A nuclear magnetic resonance-based study of the behavior of the tautomers of triclabendazole in DMSO-d6, in the presence of water

Abstract

The benzimidazole antihelmintic agent triclabendazole (TCB) exhibits tautomeric polymorphism. It has been claimed that DMSO-d6 solutions prepared from the polymorphs of TCB display different NMR behavior. Therefore, a comprehensive NMR study was undertaken, in order to unveil the causes behind this unusual observation. In a first instance, 1H and 13C spectra of TCB were unequivocally assigned. Then, isothermal and variable temperature 1H NMR experiments were conducted in order to explore the relationship between the spectral changes with system variables: time, temperature and amount of water. Solutions prepared from both polymorphs experienced water-dependent spectral changes related to a tautomerization process at 298 K, and EXSY experiments confirmed the participation of H2O in the N[sbnd]H exchange. The tautomerization constant (KT = 1.14 ± 0.07) was determined from the signals of H4 at 298 K. On the other hand, variable temperature 1H NMR enabled estimation of coalescence temperatures (Tc) for the signals of H4, H6’ and NH, where a water-dependence was found for H4 (Tc = 298→334 K when H2O:TCB = 8:1→16:1). The exchange constants at coalescence (kc = 111.8 s−1 at 303 K, H2O:TCB = 9:1 and kc = 179.7 s−1 at 334 K, H2O:TCB = 16:1) were also determined. Thermodynamic data of the tautomerization, including free energy (ΔG‡ = 15.03 Kcal mol−1 at 300 K), enthalpy (ΔH‡ = 7.75 Kcal mol−1) and entropy (ΔS‡ = –0.0244 Kcal mol−1 K at 300 K) were obtained from the Eyring graph of linewidth-corrected exchange constant values of H4, while the activation energy (Eact = 8.48 Kcal mol−1) was obtained from an Arrhenius plot. A 1H NMR/chemometric approach demonstrated the reversibility of the temperature-driven changes in the range 298–355 K. Finally, a novel mechanism was proposed to account for the observations and dismiss the original claim, taking the presence of water in the system as the agent responsible for the detected variations.