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

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Abstract

Multinuclear one (1D-) and two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopic investigations of famotidine, the most potent and widely used histamine H2-receptor antagonist, were carried out in dimethyl sulfoxide-d6 (DMSO-d6) 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 1H, 13C, and 15N 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-d6. 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 NH2 protons and the thiazole nitrogen. Our detailed 1D and 2D NMR experiments enabled complete ab initio 1H, 13C, and 15N 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-d6. The aqueous acid–base properties of famotidine were studied by 1D 1H- and 2D 1H/13C 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.

Correct amidine assignments and protonation sites of famotidine

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Notes

  1. pH* refers to the value measured in deuterium oxide by a glass electrode calibrated with aqueous buffer solutions. According to Gross-Butler-Purlee theory [29], the pD value can be calculated from pH* values by addition of a factor of 0.44.

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Acknowledgment

The authors are grateful to Professor Csaba Szántay Jr. (Gedeon Richter Plc.) for the fruitful discussions and suggestions. This work was supported by the National Scientific Research Fund of Hungary, OTKA K73804.

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Authors and Affiliations

  1. Department of Pharmaceutical Chemistry, Semmelweis University, 1092, Budapest, Hőgyes Endre u. 9, Hungary

    Attila Marosi, Szabolcs Béni, Ákos Rácz & Béla Noszál

  2. Drug Polymorphism Research, Gedeon Richter Plc., P.O. Box 27, 1475, Budapest, Hungary

    Zsófia Szalay & Ádám Demeter

  3. Spectroscopic Research, Gedeon Richter Plc., P.O. Box 27, 1475, Budapest, Hungary

    Zoltán Szakács & Tamás Gáti

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  1. Attila Marosi
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Correspondence to Ádám Demeter.

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Marosi, A., Szalay, Z., Béni, S. et al. Solution-state NMR spectroscopy of famotidine revisited: spectral assignment, protonation sites, and their structural consequences. Anal Bioanal Chem 402, 1653–1666 (2012). https://doi.org/10.1007/s00216-011-5599-6

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