Protonation of trimipramine salts of maleate, mesylate and hydrochloride observed by 1H, 13C and 15N NMR spectroscopy

Protonation of the tricyclic antidepressant drug trimipramine with maleic acid, methanesulfonic acid and hydrochloric acid was studied using 1H, 13C and 15N NMR spectroscopy at natural abundance. The effect of counter ions on the protonation was compared under identical conditions of solvent, concentration and temperature using homonuclear and heteronuclear one- and two-dimensional experiments. Differential protonation of the terminal tertiary amine nitrogen is determined from the indirect spin-spin couplings, chemical shifts, 13C relaxation data and variable-temperature experiments. In the maleate salt, only one of the acidic protons is involved in protonation, the other being associated with the anion moiety. 15N chemical shifts of the protonated nitrogens are nearly linearly related to the pK(a) of the constituent acid.