Cyclodextrins and pesticides: how molecular recognition can help control agrochemicals

What is it about?

How do pesticides interact with molecules capable of capturing them? Understanding this question at the molecular level could open new possibilities for controlling the mobility, stability and environmental fate of agrochemicals. A research study has investigated the interaction between carbaryl (CB), a widely studied carbamate pesticide, and β-cyclodextrin (β-CD), a naturally derived cyclic molecule characterized by a hydrophobic internal cavity. Cyclodextrins are particularly interesting in this context because their cavity can accommodate suitably sized organic molecules, forming so-called host–guest complexes. Such molecular recognition processes can potentially be exploited for pesticide sequestration, residue management, controlled release and environmental remediation.

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Photo by Dibakar Roy on Unsplash

Photo by Dibakar Roy on Unsplash

Why is it important?

β-Cyclodextrin can be thought of as a molecular container: its external surface is hydrophilic and therefore compatible with water, while its internal cavity provides a relatively hydrophobic environment. This peculiar architecture allows β-CD to interact with organic molecules that would otherwise have limited compatibility with an aqueous environment. In the case of carbaryl, the study found that the pesticide forms a 1:1 inclusion complex with β-CD. Fluorescence spectroscopy showed that the complex is moderately stable, with an association constant of approximately 189 M⁻¹. This value indicates that the interaction is sufficiently strong to promote complex formation in water, while still allowing the guest molecule to exchange dynamically with the surrounding solution. Where does carbaryl fit inside the cyclodextrin? One of the most important questions was not simply whether carbaryl binds to β-CD, but how it binds. NMR spectroscopy provided important structural information. In particular, changes observed for the protons located inside the β-CD cavity indicated that the internal region of the cyclodextrin is directly involved in the interaction. Even more revealing were the intermolecular ROESY correlations, which allowed the relative proximity between different portions of the two molecules to be investigated. The results indicate a preferential orientation in which the naphthalene moiety of carbaryl penetrates into the hydrophobic cavity of β-CD, whereas the carbamate group remains close to the hydroxyl rim of the cyclodextrin. In simple terms, the aromatic portion of the pesticide acts as the part that is preferentially “hosted” inside the molecular cavity, while the more polar carbamate portion remains closer to the aqueous environment. Experimental observations were complemented by density functional theory (DFT) calculations. Computational modelling independently supported the binding geometry proposed from the spectroscopic experiments. The calculations also provided information about the nature of the interactions responsible for complex stabilization. Rather than being dominated by a single strong chemical bond, the association is primarily driven by a combination of hydrophobic effects and dispersion interactions, particularly involving the aromatic region of carbaryl and the interior of the cyclodextrin cavity.

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