Showing papers in "Advances in Carbohydrate Chemistry and Biochemistry in 1976"

Hemicellulases: Their Occurrence, Purification, Properties, and Mode of Action

Abstract: Publisher Summary The terms “hemicellulases” or “hemicellulose-degrading enzymes” refer to those enzymes that specifically degrade only hemicelluloses, and do not include the glycosidases—which—in addition to their activity on glycosides of low molecular weight, are also frequently capable of hydrolyzing the short-chain or monosaccharide appendages from the backbone chain of hemicelluloses. Typical hemicellulases are the L-arabinanases, D-galactanases, D-mannanases, and D-xylanases. This chapter reviews the literature on hemicellulases from 1950 to 1973, and discusses those enzyme preparations that have employed in protein purification procedures, been shown to be homogeneous. Fungal, extracellular L-arabinanases have been shown to be inductive and constitutive. Several saprophytic fungi were found by Fuchs and coworkers to produce L-arabinanase inductively but not constitutively, whereas several phytopathogenic fungi were found capable of producing L-arabinanases by induction when grown on Larabinan, and constitutively when these organisms were grown on Dglucose as the sole carbon source.

The utilization of sugars by yeasts.

Abstract: Publisher Summary This chapter outlines the breakdown of sugars by yeasts. To continue biosynthetic processes necessary for growth, yeasts obtain energy from sugars by breaking them down. The energy set free is stored as the “high energy” phosphate derivative adenosine 5’-triphosphate (ATP) that is synthesized as the sugar is catabolized. In catabolism, glycosidic bonds are hydrolyzed to yield component monosaccharides. A low concentration of oxygen is often important for obtaining a high yield of ethanol from certain sugars. Particularly in initial and terminal reactions, differences are found among different yeasts, and such relatively minor biochemical differences are often of considerable practical importance. The information provided in this chapter is based chiefly on studies yeast such as, Saccharomyces cerevisiae , Saccharomyces uvarum , Candida utilis , and Kluyveromyces fragilis . However, the ability of yeasts to utilize sugars is not only of potential value, it can also be a nuisance. Yeasts are notorious as spoilers of foods that contain a high concentration of one or more sugars, such as honey, maple syrup, sugar cane, and confectionery.

Relative Reactivities of Hydroxyl Groups in Carbohydrates

Abstract: Publisher Summary This chapter discusses the important reactions of the hydroxyl group. Knowledge of the relative reactivities of hydroxyl groups in carbohydrates is fundamental to a thorough understanding of carbohydrate chemistry. The chapter explains esterification, which forms the largest part of the literature since 1953 on selective reactions of carbohydrates. The common methods of esterification utilize an acid chloride or an acid anhydride as the reagent, but other acid derivatives often show different selectivities. An investigation pertinent to the subject of favored reactivity concerned the reactions of certain acyl halides with ethanol, in acetone or chloroform solution. The mixed-order nature of some of these reactions suggested that every rate-determining transition-state contained the substrate, the nucleophile (ethanol), and an acceptor— X—for hydrogen bonding, suitable acceptors being an acetone molecule, a chloride ion, or another ethanol molecule. The possibility that intramolecular hydrogen-bonding does not persist in pyridine and that the advantage of a bonded hydroxyl group would be lost, invalidates the explanation of favored reactivity. But if the hydrogen-bonding to pyridine is hindered in the transition state, intramolecular hydrogen-bonding could conceivably assume importance.

Synthesis of naturally occurring C-nucleosides, their analogs, and functionalized C-glycosyl precursors.

Abstract: Publisher Summary The C-nucleosides are a group of C-glycosylated heterocycles in which the anomeric carbon atom is attached to the heterocycle by a C-C bond. C-nucleosides have been mainly from fermentation sources, and have been found to exhibit a variety of interesting biological properties. Pyrazomycin has significant antiviral activity. Formycin, formycin B, and oxazinomycins exhibit, in addition, antitumor activity. The biological importance of most of the naturally occurring C-nucleosides has prompted the exploration of synthetic routes leading to these compounds and their analogs. As synthetic targets, the C-nucleosides have deceptively simple structures, and the design of synthetic routes to them must take two important factors into account. Firstly, the methods selected for C-C bond formation at the anomeric center should be stereocontrolled because the structures known thus far are, with two exceptions, β-D-glycosyl compounds. Secondly, the carbon atom attached to the glycosyl group in a C-nucleoside precursor should be appropriately substituted or be amenable to substitution, so as to allow the elaboration of the other heterocyclic portion of the molecule.

Noncytotoxic, antitumor polysaccharides.

Abstract: Publisher Summary This chapter provides an overview of the available information on host-mediated, antitumor action of noncytotoxic polysaccharides extracted from different botanical sources. Serratia marcescens polysaccharide has been reported to be active against a number of tumors, such as Sarcoma 180, Ehrlich carcinoma, Guerin carcinoma, Sarcoma M-1, and Walker carcinosarcoma. Navashin and coworkers later found that animals develop resistance to this polysaccharide, and that continued administration is without effect. However, if the polysaccharide is given periodically, an increase in activity and a diminution in toxicity are observed. Polysaccharide complexes from Escherichia co1i , Staphylococcus aureus , Streptococcus , Klebsiella , Pseudomonas , Acetobacter xylinum , Proteus vulgaris , and others are reported to be active against solid tumors. Moreover, crude, fungal extracts from a number of fungi are active against experimental tumors, and—in some cases—they showed definite palliative action on some non-operable, human tumors. The polysaccharides so far examined, although strongly active in suppressing transplanted tumors, do not seem eligible for clinical trial in human cancer therapy. Therefore, to search for new polysaccharides more strongly antitumor-active stands as a challenge for further studies on antitumor polysaccharides.

