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Carbohydrates Carbohydrates Carbohydrates Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 1 Carbohydrates Carbohydrates are ▪ A major source of energy from our diet. ▪ Composed of the elements C, H, and O. ▪ Also called saccharides, which means “sugars.” Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 2 Carbohydrates Carbohydrates ▪ Are produced by photosynthesis in plants. ▪ Such as glucose are synthesized in plants from CO2, H2O, and energy from the sun. ▪ Are oxidized in living cells (respiration) to produce CO2, H2O, and energy. Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin Cummings 3 ▪ Carbohydrates – polyhydroxyaldehydes or polyhydroxy-ketones of formula (CH2O)n, or compounds that can be hydrolyzed to them. (sugars or saccharides) ▪ Monosaccharides – carbohydrates that cannot be hydrolyzed to simpler carbohydrates; eg. Glucose or fructose. ▪ Disaccharides – carbohydrates that can be hydrolyzed into two monosaccharide units; eg. Sucrose, which is hydrolyzed into glucose and fructose. ▪ Oligosaccharides – carbohydrates that can be hydrolyzed into a few monosaccharide units. ▪ Polysaccharides – carbohydrates that are are polymeric sugars; eg Starch or cellulose. 4 ▪ Aldose – polyhydroxyaldehyde, eg glucose ▪ Ketose – polyhydroxyketone, eg fructose ▪ Triose, tetrose, pentose, hexose, etc. – carbohydrates that contain three, four, five, six, etc. carbons per molecule (usually five or six); eg. Aldohexose, ketopentose, etc. ▪ Reducing sugar – a carbohydrate that is oxidized by Tollen’s, Fehling’s or Benedict’s solution. ▪ Tollen’s: Ag+ → Ag (silver mirror) ▪ Fehling’s or Benedict’s: Cu2+ (blue) → Cu1+ (red ppt) ▪ These are reactions of aldehydes and alpha-hydroxyketones. ▪ All monosaccharides (both aldoses and ketoses) and most* disaccharides are reducing sugars. ▪ *Sucrose (table sugar), a disaccharide, is not a reducing sugar. 5 Classification ◼ Monosaccharides (monoses or glycoses) ◼ Trioses, tetroses, pentoses, hexoses ◼ Oligosaccharides ◼ Di, tri, tetra, penta, up to 9 or 10 ◼ Most important are the disaccharides ◼ Polysaccharides or glycans ◼ Homo and Heteropolysaccharides ◼ Complex carbohydrates 6 Types of Carbohydrates 7 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Monosaccharides Monosaccharides consist of ▪ 3 to 6 carbon atoms, typically. ▪ A carbonyl group (aldehyde or ketone). ▪ Several hydroxyl groups. ▪ also known as simple sugars ▪ classified by 1. the number of carbons and 2. whether aldoses or ketoses ▪ most (99%) are straight chain compounds ▪ D-glyceraldehyde is the simplest of the aldoses (aldotriose) ▪ all other sugars have the ending ose (glucose, galactose, ribose, lactose, etc…) 8 Properties ▪ Differences in structures of sugars are responsible for variations in properties ▪ Physical Crystalline form; ▪ solubility; rotatory power ▪ Chemical Reactions ▪ oxidations, reductions, condensations ▪ Physiological Nutritive value ▪ (human, bacterial); sweetness; absorption 9 Aldoses Aldoses are monosaccharides O ▪ With an aldehyde group. ║ ▪ With many hydroxyl (─OH) C─H aldose groups. │ triose (3 C atoms) H─ C─OH tetrose (4 C atoms) │ pentose (5 C atoms) H─ C─OH hexose (6 C atoms) │ CH2OH Erythose, an aldotetrose 10 Aldose sugars H H H H H C O C O C O C O C O (H C OH)n H C OH H C OH H C OH H C OH CH OH H C OH H C OH H C OH 2 CH2OH Aldose Aldotriose CH2OH H C OH H C OH n = 1 Aldotetrose CH2OH n = 2 H C OH Aldopentose CH OH n = 3 2 Aldohexose n = 4 11 Ketoses Ketoses are monosaccharides CH2OH ▪ With a ketone group. │ ▪ With many hydroxyl (─OH) C=O ketose groups. │ OH ─ C─ H triose (3 C atoms) │ tetrose (4 C atoms) H─ C─OH pentose (5 C atoms) │ hexose (6 C atoms) H─C─OH │ CH2OH Fructose, a ketohexose 12 Ketose sugars CH2OH CH2OH CH2OH CH2OH C O CH2OH C O C O C O H C OH C O (H C OH)n H C OH CH2OH H C OH H C OH CH2OH CH2OH CH2OH H OH Ketose Ketotriose Ketotetrose Ketopentose n = 0 n = 1 H C OH n = 2 CH2OH Ketohexose n = 3 13 Enantiomers and epimers H H H H C O C O C O C O HO C H HO C H H C OH OH C H HO C H HO C H H C OH OH C H H C OH HO C H CH2OH CH2OH H C OH H C OH these two aldotetroses are enantiomers. They are stereoisomers that are mirror CH2OH CH2OH images of each other these two aldohexoses are C-4 epimers. they differ only in the position of the hydroxyl group on one asymmetric carbon (carbon 4) 14 Enantiomers ▪ Pairs of stereoisomers ▪ Designated by D- or L- at the start of the name. ▪ They are mirror images ▪ that can’t be overlapped. 