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Carbohydrates Prof. Dr. Ahmed M. El-Saghier Professor of Organic Chemistry

Carbohydrates Prof. Dr. Ahmed M. El-Saghier Professor of Organic Chemistry. Definition :. Organic compounds composed of C, H and O with H and O present in the same ratio as in water. General formula C y (H 2 O) x or C x( H 2 O) x E.g. Glucose C 6 H 12 O 6 . Exceptions:

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Carbohydrates Prof. Dr. Ahmed M. El-Saghier Professor of Organic Chemistry

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  1. CarbohydratesProf. Dr. Ahmed M. El-SaghierProfessor of Organic Chemistry

  2. Definition: • Organic compounds composed of C, H and O with H and O present in the same ratio as in water. • General formula Cy(H2O)x or Cx(H2O)x • E.g. Glucose C6H12O6. • Exceptions: • Deoxy sugars such as Rhamnose C6H12O5, digitoxoseC6H12O4 • Some non carbohydrates follow the definition: • Acetic acid C2H4O2 • Formaldehyde HCHO • Lactic acid C3H6O3 • New definition: Optically active Polyhydroxy aldehydes or ketones, or substances that hydrolyze to yield polyhydroxy aldehydes or ketones. Prof. Dr. Ahmed M. El-Saghier

  3. Carbohydrates Carbohydrates are special aldehydes and ketones that contain many OH groups. They are often referred to as sugars. • Carbohydrates are used for… • energy storage (glucose) • plant structure (cellulose) • cellular functions • building nucleic acids Prof. Dr. Ahmed M. El-Saghier

  4. Carbohydrate Synthesis Carbohydrates are made by green plants during the photosynthesis process. sunlight 6 CO2 + 6 H2O 6 O2 + C6H12O6 glucose cellulose& starch Energy From Carbohydrates Animals use carbohydrates as a source of energy. C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy glucose from plants Prof. Dr. Ahmed M. El-Saghier

  5. Prof. Dr. Ahmed M. El-Saghier

  6. Prof. Dr. Ahmed M. El-Saghier

  7. Classifications of Sugars • * According to the number of carbon atoms • The name derived from the Latin name with terminal by suffix “ose” e.g., • C3H6O3 3 carbon atoms triose • C4H8O4 4 carbon atoms tetrose • C5H10O5 5 carbon atoms pentose • C6H12O6 6 carbon atoms hexose • ** According to the type of carbonyl group in the molecule • Aldehyde group (CHO) Aldose • Keto group (C=O) Ketose Prof. Dr. Ahmed M. El-Saghier

  8. Physical Characters • Condition: Sugars are white, crystalline in shape and with sharp melting points, while polysaccharides are white amorphous solids. • Taste: Sugars have a sweet taste. Polysaccharides are tasteless. • Solubility: Monosaccharides are soluble in cold water and hot alcohol. Polysaccharides are partially soluble in hot water. Prof. Dr. Ahmed M. El-Saghier

  9. Optical activity: • A compound is optically active when, in solution, it is capable to rotate the plane of polarized light either to right (dextrorotatory, + ord) or to the left (levorotatory, - or l). • The optical activity of a compound is measured by determination of its specific optical rotation ([] Dt) using a polarimeter, and applying the following equation: []D25 =  /LC Where:  = extension of rotation, L = length of tube (light path) in decimeter, C = concentration g /ml, 25 = operating temperature (t) in 0C, D = line spectrum of sodium light (589 nm). Prof. Dr. Ahmed M. El-Saghier

  10. Simple Sugars Simple Sugars are carbohydrates that can’t be hydrolyzed (broken down) into smaller molecules. cyclic form of glucose open-chain form of glucose Prof. Dr. Ahmed M. El-Saghier

  11. Fischer Projections Fischer projections are used to represent the stereochemistry in chiral molecules. * * * * Fischer projections Prof. Dr. Ahmed M. El-Saghier

  12. Fischer Projections Fischer projections for carbohydrates are drawn so that the carbonyl group is at the top (for aldoses) or near the top (for ketoses). * * * * * * Notice that only the stereocenters in a Fischer projection appear as intersecting lines. Prof. Dr. Ahmed M. El-Saghier

