Oligosaccharides Lecture 10 PDF

◼Phytochemistry I (PG 504) ◼CARBOHYDRATES Lecture 10 Prof. Dr. Magda Tohamy Professor and Head of Pharmacognosy Department 1  Oligosaccharides 1) Like monosaccharides, oligosaccharides are: a) cryst...

◼Phytochemistry I (PG 504) ◼CARBOHYDRATES Lecture 10 Prof. Dr. Magda Tohamy Professor and Head of Pharmacognosy Department 1  Oligosaccharides 1) Like monosaccharides, oligosaccharides are: a) crystalline. b) soluble in water. c) sweet taste. 2) They are hydrolysized by acids or specific enzymes to yield 2 – 10 molecules of monosaccharides. 3) The monosaccharide units are linked through glycosidic linkages. 4) The type of glycosidic bond is based on the orientation of the OH group at anomeric position: α -glycosidic bond β -glycosidic bond Classification of Oligosaccharides: I. According to the number of monosaccharide molecules: Disaccharides Trisaccharides Tetrasaccharides II. According to the presence or absence of a free functional group: Reducing Non-reducing 1) Reducing Disaccharides: They are classified according to the position of the linkage into: C1-C3 C1-C4 C1-C6 Dihexoses Dihexoses Dihexoses e.g. turanose Most common ! e.g. maltose, e.g. gentiobiose lactose & & melibiose cellobiose A) C-3 Dihexoses: 1) Turanose: * Obtained from the trisaccharide melezitose by careful acid hydrolysis. CH2OH H O H H OH H 1 C1 – C3 α-D-glucopyranose HO H HO linkage HOH2C O O OH 2 H H 3 CH OH β-D-fructofuranose 1 2 OH H 3-O-(α-D-glucopyranosyl)-β- D-fructofuranose B) C-6 Dihexoses: Item Gentiobiose Melibiose Source From trisaccharide From gentianose & occur trisaccharide in glycoside raffinose amygdalin Mono- saccharide Two β-D-glucose α-D-Galactose units units + α-D-Glucose  6 CH2OH O CH2 H O H O OH H H OH H 1 OH H HO H HO H CH2OH H OH H OH HO H O H OH H 1 6 Gentiobiose H O CH2 H OH H O H 6-O-(β-D-glucopyranosyl)- H OH H D-glucopyranose HO OH H OH Melibiose 6-O-(α-D-galactopyranosyl)-D- glucopyranose C) C-4 Dihexoses: 1) Maltose: * It is known as malt sugar. * Not common in nature except in malt and germinating cereals. * A hydrolytic product of polysaccharides e.g. starch & dextrin. Structure: 4-O-α-(D-glucopyranosyl)-D-glucopyranose CH2OH CH2OH H O H H O OH H 4 H 1 OH H O OH H HO H H OH H OH α-D-glucopyranose + α or β-D-glucopyranose * Two glucose units, linked by α ( 1 → 4 ) linkage. * Occurs in two anomeric forms α- and β-maltose → mutarotation  Properties: 1) Reduces Fehling’s solution but not Barfoed’s solution. 2) Forms characteristic osazone → rosettes of plates. 3) Hydrolyzed by maltase enzyme (α-glucosidase). 4) Maltose is prepared from starch by: Partial acid hydrolysis β-amylase enzyme 2) Lactose: * It is the milk sugar. * The main sugar of mammalian milk (5 %). * Not present in higher plants. Preparation: Whey concentration Lactose crystals Structure: 4-O-β-(D-galactopyranosyl)-D-glucopyranose CH2OH CH2OH HO H O H O H 1 O 4 H OH H OH H H H OH H OH H OH β-D-galactopyranose + -D-glucopyranose β ( 1 → 4 ) linkage  Properties: 1) Reduces Fehling’s solution but not Barfoed’s soln. 2) Forms characteristic osazone (needles aggregated in clusters). 3) Hydrolyzed by β-glucosidases e.g. emulsin Uses: 1) Nutrient in infant food. 2) An inert diluent for other drugs. 2) Non-reducing Disaccharides: * The two important non-reducing dihexoses are: Sucrose Trehalose in plants capable of in yeast and fungi photosynthesis * They do not contain free hemiacetal or hemiketal group and thus: 1) do not reduce Fehling’s solution. 