Showing papers on "Pyranose published in 1998"

Polysaccharide elasticity governed by chair–boat transitions of the glucopyranose ring

Abstract: Many common, biologically important polysaccharides contain pyranose rings made of five carbon atoms and one oxygen atom. They occur in a variety of cellular structures, where they are often subjected to considerable tensile stress. The polysaccharides are thought to respond to this stress by elastic deformation, but the underlying molecular rearrangements allowing such a response remain poorly understood. It is typically assumed, however, that the pyranose ring structure is inelastic and locked into a chair-like conformation. Here we describe single-molecule force measurements on individual polysaccharides that identify the pyranose rings as the structural unit controlling the molecule's elasticity. In particular, we find that the enthalpic component of the polymer elasticity of amylose, dextran and pullulan is eliminated once their pyranose rings are cleaved. We interpret these observations as indicating that the elasticity of the three polysaccharides results from a force-induced elongation of the ring structure and a final transition from a chair-like to a boat-like conformation. We expect that the force-induced deformation of pyranose rings reported here plays an important role in accommodating mechanical stresses and modulating ligand binding in biological systems.

1-N-Iminosugars: Potent and Selective Inhibitors of β-Glycosidases

Abstract: A series of 1-N-iminosugars were synthesized to supply the need for glycosidase inhibitors that are both highly potent and selective for β-glycosidases. Designed on the basis of the transition-state model of the β-glucosidase reaction, these iminosugar inhibitors differ from the currently available inhibitors in possessing a nitrogen atom at the anomeric position of the pyranose ring, thereby generating a positive charge on the anomeric position rather than on the ring oxygen of the sugar. Their syntheses, starting with a readily available carbohydrate derivative, involve (i) introduction of an amino functionality as an azido group, (ii) formation of a 1-N-iminopyranose ring with reductive amination, and (iii) stereoselective introduction of a hydroxymethyl or methyl group and were accomplished in a highly stereoselective and efficient manner. The inhibitory potencies of the 1-N-iminosugars were evaluated against several α- and β-glycosidases, and they were found to be extremely potent and highly specific...

Radiation-chemical degradation of cellulose and other polysaccharides

Abstract: Results of studies on the radiation-chemical transformations of cellulose, its ethers, and some other polysaccharides (xylan, starch, dextran, chitin, chitosan, and heparin) are discussed. Ionising radiation causes the degradation of these compounds accompanied by decomposition of the pyranose ring and formation of compounds with carbonyl and carboxy groups, as well as formation of hydrogen, carbon dioxide, and carbon monoxide. The efficiency of degradation increases considerably with temperature and depends on the structure of the polysaccharide and the nature of its substituents. A mechanism of the radiation-chemical transformations of cellulose and other polysaccharides is suggested. The prospects of using radiation-chemical methods for processing of cellulose and other polysaccharides in industry and agriculture are considered. The bibliography includes 213 references.

Rare Keto‐Aldoses from Enzymatic Oxidation: Substrates and Oxidation Products of Pyranose 2‐Oxidase

Abstract: Pyranose oxidases are known to oxidise o-glucose, o-xylose and l- sorbose to keto-aldoses, biochemically interesting compounds that may also be used for synthetic purposes in a variety of reactions. In this study pyranose oxidase from the basidiomycete Penio- phora gigonteo was investigated, and it was found that this enzyme is able to oxidise a broad variety of substrates very effectively. ln analogy to its natural mode of action. most substrates are oxidised regioselectively in position 2. Certain compounds, however, are con- verted into 3-keto derivatives. and the enzyme even exhibits transfer potential, that is, disaccharides are formed from /j- glycosides of higher alcohols. Substrates that may be oxidised at C-2 in yields between 40-98"A are n-allose, o-galac- tose, 6-deoxy-o-glucose, D-gentiobiose, a-o-glucopyranosyl fluoride and the very interesting 3-deoxy-o-glucose. 1,5- Anhydro-o-glucitol ( 1 -deoxy-o-glucose) is very effectively oxidised in position 2 in 98 % yield and additionally gives a product of dioxidation at C-2 and C-3 upon prolonged reaction time. Selective oxidation at C-3 was found for 2-deoxv-

Nitrogen-containing furanose and pyranose analogues from Hyacinthus orientalis

Abstract: Aqueous methanol extracts from the bulbs of Hyacinthus orientalis were subjected to various ion-exchange column chromatographic steps to give 2(R),5(R)-bis(hydroxymethyl)-3(R),4(R)-dihydroxypyrroli...

