Carbohydrates Introduction Lecture Notes PDF

LECTURE - 1 MPHA2217 Introduction to carbohydrates BY Dr. V. Appalaraju. HOD, Lecturer, Faculty of Pharmacy, MAHSA University. 0  CARBOHYDRATES DEFINITION: They are polyhydroxy aldehydes or polyhydroxy...

LECTURE - 1 MPHA2217 Introduction to carbohydrates BY Dr. V. Appalaraju. HOD, Lecturer, Faculty of Pharmacy, MAHSA University. 0  CARBOHYDRATES DEFINITION: They are polyhydroxy aldehydes or polyhydroxy ketones and large molecules that produce the compounds on hydrolysis. The carbohydrates are an important class of naturally occuring organic compounds. These include glucose (grapes), fructose (honey), starch(potatoes), cellulose (wood). They are all composed of C, H, O. In general carbohydrates can be represented by the formula cm(H2O)m. Carbohydrates are often referred as saccharides. Carbohydrates are polyfunctional compounds, they contain the following functional groups. 1. Alcoholic hydroxyl groups. 2. Aldehyde group. 3. Ketone group.  In light of the above, the definition of carbohydrates may be improved as: A polyhydroxy compound that has an aldehyde or a ketone function present, either free or as hemiacetal or acetal. Classification of carbohydrates: The carbohydrates are divided into three major classes depending on the number of simple sugar molecules produced on hydrolysis. The molecules so obtained may be of the same or different sugars. 1. Monosaccharides (simple sugars): These are single unit carbohydrates that cannot be broken into simpler carbohydrates upon hydrolysis. Eg: glucose and fructose. C6H12O6 + H2O → No reaction.  2. Oligosaccharides: They are made of 2 to 10 units of monosaccharides or simple sugars. The oligosaccharides containing two monosaccharides units are called disaccharides. Eg: sucrose. sucrose → glucose + fructose. The oligosaccharides containing three monosaccharides units are trisaccharides. Eg: raffinose. C18H32O16 + 2 H2O → C6H12O6 + C6H12O6 + C6H12O6 Raffinose Glucose fructose galactose 3. Polysaccharides: They contain more than ten monosaccharide units in a molecule. One molecule of starch or cellulose upon hydrolysis yields a very large number of glucose units. (C6H10O5)n + H20 → n C6H12O6  Monosaccharides They are the simplest one unit sugars. Mono- one; saccharides- sugar. They have the general formula CnH2nOn, where n varies from 3 to 8. Trioses - C3H6O3 eg: glyceraldehyde. Tetroses - C4 H8 O4 eg: erythrose. Pentoses - C5H10O5 eg: ribose. Hexoses - C6H12O6 eg: glucose. Heptoses - C7 H14 O7 eg: heptulose. Octoses - C8 H16 O8 The most important naturally occuring monosaccharides are pentoses and hexoses. Of these glucose and fructose are undoubtedly the most important and typical.  Simple aldose sugars Triose carbohydrate Tetrose carbohydrate Pentose carbohydrate Hexose carbohydrate C3H6O3 C4H8O4 (Aldopentoses) (Aldohexoses) C5H10O5 C6H12O6 D-glyceraldehyde D-erythrose D-ribose D-glucose  Glucose Glucose is the most common monosaccharide. It is known as Dextrose because it occurs in nature principally as the optically active dextrorotatory isomer. The naturally occuring glucose is D(+) glucose and fructose is D(-) fructose. It is pentahydroxyhexanal. It is essential constituent of human blood. The blood normally contains 65 to 110mg of glucose per looml. In diabetic patients the level of glucose increases than the normal level. In the combined form glucose occurs in abundance in cane sugar and polysaccharides such as starch and cellulose.  Physical properties: Glucose is a white crystalline solid, M.P-146°c. Soluble in water, sparingly soluble in ethanol. It is optically active. Naming simple monosaccharides: 1 Functional group = aldehyde 6 carbons= hex.. 5 carbons = pent. Chiral centres have a fixed configuration. Changing them produces another sugar. Here there are 16 possible isomers including the choice of D,L at position 5. Chiral centre used in designating D,L: -OH on right gives D absolute configuration in comparison to D-glyceraldehyde  Aldohexose sugars D-allose D-alltrose D-glucose D-mannose D-idiose D-galactose D-talose Numbering monosaccharides The Fischer projection. Arrange C-chain vertically. For sugars with a terminal aldehyde function put the aldehyde function at top and count from the aldehyde=1: For sugars with a keto group put the keto carbon next to top and it is number 2 Projections for representing glucose 1  Carbohydrate Structure D- or L-sugars Indicates stereochemistry of lowest chrial carbon. D- sugars: hydroxyl group at lowest chiral carbon atom pointing to the right. L-sugars: hydroxyl group at lowest chiral carbon aroms pointing to the left.  