Recognition Characteristics of an Adaptive Vesicular Assembly of Amphiphilic Baskets for Selective Detection and Mitigation of Toxic Nerve Agents

Abstract

We used isothermal titration calorimetry to investigate the affinity of basket <b>1</b> (470 ų) for trapping variously sized and shaped organophosphonates (OPs) <b>2</b>–<b>12</b> (137–244 ų) in water at 298.0 K. The encapsulation is, in each case, driven by favorable entropy (<i>T</i>Δ<i>S</i>° = 2.9 kcal/mol), while the enthalpic component stays small and in some cases endothermic (Δ<i>H</i>° ≥ −1 kcal/mol). Presumably, a desolvation of basket <b>1</b> and OP guests permits the inclusion complexation at room temperature via a “classical” hydrophobic effect. The amphiphilic basket <b>1</b> shows a greater affinity (Δ<i>G</i>° ≈ −5 to −6 kcal/mol), both experimentally and computationally, for encapsulating larger organophosphonates whose size and shape correspond to VX-type agents (289 A³). Importantly, baskets assemble into a vesicular nanomaterial (<i>D</i>_H ≈ 350 nm) that in the presence of neutral OP compounds undergoes a phase transition to give nanoparticles (<i>D</i>_H ≈ 250 nm). Upon the addition of an anionic guest to basket <b>1</b>, however, there was no formation of nanoparticles and the vesicles grew into larger vesicles (<i>D</i>_H ≈ 750 nm). The interconversion of the different nanostructures is reversible and, moreover, a function of the organophosphonate present in solution. On the basis of ¹H NMR spectroscopic data, we deduced that neutral guests insert deep into the basket’s cavity to change its shape and thereby promote the conversion of vesicles into nanoparticles. On the contrary, the anionic guests reside at the northern portion of the host to slightly affect its shape and geometric properties, thereby resulting in the vesicles merely transforming into larger vesicles