are the predominant form of monosaccharides at equilibrium. Note in Figure 24-8 that the carbonyl groups are present only in the open-chain structures. In the cyclic structures, they are converted to hydroxyl groups. 

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