Interaction of the Fungal Metabolite Harzianic Acid with Rare-Earth Cations (Pr, Eu, Ho, Tm).

Rare-earth elements (REEs) are in all respect a class of new contaminants that may have toxic effects on organisms and microorganisms and information on their interactions with natural ligands should be of value to predict and control their diffusion in natural environments. In the current study, we investigate interactions of tripositive cations of praseodymium, europium, holmium, and thulium with harzianic acid (HL), a secondary metabolite produced by selected strains of fungi belonging to the genus. We applied the same techniques and workflow previously employed in an analogous study concerning lanthanum, neodymium, samarium, and gadolinium tripositive cations. Therefore, in the current study, HPLC-ESI-HRMS experiments, circular dichroism (CD), and UV-Vis spectrophotometric absorption data, as well as accurate pH measurements, were applied to characterize bonding interactions between harzianic acid and Pr, Eu, Ho, and Tm cations. Problems connected to the low solubility of harzianic acid in water were overcome by employing a 0.1 M NaClO/(CHOH + HO 50/50 /) mixed solvent. For Pr, Ho, and Tm, only the mono complexes PrL, HoL, and TmL were detected and their formation constant determined. Eu forms almost exclusively the bis complex EuL2- for which the corresponding formation constant is reported; under our experimental conditions, the mono complex EuL is irrelevant. Combining the results of the present and previous studies, a picture of interactions of harzianic acid with rare-earth cations extending over 8 of the 17 REEs can be composed. In order to complement chemical information with toxicological information, a battery of bioassays was applied to evaluate the effects of praseodymium, europium, holmium, and thulium tripositive cations on a suite of bioindicators including (Gram-negative bacterium), (green alga), and (microcrustacean), and median effective concentration (EC50) values of Pr, Eu, Ho, and Tm for the tested species were assessed.