Journal of Physical Chemistry B, Vol.124, No.1, 28-37, 2020
Interplay between Ionization and Tautomerism in Bioactive beta-Enamino Ester-Containing Cyclic Compounds: Study of Annulated 1,2,3,6-Tetrahydroazocine Derivatives
Depending on the chemical scaffold, a bioactive species could reflect the interplay between ionization and tautomerism, which is often complicated by the possibility of populating different conformational states, in the case of flexible ligands. In this context, theoretical methods can be valuable to discern the role of these factors, as shown here for beta-enamino esters of 1,2,3,6-tetrahydroazocino-fused ring systems, some of which had proven to be suitable scaffolds for designing novel acetylcholinesterase inhibitors. The compounds investigated herein form two clusters with distinctive experimental pK(a) values (i.e., alpha,beta-diesters and beta-esters ranging within 6.1-7.3 and 8.2-9.0 pK(a) intervals, respectively), which implies a drastic difference in the most populated species at physiological conditions. While chemoinformatic tools did not provide a consistent description of the actual pK(a) values, the theoretical analysis performed for the protonated and neutral species of these compounds revealed a marked change in the tautomeric preference of the tetrahydroazocine moiety upon (de)protonation. Excellent agreement between the calculated and experimental pK(a) values was found when the tautomeric preference of the protonated and neutral species was considered. Overall, this study highlights the potential use of high-level computational methods to disclose the mutual influence between ionization, tautomerism, and conformational preferences in multifunctional (bio)organic compounds.