Amino acids with basic side chains

The side chains of the basic amino acids accept protons . At physiologic pH, the R groups of lysine and arginine are fully ionized and positively charged. In contrast, the free amino acid histidine is weakly basic and largely uncharged at physiologic pH. However, when histidine is incorporated into a protein, its R group can be either positively charged (protonated) or neutral, depending on the ionic environment provided by the protein. This important property of histidine contributes to the buffering role it plays in the functioning of such proteins as hemoglobin . [Note: Histidine is the only amino acid with a side chain that can ionize within the physiologic pH range.]