Four side chains are charged at physiological pH values. The charges occur at the ends of the side chains, away from the backbone. Aspartate and glutamate are negatively charged; lysine and arginine are positively charged. The methylene groups between the charged functional group and the backbone are nonpolar.
The Acidic Residues: Asp and Glu
The side chains of and differ only in having one and two methylene groups, respectively. The carboxyl groups of the side chains have pK values of 3.9 (Asp) and 4.3 (Glu), so these side chains are ionized at pH 7.