Solution-state NMR spectroscopy of famotidine revisited: spectral assignment, protonation sites, and their structural consequences

Multinuclear one (1D-) and two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopic investigations of famotidine, the most potent and widely used histamine H 2 -receptor antagonist, were carried out in dimethyl sulfoxide-d 6 (DMSO-d 6 ) and water. Previous NMR assignments were either incomplete or full assignment was based only on 1D spectra and quantum-chemical calculations. Our work revealed several literature misassignments of the 1 H, 13 C, and 15 N NMR signals and clarified the acid–base properties of the compound at the site-specific level. The erroneous assignment of Baranska et al. ( J. Mol. Struct. 2001, 563) probably originates from an incorrect hypothesis about the major conformation of famotidine in DMSO-d 6 . A folded conformation similar to that observed in the solid-state was also assumed in solution, stabilized by an intramolecular hydrogen bond involving one of the sulphonamide NH 2 protons and the thiazole nitrogen. Our detailed 1D and 2D NMR experiments enabled complete ab initio 1 H, 13 C, and 15 N assignments and disproved the existence of the sulphonamide NH hydrogen bond in the major conformer. Rather, the molecule is predominantly present in an extended conformation in DMSO-d 6 . The aqueous acid–base properties of famotidine were studied by 1D 1 H- and 2D 1 H/ 13 C heteronuclear multiple-bond correlation (HMBC) NMR-pH titrations. The experiments identified its basic centers including a new protonation step at highly acidic conditions, which was also confirmed by titrations and quantum-chemical calculations on a model compound, 2-[4-(sulfanylmethyl)-1,3-thiazol-2-yl]guanidine. Famotidine is now proved to have four protonation steps in the following basicity order: the sulfonamidate anion protonates at pH = 11.3, followed by the protonation of the guanidine group at pH = 6.8, whereas, in strong acidic solutions, two overlapping protonation processes occur involving the amidine and thiazole moieties. Figure Correct amidine assignments and protonation sites of famotidine