Chemistry of Carbohydrates

1. Chemistry of Carbohydrates

2. Chemistry of Carbohydrates • Definition of carbohydrates: • First Definition (Old definition): • Carbohydrates are substances containing carbon, hydrogen and oxygen having the general formula CnH2nOn. • (C3 H6 O3) • Hydrogen and oxygen are present in 1:2 ratio the same ratio as water, so the French called them “Hydrates de Carbon”, i.e., carbo-hydrates Cn(H2O)n.

3. The old definition is inaccurate because: • There are substances which are not carbohydrates but have the formula CnH2nOn , e.g., acetic acid CH3COOH (C2H4O2) and lactic acid. • There are some carbohydrates, which do not have this general formula, e.g., amino sugars and deoxy sugars

4. Second Definition (new definition): Carbohydrates are aldehyde (CHO) or ketone (C=O) derivatives of polyhydric alcohols (have more than one OH group) or compounds which yield these derivatives on hydrolysis.

5. Importance of carbohydrates: • The chief source of energy. • Important structural components in animal and plant cells. • Important part of nucleic acids and free nucleotides and coenzymes. • Major antigens are carbohydrates in nature, e.g., blood group substance. • Biological role as a part of hormones and their receptors and enzymes.

6. Classification of carbohydrates • According to the number of sugar units in the molecule there are three type: • Monosaccharides (simple sugars): They contain one sugar unit, i.e., and the simplest form of sugars and cannot be further hydrolyzed. They represent the end products of carbohydrate digestion in the human body. • Oligosaccharides: They contain 2 – 10 monosaccharide units per molecule and give monosaccharides on acid hydrolysis. • Polysaccharides: They contain more than 10 monosaccharide units per molecule and give monosaccharides on acid hydrolysis.

7. Monosaccharides • They are classified according to the number of carbon atoms into five important groups. • Each of these groups is subdivided according to the type of functional chemical group into: Aldoses (sugars containing aldehyde group) and Ketoses (sugars containing ketone group).

8. 1. Bioses: are monosaccharides containing 2 carbon atoms, Glycolaldehyde

9. 2. Trioses: are monosaccharides containing 3 carbon atoms, e.g., • A- Aldotriose: Example is

10. B- Ketotriose:e.g., • Dihydroxyacetone, the hydroxylated form of Acetone

11. 3. Tetroses:are monosaccharides containing 4 carbon atoms: • Aldotetroses:

12. B- Ketotetroses:

13. 4. Pentoses: they contain 5 carbon atoms. Ribose is the most important pentose because it enters in the structure of DNA and RNA and important free nucleotides such as ATP and coenzymes

14. A- Aldopentoses:

15. B- Ketopentose:

16. 5.Hexoses: are monosaccharides containing 6 carbon atoms. • They are the most important monosaccharides particularly glucose:

17. A- Adohexoses:

18. B- Ketohexoses:

19. Asymmetric carbon atom - Asymmetric carbon atom is the carbon atom attached to which 4 different groups or atoms.

20. Ordinary light vibrates in all directions. • Ordinary light can be changed to plane polarized light by passing it through a prism made of calcium carbonate . • Plane polarized light vibrates in one plane and direction.

21. Optical activity is determined by polariscope or polarimeter that is consisted of: • A light source (usually a sodium lamp). • A polarizer that is a prism made of calcite. • A narrow slit path to bring forth a parallel beam of light. • A polarimeter tube to contain the solution of the substance tested (mostly 1.0 decimeter in length). • A scale graduated in increasing positive (+) degrees from 0.0 on its right part and in increasing negative (-) degrees from 0.0 on its left part.

22. Importance of asymmetric carbon atom: Any compound containing asymmetric carbon atom has the following two properties: • Isomerism is created around it. • It makes the compound optically active

23. Optical activity • Definition: It is the ability of the sugar to rotate the plane of the plane polarized light. • The sugar that rotates the light to the right is called dextrorotatory (d or +) such as glucose, galactose and starch and that rotating light to the left is called levorotatory (l or -) such as fructose and invert sugar.

24. Factors affecting optical activity: • Type and Concentration of the substance and type of solvent. • Type of light used and temperature. • Length of polarimeter tube in decimeters.

25. Specific rotation: • It is the observed angle of deviation of the plane polarized light in degrees from the straight path. • It is measured when the solution of the substance or the sugar dissolved in water is introduced in the path of the plane polarized light under the following conditions. • The light source used is sodium light, the temperature is 20oC, the concentration is 100 gm/ 100 mL and the polarimeter tube is one decimeter in length.

