 The peptide bond has two resonance forms that contribute some double @-@ bond character and inhibit rotation around its axis , so that the alpha carbons are roughly coplanar . 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 carboxyl group is known as the C @-@ terminus or carboxy 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 oligomers often lacking a stable three @-@ dimensional structure . However , the boundary between the two is not well defined and usually lies near 20 – 30 residues . Polypeptide can refer to any single linear chain of amino acids , usually regardless of length , but often implies an absence of a defined conformation . 
 The peptide bond <a href='got' title='1'><mark>has</mark></a> two resonance forms <a href='null_relcl' title='2'><mark>that</mark></a>contribute some double @-@ bond character and inhibit rotation around its axis , so that the alpha carbons <a href='drop_aux' title='3'><mark>are</mark></a>roughly coplanar . 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 carboxyl group is known as the C @-@ terminus or carboxy terminus , whereas the end with a free amino group is known as the N @-@ terminus or amino terminus . The words protein , polypeptide , and peptide <a href='drop_aux' title='4'><mark>are</mark></a>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 oligomers often lacking a stable three @-@ dimensional structure . However , the boundary between the two <a href='negative_concord' title='5'><mark>is</mark></a> <a href='negative_concord' title='6'><mark>not</mark></a>well defined and usually <a href='uninflect' title='7'><mark>lies</mark></a> near 20 – 30 residues . Polypeptide can refer to any single linear chain of amino acids , usually regardless of length , but often <a href='uninflect' title='8'><mark>implies</mark></a> an absence of a defined conformation . 
The peptide bond got 2 resonance forms contribute sum double @-@ bond character and inhibit rotation around its axis , so dat the alpha carbons roughly coplanar . The other tw2ihedral angles in the peptide bond determine the local shape assumed by da protein backbone . Tdaend of the protein witt a free carboxyl group is known as the C @-@ terminus or carboxy terminus , whereas the end with a free amino group is known as thda @-@ terminus or amino terminus . The words protein , polypeptide , and peptide alil 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 oligomers often lacking a stable three @-@ dimensional structure . However , the boundary between the two ain't well defined and usually lie near 20 – 30 residues . Polypeptide can refer to any single linear chain of amino acids , usually regardless of length , but often imply an absence of a defined conformation .