Abstract
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. 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. 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.
Highlights
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
Our aim was to study, by proton nuclear magnetic resonance (1H NMR) spectroscopy, the binding that occurs in aqueous solution between the CB[7] macrocyclic host molecule and three weak organic acids of the auxin family, IAA, 2-NAA, and 2,4-D (Figure 2), and their response to changes in pH
No significant pKa shifts were observed for the auxin guests upon addition of CB[7], most likely because the hormone carboxylic acid moieties, which are positioned outside the macrocyclic host, were not affected by changes in the electronic environment
Summary
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. Chemical and biochemical technologies to support crop production could play a vital role towards food security in the coming years In this context, a technology based on host-guest complexation of bioactive compounds by macrocyclic host molecules in aqueous media may represent a controlled release system that may achieve efficient and balanced regulation of plant growth and increase of crop yields [1]. A technology based on host-guest complexation of bioactive compounds by macrocyclic host molecules in aqueous media may represent a controlled release system that may achieve efficient and balanced regulation of plant growth and increase of crop yields [1] Such an innovative technology may become important in sustainable agriculture to substitute inefficient application practices of agrochemicals resulting in their hazardous and expensive dissipation in the environment [2]. Indole-3-acetic acid (IAA) is the most abundant and potent native auxin active in plants [3]. 2-Naphthalene acetic acid (2-NAA) is used as a
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