Amino acids and proteinsEdexcel A-Level Chemistry: Revision notes

A 2-amino acid has an amino group, –NH₂, and a carboxyl group, –COOH, attached to the same carbon atom, which also carries a hydrogen and a side group R:

H₂N–CHR–COOH

Glycine (R = H) is H₂NCH₂COOH and alanine (R = CH₃) is CH₃CH(NH₂)COOH. In every 2-amino acid except glycine the central carbon is bonded to four different groups, so it is chiral.

In the solid and in neutral solution, the –COOH group donates a proton to the –NH₂ group, giving a zwitterion, ⁺H₃NCHRCOO⁻: an ion with both a positive and a negative charge and no overall charge.

The strong ionic attractions between zwitterions explain why amino acids are crystalline solids with high melting temperatures that dissolve in water.

Amino acids are amphoteric:

• with acid: –COO⁻ + H⁺ → –COOH, giving ⁺H₃NCHRCOOH
• with alkali: –NH₃⁺ + OH⁻ → –NH₂ + H₂O, giving H₂NCHRCOO⁻

A chiral amino acid exists as two enantiomers, which are mirror images. Each enantiomer rotates the plane of plane-polarised monochromatic light by equal amounts in opposite directions.

• A solution of a single enantiomer rotates the plane.
• A racemic mixture (equal amounts of both) shows no net rotation.
• Glycine has two H atoms on the central carbon, so it is not chiral and does not rotate polarised light.

Laboratory synthesis normally gives a racemic mixture, whereas natural proteins contain only one enantiomer of each chiral amino acid.

Amino acids join by condensation polymerisation: the –COOH of one reacts with the –NH₂ of another, losing water and forming a peptide bond, –CONH–.

glycine + alanine → H₂NCH₂CONHCH(CH₃)COOH + H₂O (a dipeptide)

Many amino acids joined this way make a polypeptide or protein. Each chain has a free amino end and a free carboxyl end.

Peptide bonds are broken by hydrolysis, which reverses the condensation. A protein is heated under reflux with hydrochloric acid (for example 6 mol dm⁻³ for about 24 hours), giving a mixture of amino acids:

–CONH– + H₂O → –COOH + H₂N–

In the acid solution the amino acids are present as their ⁺H₃N–CHR–COOH cations (as chloride salts).

The mixture from hydrolysis is separated by chromatography (paper or thin layer). A spot is placed on the baseline next to known amino acids, and a solvent (mobile phase) rises up the plate.

Amino acids are colourless, so the dried plate is sprayed with ninhydrin (or viewed under UV) to show the spots.

Rf = distance moved by spot ÷ distance moved by solvent front

An amino acid is identified by matching its Rf value, measured in the same solvent, with a reference. The amino acids travel different distances because they differ in attraction to the stationary phase and solubility in the mobile phase. The method shows which amino acids are present, not their order.