The 20 common amino acids. The structural formula for each amino acid shows its ionization state at pH 7.0. The groups attached to the α-carbon atom that are shaded pink, blue, or gray—the carboxyl group, amino group, and single proton, respectively—are common to all the amino acids. The side chains (R groups), which are unique to each amino acid, are shaded purple. The R group of histidine is drawn as uncharged, but its pKa is such that a significant fraction of the His side chains will be protonated and thus positively charged at pH 7.0. Several important functional groups in some amino acid side chains are highlighted.