Amino acids, zwitterions and proteins

What you’ll learn

• Predict amino-acid charge in acidic and alkaline conditions.
• Identify chirality and peptide bonds.
• Relate condensation and hydrolysis to protein structure.

Understand the idea

An amino acid contains both an amino group and a carboxylic-acid group. For a simple alpha-amino acid, proton transfer can form a zwitterion with a positive ammonium group and a negative carboxylate group. Its net charge is zero, but it is not uncharged internally. In acid it gains a proton to form a cation; in alkali it loses a proton to form an anion.

Most simple alpha-amino acids have a chiral carbon bonded to H, the side chain, an amino group and a carboxyl group. Glycine is the exception because its central carbon has two hydrogens. Condensation forms a peptide bond between an amino group and a carboxyl group, with elimination of water; reversing that process by hydrolysis breaks the peptide link. Proteins are polyamides whose amino-acid order gives their primary structure, and bonding interactions contribute to their larger folded structures.

Key formulae

$$\mathrm{H_2NCH(R)COOH\rightleftharpoons{}^+H_3NCH(R)COO^-}$$

$$\mathrm{{}^+H_3NCH(R)COO^-+H^+\longrightarrow{}^+H_3NCH(R)COOH}$$

$$\mathrm{{}^+H_3NCH(R)COO^-+OH^-\longrightarrow H_2NCH(R)COO^-+H_2O}$$

$$\mathrm{-COOH+H_2N-\longrightarrow-CO-NH-+H_2O}\quad\text{(condensation link)}$$

Worked example

Explain why glycine is not optically active as a pure single molecular form, while alanine can have enantiomers.

1. Glycine has central carbon groups H, H, NH₂ and COOH: two groups are identical.
2. Alanine has H, CH₃, NH₂ and COOH around its central carbon: all four groups differ.
3. Alanine therefore has two non-superimposable mirror-image arrangements, whereas glycine has no such chiral centre.

Answer: Glycine is achiral; alanine has a chiral centre and can exist as a pair of enantiomers.