The tautomer ratio of histamine

Abstract

Abstract The tautomeric equilibrium constant Kt for the two tautomeric forms (Nτ-H and Nπ-H) of histamine mono-cation is determined from the difference between the pKa values of Nπ- and Nτ-methylhistamines, and of Nπ- and Nτ-benzylhistamines. The ratio of concentrations of tautomers [Nτ-H]/[Nπ-H] = Kt is 4ṁ2 indicating that the histamine mono-cation exists in aqueous solution approximately 80% as the Nτ-H-tautomer and 20% as the Nπ-H tautomer. From the value of Kt and pKa1 of histamine the individual ionization constants at 25° of the two tautomers are derived. Tautomer Nτ-H (pKa1 6ṁ16) is a slightly weaker base than is tautomer Nπ-H (pKa1 6ṁ79).