Salt bridges between oppositely charged amino acid side chains. In this representation of the three-dimensional structure of a polypeptide, the white tube represents the peptide backbone; dashed lines indicate salt bridges between the positively charged amino groups of an arginine side chain (nitrogens shown in blue) and the negatively charged carboxyl group of a glutamic acid side chain (oxygens shown in red). Only the side chains of the four residues involved in the salt bridges are shown.