A peptide is a chemical compound containing two or more amino acids (amino acid polymers) that are coupled by a peptide bond. This bond is a special linkage in which the nitrogen atom of one amino acid binds to the carboxyl carbon atom of another...
Peptides are often classified according to the number of amino acid residues. Oligopeptides have 10 or fewer amino acids. Molecules consisting from 10 to 50 amino acids are called peptides. The term protein describes molecules with more than 50 amino acids...
Molecules with molecular weights ranging from several thousand to several million daltons (D) are called polypeptides. The terms polypeptide and protein are frequently used interchangeably.