Biomolecules

Carbohydrates – Introduction

Earlier, carbohydrates were defined as hydrates of carbon with general formula C_n(H₂O)_y, but this definition is no longer valid.

Modern Definition

Carbohydrates are polyhydroxy aldehydes or ketones or substances which give these on hydrolysis. They generally contain at least one chiral carbon.

Classification of Carbohydrates

Type 1 – Based on Hydrolysis

• Monosaccharides: Cannot be hydrolysed further; general formula (CH₂O)_n, n = 3–7
• Oligosaccharides: On hydrolysis give 2–10 monosaccharide units
• Polysaccharides: On hydrolysis give a large number of monosaccharides
  General formula: (C₆H₁₀O₅)_n, n ≈ 100–3000

Type 2 – Sugar & Non-Sugar

• Sugars: Mono- and oligosaccharides; sweet and crystalline
• Non-sugars: Polysaccharides; tasteless and amorphous

Type 3 – Reducing & Non-Reducing

• Reducing sugars: Have free aldehydic or ketonic group
  All monosaccharides are reducing sugars
• Non-reducing sugars: Cannot reduce Tollens’ or Fehling’s reagent
  All polysaccharides are non-reducing

Monosaccharides

Simplest Monosaccharides (Trioses)

• Glyceraldehyde: Aldotriose
• Dihydroxyacetone: Ketotriose

Glucose

• Also called dextrose or grape sugar
• Molecular formula: C₆H₁₂O₆
• An aldohexose

Preparation of Glucose

• From sucrose by acidic hydrolysis → glucose + fructose
• From starch by hydrolysis (commercial method)

Chemical Reactions of Glucose

• Acetylation: Forms glucose pentaacetate
• With hydroxylamine: Forms glucose oxime
• With HCN: Forms glucose cyanohydrin
• Oxidation:
  • Mild → gluconic acid
  • Strong (HNO₃) → saccharic acid
• Reduction: Forms n-hexane
• Osazone formation: With phenylhydrazine

Structural Features of Glucose

• Does not show typical aldehyde reactions (no Schiff’s test)
• Exists mainly in cyclic hemiacetal forms
• Two anomers: α-D-glucose and β-D-glucose

Anomers & Mutarotation

• Anomers differ in configuration at anomeric carbon (C1)
• Mutarotation: change in optical rotation until equilibrium is reached (+52.5°)

Oligosaccharides

Disaccharides

• General formula: C₁₂H₂₂O₁₁
• Sucrose: Glucose + Fructose (non-reducing)
• Maltose: Glucose + Glucose (reducing)
• Lactose: Glucose + Galactose (reducing)

Inversion of Sugar

• Hydrolysis of sucrose gives laevo-rotatory solution
• Due to higher laevorotation of fructose

Polysaccharides

Starch

• Plant storage polysaccharide
• Mixture of amylose (10–20%) and amylopectin (80–90%)
• Non-reducing; does not form osazone

Cellulose

• Major component of plant cell walls
• Linear polymer of β-D-glucose (β-1,4-glycosidic bonds)
• Humans cannot digest cellulose

Proteins

• Condensation polymers of α-amino acids
• Contain C, H, O, N and often S

α-Amino Acids

• General structure: NH₂–CH(R)–COOH
• Total 20 amino acids found in proteins
• Exist as zwitterions in solution

Classification of Amino Acids

• Essential: Cannot be synthesised by human body (10)
• Non-essential: Synthesised in human body (10)
• Neutral, Acidic, Basic (based on –NH₂ and –COOH groups)

Isoelectric Point

• pH at which amino acid has no net charge
• Least soluble at isoelectric point
• Neutral ≈ 6.1 | Acidic ≈ 3.2–3.5 | Basic ≈ 7.6–10.8

D- and L- Nomenclature

• All amino acids except glycine are chiral
• All naturally occurring amino acids belong to L-series

Peptides & Proteins

• Peptide bond: –CO–NH– linkage
• Formed by condensation of two amino acids with loss of water
• Polypeptides are formed when many amino acids join together