15 Enantiomers 16 17 Aldotetrose 18 Aldopentoses: C5, three chiral carbons, eight stereoisomers 19 Learning Check Identify each as aldo- or keto- and as tetrose, pentose, or hexose: O CH2OH C H C O H C OH HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH A B 20 Solution A. aldohexose B. ketopentose 21 Carbohydrates Structures of Monosaccharides Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 22 Structural representation of sugars ▪ Fisher projection: straight chain representation ▪ Haworth projection: simple ring in perspective ▪ Conformational representation: chair and boat configurations 23 Fischer Projections A Fischer projection ▪ Is used to represent carbohydrates. ▪ Places the most oxidized group at the top. ▪ Shows chiral carbons as the intersection of vertical and horizontal lines. Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin Cummings 24 D and L Rotations In a Fischer projection, the −OH group on the ▪ Chiral carbon farthest from the carbonyl group determines an L or D isomer. ▪ Left is assigned the letter L for the L-isomer. ▪ Right is assigned the letter D for the D-isomer. Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin Cummings 25 ▪ (+)-glucose? An aldohexose Emil Fischer (1902) Four chiral centers, 2n=24= 16 stereoisomers 26 aldohexoses CHO CHO CHO CHO CHO CHO H OH HO H HO H H OH H OH HO H H OH HO H H OH HO H HO H H OH H OH HO H H OH HO H H OH HO H H OH HO H H OH HO H H OH HO H CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CHO CHO CHO CHO CHO CHO HO H H OH H OH HO H HO H H OH HO H H OH H OH HO H H OH HO H H OH HO H HO H H OH HO H H OH H OH HO H H OH HO H H OH HO H CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CHO CHO CHO CHO H OH HO H HO H H OH HO H H OH HO H H OH HO H H OH HO H H OH H OH HO H H OH HO H CH2OH CH2OH CH2OH CH2OH 27 Optical isomerism ▪ A property exhibited by any compound whose mirror images are non-superimposable ▪ Asymmetric compounds rotate plane polarized light POLARIMETRY Measurement of optical activity in chiral or asymmetric molecules using plane polarized light Molecules may be chiral because of certain atoms or because of chiral axes or chiral planes Measurement uses an instrument called a polarimeter Rotation is either (+) dextrorotatory or (-) levorotatory 28 POLARIMETER 29 Examples of D and L Isomers of Monosaccharides O O C H C H O HO H H OH C H H OH HO H H OH H OH H OH H OH HO H H OH H OH CH OH CH2OH CH2OH 2 D-glucose D-ribose L-galactose 30 Learning Check Identify each as the D or L isomer. A. B. C. O CH2OH C H O O HO H C H HO H HO H H OH H OH HO H HO H H OH CH OH CH2OH CH2OH 2 __-ribose __- threose __- fructose 31 Solution Identify each as the D or L isomer. A. B. C. O CH2OH C H O O HO H C H HO H HO H H OH H OH HO H HO H H OH CH OH CH2OH CH2OH 2 L-ribose L-threose D-fructose 32 Epimers – stereoisomers that differ only in configuration about one chiral center. CHO CHO H OH HO H HO H HO H H OH H OH H OH H OH CH2OH CH2OH D-glucose D-mannose epimers 33 D-Glucose D-glucose is ▪ Found in fruits, corn syrup, and honey. ▪ An aldohexose with the formula C6H12O6. ▪ Known as blood sugar in the body. ▪ The monosaccharide in polymers of starch, cellulose, and Copyright © 2007 by Pearson Education, Inc glycogen. Publishing as Benjamin Cummings 34 Blood Glucose Level In the body, ▪ Glucose has a normal blood level of 70-90 mg/dL. ▪ A glucose tolerance test measures blood glucose for Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin several hours Cummings after ingesting glucose. 35 D-Fructose D-fructose ▪ Is a ketohexose CH2OH C6H12O6. C O ▪ Is the sweetest HO C H carbohydrate. H C OH ▪ Is found in fruit juices and honey. H C OH ▪ Converts to CH2OH glucose in the body. Copyright © 2007 by Pearson Education, Inc D-Fructose Publishing as Benjamin Cummings 36 D-Galactose D-galactose is O ▪ An aldohexose C6H12O6. C H ▪ Not found free in nature. H C OH ▪ Obtained from lactose, a HO C H disaccharide. HO C H ▪ A similar structure to glucose except for the H C OH –OH on C4. CH2OH D-Galactose 37 Learning Check Draw the structure and Fischer projection of D-fructose. 38 Solution CH OH CH2OH 2 │ C O C=O HO C H HO H H C OH H OH H C OH H OH CH2OH CH2OH D-Fructose Fischer projection 39 Carbohydrates Cyclic Structures of Monosaccharides CH2OH O OH OH OH OH Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin Cummings 40 Cyclic Structures Cyclic structures ▪ Are the prevalent form of monosaccharides with 5 or 6 carbon atoms.
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