Functional Group-Level Ligand Exchange of Nucleotide and Nucleic Acids on Goethite

Abstract

Ligand exchange (LE), comprising Fe-O-P formation and OH- release, regulates phosphate-iron (oxyhydr)oxide interactions and their environmental effects in water and soil. Although their multiple P-OH groups render many organic phosphates more favorable than orthophosphate for LE on iron (oxyhydr)oxides, the question remains of how many organic P-O(H) groups (including P-OH and P-O-) contribute to LE and phosphorus adsorption. Here, we confirmed that the LE sites on goethite at pH7.0 for monomeric, oligomeric and nucleic acid organophosphate are also those for orthophosphate, and the stoichiometric relationship between P-O(H) engaged in LE and OH- exchanged from goethite (SP-OH~OH-) is the same for both types of phosphate. Accordingly, organic P-O(H) groups involved in LE can be discriminated by the SP-OH~OH- probed by orthophosphate. For nucleotide monophosphates, diphosphates, triphosphates, and nucleic acids, the number of P-O(H) groups engaged in LE per adsorbed molecule increased from 2 to 2110-29600 because of increased molecular P-O(H) groups, while LE versus adsorbed P-O(H) decreased from 100% to 24.7-40.2% due primarily to nucleic acid clustering-caused shielding of P-O(H). The quantified amount of P-O(H) involved in LE per adsorbed molecule and relative to adsorbed mass elucidated the variations in phosphorus adsorption, as these parameters reflect the LE-motivated adsorption force and molecular mass burden of LE in adsorption, respectively. This LE-based stoichiometry will promote the understanding of environmental organophosphorus-iron coupling.

Keywords: phosphate, ligand exchange, goethite, Adsorption, nucleotide, nucleic acids