Recognition of bio-relevant dicarboxylate anions by an azacalixarenetriazine derivative decorated with urea moieties†
A new dichloroazacalixarenetriazine receptor (1) with two chiral urea binding moieties is reported. The binding affinity of this macrocycle was evaluated by 1H NMR titrations in CDCl3 for the dicarboxylate anions oxalate (ox2−), malonate (mal2−), succinate (suc2−), glutarate (glu2−), diglycolate (dg2−), fumarate (fum2−), maleate (male2−), and (R,R)- and (S,S)-tartarate (tart2−) enantiomers. Among the first five linear anions, the higher association constants were calculated for the larger anions glu2− and dg2− and for the smallest anion ox2−, with Kass values following the sequence dg2− > glu2− > ox2− > suc2− > mal2−. Despite the high binding affinity of 1 for both tart2− enantiomers, no enantioselectivity was observed. By contrast, Kass for fum2− is ca. 8.9 times greater than that for male2−, showing the selectivity of 1 for the trans isomer. These binding preferences were further elucidated by theoretical methods. Molecular dynamics simulations showed that the linear anions are lodged between both pendant arms and that each anion can assume two distinct binding poses, with one or two carboxylate groups establishing intermittent hydrogen bonds with both urea binding units. On the other hand, the recognition of male2− ensues in an alternative scenario, characterised by the interaction between a carboxylate group and a single urea binding unit, mirroring the lower experimental binding affinity relatively to fum2−. A linear increase of the receptor's Nurea⋯Nurea and the anions’ −O2C⋯CO2− distances versus experimental Kass was established for mal2−, suc2−, glu2− and dg2− associations, indicating that the match between these two distances determines the anion binding strength. The affinity for ox2− was associated with the most negative values of electrostatic potential positioned near carboxylate groups.