Amino acids are the building blocks of proteins. They contain both a basic amine group and an acidic carboxylic acid group, giving them unique amphoteric chemical properties.
🔑 Key Principle
The general formula of an alpha-amino acid is \\( \text{H}_2\text{NCHRCOOH} \\), where the amine group and the carboxylic acid group are attached to the same central carbon atom (the alpha-carbon).
An organic compound containing both an amine (\\( \text{-NH}_2 \\)) and a carboxylic acid (\\( \text{-COOH} \\)) group attached to the same central carbon atom.
Amphoteric Nature of Amino Acids
Because they contain both basic amine groups (proton acceptors) and acidic carboxylic acid groups (proton donors), amino acids are amphoteric and react with both acids and bases.
pH-Dependent Structure
The chemical structure of an amino acid change dramatically depending on the pH of its environment:
A dipolar ion containing both positive (\\( \text{-NH}_3^+ \\)) and negative (\\( \text{-COO}^- \\)) charged functional groups, carrying a net overall electrical charge of zero.
The specific pH value at which a molecule (such as an amino acid) carries no net electrical charge and exists predominantly as a zwitterion.
Physical Properties of Zwitterions
Because they form zwitterions, amino acids display physical properties that are very different from standard neutral organic molecules of similar molecular mass:
- High melting points: Amino acids are crystalline solids at room temperature. The electrostatic attractions between the oppositely charged parts of adjacent zwitterions (ionic bonds) require large amounts of energy to overcome.
- Solubility: They are highly soluble in water because the ionic charges can form strong hydrogen bonds and electrostatic interactions with polar water molecules. They are insoluble in non-polar organic solvents like hexane or ether.
Buffer Solutions
Because amino acids are amphoteric, they can act as buffer solutions, resisting changes in pH when small amounts of acid or alkali are added:
- If acid (\( \text{H}^+ \)) is added, the zwitterion carboxylate end accepts the proton: \[ \text{H}_3\text{N}^+\text{-CHR-COO}^- + \text{H}^+ \rightarrow \text{H}_3\text{N}^+\text{-CHR-COOH} \]
- If alkali (\( \text{OH}^- \)) is added, the zwitterion ammonium end donates a proton to form water: \[ \text{H}_3\text{N}^+\text{-CHR-COO}^- + \text{OH}^- \rightarrow \text{H}_2\text{N-CHR-COO}^- + \text{H}_2\text{O} \]
Optical Isomerism in Amino Acids
A molecule is optically active if it contains a chiral carbon atom: a carbon atom bonded to four different groups.
- Except for glycine, all alpha-amino acids are optically active and exist as two non-superimposable mirror image forms (enantiomers).
- In glycine, the R group is a hydrogen atom (\\( \text{R = H} \\)). This means the central carbon atom is bonded to two hydrogen atoms, making it symmetric and achiral.
When asked to draw the 3D structures of amino acid optical isomers, always start by drawing the central carbon atom. Bond it tetrahedrally to the four different groups: the hydrogen atom (\\( \text{-H} \\)), the carboxyl group (\\( \text{-COOH} \\)), the amine group (\\( \text{-NH}_2 \\)), and the specific R group. Then, draw a vertical dashed mirror line and construct the mirror image, ensuring the bonds point towards the correct atoms.
- Excess hydrochloric acid
- Excess sodium hydroxide
- Methanol in the presence of concentrated sulfuric acid catalyst
1. Reaction with Acid (HCl): The amine group accepts a proton to form a cation. \[ \text{H}_3\text{N}^+\text{-CH(CH}_3)\text{-COOH} \text{ Cl}^- \]
2. Reaction with Base (NaOH): The carboxylic acid group loses a proton to form a carboxylate salt. \[ \text{H}_2\text{N-CH(CH}_3)\text{-COO}^- \text{Na}^+ \]
3. Reaction with Methanol / H₂SO₄ catalyst: This is a standard esterification. The carboxylic acid group reacts with the alcohol to form an ester, while the basic amine group is protonated in the acidic conditions. \[ \text{H}_3\text{N}^+\text{-CH(CH}_3)\text{-COOCH}_3 \text{ HSO}_4^- \]
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