Theoretical Studies of the Potential Energy Surfaces and Compositions of the d-Aldo- and d-Ketohexoses

Abstract: The potential energy surfaces of all eight d-aldohexoses and four d-ketohexoses have been extensively studied, employing quantum mechanical and molecular mechanical calculations. Anomeric preferences for the axial OH positions were observed for all of the hexoses studied. Several stability factors determining the potential energy surface were examined, and we found that the Hassel−Ottar effect, the delta-two effect and the cis−trans effect are not discernible on the gas-phase potential energy surfaces of the cyclic hexoses. Instead, the anomeric effect and intramolecular hydrogen bonding effects dominate. The most important finding in this study is that in the gas phase, furanose forms are more stable than pyranose forms for all of the hexoses except galactose, idose, psicose, and sorbose, in contrast to the generally greater stability of pyranose forms observed in solution. The decrease of intramolecular hydrogen bonding stabilization in solution was found to be primarily responsible for the composition ...

The structure of a glycogen phosphorylase glucopyranose spirohydantoin complex at 1.8 A resolution and 100 K: the role of the water structure and its contribution to binding.

Abstract: A glucopyranose spirohydantoin (a pyranose analogue of the potent herbicide, hydantocidin) has been identified as the highest affinity glucose analogue inhibitor of glycogen phosphorylase b (GPb). In order to elucidate the structural features that contribute to the binding, the structures of GPb in the native T state conformation and in complex with glucopyranose spirohydantoin have been determined at 100 K to 2.0 A and 1.8 A resolution, respectively, and refined to crystallographic R values of 0.197 (R[free] 0.248) and 0.182 (R[free] 0.229), respectively. The low temperature structure of GPb is almost identical to that of the previously determined room temperature structure, apart from a decrease in overall atomic temperature factors ((B) room temperature GPb = 34.9 A2; (B) 100 K GPb = 23.4 A2). The glucopyranose spirohydantoin inhibitor (Ki = 3.0 microM) binds at the catalytic site and induces small changes in two key regions of the protein: the 280s loop (residues 281-286) that results in a decrease in mobility of this region, and the 380s loop (residues 377-385) that undergoes more significant shifts in order to optimize contact to the ligand. The hydantoin group, that is responsible for increasing the affinity of the glucose compound by a factor of 10(3), makes only one hydrogen bond to the protein, from one of its NH groups to the main chain oxygen of His377. The other polar groups of the hydantoin group form hydrogen bonds to five water molecules. These waters are involved in extensive networks of hydrogen bonds and appear to be an integral part of the protein structure. Analysis of the water structure at the catalytic site of the native enzyme, shows that five waters are displaced by ligand binding and that there is a significant decrease in mobility of the remaining waters on formation of the GPb-hydantoin complex. The ability of the inhibitor to exploit existing waters, to displace waters and to recruit new waters appears to be important for the high affinity of the inhibitor.

On the mechanism and specificity of soluble, quinoprotein glucose dehydrogenase in the oxidation of aldose sugars.

Abstract: Kinetic and optical studies were performed on the reductive half-reaction of soluble, quinoprotein glucose dehydrogenase (sGDH), i.e., on the conversion of sGDHox plus aldose sugar into sGDHred plus corresponding aldonolactone. It appears that the nature and stereochemical configuration of the substituents at certain positions in the aldose molecule determine the substrate specificity pattern: absolute specificity exists with respect to the C1-position (only sugars being oxidized which have the same configuration of the H/OH substituents at this site as the β-anomer of glucose, not those with the opposite one) and with respect to the overall conformation of the sugar molecule (sugars with a 4C1 chair conformation are substrates, those with a 1C4 one are not); the nature and configuration of the substituents at the 3-position are hardly relevant for activity, and an equatorial pyranose group at the 4-position exhibits only aspecific hindering of the binding of the aldose moiety of a disaccharide. The pH o...

Pyranose Ring Flexibility. Mapping of Physical Data for Iduronate in Continuous Conformational Space

Abstract: A method for relating pyranose ring conformation to experimental measurements is presented. A three-dimensional conformational space (Q, θ, P2) based on a torsion angle formalism is projected in tw...

