Structural complexity of glyphosate and aminomethylphosphonate metal complexes

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 = Mg , Ca , Sr , Ba , Mn , Co , Cu , and Zn ; 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 ( CCS ) 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 ( infrared multiple photon dissociation, IRMPD). Theoretical nitrogen collisional cross sections ( CCS ) of these confirmed structures tended to underestimate the CCS 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 ( CCS and CCS ) 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.