Essential of Carbohydrate Chemistry

Essential of Carbohydrate Chemistry Wood Chemistry Carbohydrates • Photosynthesis is probably the most fundamental of all life processes and provides a means of converting “inorganic carbon”, in the form of carbon dioxide, into carbohydrates and then into other organic compounds. • The early photosynthetic bacteria first appeared about 3,000 million years ago, and were joined much later by blue-green algae (2,000 million years ago) and the first vascular land plants (400 million years ago). Carbohydrates • Together these organisms now produce approximately 14 × 1010 tones of organic matter every year according to the process shown in hυ CO ＋ H O Carbohydrates ＋ O 2 2 Photosynthetic 2 organism Carbohydrates • Definitions – The name carbohydrate was originally derived from the general formula Cx(H2O)y formally to hydrates of carbon, but this type of sample definition does not cover the broad class of carbohydrates. • The sugars in a plant usually function as a source of energy while polysaccharides, such as starch, fulfill the need for the storage of reserve food or they contribute mechanical strength to the plant cell. Carbohydrates A variety of carbohydrates are included as essential building elements in natural compounds performing vital functions in living organisms. Carbohydrates may be classiﬁed into following three large groups Monosaccharide Oligosaccharides Polysaccharides • Monosaccharide is the simple sugars. • Oligosaccharides – Oligosaccharide consist of several monosaccharide residues joined together by glycosidic linkages, namely di-, tri-, tetra-saccharide…. The name oligosaccharide is usually restricted to the group of carbohydrates in which the number of monosaccharide units less than 10. • Polysaccharides – Polysaccharides are complex molecules composed of a large number of monosaccharide unit joined together by glycosidic linkages. Some Terms Used in Carbohydrate Chemistry • Aglycone – The nonsugar part of a glycoside; usually an alcohol, phenol or amine. • Aldoses – Monosaccharide contain aldehyde function. • Aldopentose – A five-carbon sugar with an aldehyde or cyclic hemiacetal functional group. • Aldohexose – A six-carbon sugar with an aldehyde group or cyclic hemiacetal group. Some Terms Used in Carbohydrate Chemistry • Ketoses – Monosaccharide contain keto function. • Furanose – Five-membered cyclic sugar from furan. • Pyranose – Six-membered cyclic sugar from pyran. • Septanose Glycosans – Seven-membered cyclic sugar. Anhydro sugars are from sugars by the elimination of water from a pair of • Glycosans hydroxyl groups. Glycosan are strictly – Intramolecular glycoside intramolecular gycosides. Some Terms Used in Carbohydrate Chemistry α- or β-Glucoside Acetal derived from the cyclic hemiacetal from α- or β-glucoside glucose. α- or β-Glycoside General term for the acetal derivative of any suger. Anomers Sugar isomers differing only in the configuration of the hemiacetal carbon atom; they are designated asαor β. anomers，異位異構物，異位碳原子上立體 Epimers 構造的差異所形成的 α 和 β型異構物 Sugars that differ only in the configuration at 1 carbon . 表異構物 (epimers) Mutarotation Change in optical rotation as a fresh solution stands. Configuration of Monosaccharide Isomers Compounds which have the same molecular formula but differ in some way in the arrangement of atoms. Type of Isomers Structural isomers Stereoisomer Structural isomers Isomers in which the bonding arrangement of atoms differ; they include chain isomers, position isomers, and functional group isomers Chain isomers (Sketal isomers) Butane Isobutane Position isomers 1-Propanol 2-Propanol Functional group isomers Propionic acid Methyl acetate Stereoisomer Isomers in which the bounding of the atoms in the same but the spatial arrangements of the atoms differ; they include geometric isomers and optical isomers, etc. Geometric isomers cis-2-butene trans-s-butane Optical isomers Enantiomers vs. Diastereoisomers Diastereomers If a compound contains two chiral atoms, it may exist in four stereoisomeric forms. Since the configuration at each chiral carbon may be either R or S, there are four stereochemical possibilities: RR, SS, RS, and SR. The RR and SS stereoisomers are enantiomers. The RS and SR stereoisomers are also enantiomers. The RR stereoisomer is a diastereomer of both the RS and the SR stereoisomers. The SS stereoisomer is a diastereomer of both the RS and SR stereoisomers. Configuration of Aldoses For many system which “n” chiral carbon atoms, there are 2n stereoisomers, composed 2n-1 enatiomeric (mirror-image) pairs. An aldotetrose is a four-carbon sugar that has two chiral centers. There are 22 = 4 possible stereoisomers, or two D, L pairs of enatiomers called Erythrose and Threose. D, L System of Carbohydrate Configuration Nomenclature