Amino acids and proteinsEdexcel International A Level Chemistry: Revision notes

An amino acid has both an amine group, –NH₂, and a carboxylic acid group, –COOH. In the 2-amino acids found in proteins both are attached to the same carbon, giving the general formula RCH(NH₂)COOH. Glycine has R = H, alanine R = CH₃. The two groups mean an amino acid can behave as both an acid and a base.

The –COOH group can donate H⁺ (acid) and the –NH₂ group can accept H⁺ (base), so amino acids are amphoteric.

• In strong acid: the amine is protonated, giving a cation ⁺H₃NCH(R)COOH.
• In alkali: the acid group loses H⁺, giving an anion H₂NCH(R)COO⁻.
• In neutral solution and in the solid: a zwitterion, ⁺H₃NCH(R)COO⁻, with no overall charge.

Strong electrostatic attractions between zwitterions explain why amino acids are solids with high melting temperatures and are soluble in water.

Examples: H₂NCH₂COOH + HCl → ⁺H₃NCH₂COOH Cl⁻, and H₂NCH₂COOH + NaOH → H₂NCH₂COO⁻Na⁺ + H₂O.

Every amino acid except glycine has a chiral carbon, bonded to four different groups (H, NH₂, COOH and R). It exists as two enantiomers that are non-superimposable mirror images. Each rotates the plane of plane-polarised monochromatic light in opposite directions. A racemic mixture (equal amounts) gives no net rotation. Glycine has two H atoms on the central carbon, so it is not chiral and has no effect on plane-polarised light. Proteins contain only one enantiomer of each amino acid.

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

Gly + Ala → H₂NCH₂CONHCH(CH₃)COOH + H₂O

The dipeptide still has –NH₂ at one end and –COOH at the other, so it can react further to give a polypeptide. A protein is a long polypeptide. Gly–Ala and Ala–Gly are different dipeptides because the order of joining differs.

Unknown substances are identified by a series of tests, recording observations and ruling out options.

Inorganic:

• Carbonate: add dilute acid; effervescence, CO₂ turns limewater milky.
• Ammonium: add NaOH(aq), warm; NH₃ turns damp red litmus blue.
• Halide: add dilute HNO₃ then AgNO₃(aq); chloride white, bromide cream, iodide yellow precipitate.
• Sulfate: add BaCl₂(aq) with dilute HCl; white precipitate.
• Metal ions: NaOH(aq) gives, e.g., Cu²⁺ pale blue and Fe³⁺ brown precipitates.

Organic:

• Alkene: bromine water decolourised.
• Carbonyl: 2,4-DNPH gives an orange precipitate; aldehydes also give a silver mirror with Tollens' reagent.
• Primary/secondary alcohol or aldehyde: acidified dichromate(VI) goes orange to green.
• Carboxylic acid: effervescence with sodium carbonate.

Glycine gives none of these inorganic results and is identified by elimination.