Chemistry of carbohydrates-II. Chemical properties, Monosaccharides, Disaccharides

This ppt discusses about the chemical properties of carbohydrates. Also it tells about individual monosaccharides and disaccharides. Clinical significance is mentioned in each respective section. Suitable for MBBS, MD and BDS students.

1. Chemistry of carbohydrates-II Chemical properties, Disaccharides Prof Dr Viyatprajna Acharya, MD, PhD Biochemistry

2. Chemical properties of glucose Action of strong acids – Furfural formation Action of alkalies – Enolization Oxidation- sugar acid formation Reduction- sugar alcohol formation Action of phenylhydrazine – Osazone formation

3. Action of strong acids – Furfural formation Monosaccharides lose 3 mol of water on heating with Conc. sulfuric acid and form furfural derivatives which may combine with α - naphthol, resorcinol or thymol to produce a coloured complex Basis of Molisch’s test, Seliwanoff’s test and Bial’s test C₆H₁₂O₆+ Conc. H₂SO₄ ​ → C₆H₆O₃ (HMF)+3H 2 ​O Hydroxymethyl furfural derivative

4. Action of alkalies – Enediol formation In dilute alkaline solution both Aldoses and ketoses are changed to enediols Ene-di- ol Involves the free aldehyde or keto group Glucose is converted into fructose and mannose in mild alkaline solution Lobry de Bruyn Van Enkenstein transformation Enediols – good reducing agent Basis of Benedict’s & Fehling’s test

5. By formation of a common enediol , glucose, fructose and mannose can isomerize Benedict’s test – MC test done for diagnosing DM In alkaline medium sugars form enediols, cupric ions are reduced, correspondingly sugar is oxidized. Any reducing sugar will give +ve reaction

6. Oxidation (Sugar acid formation) Mild oxidation conditions (Hypobromous acid)- aldehyde gr is oxidized to produce aldonic acid C-1 is oxidized Glucose → Gluconic acid Galactose → galactonic acid

7. 2. When aldehyde gr is protected and the molecule is oxidized, the last C atom becomes COOH gr – Uronic acid; C-6 is oxidized Glucose – glucuronic acid Mannose – Mannuronic acid Used in synthesis of GAGs and glycoproteins

8. 3. Under strong acidic conditions (Nitric acid + heat) – the first and last C atoms are simultaneously oxidized to form a dicarboxylic acid – Saccharic acid Glucose- Glucosaccharic acid Mannose – Mannaric acid Galactose – Mucic acid Identification of sugars

9. Reduction (Sugar alcohol) When treated with reducing agents like sodium amalgam, hydrogen can reduce sugars, aldose yields corresponding alcohol Ketoses yield 2 alcohols – a new asymmetric C atom is generated in the process Glucose – sorbitol Fructose – Sorbitol & Mannitol Galactose – dulcitol Ribose - Ribitol Clinical significance: Mannitol- Forced diuresis Sorbitol, dulcitol- syrup Sorbitol- cataract

10. Action of phenylhydrazine (Osazone formation) All reducing sugars – osazones With excess of phenylhydrazine when boiled at 100 c – phenylhydrazone formed 2 nd mol of phenylhydrazine – creates a new carbonyl C 3 rd mol reacts with the new carbonyl gr and osazones are formed

11. Clinical significance: To differentiate between different sugars in urine

12. Glycoside formation When the hydroxyl gr of the anomeric C atom reacts with an alcohol (-OH) or phenol (-NH) gr of another compound (may or may not be a monosaccharide) glycosides are formed. Non-carbohydrate part- aglycone Ex- Streptomycin, Cardiac glycosides (Digitonin), anthracyclin, Daunorubicin and Doxorubicin, Phlorhizin - used to produce renal damage in experimental animals Digitalis

13. Sugar derivatives Phosphoric acid ester- Glucose -6-P, Glucose-1-P; -OH groups are esterified to form acetates, propionates, benzoates, phosphates Amino sugar – OH gr replaced by amino or acetylamino gr Usually C2 is involved. Glucosamine, galactosamine, Mannosamine Further acetylation- N-acetyl glucosamine ( GluNac ), N-acetyl galactosamine ( GalNac ) Deoxysugars - in place of OH gr H is replaced 2-deoxy ribose in DNA, L- fucose in glycoprotein Glucose-1-P; Cori ester

14. Pentoses Ribose- RNA Deoxyribose- DNA Ribulose- intermediary of HMP shunt Arabinose- glycoproteins Xylose- proteoglycans Xylulose- intermediate of the uronic acid pathway

15. Disaccharides 2 sugar units joined by glycosidic bonds Crystalline, water soluble and sweet to taste Sucrose, Lactose, Maltose & Isomaltose

16. Maltose α -D- Glucose + α- D-Glucose C-1 & C4 glycosidic linkage Free anomeric C atom- hence reducing Malt product Intermediate in digestion of starch and glycogen

17. Isomaltose 2 glucose molecules linked by α 1 →6 glycosidic bond Intermediary product from digestion of starch and glycogen

18. Lactose Glucose + Galactose β 1-4 glycosidic bond joins a β -D- galactose to β -D- glucose Nowhere else found in the nature Lactase in man and β -D- galactosidase in bacteria hydrolyze lactose to glucose and galactose

19. Sucrose Table sugar / cane sugar α - D glucose + β -D fructose α 1- β -2 glycosidic linkage No free anomeric C atom- non-reducing Sucrase / invertase hydrolyses

20. Sucrose is also known as invert sugar Optical rotation of sucrose (+66.5 ) On hydrolysis- becomes levorotatory (-20 ) Glucose (+52.5 ) and fructose (-92 ) Honey contains invert sugar (equimolecular mixture of glucose and fructose) – sweeter than sucrose Digested by sucrase

21. Trehalose Glucose + Glucose ( α1→α1) Found in mushrooms and insects. Trehalase – digests Trehalose Deficiency may lead to gastric upset after mushroom ingestion

22. Clinical significance Sucrase, Trehalase deficiency- gastric upset, bloating Sucrose- ORS solutions; disaccharidase deficiency evaluation Lactose intolerance Malabsorption syndrome Dental caries- Sucrose feeds the bacteria that forms a plaque made of a network of dextran

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