Carbohydrate Structure Mirror CH2OH CH2OH C O C O H OH HO H HO H H OH HO H H OH CH2OH CH2OH L-Fructose D-Fructose Configuration is reversed at each chiral centre  Acetal and ketal formation Ketone + alcohol hemiketal+ alcohol ketal* *Ketals are sometimes just described as a sub-group of “acetals”. Ring formation in monosaccharides The -OH group on C5 is well placed to react with the C1 aldehyde function. The resulting 6-membered pyranose ring is relatively unstrained with bond angles near their normal values. -OH groups on C4 and C6 are less favourably oriented. FISHER PROJECTION FORM H O H OH HO H C C C H C OH H C OH H C OH HO C H HO C H O HO C H O or H C OH H C OH H C OH H C OH H C H C CH2 OH CH2 OH CH2 OH D-glucose α-D-glucose -D-glucose Anomeric Carbon --- The carbon atom which is involved in hemiacetal or acetal formation Chain ring interconversion H D-glucose a -D-glucose a -D-glucose H b -D-glucose H HAWORTH PROJECTION FORMULAS FOR SUGARS H OH C CH2 OH H C OH O HO C H O OH HO H C OH OH H OH C CH 2 OH a - D - Glucopyranose  When either of these forms of D-glucose is dissolved in water and allowed to stand, a gradual change in specific rotation occurs. The specific rotation of α-form falls and that of ß-form rises until a constant value of +52° is reached. anomer: 112º decreases to 52° at equalibrium. anomer: 19º increase to 52° at equalibrium. At eq’m: 36% : 64% , and (0.003% open- chain). This change in optical rotation of a solution of either form of glucose until a constant value is obtained is called mutarotation.  Monosaccharide Anomers Anomeric carbon Anomers: or anomer has hydroxyl down anomer has hydroxyl up Anomers undergo mutarotation:  Chemical properties 1. Oxidation: A) glucose on treatment with a weak oxidising agent such as bromine water, it is oxidised to gluconic acid. B) glucose on oxidation with strong oxidising agent like nitric acid give glucaric acid.  2. Reduction: Glucose on reduction with sodium borohydride gives D-glucitol (sorbitol). 3. Acetylation: Glucose reacts with acetic anhydride in the presence of anhydrous zinc chloride forms penta acetyl glucose. 4. Reaction with hydroxylamine: glucose condenses with hydroxylamine,NH2OH to form glucose oxime. 5. reaction with phenylhydrazine: glucose when warmed with excess phenyl hydrazine it forms phenyl hydrazone by condensation with CHO group this reaction does not stop at this stage. The adjacent CHOH group is then oxidised by a second molecule of phenylhydrazine which it self is reduced to aniline and ammonia. The resulting carbonyl group reacts with a third molecule of the reagent to yield the final product glucosazone.  Reactions of Glucose HC O HC O CHOH acetylation CHOAc CHOH CHOAc Glucose penta-acetate CHOH CHOAc CHOH CHOAc CH2OH CH2OAc mild oxidation hydroxylamine NH2OH COOH gluconic acid HC NOH oxime = rest of molecule unchanged  HC O CH2OH CHOH CHOH reduction CHOH CHOH Glucose sorbitol CHOH CHOH CHOH CHOH CH2OH CH2OH Reactions of vigorous oxidation Glucose COOH CHOH CHOH glucaric acid CHOH CHOH COOH 3. Fructose (levulsoe) --- Rotation in polarimeter is left CH2 OH CH2 OH CH2 OH OH C O HO C C HO C H HO C H HO C H O or O H C OH H C OH H C OH H C OH H C H C CH2 OH CH2 OH CH2 OH D-Fructose b-D-Fructose a-D-Fructose Fructose (levulsoe) HOCH2 O OH HOCH2 O HO HO CH2 OH OH OH b - D - Fructofuranose a - D - Fructofuranose CH 2OH CH 2OH OH H C O C O CH2 OH HO C H HO C H OH H C OH H C OH O OH OH H C OH H C OH OH CH 2OH H C H Naturally-occurring free form α - D – Fructose α - D - Fructopyranose Ring for b -D-fructose Ring isomers for D-fructose  CH2OH CH2OAc acetylation C O C O CHOH CHOAc fructose CHOH CHOAc CHOH CHOAc CH2OH CH2OAc hydroxylamine mild oxidation Reactions of no reaction Fructose CH2OH vigorous oxidation C NOH CH2OH COOH + COOH CHOH glycolic acid CHOH CH2OH glycaric acid  Reactions of Fructose CH2OH CH2OH CH2OH C O reduction H C OH + HO C H CHOH Fructose mannitol sorbitol CHOH CHOH CH2OH Glycosidic bond Disaccharides Polysaccharides Glycogen both α(1→4), α(1→6) linkages are possible. (up to 100 000 glucose units)  Major form of storage in animals. Found in the liver. Highly branched glucose chains – branches every 8-12 glucose units. Linear linkages - (1 4); branch linkages - (1 6). Non-reducing – red colour with iodine. cellulose Found in cell walls of plants. Linear polymer of glucose - (1 4) linkages – hydrogen bonding conveys strength. Repeating disaccharide is cellobiose. Uses – methylcellulose/carboxymethylcellulose are used as disintegrants, laxatives, stabilisers  Structure of cellulose CH2OH CH2OH CH2OH O O etc O O O O O cte HO HO HO HO HO HO  Starch is mainly composed of amylose and amylopectin. starch  Starch is the best antidote for poisoning by iodine. starch consists of hundreds of glucose molecules linked together by α (1→4) and also α(1 → 6) glycoside bonds. Amylopectin contains branches (nonlinear), approximately one in every 20 to 25 glucose units. Hydrolysis of amylopectin yields maltose. Thank you