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Carbohydrates Monosaccharides: 1 sugar unit Disaccharides: 2 sugar units Oligosaccharides: 3 – 10 sugar units Polyscaccharides: More than 10 sugar units * The sugar units are connected by a bridging oxygen atom known as the glycosidic link Monosaccharide Nomenclature Classification based on location of the double bond C=O The family-name ending -ose indicates a carbohydrate Monosaccharide Classification using # of C atoms in the chain Can be either a ketose or an aldose sugar Enantiomers - Non-superimposable mirror images of a Chiral molecule * A chiral carbon is attached to 4 different groups adapted from ull.chemistry.uakron.edu The D and L classification is from optical activity Physical Properties Optical Activity Ability to rotate planar polarized light Dextrorotatory (D- isomer) -Also written as (+) isomer -Rotates light to the right Levorotatory (L-isomer) -Also known as (-) isomer -Rotaes light to left - Usually white in color. Tastes sweet. Soluble in water (why?) Note- Each pair of enantiomers have a different name Using Fischer projections In a carbohydrate Fischer projection the Aldehyde CHO or the Ketone C=O Is always placed on top. The dotted lines are connected to atoms that are Projected above the plane, while the solid wedges are atoms that are behind the plane The result is that -H and -OH groups projecting above the page are on the left and right of the chiral carbons, and groups projecting behind the page are above and below the chiral carbons Some Important Monosaccharides • D- Glucose- Most common. Also known as blood sugar or grape sugar. Main energy source and the brain requires a constant concentration in blood. • D-Fructose- Usually found in fruits and sweeter than table sugar • D-Ribose- Used in the biosynthesis of RNA. Cyclic Monosaccharides Monosaccharides have a aldehyde/ketone and an alcohol in the same molecule In the same molecule, so it forms an intramolecular hemiacetal linkage Definitions Anomers: Cyclic sugars that differ only in positions of substituents at the hemiacetal carbon (the anomeric carbon) the α form has the -OH on the opposite side from the -CH2OH (below the plane of the ring) the β form has the -OH on the same side as the -CH2OH (above the plane of the ring) Q: Is the hemiacetal carbon a ‘chiral carbon’ ? Haworth Projections The monosaccharides are shown as cyclic and planar rings in a Haworth projection unlike Fischer projections (the six-membered ring are generally in a chair conformation) Diagram adapted from www.rpi.edu/molbiochem D and L forms How to distinguish between D and L forms of Glucose ? Diagram adapted from www.rpi.edu/molbiochem Monosaccharide: Fructose A ketohexose, can also form a 5-membered ring in solution Monosaccharide: Ribose A 5 carbon aldehyde sugar, i.e aldopentose Both molecules are part of larger biomolecules, such as, coenzymes. Ribose is part of RNA and Deoxyribose is a part of DNA (Deoxyribo-) Reactions of Monosaccharide We will look at three types: • Reducing sugar reactions (Lab)!!! • Phosphate ester formation • Glycoside bond formation Reducing sugar reactions Aldehyde sugars get oxidized to carboxylic acid with the help of an oxidizing agent.* Note: we refer to the open-chain form of the aldose. These sugars are known as reducing sugars The hemiacetal group must be present in a reducing sugar Phosphate esters The –OH groups of sugars can add phosphate groups to form phosphate esters. ** Will see these esters a lot when we look at carbohydrate metabolism ! Glycoside bond formation This reaction can also happen between two monosaccharide sugars methyl glucoside (methyl-glucopyranose). Disaccharides 1,4-Glycosidic Links Polysaccharides 1,4-glycoside linkage between glucose molecules Q. Which end is the reducing sugar ? Q. Is it a a galactosidic linkage or a β linkage ? Branched Polysaccharides Distinguish between 1,6 and 1,4 glycosidic links Glucosamines An Amino group replaces the Hydroxyl (-OH) group. The amine is ‘acetylated’ on the right In Summary Monosaccharides are simple sugars -An ‘aldose’ is an aldehyde, a ‘ketose’ is a ketone -Triose: 3 carbons ; Tetrose: 4 carbons etc… -Aldohexose: an aldehyde sugar with 6 carbons When writing D sugars: The lowest –OH group goes to the right, L-sugar the lowest –OH is on left Cyclic sugars: Cyclization by internal hemiacetal formation. Forms between C1 and C5. Haworth projections: Showing the ring sugars in a flat structure. Anomers: α and β anomers from Haworth (ring) structures. In the α form the –OH on C1 is on the opposite of –CH2OH (C6). In the β form they are on the same side of the ring. A reducing sugar: A carbohydrate that gets oxidized itself to carboxylic acid. It reduces the oxidizing agent used. (Generally aldoses are reducing sugars). Mutarotation: change in the specific rotation of a cyclic monosaccharide as it reaches an equilibrium between its α and β anomeric forms. Practice: -Distinguishing acetals and hemiacetals in sugars -Identifying 1,4-, 1-2 Glycosidic linkages -Writing hydrolysis reactions of disaccharides - Writing Structures of phosphate esters - Oxidation/reduction reactions of sugars The monosacchararide shown below is classified as a(an) 1. 2. 3. 4. aldotriose aldopentose ketotriose ketopentose What is the maximum number of aldotetrose stereoisomers? 1. 2. 3. 4. The cyclic structure below is 1. 2. 3. 4. α-D-galactose α-L-galactose β-D-galactose β -L-galactose One Two Three Four The cyclic structure below is 1. 2. 3. 4. α-D-galactose α-L-galactose β-D-galactose β -L-galactose The disaccharide maltose is shown below. Classify the connection between the two α-D-glucose units as an α1,4 link or a β-1,4 link, and indicate whether maltose is a reducing or a nonreducing sugar. 1. 2. 3. 4. α-1,4 link, nonreducing α-1,4 link, reducing β -1,4 link, nonreducing β -1,4 link, reducing In N-linked glycoproteins, the sugar is usually attached to the protein by a bond to the N-atom in a side-chain amide. Which amino acid below can form such a bond? 1. 2. 3. 4. Cysteine Glutamine Threonine Tyrosine The polysaccharide that provides structure in plants is 1. Amylopectin 2. Amylose 3. Cellulose 4. Glycogen