Quantitative Evaluation of Noncovalent Interactions between Glyphosate and Dissolved Humic Substances by NMR Spectroscopy

Abstract

Interactions of glyphosate (N-phosphonomethylglycine) herbicide (GLY) with soluble fulvic acids (FAs) and humic acids (HAs) at pH 5.2 and 7 were studied by (1)H and (31)P NMR spectroscopy. Increasing concentrations of soluble humic matter determined broadening and chemical shift drifts of proton and phosphorus GLY signals, thereby indicating the occurrence of weak interactions between GLY and humic superstructures. Binding was larger for FAs and pH 5.2 than for HAs and pH 7, thus suggesting formation of hydrogen bonds between GLY carboxyl and phosphonate groups and protonated oxygen functions in humic matter. Changes in relaxation and correlation times of (1)H and (31)P signals and saturation transfer difference NMR experiments confirmed the noncovalent nature of GLY-humic interactions. Diffusion-ordered NMR spectra allowed calculation of the glyphosate fraction bound to humic superstructures and association constants (K(a)) and Gibbs free energies of transfer for GLY-humic complex formation at both pH values. These values showed that noncovalent interactions occurred most effectively with FAs and at pH 5.2. Our findings indicated that glyphosate may spontaneously and significantly bind to soluble humic matter by noncovalent interactions at slightly acidic pH and, thus, potentially pollute natural water bodies by moving through soil profiles in complexes with dissolved humus.

Distribution of Glyphosate in Chicken Organs and its Reduction by Humic Acid Supplementation

“…Many studies have reported that glyphosate can be sorbed to humic acids (Piccolo et al, 1996;Banta et al, 2009;Mazzei and Piccolo, 2012). In the present study the humic acid RB4 neutralized the antimicrobial effect of glyphosate in vitro.…”

“…Humic acids are classified among the polyvalent weak organic acids and possess the ability to form com-plexes. Humic substances could adsorb glyphosate in vitro (Piccolo et al, 1996;Banta et al, 2009;Mazzei and Piccolo, 2012). Also Chlorella (microalgae) and yeast showed detoxifying properties (Shim et al, 2008;Baptista et al, 2008).…”

Oral Application of Charcoal and Humic acids to Dairy Cows Influences Clostridium botulinum Blood Serum Antibody Level and Glyphosate Excretion in Urine

“…and Piccolo found that glyphosate may spontaneously and significantly bind to soluble humic matter by non-covalent interactions at slightly acidic pH [39]. Binding to matrices such as soluble fulvic and humic acids could be the reason.…”

Water-mediated NOE: a promising tool for interrogating interfacial clay–xenobiotic interactions

“…In environmental research, NMR is becoming an important tool used to highlight specific interactions between different constituents in soils [21,[29][30][31][32]. Over the years, numerous detailed NMR studies have provided valuable information on the underlying factors regarding the different binding properties of a number of anthropogenic contaminants [31][32][33][34][35]. Recently, a novel NMR technology was introduced, termed as comprehensive multiphase (CMP) NMR [38].…”

“…Recently, a novel NMR technology was introduced, termed as comprehensive multiphase (CMP) NMR [38]. This approach combines hardware from solution, gel, and solid-state NMR into a single probe and permits environmental samples to be investigated in situ, in their fully hydrated state, and in all phases, allowing for all phases to be simultaneously investigated [28,[35][36][37][38]. A number of NMR techniques can be used to investigate interactions between organic soil constituents [31,36,39].…”