Photochemical transformation of the plant activator Acibenzolar-S-methyl in solution

•Plant elicitors like BTH are an alternative to conventional pesticides.•BTH absorbs solar light strongly and undergoes fast photolysis in a wide range of solvents.•Loss of N2 by initial cleavage of the S-N bond is highly favored as confirmed by DFT calculations.•Cleavage of the thioester function also takes place.•Numerous photoproducts are formed, in particular those arising from S and ring oxidation.

Acibenzolar-S-methyl (BTH) is used to protect plants from pathogens by triggering Systemic Acquired Resistance. While this compound absorbs solar light strongly, little is known on its ability to undergo photodegradation and on the generated photoproducts. In the present work, we studied the photolysis of BTH dissolved in solvents of different polarities: n-heptane to mimic the hydrophobic surface of leaves and more polar solvents to simulate polar environmental compartments. We found that BTH is easily photodegraded in simulated solar light. The quantum yield of photolysis at 313 nm ranges from 0.048 to 0.092 depending on the solvent. LC-ESI-HRMS analyses in negative and positive modes revealed the presence of numerous photoproducts arising from two initial reaction pathways. As confirmed by DFT calculations, the main pathway involves the SN bond cleavage followed by N2 loss. The scission of the thioester bond with the generation of acyl and SCH3 radicals also takes place. Based on these findings, it can be predicted that BTH should undergo fast photodegradation once released in the environment and lead to numerous new compounds.

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