The Cetus Process Revisited: A Novel Enzymatic Alternative for the Production of Aldose-Free D-Fructose

Abstract: In the Cetus process crystalline D-fructose is produced from D-glucose via the intermediate 2-keto-D-glucose. Whereas the first step in the traditional process is catalyzed by the immobilized enzyme pyranose 2-oxidase, the ensuing reduction is performed by catalytic hydrogenation. In an entirely enzymatic variation of this process, soluble pyranose 2-oxidase from Trametes multicolor was employed. This biocatalyst could be efficiently stabilized under operational conditions by the addition of bovine serum albumin (BSA) together with catalase which decomposes hydrogen peroxide formed as a by-product. D-Glucose could be converted into 2-keto-D-glucose in yields above 98%. When the biocatalyst together with both stabilizing agents was separated from the product solution by ultrafiltration, it could be reutilized for several subsequent batch operation cycles. 2-Keto-D-glucose thus obtained was quantitatively reduced to D-fructose by NAD(P)-dependent aldose reductase from Candida tenuis. Two different enzymatic...

Conversion of Pyranose Glycals to Furanose Derivatives: A New Route to Oligofuranosides

Abstract: Acetylated pyranose glycals have been converted through a convenient three-step process into protected furanose reducing sugars. Ozonolysis of 2,3,5-tri-O-acetyl-glucal or 2,3,5-tri-O-acetyl-galactal, followed by treatment with dimethyl sulfide and then hydrolysis gave respectively protected arabinofuranose (6) and lyxofuranose (7) derivatives. Conversion of these hemiacetals to oligosaccharides was explored using a number of methods. Activation of 6 or 7 in situ afforded glycosides in modest yield and stereoselectivity. Glycosylation of tetraacetates 16 and 18, obtained from 6 and 7, gave similar results. However, thioglycosides 17 and 19, also derived from 6 and 7, were found to be effective glycosyl donors, producing products in good to excellent yields and with high stereoselectivities. The method was also used to synthesize a disaccharide in which one residue contained uniform 13C enrichment.

Production of a Novel Pyranose 2-Oxidase by Basidiomycete Trametes multicolor

Abstract: During a screening for the enzyme pyranose 2-oxidase (P2O) which has a great potential as a biocatalyst for carbohydrate transformations, Trametes multicolor was identified as a promising, not-yet-described producer of this particular enzyme activity. Furthermore, it was found in this screening that the enzyme frequently occurs in basidiomycetes. Intracellular P2O was produced in a growth-associated manner by T. multicolor during growth on various substrates, including mono-, oligo-, and polysaccharides. Highest levels of this enzyme activity were formed when lactose or whey were used as substrates. Peptones from casein and other casein hydrolysates were found to be the most favorable nitrogen sources for the formation of P2O. By applying an appropriate feeding strategy for the substrate lactose, which ensured an elevated concentration of the carbon source during the entire cultivation, levels of P2O activity obtained in laboratory fermentations, as well as the productivity of these bioprocess experiments, could be enhanced more than 2.5-fold.

The First Platinum(IV) Complexes with Glucopyranoside Ligands. A New Coordination Mode of Carbohydrates.

Abstract: [(PtMe3I)4] reacts with AgBF4 in acetone to give fac-[PtMe3(Me2CO)3]BF4 (2) which was isolated as strongly moisture and air sensitive, colorless crystals and characterized by microanalysis and NMR spectroscopy (1H, 13C, 195Pt). The X-ray structure analysis (orthorhombic, Pcab, a = 15.599(3) A, b = 15.685(2) A, c = 15.763(3) A, Z = 8) reveals that 2 is monomeric and that the cation exhibits local C3v symmetry. The reaction of 2 with glucopyranoses (1,6-anhydro-β-d-glucopyranose (3a), 1-methyl-α-d-glucopyranoside (3b), and 1-phenyl-β-d-glucopyranoside (3c)) yields carbohydrate complexes [PtMe3L]BF4 (4a−4c, L = 3a−3c). The complexes were characterized by microanalysis, ESI mass spectrometry and by NMR spectroscopy (1H, 13C, 195Pt) displaying that the carbohydrates act as tridentate chelating ligands coordinated by the hydroxyl groups 2-OH and 4-OH and by the acetal oxygen atom of the pyranose ring. This structure was also confirmed by X-ray structure analysis of 4a (orthorhombic, P212121, a = 6.404(1) A, b =...