  13. Sugar isomers: Configuration in Monosaccharides • Hexoses like glucose have 4 asymmetric (chairal) carbons. • Number of isomers can be calculated from the formula: Number of isomers = 2n =24= 16 Prof. Dr. Ahmed M. El-Saghier

  14. D & L Sugars • A monosaccharide in which the OH group attached to the carbon atom next to the CH2OH (farthest asymmetric carbon atom from the carbonyl group) is always to the right is designated as a “D-sugar” and that with the same OH to the left as “L -sugar”. • D and L designations are like (R) and (S) designations in that they are not necessarily related to the optical rotations of the sugars to which they are applied. Thus, one may encounter sugars that are D (+) or D (-) and others that are L (-) or L (+). Prof. Dr. Ahmed M. El-Saghier

  15. The monosaccharide (R)-(+)-glyceraldehyde is commonly referred to as D-glyceraldehyde. D-glyceraldehyde is a D sugar and its mirror image (L-glyceraldehyde) is an L sugar. Most naturally occurring monosaccharides are D sugars. Prof. Dr. Ahmed M. El-Saghier

  16. D Sugars Any sugar that has the OH group on the lowest stereocenter pointing to the right is a D sugar. D-glyceraldehyde D-glucose Prof. Dr. Ahmed M. El-Saghier

  17. L Sugars Any sugar that has the OH group on the lowest stereocenter pointing to the left is an L sugar. D-glucose L-glucose Prof. Dr. Ahmed M. El-Saghier

  18. D & L Notation Except for D-glyceraldehyde, the D & L notations do not indicate whether a sugar is dextrorotatory or levorotatory. D & L only show that the stereochemistry at the lowest stereocenter in the Fischer projection is the same as D- or L-glyceraldehyde. For example, there is D-(+)-glucose and D-(-)-fructose. Prof. Dr. Ahmed M. El-Saghier

  19. (1)= D-Glyceraldehyde , (2) = D-Erythrose , (3) = D-Threose ,(4) = D-Ribose , (5) = D-Arabinose(6) = D-Xylose , (7) = D-Lyxose , (8) = Allose , (9) = Altrose , (10) = D-Glucose , (11) = D-Mannose(12) = D-Idose , (13) = D-Gulose , (14) = D-Galactose , (15) = D-Talose Prof. Dr. Ahmed M. El-Saghier

  20. = Dihydroxyacetone , (2) = D-Erythulose , (3) = D-Ribose , (4) = Xylose , (5) = D-Psicose , (6) = D-Fructose , (7) = D-Sorbose , (8) = D-Tagastose , (9) = D-Alloheptulose , (10) =D-Allroheptulose , (11) = D-Glucoheptulose , (12) = DMannoheptulose , (13) = D-Guloheptulose , (14) = D-Iodoheptulose , (15) = D-Galactoheptulose , Prof. Dr. Ahmed M. El-Saghier

  21. Hemiacetals A hemiacetal is formed by the nucleophilic addition of an aldehyde with an alcohol. acid catalyst alcohol aldehyde hemiacetal Intermolecular nucleophilic addition leads to the formation of an unstablehemiacetal. Prof. Dr. Ahmed M. El-Saghier

  22. Hemiacetals A stablehemiacetal is formed if intramolecularnucleophilic addition takes place. open-chain molecule containing an aldehyde & alcohol cyclic hemiacetal Most carbohydrates exist as stablecyclichemiacetals rather than open-chain molecules. Prof. Dr. Ahmed M. El-Saghier

  23. Definitions AFuranoseis a five-member cyclic hemiacetal form of a monosaccharide. APyranoseis a six-member cyclic hemiacetal form of a monosaccharide. The terms furanoseand pyranoseare used because of the resemblance tofuranandpyran. furan pyran Prof. Dr. Ahmed M. El-Saghier

  24. Review We’ve see that a monosaccharide can be drawn as an open-chain molecule or a cyclic structure that shows the chair conformation of the ring. D-glucose as a chair conformation D-glucose as an open-chain Prof. Dr. Ahmed M. El-Saghier

