An amino acid is an organic molecule containing both an amino group (–NH₂) and a carboxyl group (–COOH) attached to a central α-carbon that also bears a hydrogen and a side-chain (R group). The identity of the side chain distinguishes the twenty proteinogenic amino acids encoded by the standard genetic code and defines their polarity, charge and reactivity.
Amino acids polymerise through condensation to form peptide bonds, releasing water and generating chains whose sequence constitutes the primary structure of a peptide or protein. Non-standard and D-configured amino acids are frequently incorporated into research peptides to modulate proteolytic stability, receptor selectivity or conformation.
Every downstream concept — sequence, molecular weight, modification, receptor interaction — is defined in terms of the constituent amino acids.