2) do not form osazone. 3) do not undergo mutarotation in solution. 1) Sucrose: Saccharose, cane sugar or beet sugar CH2OH * Most common H O H H α-D-gluco- disaccharide. OH H 1 HO pyranose H HO O * Sugar cane and beet, are up to 20 % by mass HOH2C O β-D-fructo- sucrose. H HO 2 furanose H CH2OH OH H * Found in many fruit juices, seeds, leaves, α ( 1 → 2 ) linkage and honey. α-D-glucopyranosyl-β-D- fructofuranose Properties: 1) Readily soluble in water. 2) Crystallizes readily from water & crystals exist in anhydrous form. 3) Sweeteness = 1.00 > Glucose & < Fructose 4) It does not: a) reduce Fehling’s. b) form Osazone. c) undergo mutarotation. 5) Sucrose Heating from 200 – 250 ºC amorphous brown substance (caramel) used as Flavouring and colouring agent 6) Cobalt nitrate test: + ve Sucrose solution + 2 dps. Co nitrate + excess NaOH → Violet colour 7) Seliwanoff’s test (rapid furfural): + ve  Preparation of Sucrose: from sugar-cane from sugar-beets (Saccharum officinarum) (Beta vulgaris) Hydrolysis of Sucrose: Methods of hydrolysis: by dil. acids by enzymes Invertase α-Glucosidase Hydrolysis of sucrose (both methods) in called: Inversion Why ?  Sucrose [α]25D = + 66.5 º After hydrolysis → mixture of D-(+)-Glucose D-(-)-Fructose [α]25D = + 52.7 º [α]25D = - 92.4 º Final [α]25D = - 20.4 º Since the sign of rotation: (+) of sucrose → (-) hydrolyzed solution  Inversion & the mixture formed in known as: Invert sugar Uses of Sucrose: 1) In syrup preparation & tablet manufacture. 2) As nutrient & demulcent. 3) In high concentration: bacteriostatic & preservative e.g. jams. 4) In preparation of dextran (polysaccharide used as plasma substitute). 2) Trehalose: * Found in yeast and fungi as reserve food material. * Hydrolyzed by HCl or trehalase into two molecules of D-Glucose. * Linkage is (C1 – C1).  II. Trisaccharides Reducing Non-reducing e.g. Raffinose &Gentianose Non-reducing trihexoses 1) Raffinose: melitriose or gossypose * Found in sugar beet, cottonseed and soyabean. α-D-galactopyranose Emulsin Mixture of (1 – 6) linkage Melibiose α-galactosidase & β- glucosidase α-D-glucopyranose Sucrose Invertase (1 – 2) linkage enzyme β-D-fructofuranose A β-fructofurano- sidase Different conditions of hydrolysis → different products: 1) Invertase enzyme → Melibiose & D-fructose 2) Emulsin enzyme → Sucrose & D-Galactose 3) Hydrolysis by bottom yeast (both enzymes) or acid hydrolysis → D-galactose & D-glucose & D-fructose  CH2OH HO H O H 1 OH H 6 H O CH2 H OH H O H H 1 OH H HO H HO O Raffinose HOH2C O H HO 2 H CH2OH OH H α-D-galactopyranosyl (1 – 6)-O- α-D-glucopyranosyl-(1 – 2)-β-D- fructofuranose  2) Gentianose: found in Gentian roots. β-D-glucopyranose Emulsin Mixture of (1 – 6) linkage Gentiobiose α-galactosidase & β- glucosidase α-D-glucopyranose Invertase Sucrose (1 – 2) linkage enzyme β-D-fructofuranose Different conditions of hydrolysis → different products: 1) Invertase enzyme → Gentiobiose & D-fructose 2) Emulsin enzyme → Sucrose & D-Glucose 3) Complete acid hydrolysis: → Two molecules of D-glucose & D-fructose III. Tetrasaccharides * Stachyose is the only known tetrasaccharide. * It is a non-reducing tetrahexose. * It is isolated from the tubers of Stachys tuberifera (Japanese artichoke) & found in soyabean. * Complete acid hydrolysis 2 molecules of D-galactose + D-glucose + D-fructose  THANK YOU For any questions feel free to contact me by mail [email protected] Dr. Magda Tohamy Ibrahim Professor of Pharmacognosy 30