 The peptide bond has two resonance forms that contribute some double @-@ bond character and inhibit rotation around its axis , so that the alpha <unk> are roughly <unk> . The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone . The end of the protein with a free <unk> group is known as the C @-@ terminus or <unk> terminus , whereas the end with a free amino group is known as the N @-@ terminus or amino terminus . The words protein , polypeptide , and peptide are a little ambiguous and can overlap in meaning . Protein is generally used to refer to the complete biological molecule in a stable conformation , whereas peptide is generally reserved for a short amino acid <unk> often lacking a stable three @-@ dimensional structure . However , the boundary between the two is not well defined and usually lies near 20 – 30 residues . <unk> can refer to any single linear chain of amino acids , usually regardless of length , but often implies an absence of a defined conformation . 