  25. Haworth Projections A Haworth projection is another way to represent the cyclichemiacetal form of a monosaccharide. Haworth projection of D-glucose A Haworth projection shows the cyclic hemiacetal as a flat ring, viewed edge-on, with the oxygen at the upperright corner, usually with only ring OH’s. Prof. Dr. Ahmed M. El-Saghier

  26. Haworth and Conformational Formulas عند رسم الحلقات المسطحة، في صيغ هاروث، يُفترض أن مستواها واقعاً عمودياً على مستوى الورقة، ويبين ذلك، برسم الحافة القريبة من المشاهد لهذه الحلقات بخطوط غليظة . Furanose Pyranose وإصطلاحاً، أتُفق على أن ترسم صيغة هاورث، بحيث يكون أكسجين الحلقة إلى الأعلى و في الجانب البعيد من الحلقة، والكربون الأنوميري في جهة اليمين. أما مجموعة –CH2OH الطرفية، فتوضع فوق مستوى الحلقة في السلسلة – D، وتحت مستوى الحلقة في السلسلة – L، وكما هو الحال، في مساقط فيشر فإن ذرات الهيدروجين حول المراكز الكيرالية لا تبين عادة . Anomer carbon Anomer carbon Anomer carbon Sugar– D Sugar–L 26 Prof. Dr. Ahmed M. El-Saghier

  27. Drawing Furanoses Draw the two furanose forms of D-ribose as Haworth projections. rotate tip & coil D-ribose Prof. Dr. Ahmed M. El-Saghier

  28. Drawing Furanoses close ring close ring Prof. Dr. Ahmed M. El-Saghier

  29. anomeric center Anomeric Center The Anomeric Center is the hemiacetalcarbon of the cyclic form of a monosaccharide. It’s the carbon in the ring that’s attached to twooxygens. * The anomeric center is a stereocenter. Prof. Dr. Ahmed M. El-Saghier

  30. cis trans Anomers Anomersare monosaccharides that differ in configuration only at their anomeric centers. Anomers are cis-transdiastereomers. Prof. Dr. Ahmed M. El-Saghier

  31. Alpha & Beta Anomers The cisdiastereomer is referred to as the beta (b) anomer and the transdiastereomer is referred to as the alpha (a) anomer. cis trans beta anomer alpha anomer (OHup) (OHdown) b-D-ribofuranose a-D-ribofuranose Prof. Dr. Ahmed M. El-Saghier

  32. - and - anomers of glucose: • When sugars under go cyclization C-1 became a new chairal carbon and two isomers exist. They are called “ Anomers”. • In the -anomer the OH group is directed downside and in the -anomer is directed to the upper side. • These two forms have different specific rotation, in solution an equilibrium exsit between the two forms (mutarotation phenomenon). ويلاحظ كذلك، أن أي مجموعة تقع في جهة اليمين في إسقاط فيشر، تقع أسفل الحلقة في صيغ هاروث، وأي مجموعة واقعة في جهة اليسار في إسقاط فيشر فإنها تقع أعلى مستوى الحلقة في صيغ هاورث . 32 Prof. Dr. Ahmed M. El-Saghier

  33. a–D-Glucose Methy-a-Glucose D-Glucose Open Chain b-D-Glucose Methl-b-Glucose Prof. Dr. Ahmed M. El-Saghier

  34. Mutarotation When a sugar is dissolved in water, the specific rotation of the solution gradually changesuntil it reaches to a constant value due to equilibrium between the aand bforms. e.g. freshly prepared solution of b-glucose has a specific rotation +112o. When this solution is allowed to stand the rotation falls till reach + 52.7o. The equilibrium reached is: 36% a–D- glucose [a]D = + 18.7 0 64% b–D- glucose [a]D = + 112 0 The sum is + 52.7o crystalization from H2O at 25 oC  -D-Glucose Specific rotation +112 D(+)Glucose crystalization from H2O at 98 oC  -D-Glucose Specific Rotation +18.7 Prof. Dr. Ahmed M. El-Saghier

  35. Mutarotation Mutarotation is the conversion of the a and banomers into an equilibrium mixture of the two. D-ribose b-D-ribofuranose a-D-ribofuranose Prof. Dr. Ahmed M. El-Saghier

  36. Drawing Pyranoses Draw the two pyranose forms of D-ribose as Haworth projections. close ring tip & coil D-ribose Prof. Dr. Ahmed M. El-Saghier

