Amino acids contains both an amino group (-NH₂) and carboxyl group (-COOH). Cysteine is a polar amino acid with a sulfur-containing (thiol) group. Methionine has also a sulfur-containing functional group, a thioether group. In contrast, Proline is lacking a sulfur-containing group. Proline is also called an imino acid since it lacks a primary amino group, in stead it contains a secondary amine group (imino; >C=NH) and a carboxyl group (-C(=O)-OH). Both methionine and proline are non polar amino acids.