Carbohydrate Chemistry Basics

Carbohydrate Chemistry

• Carbohydrates have the general formula C₆H₁₂O₆
• Carbohydrates are classified into four groups: monosaccharides, disaccharides, polysaccharides, and oligosaccharides.
• Monosaccharides are simple sugars that cannot be broken down into smaller groups.
• Monosaccharides have the general formula (CH₂O)_n.
• Monosaccharides can be further classified based on the number of carbon atoms (triose, tetrose, pentose, hexose, heptose), the position of the carbonyl group (aldose, ketose), and chirality.
• The names of monosaccharides depend on the number of carbon atoms present and the suffix -ose.
• Examples of monosaccharides include trioses such as glyceraldehyde and dihydroxyacetone, pentoses such as ribose and ribulose, and hexoses such as glucose, galactose, and fructose.
• Chirality refers to an object that is not superimposable on its mirror image.
• The letter D- or L- is used before the name of the monosaccharide to distinguish isomers.
• A chiral carbon has four different groups bonded to it.
• For longer monosaccharides, there will be more than one asymmetric carbon.
• Isomerism occurs due to the presence of asymmetric carbon
• Two main types of isomers include constitutional and stereoisomers.
• Stereoisomers include geometric and optical isomers.
• Geometric isomers, cis-trans isomers, are not applicable to monosaccharides, usually occurs in unsaturated compounds having double bonds.
• Optical isomers, enantiomers, are non-superimposable mirror images. D (+) and L (-) enantiomers.
• When equal amounts of D and L forms are present, the resulting mixture is optically inactive and called racemic.
• Monosaccharides can exist in linear and ring forms.
• Linear - Fischer projection, Ring - Haworth Projection.
• For hexoses (6 carbons), the predominant form is the ring structure.
• Six-membered rings are called pyranose, examples are glucose and galactose and five-membered rings are called furanose, example is fructose.
• Chemical properties of monosaccharides include the addition of alcohols to ketones and aldehydes to form hemiacetals (a hemiacetal) or hemiketal
• Hemiacetals/hemiketals are unstable and only present in small amounts in equilibrium mixtures; except when the hydroxyl group is part of the same molecule as the carbonyl group, then a 5 or 6-membered ring can be formed.
• Cyclization yields a and ȣ anomers which can be obtained, and more stable for six member ring.
• Conversion from Fischer to Haworth projection.
• Mutarotation: gradual change in specific rotation in all reducing sugars (except a few ketoses)
• Mutarotation converts between a and ȣ forms.
• a and ß- anomers are typically stable solids in solution, and they quickly equilibrate to an equilibrium mixture of the two forms.
• Example, glucose exists in aqueous solution as a mixture of 36% a- and 64% ß- forms.
• Formation of Glycosides (acetals): Further treatment of a hemiacetal with an alcohol yields an acetal(or ketal)
• The bond from the anomeric carbon to the OR group is called a glycosidic bond or O-glycosidic bond.
• Sugars can be linked by O-glycosidic bonds to form disaccharides and polysaccharides.
• Mutarotation is not possible in a glycoside.
• Glycosides are stable in water and aqueous base.
• Glycosides can be hydrolyzed in aqueous acids yielding an alcohol and a monosaccharide.
• Naming Glycosides – alkyl group bonded to oxygen followed by the name of the carbohydrate then replace ending -e with -ide.
• Reduction of monosaccharides to alditols – the carbonyl group can be reduced to a hydroxyl group by using H₂ in the presence of a catalyst and sodium borohydride.
• Reduction of ketones usually produces two diastereoisomeric alditols.
• Examples of naming alditols, drop 'ose' and add 'itol'. D-glucose → D-glucitol (sorbitol)
• Oxidation to aldonic acids – aldehydes in aldoses is oxidized to carboxylic acid by oxidizing agents such as O₂ under basic conditions to a carboxylate group, and the carbohydrate is known as a reducing sugar.
• Oxidation to aldaric acids - both ends of the aldose chain are oxidised to carboxylic acids gives aldaric acid.
• Oxidation to uronic acid – An enzyme catalyzed oxidation of the primary alcohol at C-6 of a hexose yields a uronic acid.
• Reduction and oxidation reactions with examples.
• Reducing sugars – monosaccharides with a carbonyl group that oxidizes to give a carboxylic acid; react with Benedict's reagent to give the corresponding carboxylic acid.
• Common monosaccharides – glucose, fructose, galactose.

Disaccharides

• Two sugar molecules linked together by an O-glycosidic bond
• Examples: Sucrose, Lactose, Maltose
• Sucrose – table sugar; glucose and fructose; not a reducing sugar
• Lactose – milk sugar; glucose and galactose; a reducing sugar
• Maltose – malt sugar; two glucose molecules; a reducing sugar

Polysaccharides

• Complex sugars; high molecular weight.
• Can be straight-chain or branched.
• Two main types: homopolysaccharides and heteropolysaccharides.
• Homopolysaccharides produce only one type of monosaccharide on hydrolysis
• Examples of homopolysaccharides include starch, glycogen and cellulose.
• Starch is an important food storage carbohydrate in plants, composed of amylose (straight chain) and amylopectin (branched).
• Glycogen is the storage form of carbohydrates in animals. Structurally similar to amylopectin, but with more branching.
• Cellulose is the most abundant extracellular polysaccharide in plants; structural carbohydrate; glucose units linked by β-1,4-glycosidic linkages.
• Heteropolysaccharides produce more than one type of monosaccharide on hydrolysis.
• Examples of heteropolysaccharides (with biological significance) are hyaluronic acid, chondroitin sulfate, and heparin.
  • Hyaluronic acid – found in various tissues; a straight chain polymer of D-glucuronic acid and N-acetyl-D-glucosamine.
  • Chondroitin sulfate – found in cartilage and cell coat; straight chain polymer of D-glucuronic acid and N-acetyl-D-galactosamine.
  • Heparin – found in the liver; made up of D-glucuronic acid and D-glucosamine-N-sulphate.
• Chitin is a polysaccharide found in the outer skeletons of insects, crabs, shrimps, and lobsters; composed of β (1-4) linked units of the amino sugar N-acetyl-glucosamine; a main source of chitosan.

Oligosaccharides

• 3-10 monosaccharide molecules (same or different)
• Often conjugated to proteins or lipids, functioning as receptors or aiding in cell interaction.
• Found in plants as soluble fiber, may help prevent constipation.
• Examples: Raffinose and Stachyose
• Raffinose – trisaccharide, found in beans, vegetables, and whole grains.
• Stachyose – tetrasaccharide, a normal human metabolite present in human milk and many vegetables.
• Oligosaccharides on blood cells give the groups A, B, O and AB.
• Three types of oligosaccharides are found on the surface of red blood cells, with chains of sugars including N-acetyl galactosamine, N-acetylglucosamine, galactose and L-fucose.