Stephen J. Angyal

The Composition of Reducing Sugars in Solution

Publisher Summary This chapter discusses the composition of reducing sugars in solution. Reducing sugars differ from most other organic compounds in one characteristic property. When a pure organic compound that is not a reducing sugar is dissolved in a solvent, it is certain that the solution will usually contain only one compound; but when a reducing sugar, above an aldotetrose or a 2-pentulose, is dissolved in water, a solution is obtained that always contains at least six compounds: two pyranoses, two furanoses, and an acyclic (open-chain) carbonyl form and its hydrate. There are also minute proportions of septanoses and dimers. Very few of these many compounds that are present in the equilibrium solutions of sugars have ever been isolated. The only method for separating them from the equilibrium mixture is by crystallization and that depends on the fortuitous presence of seed crystals. The chapter details about the pyranose form, the furanose form, and the septanose form. It also discusses the composition of aldoses, ketoses, and substituted and derived sugars in aqueous solution.

Complexes of Metal Cations with Carbohydrates in Solution

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 separation of sugars and of polyols on cation-exchange resins in the calcium form☆☆☆

Abstract Chromatography on a column of a cation-exchange resin in its calcium form, with water as the eluant, is a convenient method for the separation of many sugars and polyols and their derivatives. The advantages and limitations of this method are discussed. Addition of methanol to the eluant and lowering of the temperature improve the separation.

A Symmetry rule for the Circular Dichroism of reducing sugars, and the proportion of Carbonyl forms in Aqueous solutions thereof

Abstract From the circular dichroisrm spectra of 23 aldoses and 10 ketoses in aqueous solution, a symmetry rule was derived relating the sign of the n- band of the carbonyl form to the absolute configuration of the sugar. The sign is positive when the configuration of the adjacent chiral center is (S), and it is negative when the configuration is (R). The rule also holds for aldehydo- and keto-sugar peracetates, and for y-lactones substituted with oxygen-containing groups. The magnitude of a mean, molar, dichroic-extinction coefficient for the pure carbonyl forms of aldoses and ketoses was estimated from published c.d. data on the aldehydo- and keto-sugar peracetates and related compounds, which permitted estimation of the proportion of the carbonyl form in the equilibrium mixture for each sugar. D-Glucose had the smallest carbonyl concentration, 0.002% at 20°, in agreement with an earlier, polarographic value. The relationship between carbonyl content and sugar structure and configuration is discussed.

Hudson's rules of isorotation as applied to furanosides, and the conformations of methyl aldofuranosides

Abstract Molecular rotations and molecular-rotation differences are tabulated for the methyl furanosides of all of the aldopentoses, aldohexoses, and hexuloses. The n.m.r. spectra of most of the methyl aldofuranosides have been determined and analyzed, and from the data, the conformations of the glycosides have been deduced. In the d series, the β-pentofuranosides are found to be in the 3 T 2 form, with the methoxyl group quasi -axial and the side-chain quasi -equatorial. The α- d -pentofuranosides are mixtures of two twist forms, or assume an envelope conformation. The correlation between conformation and optical rotation of the methyl furanosides is discussed.

The composition of reducing sugars in solution : current aspects

Publisher Summary This chapter analyzes the composition of reducing sugars in solution. It elaborates the concepts of relative stabilities of various forms of sugars and elaborates the composition of aldoses, ketoses, and substituted and derived sugars in aqueous solution. Neuclear Magnetic Resonance (NMR) spectrometers have considerably improved over the past few years, and important advances now allow the detection and measurement of components that occur in the range of 0.01 -0.1%. Gas-liquid chromatography (GLC) of trimethylsilyl derivatives has been used to determine the composition of some dozen sugars in water and in pyridine. In the latter solvent, the results agree well with previous determinations; in aqueous solution, however, some of the values for furanoses prove to be too high and the values for idose (presumably mutarotating) rapidly differ considerably from those obtained by NMR spectroscopy. GLC of the trimethylsilyl derivatives is also used for studying the mutarotation of D-fructose. The composition data obtained for the major components agree well with those given by NMR spectroscopy.

The Lobry de Bruyn-Alberda van Ekenstein Transformation and Related Reactions

Recent examples of the use of a very old synthetic method are discussed. Discovery of the cation-catalyzed epimerization explains some of the previously controversial results. The mechanism of the reaction is discussed. Suggestions are made for improvements in yield and in the method of work-up.

The 13C-N.M.R. spectra of alditols☆

Abstract The 13 C-n.m.r. signals of the pentitols and hexitols in aqueous solution, and of their acetates in chloroform solution, have been assigned by the use of specifically deuterium-substituted compounds. Qualitative correlations have been established between the chemical shifts and the configuration and preponderant conformation of each of the alditols.

Conformations of acyclic sugar derivatives : Part II. Determination of the conformations of alditol acetates in solution by the use of 250-MHz n.m.r. spectra

Abstract The p.m.r. spectra of all the fully acetylated pentitols and several fully acetylated hexitols have been analysed. Computation by iterative analysis and recourse to 250-MHz spectra were required in several cases. The vicinal coupling constants were used to determine the conformations of these compounds in solution. The planar zigzag conformation was found to be predominant only in those configurations ( arabino, manno ) which do not possess parallel 1,3-interactions between acetoxyl groups on alternate carbon atoms. The other compounds were found to be mixtures of two or more conformers, none of which has the planar zigzag conformation, except in the case of hexa- O -acetylallitol. The conformations of alditols in the crystalline state and of the alditol acetates in solution are compared.

Complex formation between polyols and rare earth cations. The crystal structure of galactitol · 2PrCl3 · 14 H2O

Abstract Alditols form stronger complexes with Nd 3+ , Sm 3+ , and Eu 3+ than with La 3+ cations, but weaker ones with the higher lanthanides. There is a clear relationship between ionic radii and stability constants. Chromatographic ion-exchange columns can be smaller when prepared with Nd 3+ rather than the usual Ca 2+ columns. Galactitol, alone amongst the hexitols, can form 1:2 complexes with lanthanide ions. The crystal structure of galactitol·2PrCl 3 ·14H 2 O has been determined.