The Chemistry of Sucrose

Abstract: Publisher Summary This chapter brings together information on the reactions of sucrose and illustrates some of the physical methods that have contributed to the characterization of sucrose derivatives. In addition, some of the potential commercial applications of compounds derived from sucrose have also been discussed in the chapter. Sucrose, systematically named β-D-fructofuranosyl α- Dglucopyranoside, is a nonreducing disaccharide. The synthesis of sucrose by enzymic methods has been achieved through glycosylation involving free D-fructose and either D-glucose 6-phosphate or D-glucose. Physical methods as X-ray crystallography, neutron diffraction, and nuclear magnetic resonance (n.m.r.) spectroscopy have proved of immense value in determining the configuration and the conformation of the sucrose molecule. Furthermore, transesterification reaction of sucrose with long-chain fatty acid esters performed in a melt as the reaction medium. According to this process, sucrose and sodium stearate are mixed with a small proportion of water to afford a (homogeneous) solution.

The Pneumococcal Polysaccharides: A Re-Examination

Abstract: Publisher Summary The pneumococcal polysaccharides and their antisera are used in the immunological characterization of other polysaccharides. Structural studies of these polysaccharides are of special importance, and this chapter summarizes the results of such studies. Studies of a great number of extracellular polysaccharides from Gram-positive and Gram-negative bacteria indicate that they are composed of oligosaccharide repeating-units. The synthesis of a bacterial polysaccharide requires a considerable number of highly specific enzymes, increasing in number with the complexity of the repeating unit. The purification of a pneumococcal polysaccharide may be difficult, and the polysaccharide material is sometimes contaminated by a cell-wall component known as the “C-substance.” The methods for structural analysis of polysaccharides are not always adequate, especially when results of quantitative significance are needed. Sometimes, it may even be difficult to obtain satisfactory, quantitative analysis of the component sugars. Furthermore, considering the importance of these compounds in immunochemistry further work in this field is needed.

Dithioacetals of Sugars

Abstract: Publisher Summary This chapter provides an overview of dithioacetals and their direct chemical transformations. Dithioacetals and their derivatives comprise the largest recorded class of acyclic carbohydrate derivatives. They are versatile intermediates in synthesis not only because a multitude of transformations are possible at the dithioacetal group, but also because all of the hydroxyl groups of the sugar chain are available for chemical transformation or selective protection. Of particular importance is the fact that they provide a route for kinetically controlled ring-closure of the sugar under neutral conditions by the action of mercury (II) salts, thus providing a useful route of access to furanosides. Although dithioacetals are odorless, an inherent practical disadvantage of their preparation is the generally disagreeable scent of the lower thiols. Furthermore, appended to this chapter is a set of tables that lists the melting points and specific rotations of sugar dithioacetals and certain derivatives.

Reactions of D-Glucofuranurono-6,3-Lactone

Abstract: Publisher Summary This chapter discusses the reactions of D-glucofuranurono-6,3-lactone as they have been the subject of extensive investigations. Fundamentally, D-glucofuranurono-6,3-lactone contains the 2,6-dioxabicyclo [3.3.0]octane structure. The chapterdescribes its properties. D-glucofuranurono-6,3-lactone halides exhibit certain properties, such as (1) thermodynamic and solvolytic stability decreases in the series: fluoride, chloride, bromide, (2) 2,5-Di-O-acetyl-D-glucofuranosylurono-6,3-lacton halides are less stable than their 2,5-di-O-benzoyl or their 2-O-acetyl-5-O-benzoyl counterparts, (3) β-D-Glucofuranosylurono-6,3-lactone chlorides are subject to alcoholysis, with formation of alkyl β-D-glucofuranosidurono-6,3-lactones; under the same conditions the α-D-anomers are nonreactive, (4) like methyl 2,3,4-tri-O-acetyl-β-D-glucopyranosyluronate chloride,2,5-di-O-acyl- β-D-glucofuranosylurono-6,3-lactone chloride are quite resistant to anomerization. Furthermore, the reaction of per-O-acylated D-glucosyl bromides and chlorides with alcohols by heterogeneous, or homogeneous, base catalysis is commonly used for the preparation of D-glucosides. By an SN2 type of mechanism under Koenigs-Knorr conditions, α-D-glucopyranosyl halides form β-D-glucopyranosides, which are also obtained from the “unstable” β -D-glucopyranosyl chlorides through neighboring group participation.

Bibliography of Crystal Structures of Carbohydrates, Nucleosides, and Nucleotides 1974

Abstract: Publisher Summary This chapter provides projection diagrams to illustrate the conformation of the organic molecule or ion These diagrams are produced, by means of computer graphics, directly from the unit-cell dimensions and the atomic co-ordinates of the molecules in the crystal structures For the carbohydrates, carbon and hydrogen atoms are shown as small, solid circles and other atoms by the appropriate symbol The CRYSNET software is used to rotate the molecular diagrams so as to correspond as closely as possible to the conventional, conformational diagrams used in the carbohydrate literature For the nucleosides and nucleotides, the ORTEP programs are used, with the following atom designations: carbon, open circle; hydrogen, small open circle; oxygen, solid circle; nitrogen, hatched circle; sulfur, dotted circle; and halogen, cross-hatched circle