Calcium L-Malate and D-Tartarate Frameworks as Adjuvants for the Sustainable Delivery of Fungicide

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Abstract
Agrichemical adjuvants that combine a highly selective, efficient, and active mode of operation are critically needed to realize a more sustainable approach to their usage. Herein, we report the synthesis and full characterization of two new metal-organic frameworks (MOFs), termed UPMOF-1 and UPMOF-2, that were constructed from eco-friendly Ca2+ ions and naturally-occurring, low molecular weight plant acids, L-malic and D-tartaric acid, respectively. Upon structural elucidation of both MOFs, a widely-used fungicide, hexaconazole (Hex), was loaded on the structures reaching binding affinities of -5.0 and -3.5 kcal mol-1 and loading capacities of 63 and 62% for Hex@UPMOF-1 and Hex@UPMOF-2, respectively, as a result of the formation of stable host-guest interactions. Given the MOFs’ framework chemistry and predisposition to disassembly in relevant agricultural conditions, the sustained release kinetics were determined to show near quantitative release (98 and 95% for Hex@UPMOF-1 and Hex@UPMOF-2, respectively) after >500 h – a release profile drastical-ly different than the control (>80% release in 24 h), from which the high efficiency of these new systems was established. To confirm their high selectivity and activity, in vitro and in vivo studies were performed to illustrate the abilities of Hex@UPMOF-1 and Hex@UPMOF-2 to combat the known, aggressive pathogen Ganoderma boninense (G. boninense) that causes basal stem rot disease in oil palm. Accordingly, at an extremely low concentration of 0.05 μg mL-1, both Hex@UPMOF-1 and Hex@UPMOF-2 were demonstrated to completely inhibit (100%) G. boninense growth and during a 26-week in vivo nursery trial, the progression of basal stem rot infection was not only completely halted upon treatment with Hex@UPMOF-1 and Hex@UPMOF-2, but seedling growth accelerated given the additional nutrients supplied via the disassembly of the MOFs. This study represents a significant step forward in designing adjuvants to support the environ-mentally responsible use of agrichemical crop protection.

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