  37. Drawing Pyranoses close ring close ring b-D-ribopyranose a-D-ribopyranose Prof. Dr. Ahmed M. El-Saghier

  38. Cyclization Cyclization hydrolysis hydrolysis  -Glucose  -Glucose D-Glucose %0.02 Prof. Dr. Ahmed M. El-Saghier

  39. وحلقة البيرانوز، وهي حلقة سداسية، مثل حلقة السايلكوهكسان، هي في الواقع غير مسطحة، وهي تتواجد أساساً في هيئة الكرسي ( Chair conformation) . Pyranose Ring Cyclohexane Ring Chair Conformation Chair Conformaion a-D-glucose Chair conformation fisher projection b-D-glucose Prof. Dr. Ahmed M. El-Saghier

  40. Identify the Sugar open ring a-D-xylofuranose rotate tip & uncoil D-xylose Prof. Dr. Ahmed M. El-Saghier

  41. Identify the Sugar open ring b-D-galactopyranose rotate tip & uncoil D-galactose Prof. Dr. Ahmed M. El-Saghier

  42. anomeric hydroxyl group Glycosides A Glycoside is a carbohydrate derivative that is formed by replacing the anomerichydroxyl group with a different substituent. methyl b-D-glucopyranoside b-D-glucopyranose hemiacetal acetal Prof. Dr. Ahmed M. El-Saghier

  43. Glycosides What glycoside is formed upon the addition of ethanol to b-D-ribofuranose? ethyl b-D-ribofuranoside b-D-ribofuranose acetal hemiacetal Prof. Dr. Ahmed M. El-Saghier

  44. Glycosides & Mutarotation Glycosides are not in equilibrium with an open-chain form of the molecule containing an aldehyde. X In general, glycosides do not undergo mutarotation because the ring does not open. Prof. Dr. Ahmed M. El-Saghier

  45. أرسم الهيئة الأكثر ثباتاً لكل من (أ) -D-جلاكتوبايرانوز , (ب) -D-أيودوبايرانوز Draw the more stable conformational structure for: a) b-D-Galactopyranose, b) a-D-Iodopyranose في كل الحالتين , فإن الهيئة الأكثر ثباتاً هي الذي يكون فيها أكبر عدد من المجموعات في أوضاع إستوائية . The more stable is the compound have the more equatorial groups • مثال :أكتب معادلات (بإستخدام صيغ هاورث) التي توضح ما يلي : write the equation (use Hawarth Projection) which showing the fllwing • الدوران التلقائي ﻟ-D-أرابينوفيورانوز النقي في الماء . A) specific rotation for purified b-D-arabinofuranose in water Prof. Dr. Ahmed M. El-Saghier

  46. تحول -D-فركتوفيورانوز إلى -D-مركتوبايرانوز . B) conversion of b-D-fractofuranose to b-D-fractopyranose (ﺟ) الدوران التلقائي ﻟ-مالتوز (سكر ثنائي , أنظر السكريات الثنائية) C) specific rotation of b-maltose Prof. Dr. Ahmed M. El-Saghier

  47. Galactose and Mannose are different from each other in the stereochemistry of carbons 2 and 4. They are described as “Diastereoisomers””Diastereomers”. Prof. Dr. Ahmed M. El-Saghier

  48. Glucose and Fructose have the same molecular formula C6H12O6. They have different structures with different function groups. They are described as “Structural Isomers”. Prof. Dr. Ahmed M. El-Saghier

  49. Chemical Reaction Related to Colour Tests for Carbohydrates • Effect of conc. acids: Treatment with conc. mineral acid (HCl or H2SO4) leads to dehydration of sugars and formation of the corresponding furfural. Prof. Dr. Ahmed M. El-Saghier

  50. Reducing Sugars A reducing sugar is a sugar with an aldehyde group that reduces a metallic oxidizing agent. A reducing sugar is oxidized to the corresponding carboxylicacid. • Typical oxidizing agents are… • Tollens’ reagent (Ag+ in NH3(aq)) • Fehling’s reagent (Cu2+ with Na tartrate) • Benedict’s reagent (Cu2+ with Na citrate) Prof. Dr. Ahmed M. El-Saghier

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