Chemical Properties of Monosaccharides

Problem 22.6 What are the products of reaction of HOCHj(CHOH) CHO (I) and 

Problem 22.7 (a) Explain how in basic solution an equilibrium is established between an aldose, its C epimer (a diastereomer with a different configuration at one chiral C) and a 2-ketose. (b) Will fructose give a positive Fehling s test which is done in a basic solution 4 

When the aldose is reformed from the enediol, H can attack the now achiral C from either side of the double bond to give C epimers. 

Problem 22.8 Glucose is reduced to a single alditol fructose is reduced to two epimers, one of which is identical to the alditol from glucose. Explain in terms of configurations.  

In glucose no new chiral C is formed on going from CHO to CH,OH. In fructose the carbonyl C (C ) becomes chiral there are two configurations and we get epimers. 

STEP Q Draw the new —OH group on carbon 1 above the ring to give the /3 isomer. 

LEARNING GOAL Draw and identify the Haworth structures 

31 What are the kind and number of atoms in the ring portion of the Haworth structure of glucose  

Identify the products of oxidation or reduction of monosaccharides determine whether a carbohydrate is a reducing sugar. 

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The chemical properties of monosaccharides are further complicated by the fact that they can exhibit tautomerism in aqueous basic solutions (Figure 1.15). This means that after a short time a basic aqueous solution of a monosaccharide will also contain a mixture of monosaccharides that will exhibit their characteristic chemical properties. For example, a solution of fructose will produce a silver mirror when treated with an ammoniacal solution of silver nitrate (Tol-len s reagent). This is because under basic conditions fructose undergoes tautomerism to glucose, whose structure contains an aldehyde group, which reduces Tollen s reagent to metallic silver. 

The physical properties and many chemical properties of monosaccharides depend on the molecular shape. Equilibria of pyranoid compounds depend largely upon the axial-equatorial relationship between substituents on the rings. Thus, the alp ratio of pyranoses is governed to some extent by the tendency of the anomeric hydroxyl group to occupy the less hindered equatorial substituent orientation. 

During the past ten years, the l,6-anhydro-/3-D-hexopyranoses have found wide application in the synthesis of hexoses and their derivatives, and of oligosaccharides, and for polymerization to polysaccharides. They can be used as model compounds in studying the physical and chemical properties of monosaccharides, as has been shown, for example, by nuclear magnetic resonance (n.m.r.) studies, and also by studies of (a) their chiroptical properties, (b) the partial reactivity of hydroxyl groups, and (c) their ability to form complexes. 

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

Structural elucidation of natural macromolecules is an important step in understanding the relationships between the chemical properties of a biomolecule and its biological function. The techniques used in organic structure determination (NMR, IR, UV, and MS) are quite useful when applied to biomolecules, but the unique nature of natural molecules also requires the application of specialized chemical techniques. Proteins, polysaccharides, and nucleic acids are polymeric materials, each composed of hundreds or sometimes thousands of monomeric units (amino acids, monosaccharides, and nucleotides, respectively). But there is only a limited number of these types of units from which the biomolecules are synthesized. For example, only 20 different amino acids are found in proteins but these different amino acids may appear several times in the same protein molecule. Therefore, the structure of... 

Fischer projections are however, unsatisfactory when considering the physical properties and chemical reactivity of monosaccharides for which definitive spatial formulations are necessary. These are given below for D-glyceraldehyde, D-erythrose and D-threose, for which the (/ ,S configuration may be readily assigned at the appropriate chiral carbons. 

This chapter is exclusively devoted to the occurrence, significance, and physical and randomly selected chemical properties of various groups of sugars and their implications in synthetic applications of these specified classes of monosaccharides. 

Sucrose consists of two monosaccharides, glucose and fructose, joined by a glycosidic bond between carbon atom 1 of the glucose unit and carbon atom 2 of the fructose unit (O Fig. 3). Since it contains no free anomeric carbon atom (O Fig. i), it is a non-reducing sugar. Some basic chemical, physical, and physico-chemical properties of pure sucrose are listed in O Table 3. 

There is a great variety in structure as well as chemical and physical properties of monosaccharides. There is no single method that is applicable to the qualitative and quantitative analysis of all monosaccharides instead, the method must be chosen according to the chemical and physical characteristics of the solutes of interest. Borate complexes of monosaccharides can be separated using strong (quaternary amine, Q-type) anion-exchange columns. Alternatively, if the monosaccharide is acidic... 

Since in aqueous solutions the cyclic form of monosaccharides is in equilibrium with their corresponding open forms, the a. and p structures continually interconvert. At equilibrium, one form usually predominates. For instance, glucose dissolved in water consists of about a 2 1 ratio of p-D-glucose to a-D-glucose. Although their chemical constituents are identical, the biochemical properties between the a and the P forms can be quite different. Monosaccharides linked together to form disaccharides and polysaccharides cannot continue to interconvert and are therefore frozen in the a or p forms. Changing one monosaccharide in a complex carbohydrate to its opposite... 

One recent trend has been away from using a photochromic dye itself merely as an individual component of a solution, polymer film or bulk polymer matrix. Instead, the photochromic is chemically linked to a polymer, which may be a natural polymer such as a cellulose derivative, an enzyme, a protein, or synthetic polymers from acrylates, urethanes, and vinyl compounds. The properties of the polymer can then be modified by external irradiation, and conversely, the properties of the photochromic are modified by the polymer. A recent biochemical example is the photocontrolled binding of monosaccharides to concanavalin A (Con A) modified with spiropyran units.208... 

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