Protonation of the strong base methylamine CH3NH2 by carbonic acid H2CO3 in aqueous solution, HOCOOH···NH2CH3 → HOCOO–···+HNH2CH3, has been previously studied ( J. Phys. Chem. B 2016, 109, 2271−2280; ...

Unnatural amino acids have been incorporated and used in several protein systems to incisively probe cation–pi interactions, radical transfer reactions, general acid–base catalysis, hydrogen bonds, ...