The end of a polypeptide chain with the unattached carboxyl group (a -COOH group). Each amino acid in the middle of the chain has an amino group (a -NH2 group) on one side (which is attached to the carboxyl group of the previous amino acid) and a carboxyl group on the other side (which is attached to the amino group of the next amino acid). The other end of the polypeptide chain is called the amino terminal.