Structural complexity of glyphosate and aminomethylphosphonate metal complexes.

Abstract

Small differences in the structure and subsequent reactivity of glyphosate complexes can have a highly consequential impact due to the enormous quantities of glyphosate used globally. The gas phase metal speciation of glyphosate and its abundant metabolite, aminomethylphosphonic acid (AMPA), were determined using cross-platform electrospray ionisation ion mobility mass spectrometry. Monomeric [M + L - H]+ complexes, and both larger, and/or higher order clusters formed with divalent metals (M = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Cu2+, and Zn2+; and L = glyphosate and AMPA). Complexation occurred at more than one ligand donor site for [M + L - H]+, resulting in multidentate complexes. The type of complex depended on M, with central positions maximizing the interactions of the M with donor sites of the L preferred. The isomers were separated by ion mobility and experimental collisional cross sections (N2CCSexp) were derived for all isolated species. An energy threshold DFT approach located the structural families and potential lowest energy forms; these were found to be consistent with confirmed condensed phase (reported crystal structures) and gas phase structures (via infrared multiple photon dissociation, IRMPD). Theoretical nitrogen collisional cross sections (N2CCScalc) of these confirmed structures tended to underestimate the N2CCSexp for both [M + glyphosate - H]+ and [M + AMPA - H]+ complexes. Underestimation ranged between 1-20%, and was not uniform between species. By comparison, helium collisional cross sections (HeCCSexp and HeCCScalc) were in better agreement (within 1-3%). These findings suggest further refinements are needed to collisional cross section modelling for metal containing species, in particular for nitrogen drift gas.