26. Each optically active substance has a characteristic specific rotation such as, -glucose is +112, -glucose is +19 and fructose is - 92.5.

27. Uses of polariscope and importance of optical activity: • Identify an unknown optically active substance. • Identify whether the substance is optically active or not. • Identify whether the substance is levorotatory or dextrorotatory. • Determine the concentration of the substance. • Differentiate between glucosuria and lactosuria. This is important in late pregnancy to differentiate between diabetes mellitus (glucose) and the normal appearance of lactose produced by the mammary glands in urine.

28. Mutarotation: • It is a temporary change in the specific rotation of the sugar when it is freshly prepared. • Mutarotation is due to the presence of a free anomeric carbon (C1 in aldoses, C2 in ketoses).

29. For example, -glucose when freshly prepared has a specific rotation of +112, then the specific rotation decreases gradually till it stabilizes at +52.5 (point of equilibrium) • -glucose when freshly prepared has a specific rotation of +19, then the specific rotation gradually increases till it is stabilizes at +52.5 (point of equilibrium)

30. In solution, -glucose changes to -glucose through the straight chain form and vice versa till the point of equilibrium is reached, where the solution is composed of: 2/3 -glucopyranose, 1/3 -glucopyranose, 1% -glucose ( and -glucofuranose) and 0.0025% open chain form (Reactive form).

31. Cyclic structure of monosaccharides

32. Steps of the cyclic form construction: • Condensation of a molecule of H2O with the aldehyde or keto group of the sugar to form aldenol or ketonol group. • The OH group from the aldenol group condenses with the OH on C4 (Furanose) or C5 (Pyranose) of the aldo-sugar to forms a ring or hemi-acetal structure with the liberation of H2O again. Keto-sugar condenses only with C5 (Furanose) or with C6 (Pyranose).

33. When the remaining OH on the aldehyde or the keto carbon atom in the cyclic form is located on the right side, the sugar form is called -sugar and if it is located on the left side the sugar is called -sugar.

36. Haworth's projection formula: • Because Fisher’s formula could not explain some of the chemical and physical characteristics of sugars, Haworth put forth his projection formula. • C and O atoms of the ring are drawn in the plane of the page. • H and OH or other side groups are written on perpendicular plane. • All groups located on the left side of fisher’s are written upwards. All groups located on the right side of fisher’s are written downwards. • The radical of the molecule (the extra-cyclic part) is written upwards in D sugar and inside the ring (or down wards) in L-sugar.

40. Isomerism • Isomers are substances which have the same molecular formula but differ in distribution of their atoms into groups or distribution of these groups and atoms in the space around carbon atoms. There are 2 types of isomerism; • Structural isomerism. • Stereo-isomerism.

41. 1. Structural isomerism: • They are isomers that have different structure due to different ways of arrangement of atoms and groups forming the molecule. Types of structural isomerism are four as follows:

42. a. Chain isomerism. • These are isomers that have different structures due to different ways of attachment of carbon atoms forming the molecule,

43. b. Positional isomerism: • These are isomers that have the same carbon skeleton but differ in the position of the substituent groups, e.g.,

45. c C.Functional group isomerism: • - These are isomers that have the same carbon skeleton, the same position of substituent group but have different functional group (aldo-/keto-).

46. d. Ring isomerism: • Pyran/furan forms such as glucopyranose and glucofuranose, and fructopyranose and fructofuranose.

47. 2. Stereo-isomerism: • They are molecules having the same structure but differ in position of their different groups and atoms in the space, • The number of stereoisomers = 2n, where n is the number of asymmetric carbon atoms. There are four types of stereo-isomerism as follows:

48. a. D and L isomerism • They differ in distribution of H and OH groups around the sub-terminal asymmetric carbon atoms. • D form has the OH group to the right of the sub-terminal carbon atom .whereas, it is on the left in L form. • This difference make the two forms (D and L) mirror image to each other due to the change of the position of all H and OH groups into the opposite direction of D form in L form.

50. b. Epimers: • They are stereoisomers which differ in distribution of H and OH groups around a single asymmetric carbon atom other than the anomeric and DL-form creating carbon before the last i.e., without difference on other carbon atoms. • Ribose is an epimer to each of arabinose and xylose. Glucose is an epimer to each of mannose and galactose.