Development of a ReaxFF force field for glyphosate and investigation of its abiotic degradation pathways
Date
2021
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
A ReaxFF force field has been developed for the widely used herbicide glyphosate (GLP), which degrades in the environment via C-P and C-N bond dissociation. Depending on the specific reaction pathway, toxic or non-toxic metabolites may form, however, the factors that influence the selection or predominance of a given degradation pathway are not entirely understood. The ReaxFF force field developed in this work has been used to perform molecular dynamics simulations of glyphosate degradation and the umbrella sampling method has been used to determine the free energy barriers opposing C-P and C-N bond dissociation. Approximately fifty carbon-phosphorus interaction parameters were fitted against a density functional theory (DFT) derived training set that included configurational energies of glyphosate for various C-P distances, C-P-O angles, and C-P-O-H dihedral angles. The force field was further validated using an extensive set of DFT energies calculated for glycine (GLY), aminomethylphosphonic acid (AMPA), and methylphosphonic acid (MPA). Umbrella sampling calculations were carried out using two conformations of monoanionic (GLP-) and two conformations of dianionic (GLP2-) glyphosate. The GLP2- simulations yielded free energy values of (24.2, 24.7) kcal/mol, (35.1, 36.5) kcal/mol, and (44.1, 44.3) kcal/mol for the C-P bond breaking sarcosine pathway, C-N bond breaking GLY/MPA pathway, and C-N bond breaking AMPA pathway respectively. The C-P bond breaking energies are in close agreement with the previously reported DFT value of 21.8 kcal/mol. The GLP- simulations did not reveal a well-defined energy barrier for C-P bond breaking as a spurious carbon-oxygen interaction manifested during umbrella sampling. However, the GLP- simulations yielded C-N bond breaking energies of (17.6, 22.4) kcal/mol for the GLY/MPA acid pathway, and (26.3, 27.3) kcal/mol for the AMPA pathway respectively. In every case, it was observed that GLY/MPA pathway was the preferred pathway over the AMPA pathway for C-N bond dissociation. This could explain the experimental observation that glyphosate can degrade directly to glycine bypassing the formation of intermediate sarcosine after C-N bond cleavage.
Description
Keywords
Molecular dynamics simulations, Glyphosate degradation, Free energy barriers, C-P and C-N bond dissociation