Basic Amino Acids
Three of the common amino acids have side chains with net positive charges at neutral pH: histidine, arginine, and lysine. The ionized group of histidine is an imidazolium, that of arginine is a guanidinium, and lysine contains a protonated alkyl amino group. Arginine and lysine side chains, which are protonated under physiological conditions, participate in electrostatic interactions in proteins.
