Showing papers in "Advances in Carbohydrate Chemistry and Biochemistry in 1997"

Thioglycosides as glycosyl donors in oligosaccharide synthesis.

Abstract: Publisher Summary This chapter analyzes the importance of thioclycosides as the glycosyl donors in oligosaccharide synthesis. The presence and biological importance of oligosaccharide structures, usually as the components of glycolipids and glycoproteins, in bacterial capsular and cell-wall polysaccharides, in mammalian cell membranes, in cytoplasm, and in extra-cellular fluids are well documented. Acylated glycosyl thiocyanates are made by the treatment of acylated glycopyranosyl halides with potassium thiocyanate. An attractive feature of thioglycosides as the glycosyl donors in oligosaccharide synthesis is that they are readily converted into all of the other glycosyl donors. The glycosyl bromide can also be converted into a 4-pentenyl glycoside, which becomes a glycosyl donor by promotion with an iodonium compound. Tris(4-bromophenyl)ammoniurnyl hexachloroantimonate (TBPA) differs from the other promoters in the context that its cation is a radical and produces radical cationic sulfonium ions such as glycosylating species from thioglycosides. In acetonitrile as solvent, and with a nonparticipating 2-substituent in the glycosyl donor, 1,2-trans glycosides are formed as a result of solvent participant. The transfer of the Armed–Disarmed concept to thioglycoside glycosylation reactions is also elaborated in the chapter.

Sucrose Decomposition in Aqueous Solution, and Losses in Sugar Manufacture and Refining

Abstract: Publisher Summary This chapter elaborates sucrose decomposition in aqueous solution and losses in sugar manufacture and refining. D -Glucose and D -fructose are present in both cane and beet juice and decompose on the addition of lime at high temperatures during purification of the juice. In addition to direct sucrose loss to invert sugar, undesirable dark-colored products can form in the juice through the Maillard reaction of amino acids and reducing sugars. The reversible reactions are initiated by the equilibrium between neutral and ionized forms of the monosaccharides. The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization through an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar. An important consequence of sucrose degradation is the development of color from degradation products. An equation for the prediction of color development in model sucrose solutions is developed in the chapter. It is found that the number of water molecules in the sucrose solvation shell depends on the extent of intramolecular hydrogen-bonding in the sucrose molecule.

Molecular Architecture of Polysaccharide Helices in Oriented Fibers

Abstract: Publisher Summary This chapter elaborates the molecular architecture of polysaccharide helices in oriented fibers. It describes the fiber-preparation techniques and the types of diffraction patterns from fibrous specimens. It also presents the methodology for solving and refining helical structures using X-ray data. Fiber diffraction analyses have confirmed that the ribbon-like chain conformation is conserved in several chemically modified cellulose derivatives. It is found that the variety of packing arrangements originates through the exploitation of the full range of C-5 -C-6 rotations to maximize intermolecular interactions. The parallel chain packings for cellulose I and chitin I are compatible with concepts of the biosynthetic mechanisms that produce the very long microfibrils and spines. The gellan family of polysaccharides includes a set of anionic polysaccharides secreted by unrelated bacteria. It is observed that well-defined crystal structures are available for gellan and welan and they can be correlated with the physical properties of the polysaccharides. Chitin is the most abundant polysaccharide composed exclusively of amino sugars and has distinct fundamental characteristics. Chitin adopts a 2-fold helical conformation similar to that of cellulose. Three crystalline modifications—α, β, and γ—are known from X-ray diffraction studies. Of these, the α and β forms are analogous to cellulose II and I, respectively, in terms of chain packing, and their structural details have been characterized to varying extents.

Sugars and nucleotides and the biosynthesis of thiamine.

Abstract: Publisher Summary This chapter describes the various aspects of sugars and nucleotides and the biosynthesis of thiamine. Thiamine is present in cells as a free form, as the diphosphate and as the diphosphate of the hydroxyethyl derivative, in variable ratio. Thiamine can be considered to be the product of the quaternization of 4-methyl-5-(2-hydroxymethyl)thiazole by an active derivative of 4-amino-5-(hydroxymethyl)-2-methyl pyrimidine. The treatment of thiamine in aqueous solution with sodium hydrogensulfite achieves a quantitative separation of the two heterocyclic moieties to produce thiazole and the sodium salt of the sulfonic acid. It is found that if the pentose phosphates are indeed the precursors of the five-carbon chain of thiazole, it seems fairly obvious that the carbonyl group will participate in building the heterocyclic ring. Thus, carbon atoms C-1, C-2, and C-5 of the sugar would be the respective precursors of the methyl, C-2, and C-8 carbon atoms of the thiamine thiazole. Much less is known about the participation of sugars in the biosynthesis of pyramine in yeasts, and although it has been proven that sugars can provide some carbon atoms, the exact nature of the more advanced intermediates of sugar origin is not yet clear. In the case of Escherichia coli , it is found that adenine and adenosine are inhibitory and the synthesis of thiamine can be derepressed by culture in their presence.