pH-controlled release of auxin plant hormones from cucurbit[7]uril macrocycle

Abstract

Background: The influence of pH on the formation of host-guest complexes between the cucurbit[7]uril (CB[7]) macrocyclic host and three auxin plant hormones, namely indole-3-acetic acid (IAA), 2-naphthalene acetic acid (2-NAA), and 2,4-dichlorophenoxyacetic acid (2,4-D), was studied by 1H NMR and relaxation experiments. Results: Only protonated auxins formed inclusion complexes with CB[7], exhibiting preferential encapsulation of the aromatic part inside the host cavity, and orientation of the carboxyl group towards the carbonyl-laced portals of CB[7]. At pH values above the auxin pKa values, the guest molecules were negatively ionized and were no longer retained within the macrocyclic host, suggesting that a pH-controlled release of auxin guests from the CB[7] host is possible. Conclusions: The development of a technology based on the use of cucurbit[n]urils for the pH-controlled release of auxin molecules in plant systems represents an opportunity to exploit these macrocyclic compounds in a variety of agricultural applications.