Vanadium-fulvic acid chemistry: conformational and binding studies by electron spin probe techniques

Two fractions of soil fulvic acid (FA) were separated by gel filtration chromatography. An observed increase in volume of the heavier fraction (FA I) with increasing pH was attributed to aggregation, intramolecular negative charge repulsions and the rupture of hydrogen bonds, which control molecular conformation. Optical absorption properties and elemental analyses of both fractions were determined. The stability constants and stoichiometries of FA complexes with vanadyl, VO²⁺, at pH 5.0 and ionic strength of 0.04 M were measured by electron paramagnetic resonance (EPR) spectroscopy. EPR spectra of model VO²⁺ complexes with phthalic and salicylic acids, which are the probable functional groups present in FA, are identical to those of the VO²⁺-FA complexes. Aggregation of FA I occurs in the presence of VO²⁺ to form a complex that can be approximated as ‘(VO)₂(FA I)₆’. The average site distance between vanadyl ions in this complex is shown to be greater than 1.2 nm. EPR parameters for FA I suggest binding by carboxylate groups. These parameters are compared with those of other vanadyl complexes with fulvic and humic acids reported by others. Reduction of VO₃^? to VO²⁺ by these materials is discussed.