Characterization of Solar Radiation-Induced Degradation Products of the Plant Sunscreen Sinapoyl Malate

Abstract

Agricultural activities at lower temperatures lead to lower yields due to reduced plant growth. Applying photomolecular heater agrochemicals could boost yields under these conditions, but UV-induced degradation of these compounds needs to be assessed. In this study, we employ liquid chromatography−mass spectrometry (LC−MS) coupled with infrared ion spectroscopy (IRIS) to detect and identify the degradation products generated upon simulated solar irradiation of sinapoyl malate, a proposed photomolecular heater/UV filter compound. All major irradiation-induced degradation products are identified in terms of their full molecular structure by comparing the IRIS spectra obtained after LC fractionation and mass isolation with reference IR spectra obtained from quantum-chemical calculations. In cases where physical standards are available, a direct experimental-toexperimental comparison is possible for definitive structure identification. We find that the major degradation products originate from trans-to-cis isomerization, ester cleavage, and esterification reactions of sinapoyl malate. Preliminary in silico toxicity investigations using the VEGAHUB platform suggest no significant concerns for these degradation products' human and environmental safety. The identification workflow presented here can analogously be applied to break down products from other agrochemical compounds. As the method records IR spectra with the sensitivity of LC−MS, application to agricultural samples, e.g., from field trials, is foreseen.

Structural Elucidation of Agrochemical Metabolic Transformation Products Based on Infrared Ion Spectroscopy to Improve In Silico Toxicity Assessment

“…Hence, since the diganoiol tails are not symmetric, their ester cleavage may lead to two distinct isomeric forms of the m / z 499 product. Further, we will consider both cis - and trans -isomerized (upon which the UV-induced heating activity of DGSM is based) ,, products for the m / z 499 ion, giving four possible isomers.…”

“…The Bruker AmaZon ion trap was connected to the LC and equipped with a two-position, six-port divert valve for fractionation. ,, The elution time of the metabolite of interest was determined using a mass spectrometer to fractionate it from the biological matrix. Five injections were fractionated by programming the divert valve to divert the flow to a sample vial at the observed elution time.…”

“…Our computational workflow uses the cheminformatics toolbox RDKit in Python 3 and the Gaussian16 quantum chemistry software suite. For an input structure, we assume that all oxygen and nitrogen atoms can be (de)­protonation sites or coordinate with Na + ions. ,,, 500 random 3D conformations are then generated and minimized using the MMFF94 force field. The random conformation set is optimized at the semiempirical PM6 level, followed by vibrational analysis .…”

“…Hence, since the diganoiol tails are not symmetric, their ester cleavage may lead to two distinct isomeric forms of the m/z 499 product. Further, we will consider both cis-and trans-isomerized (upon which the UV-induced heating activity of DGSM is based) 29,41,47 products for the m/z 499 ion, giving four possible isomers. It appears that the m/z 499 ion (MA) is an intermediate metabolic product, as the accurate masses and corresponding molecular formulas of the other ions in Table 1 suggest further derivatization of this species.…”

“…The analytical chemistry toolbox offers various methods to elucidate the chemical structure of metabolites, including tandem mass spectrometry, nuclear magnetic resonance (NMR) spectroscopy, and radiolabeled analysis of samples. − Tandem mass spectrometry (MS/MS) is sensitive and fast, but data interpretation relies strongly on external standards collected in various libraries. On the other hand, NMR spectroscopy can resolve chemical structures without database references, − ,− but extensive purification is usually required, which is challenging, especially in multiwell plate in vitro studies. ,,,− Much progress has recently been achieved using infrared ion spectroscopy (IRIS), where IR spectra of mass-to-charge selected species in an LC–MS workflow are measured. − IRIS combines the sensitivity and selectivity of MS with the structural diagnostics of IR spectroscopy and has been successfully applied to identify, for instance, drug and plant metabolites. − ,,− …”