A peptide comprising 20 or more amino acids. Polypeptides that constitute proteins may contain up to 1000 amino acids, typically 100 to 300. Shorter ones include certain antibiotics, e.g. gramicidin, and some hormones, e.g. ACTH, which has 39 amino acids. The properties of a polypeptide are determined by the type, sequence, and three-dimensional arrangement of its constituent amino acids. One end, the C terminus, has a free carboxyl group, while the other end has a free amino group, the N terminus. See protein.