Showing papers in "Advances in Carbohydrate Chemistry and Biochemistry in 1989"

Anomeric and Exo-Anomeric Effects in Carbohydrate Chemistry

Abstract: Publisher Summary This chapter discusses the several possibilities regarding the specification of the energy associated with the anomeric effect, and the experimental data concerning the configurational and conformational equilibria of the anomeric center and the related variations of valence geometry. The free-energy aspect of the anomeric effect as displayed by the equilibria of isomers is the area wherein the very concept was incepted and developed, and which still dominates its investigation. A considerable amount of data has been accumulated during the past two decades on the anomeric effect in terms of structure, energy, reactivity, and other properties. The description and understanding of the nature of stereoelectronic effects is an appropriate field for the application of organic quantum chemistry. Although several molecular orbital (MO) methods successfully describe the stereochemical behavior of pyranose models, their application to the more complex carbohydrates and oligosaccharides is limited at present due to economical reasons. The various rationalizations of the anomeric effect have been offered over the past years; they may be roughly divided into two main groups—namely, the electrostatic and the delocalization rationalization of the anomeric effect. It may be expected that the intramolecular interactions involved in the anomeric effect also influence the other points on the generalized reaction hypersurface.

Complexes of Metal Cations with Carbohydrates in Solution

Abstract: Publisher Summary This chapter describes the complexes (briefly designated as metal-sugar complexes) that are formed, in detectable proportions, in aqueous solution from cations and unionized carbohydrates. Complex formation between cations and carbohydrates can be readily detected by paper electrophoresis. Thin-layer ligand-exchange chromatography makes the use of commercially available, thin-layer plates coated with a cation-exchange resin in the sodium form; the sodium ions can readily be exchanged for other cations by immersing the plate in an appropriate salt solution. Complex-formation causes the changes of the chemical shifts in the nuclear magnetic resonance spectra of sugars and polyols. Complex-formation with a cation does not, in itself, affect the optical rotatory power of a carbohydrate. However, complex-formation is often accompanied by a change of conformation that causes a change in the optical rotation. The proposal that strong complexing occurs between cations and a contiguous a,e,a sequence of hydroxyl groups in carbohydrates was first based simply on the observation that many compounds having such an arrangement complex well and many others lacking such an arrangement complex poorly, as shown by electrophoretic mobilities. The nature of complex- formation between cations and monosaccharides is now well understood; this knowledge should form a firm basis for the study of cation-polysaccharide interactions.

The Thermal Decomposition of Carbohydrates. Part I. The Decomposition of Mono-, Di-, and Oligo-Saccharides

Abstract: Publisher Summary This chapter explains the decomposition of saccharides by heat devoted to the caramelization of saccharides and their pyrolysis above 250°. Caramelization belongs to the group of so-called “browning reactions.” Caramel has a brown color and it originates from various sugars when they are heated, either dry, or in concentrated solutions. The quality of a caramel depends on its source as well as on the parameters of the process of its manufacture. Caramel is manufactured to change and improve the appearance (color) of many food products; the enhancement of the flavor of food is also important. The chemical nature of caramel depends on the conditions of the caramelization and the source, at least in the terminal period of their processing. Ever since it was suspected that caramels are biologically active, the daily uptake levels for them have been calculated and announced. The pyrolysis of sugars has been studied because carbohydrates could be considered as a source of technically important products. Moreover, these studies were intended to contribute to the knowledge of the chemistry of caramelization.

The Thermal Decomposition of Carbohydrates. Part II.* The Decomposition of Starch

Abstract: Publisher Summary This chapter focuses on the thermal treatment of dry starch, which usually leads exclusively to its depolymerization, unless the temperature applied exceeds 300°. Dextrins are the mixtures of various products resulting from the depolymerization of the starting material and recombination. The physical properties of pyrolyzed starch vary depending on the conditions applied in the process. The first chemical changes in heated starch begin when the water present in starch is forced to be evolved. The origin of the starch is very important for the manufacture of dextrins and British gums, as well as for the quality and properties of the final products. Increased interest is observed in the manufacture and use of dextrins from the enzymic hydrolysis of starch. Dextrin is a convenient carbon source for the production of neomycin B by Streptomyces marinensis . Ethylenically unsaturated dextrin prepared by reaction with the derivatives of acrylamide is a durable, hydrophilic photopolymer.

The macrostructure of mucus glycoproteins in solution.

Abstract: Publisher Summary This chapter describes the various models for mucus glycoprotein “macrostructure” (namely, assembly, gross conformation, and heterogeneity) in solution, which have been based on various interpretations of the data provided by various techniques. Native mucus secretions normally contain ∼1% of salts and other dialyzable components, 0.5%–1% of proteins, a similar proportion of carbohydrate-rich glycoprotein, and ∼95% of water. The molecular weights of mucus glycoproteins range from ∼0.5×106 to 16.0×106, and it is now widely accepted that the mucins of higher molecular weight are made up of the multiples of a basic unit having a molecular weight of (0.4–0.6) ×106. Although the electron-microscopic technique of air drying onto mica prior to platinum shadowing has yielded valuable information about the mode of assembly or secondary structure, it can yield misleading information about the gross conformation in solution. Selfassociation phenomena in solution do not appear to be significant, but bronchial mucins have been shown to interact with other components of mucus such as lysozyme and alginates.

Application of Anhydrous Hydrogen Fluoride for the Structural Analysis of Polysaccharides

Abstract: Publisher Summary This chapter presents an overview of data regarding the application of hydrogen fluoride for the structural analysis of polysaccharides, primarily those of bacterial origin. The reactions of amylose, cellulose, inulin, starch, and xylans, and their constituent monosaccharides have been well studied. Before conducting the reaction, the hydrogen fluoride is often dried by distilling it from cobalt(III) trifluoride or filtering it through a layer of this salt. The use of hydrogen fluoride in establishing the monosaccharide composition of carbohydrates was studied for the first time in 1977. This reagent was shown to be applicable for the quantitative determination of monosaccharides in neutral and acidic plant polysaccharides (arabinogalactan from larch and whole cell-walls from tomato). A comparison of the recoveries of sugars from tomato suspension–culture cell walls after hydrolysis with 2 M trifluoroacetic acid, with and without previous solvolysis with hydrogen fluoride, showed a great increase in the recovery of D -glucose (from cellulose) after the solvolysis, but some decrease in the recovery of pentoses.

13C-nuclear magnetic resonance-spectral studies of the interactions of metal ions with carbohydrates: use of relaxation probes.

Abstract: Publisher Summary This chapter elaborates the technique of 13 C-nuclear magnetic resonance (NMR) spectroscopy that has been used to study the binding of the metal ions to various carbohydrate residues and glycopeptides and it discusses the ways to extract information on the specific binding sites of the carbohydrate residues to define further how and why metal ions interact with certain residues in biological systems. It had been shown that as few as three neutral oxygen atoms suffice to define a binding site for metal ions if oxygen atoms are in the correct steric arrangement. It might be interesting to compare the tendencies in bonding because the metal ions typically used to study binding to carbohydrates are from the first-row transition-metals and from the lanthanides. Because of their large numbers of unpaired electrons, Gd 3+ and Mn 2+ have been considered as relaxation agents to be used in magnetic resonance imaging (MRI). Inositols provide a good starting point for studying the binding of metal ions such as Gd 3+ and Mn 2+ to carbohydrates because each is unique, they are structurally related to carbohydrates, and they do not contain other functional groups (carboxyl, amino) that may also interact with the metal ions.