The aromatic sidechains of amino acids as neutral donor groups for alkali metal cations.

An aromatic residue that can serve as a pi-donor occurs in all known protein sequences about one out of every 11 amino acids. Benzene, phenol, and indole, the sidechains of phenylalanine, tyrosine, and tryptophan, are particularly important in protein structure. Solid state structures confirm the interactions of these neutral arenes, along with double and triple bonds, with sodium and potassium cations.