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Glycosidic Bond Or O-Glycosidic Bond, at Need

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Glycosidic Bond Or O-Glycosidic Bond, at Need Seminar 5 Carbohydrates Definition Saccharides (glycids) are polyhydroxyaldehydes, polyhydroxyketones, or substances that give such compounds on hydrolysis 2 Classification Basal units Give monosaccharides when hydrolyzed MONOSACCHARIDES OLIGOSACCHARIDES POLYSACCHARIDES polyhydroxyaldehydes polyhydroxyketones 2 – 10 basal units polymeric GLYCOSES (sugars) GLYCANS water-soluble, sweet taste The historical misleading term carbohydrates. It was primarily derived from the empirical formula Cn(H2O)n and currently is taken as incorrect, not recommended in the IUPAC nomenclature (even though it can be found in numerous textbooks till now) 3 Saccharides • occur widely in the nature, present in all types of cells – the major nutrient for heterotrophs – energy stores (glycogen, starch) – components of structural materials (glycosaminoglycans) – parts of important molecules (nucleic acids, nucleotides, glycoproteins, glycolipids) – signalling function (recognition of molecules and cells, antigenic determinants) 4 Monosaccharides are simple sugars that cannot be hydrolyzed to simpler compounds Aldoses Ketoses Simple derivatives (polyhydroxyaldehydes) (polyhydroxyketones) modified monosaccharides are further classified according to the deoxysugars number of carbon atoms in their chains: amino sugars glyceraldehyde (a triose) dihydroxyacetone uronic acids tetroses tetruloses other simple derivatives pentoses pentuloses alditols hexoses hexuloses glyconic acids heptoses … heptuloses … glycaric acids Trivial names for stereoisomers glucose (i.e. D-glucose) Systematic names fructose (i.e. D-fructose) (not used in biochemistry) comprise L-idose trivial prefixes according to the configuration: e.g., for glucose D-gluco-hexose, L-xylulose, etc. for fructose D-arabino-hexulose 5 Stereoisomerism in monosaccharides Secondary alcoholic groups CH-OH in monosaccharides are stereogenic centres. Monosaccharides are chiral compounds and, therefore, most of them are optically active Stereogenic centres are mostly carbon atoms that bind four different groups; those atoms are often called "asymmetric" carbon atoms If there are more (n) stereogenic centres in the given molecule, the maximal number of stereoisomers equals 2n Each of those stereoisomers has its enantiomer (mirror image) so that there will be a maximum of 2n / 2 pairs of enantiomers Stereoisomers that differ from the particular pair of enantiomers are diastereomers of the pair In contrast to enantiomers, diastereomers differ in their properties and exhibit different values of specific optical rotation 6 Fischer projections formulas are structural formulas that describe the configuration of particular stereoisomers When a plane formula of an aldose with four stereogenic centres is drawn anywhere it is necessary to see a spatial arrangement of the atoms and assess it according to the established rules: • the least number carbon (carbonyl group in monosaccharides) is drawn upwards • the carbon chain is directed downwards then on each an hexose stereogenic centre • the bonds to neighbouring carbon atoms written above and below are projected from beneath the plane of drawing (the carbons are behind the plane) • the horizontal bonds written to the left and right are projected from above the plane of drawing, they are in front of plane 7 Without changing the configuration, Fischer formulas may only be turned 180° in the plane of the paper. Assigning configurations D- and L- (from Latin dexter and laevus) at stereogenic centres is carried out by comparison with the configurations of D- and L-glyceraldehyde Monosaccharides are classified as D- or L-sugars according to configuration at the configurational carbon atom – the chiral carbon with the highest numerical locant (i.e. the asymmetric carbon farthest from the aldehyde or ketone group): D-aldose L-ketose 8 What is that? Enantiomers, diatereomers, epimers • L-glucose is enantiomer of D-glucose because of having opposite configuration at all centres of chirality • Are there, among the following sugars, some diastereomers of D-allose that are not epimers of it? • Is there any epimer of D-mannose? D-allose D-glucose L-glucose D-mannose 9 Configurations at stereogenic centres other than configurational carbon cannot be deduced from the assignment to D- or L-sugars. Unfortunately, configurations of several most important monosaccharides have to be remembered Stereogenic centres in molecules of monosaccharides are the cause of their optical activity Solutions of mono- and oligosaccharides turn the plane of polarized light Optical activity is measured by using polarimeters and D usually expressed as specific optical rotation [α] 20. Dextrorotatory substances are marked (+), laevorotatory (–) There is no obvious relation between the assignment D- or L- and either the values or direction of optical activity 10 D- Aldoses stereochemical relations D-glyceraldehyde D-erythrose D-threose D-ribose D-arabinose D-xylose D-lyxose D-allose D-altrose D-glucose D-mannose D-gulose D-idose D-galactose D-talose 11 D- Aldoses (+) dextrorotatory optical rotation (–) laevorotatory D-(+)-glyceraldehyde D-(–)-erythrose D-(–)-threose D-(–)-ribose D-(–)- arabinose D-(+)-xylose D-(–)-lyxose D-(+)-allose D-(+)-altrose D-(+)-glucose D-(+)-mannose D-(–)-gulose D-(–)-idose D-(+)-galactose D-(+)-talose 12 D- Ketoses stereochemical relations dihydroxyacetone D-(–)-erythrulose D-(–)-ribulose D-(+)-xylulose D-(+)-psicose D-(–)-fructose D-(+)-sorbose D-(+)-tagatose 13 Cyclic forms of monosaccharides Monosaccharides (polyhydroxyaldehydes and polyhydroxy- ketones) undergo rapid and reversible intramolecular addition of some properly located alcoholic group to carbonyl group so that they form cyclic hemiacetals Monosaccharides exist mainly in cyclic hemiacetal forms, in solutions the acyclic aldehydo- or keto-forms are in minority. al-D-glucose a hemiacetal, pyranose ring 14 In this way, six- or five-membered rings can originate. In pyranoses, there is the tetrahydropyran (oxane) ring, tetrahydrofuran (oxolane) ring in furanoses. In the acyclic forms, carbon of the carbonyl group is achiral, but this carbon becomes chiral in the cyclic forms. Two configurations are possible on this new stereogenic centre called anomeric (or hemiacetal) carbon so that the cyclization results in two epimers called α or β anomers: α-anomer β-anomer 15 The hemiacetal hydroxyl group is called the anomeric hydroxyl • the configuration of - anomer is the same as the configuration at anomeric reference carbon • in monosaccharides comprising five and six carbon atoms (pentoses and hexoses, pentuloses and hexuloses), the anomeric reference carbon is the configurational carbon α-anomers in Fischer formulas of D-sugars have the anomeric hydroxyl localized on the right • the configuration of β-anomers is opposite, the anomeric hydroxyl is written on the left in Fischer formulas of D-sugars 16 In solutions, all five forms of a hexose or hexulose occur; the cyclic forms usually prevail E.g., in the aqueous solution of D-glucose equilibrated at 20 °C, there is approximately 62 % -D-glucopyranose, 36 % -D-glucopyranose, < 0.5 % -D-glucofuranose, < 0.5 % -D-glucofuranose, and < 0.003 % aldehydo-D-glucose. If D-glucose is crystallized from methanol or water, the pure α-D-glucopyranose is obtained; crystallization of D-glucose from acetic acid or pyridine gives the β-D-glucopyranose. These pure forms exhibit mutarotation, when dissolved: D α-D-Glucopyranose just after dissolution exhibits [α] 20 = + 112°, the β-form D D [α] 20 = + 19°. After certain time period, [α] 20 of both solutions will settle at the same equilibrium value of + 52°. This change can be explained by opening of the cyclic homicidal to the acyclic aldehyde. which can then recyclize to give either the α or the β form till an equilibrium is established. 17 Don't confuse: Enantiomers (optical antipodes) – stereoisomers that are not superimposable mirror images of each other, the configurations at all stereogenic centres are exactly opposite. All their chemical and physical properties are the same but the direction of optical rotation. Diastereomers – stereoisomers that are not enantiomers of one another. They have different physical properties (melting points, solubility, different specific optical rotations) so that they are viewed as different chemical substances. Epimers – are those diastereomers that differ in configuration at only one centre of chirality, they have the same configuration at all stereogenic centres except one. Anomers (α or β) represent a special kind of epimers, they have identical configuration at every stereogenic centre but they differ only in configuration at anomeric carbon atom. 18 Haworth projection formulas – the rings are projected as planes perpendicular to the plane of drawing, – carbon atoms of the rings and hydrogens attached to them are not shown, – each of the formulas can be drawn in four positions, one of which is taken as the basal position (used preferentially) α-D-glucopyranose Fischer projection Haworth projetion (the usual basal position) 19 Rules for drawing Haworth projection formulas (the basal position): – The anomeric carbon atom (C-1, in ketoses C-2) on the right; – oxygen atom in the ring is "behind", i.e. carbon atoms are numbered in the clockwise sense; Then, – hydroxyl groups and hydrogens on the right in the Fischer projection are down in the Haworth projection (below the plane of the ring), and conversely, hydroxyls on the left in Fischer formulas means up in Haworth formulas; – the terminal –CH2OH group
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