Liquid crystalline derivatives of galactose and galactitol: dependence of thermotropic mesomorphism on carbohydrate form

Abstract: An almost complete homologous series of the acyclic, open-chain 6- O - n -alkyl-D-galactitols has been prepared. Most homologues in this series exhibit an enantiotropic smectic A* phase. The liquid crystal transition temperatures of these open-chain carbohydrates are compared with those of the corresponding homologues in the cyclic form, i.e., with those of the 6- O - n alkyl-alpha-D-galactopyranoses. In general the carbohydrates in the open-chain form exhibit higher clearing points than those of the analogous cyclic pyranoses. However, the melting point of the galactitols is generally significantly lower than that of the corresponding galactopyranoses. This leads to a much wider temperature range of the smectic A* phase for the carbohydrates in the pyranose form. The lowest clearing point was found for related galactoses in the furanose form. Alternation was found for the clearing point of the 6-O-n alkyl-alpha-D-galactopyranoses and the 6- O - n -alkyl-D-galactitols. Powder X-ray diffraction studies ind...

Synthesis and hydrolytic stability of the α and β anomers of 4′-thio-2′-deoxyuridine and their 5-substituted analogs. Competition between the acid-catalysed depyrimidination and isomerisation to a 5-thiopyranoside nucleoside

Abstract: The α and β anomers of 4′-thio-2′-deoxyuridine were readily synthesised as an anomeric mixture using adapted methodology and separated chromatographically as their 3′,5′-di-O-benzyl-N 3-benzoyl derivatives. The 5-fluoro analogs were prepared in a similar manner, but the anomers could be separated simply as their 3′,5′-di-O-benzyl derivatives. The kinetics of acid-catalysed hydrolysis for the four compounds and their 5-alkylated analogs are reported. Under these conditions, cleavage of the N-glycosidic bond competes with the reversible isomerisation between the furano (4′-thio) and pyrano (5′-thio) ring systems. This was confirmed by isolation and NMR characterisation of the α and β pyranose intermediates of the parent compounds.

Crystallographic studies of some cyclic benzylidene acetals: Key synthons for o-glycoamino acid building blocks and solid-phase oligosaccharide synthesis

Abstract: Synthesis of methyl 2-azido-2-deoxy-4,6-O-benzylidene-β-D-galactopyranoside (1), one of the key components in the synthesis of O-glycoamino acids, was undertaken in order to synthesize Tn and TF(3) antigen building blocks. In pursuit of an alternative approach, benzylidenation of the crude D-galactal (2a) afforded methyl 2-deoxy-4,6-O-benzylidene-α-D-galactopyranoside (3c) and 3,4-O-benzylidene-D-galactal (3b) besides the expected 4,6-O-benzylidene-D-galactal (3a). Formation of compound 3c was explained based on the presence of methyl 2 deoxy-α-D-galactopyranoside (2b) isomer and/or trace amount of methanol in the crude mixture of deacetylated product of 2 prior to benzylidenation. On the other hand, formation of 3b in substantial quantities appears to be a thermodynamically controlled product and its formation is found to be common during prolonged Lewis-acid catalyzed benzylidenation reaction. Crystal structures of these important and useful precursors were deduced by X-ray diffraction methods to enumerate their complete molecular structure as well as to understand the effect of the cyclic acetal on the pyranose ring conformation. Compound 1 crystallizes in the orthorhombic space group P212121 with cell dimensions a = 5.058(7), b = 12.766(7), c = 22.557(7) A 3b crystallizes in the hexagonal space group P61 with cell dimensions a = 18.265(4), b = 18.265(3), c = 6.323(2) A 3c crystallizes in the monoclinic space group P21 with cell dimensions a = 10.614(3), b = 4.963(2), c = 12.730(3) A, and β = 95.47(3)°.

Sugar mimics from sugar lactones

Abstract: The potential of carbohydrate mimics which incorporate an a-amino acid moiety at the anomeric position of the sugar is illustrated by the synthesis of both furanose and pyranose analogues of L-rhamnose in a study related to inhibition of the biosynthesis of mycobacterial cell walls. Some amino acid analogues of glucopyranose are inhibitors of glycogen phosphorylase wd may provide an approach to the treatment of late-onset diabetes; bicyclic lactone intermediates provide relatively easy access to both oper, chain and spiro derivatives of glucose in which both nitrogen and carbonyl functions are present at the anomeric position, and in which the anomeric configuration is defined. Other glucose analogues were found to be inhibitors of glucokinase and glucose-6-phosphatase; such materials may allow the investigation of metabolic control by the levels of sugar phosphates.