Amino Acids and Chirality

Amino acids occupy a central place in biology — they are the building blocks of every protein in every living cell. From a chemical point of view they are also fascinating because they contain two functional groups in one molecule: an amine (–NH₂) and a carboxylic acid (–COOH). This combination makes them act as both acids and bases at the same time. Amino acids are also (almost always) chiral, which makes them an ideal vehicle for introducing optical isomerism.

This lesson covers the OCR A-Level Chemistry A (H432) specification points 6.2.4 (a) (amino acids) and 4.2.5 (c) (optical isomerism / stereochemistry applied to amino acids).

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1. The Structure of an α-Amino Acid

An α-amino acid is one in which the amino group (–NH₂) is attached to the carbon immediately adjacent to the carboxylic acid group. The general structure is:

       H
       |
   H2N-C-COOH
       |
       R

• The central C is known as the α-carbon.
• R is the side chain, which can be as simple as –H (glycine) or as complex as a ring (tryptophan).
• The 20 "proteinogenic" amino acids that make up proteins all share this structure and differ only in the side chain R.

Examples

Amino acid	R group	Simple?
Glycine	–H	Simplest, achiral
Alanine	–CH₃	Simplest chiral
Valine	–CH(CH₃)₂	Branched
Serine	–CH₂OH	Contains OH
Cysteine	–CH₂SH	Contains S
Glutamic acid	–CH₂CH₂COOH	Acidic side chain

OCR does not expect you to memorise all 20 amino acids by name, but you should be able to recognise the α-amino acid structure and draw glycine, alanine and similar simple examples.

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2. Amphoteric Behaviour — Acting as Both Acid and Base

Because an amino acid has both an acidic –COOH and a basic –NH₂ group, it can react as both an acid and a base. Compounds that do this are called amphoteric.

2.1 With acids (amino acid acts as a base)

The basic –NH₂ is protonated to give an ammonium salt:

$H_2N-CHR-COOH + HCl \longrightarrow ^+H_3N-CHR-COOH + Cl^-$H2​N−CHR−COOH+HCl⟶+H3​N−CHR−COOH+Cl−

2.2 With bases (amino acid acts as an acid)

The acidic –COOH is deprotonated to give a carboxylate salt:

$H_2N-CHR-COOH + NaOH \longrightarrow H_2N-CHR-COO^-Na^+ + H_2O$H2​N−CHR−COOH+NaOH⟶H2​N−CHR−COO−Na++H2​O

2.3 Combined — the zwitterion

In solid and in neutral aqueous solution, an amino acid exists as a zwitterion — a molecule with a positively charged N and a negatively charged carboxylate at the same time:

         H
         |
  +H3N - C - COO-
         |
         R

Key Definition — Zwitterion: A neutral molecule that carries both a positive and a negative charge on different atoms, with the net charge zero.

The zwitterion form explains some odd properties of amino acids:

• Very high melting points for small molecules (e.g. glycine melts at 233 °C, well above a normal carboxylic acid or amine) — the zwitterion form means the solid behaves almost like an ionic lattice.
• High solubility in water (polar water molecules solvate both charges) but low solubility in organic solvents.
• No distinct smell despite the amine content — the –NH₂ is locked away as –NH₃⁺.

2.4 Isoelectric point

The isoelectric point (pI) is the pH at which the amino acid is net neutral — exactly balanced between the cation, the zwitterion and the anion.

• At pH < pI, the amino acid is cationic (+H₃N-CHR-COOH).
• At pH ≈ pI, the amino acid is the zwitterion (net charge zero).
• At pH > pI, the amino acid is anionic (H₂N-CHR-COO⁻).

graph LR
  A[Low pH: cation +H3N-CHR-COOH] --> B[pH = pI: zwitterion +H3N-CHR-COO-]
  B --> C[High pH: anion H2N-CHR-COO-]

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3. Chirality — Why Amino Acids Matter for Stereochemistry

Except for glycine (R = H), every α-amino acid has four different groups attached to the α-carbon: –H, –NH₂, –COOH and –R. Four different groups means the α-carbon is a chiral centre (or stereocentre).

Key Definition — Chiral centre: A carbon atom (or other atom) bonded to four different groups.

3.1 Enantiomers

Two molecules that are non-superimposable mirror images of each other are called enantiomers. Enantiomers have: