THe binding of fulvic acid (FA) to paramagnetic ions, such as Mn2+ , Cu2+ and Fe3+ was investigated by an NMR method, to explore the nature of complexes formed. The method involves determination of nuclear relaxation times of the solvent water protons. These are sensitive to any change in the coordination sphere of the paramagnetic ion and thus provide information about possible binding between the paramagnetic ions under investigation and FA. Measurements of relaxation rates of the paramagnetic ion solutions with FA were compared with results obtained from solutions of the same metal ion and model ligands to study the type of complexation and the binding site of the metal-FA complex.
The experimental results suggest the following conclusions:
Among the model ligands, sulfosalicylic acid showed the closest similarity in complexation to FA.
Fe3+ and Cu2+ bind to FA with displacement of water from the first coordination zone of the metal, forming inner sphere complexes. This deduction was reached since the Fe3+ -FA solutions showed marked reduction in relaxation rates compared with the aquocomplex. It can be explained with the reduction of the number of coordinated waters along with the variation in the correlation times. The solutions containing Cu and FA showed enhanced longitudinal relaxation rates, which are the result of slower rotations upon inner sphere complexation with macromolecules.
Slight changes in the relaxation rates of the Mn2+ solutions upon FA complexation suggest that Mn2+ forms an outer sphere assymmetric complex, where the metal rotates with the water molecules around it in a fulvic acid "cage". The small variation in the relaxation rates could arise from symmetry distortion in the complex or from outer sphere contributions.