![]() The disaccharide lactose contains a galactose unit and a glucose unit joined by a β-1,4-glycosidic linkage. The disaccharide maltose contains two glucose units joined in an α-1,4-glycosidic linkage. Any mono- or disaccharide containing a free anomeric carbon is a reducing sugar. The carbonyl group present in monosaccharides is easily oxidized by Tollens’ or Benedict’s reagents (as well as others). These interconversions occur back and forth until a dynamic equilibrium mixture is achieved in a process called mutarotation. Once the sample is dissolved in water, however, the ring opens up into the open-chain structure and then closes to form either the α- or the β-anomer. Any solid sugar can be all alpha or all beta. In Haworth projections, the alpha form is drawn with the OH group on the “former” carbonyl carbon atom ( anomeric carbon) pointing downward the beta form, with the OH group pointing upward these two compounds are stereoisomers and are given the more specific term of anomers. Consequently, glucose in solution exists as an equilibrium mixture of three forms, two of them cyclic (α- and β-) and one open chain. Monosaccharides of five or more carbons atoms readily form cyclic structures when the carbonyl carbon atom reacts with an OH group on a carbon atom three or four carbon atoms distant. ![]() If the structure at this carbon atom is the same as that of D-glyceraldehyde (OH to the right), the sugar is a D sugar if the configuration is the same as that of L-glyceraldehyde (OH to the left), the sugar is an L sugar. Enantiomers have the same physical properties, such as melting point, but differ in the direction they rotate polarized light.Ī sugar is designated as being a D sugar or an L sugar according to how, in a Fischer projection of the molecule, the hydrogen atom and OH group are attached to the penultimate carbon atom, which is the carbon atom immediately before the terminal alcohol carbon atom. A molecule containing a chiral carbon is nonsuperimposable on its mirror image, and two molecules that are nonsuperimposable mirror images of each other are a special type of stereoisomer called enantiomers. These particular stereoisomers contain at least one chiral carbon, a carbon atom that has four different groups bonded to it. Many carbohydrates exist as stereoisomers, in which the three-dimensional spatial arrangement of the atoms in space is the only difference between the isomers. Cellulose, glycogen, and starch are polysaccharides. Most sugars are either monosaccharides or disaccharides. Those with two monosaccharide units are disaccharides, and those with many monosaccharide units are polysaccharides. The simplest carbohydrates are monosaccharides. All carbohydrates contain alcohol functional groups, and either an aldehyde or a ketone group (or a functional group that can be converted to an aldehyde or ketone). To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter.Ĭarbohydrates, a large group of biological compounds containing carbon, hydrogen, and oxygen atoms, include sugars, starch, glycogen, and cellulose.
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