Compounds are assigned to the D-family or the L-family according to the projection of the –OH group at the lowest chiral carbon atom. If the –OH group projects to the right in a plane projection structure, the compound is in the D-family. If this –OH projects to the left, the substances is in the L- family. D-Glyceraldehyde L-Glyceraldehyde D-Erythrose D-Threose Fischer Projection Formulas for Acyclic Forms of D-aldoses Mutarotation On dissolution of sugars in water, the optical rotation of the solution changes continuously until an equilibrium is reached. This phenomenon, termed Mutarotation. Conformation of the Six-membered Ring Sysem Chair Boat Skew boat Half-chair 4C 1C 1 4 Monosubstituted molecules the substituent favors the equatorial position Stable chair conformation of β-D-glucopyranose and β-D-fructopyranose Strong hydrogen- bonding interactions More stable Conformation of the Five-membered Ring System Envelope conformation Twist conformation Pseudoequatorial carbon-hydrogen bonds Monosaccharide Most of the monosaccharides occur as glycosides and as units in oligosaccharide and polysaccharide and only comparatively few of them are present free in plants. D-Glucose is the most abundant monosaccharide in nature. It occurs in a free state in many plants, especially in fruits and can be prepared from cellulose and starch by acidic or enzymic hydrolysis. D-Glucose α-D-Glucose Haworth perspective formulas Fischer-Tollens projection α: -CH2OH and -OH at the same side Monosaccharide galactose: 半乳糖 D-Mannose and D-galactose, which are aldohexoses, are important components in D-mannose D-galactose hemicellulose. The most comment aldopentose, abundant members of the hemicellulose, are D-xylose and L-arabinose. D-xylose L-arabinose D-Ribose is a constituent of nucleosides. D-ribose Monosaccharide No tetroses or triose have been detected free in plants, but D-erythrose D-erythrose 4-phosphate 4-phosphate is an important intermediate in many transformation, and D-glyceraldehyde and D-glyceraldehyde dihydroxyacetone are essential components in cellular metabolism. dihydroxyacetone Monosaccharide Deoxysugar L-rhmnose (6-deoxy-L-mannose) occurs as a constituent in gum polysaccharides and traces of it are present in hemicellulose L-rhmnose (xylan) D-Fructose, which represents the only abundant ketose in plants, is present both free and in a combined state. Compositae and Gramineae families store polymers of D-fructose such as insulin, as serve material rather than starch. Monosaccharide Derivatives In principal, the sugar derivation are formed by: Reaction of the free carbonyl or the anomeric hydroxyl at C-1. Reaction of the hydroxyl groups at other positions. Glycosides Sugars react as hemiacetals with hydroxyl compounds, such as alcohols and phenols forming glycosides. The glycosides are easily hydrolyzed by aqueous acids to free sugars but they are fairly stable toward alkali. Acetals aldehyde Cyclic acetal Acetal formation Ethers Etherification is often used in the determination of structure and types of linkages between sugars in oligo and polysaccharides. Ethers are very stable against both acids and bases. Carbohydrates can be converted into ethers by treatment with an alkyl halide in the presence of base (Williamson ether synthesis) Ag2O CH3I Cellulose Etherification Cellulose etherification: by treating alkali cellulose with Alkyl or aryl halides (or sulfate) Alkene oxides Unsaturated compounds activated by electron-attracting group - - Cell-OH + OH Cell-O + H2O - - Cell-O + R-Cl Cell-OR + Cl Anhydro sugars • Anhydro sugars are formed from sugars by the elimination of water from a pair of hydroxyl groups • Glucosans are strictly intramolecular glycosides. Its anhydro linkage is readily by action of acids, sometimes also by bases. Anhydro sugars Epoxides Ethers are derived only from alcoholic hydroxyls and the hydroxyl group in the anomeric center does not participate. Epoxides are formed when the sugar molecule contains both a good leaving group and a suitable located ionized hydroxyl group. (SN2 reaction). Anhydro sugars The ring size of epoxides can vary from three- to six- membered rings. Oxiranes (three membered derivatives) intermediates are probably formed during alkaline hydrolysis of polysaccharides such as cellulose and starch. Formation of methyl 3,4-anhydro-β-D-galactopyranosides Esters Hydroxyl groups of sugars can form esters both with organic and inorganic acids. The phosphate esters, such as D-glucose 6- phosphate, are important natural products and key intermediates in the biosynthesis and bioconversion of various carbohydrates. Esters 1: D-glucose 1-phosphate 2: 2-O-acetyl-β-D-xylopyranosides 3: 3-O-acetyl-β-D-xylopyranosides 4: β-D-galactopyranose 4-sulfate 5:

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