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Uptake and Exchange Mechanisms of Toxic Metals by Fungal Pigments : a Clue for a Better Understanding of the Accumulation Processes by Mushrooms ?

Final Activity Report Summary - FUNGI AND METALS (Uptake and exchange mechanisms of toxic metals by fungal pigments: a clue for a better understanding of the accumulation processes by mushrooms)

My pluridisciplinary research project was dealing with pigments of mushrooms in order to provide a better understanding of their role in the uptake and accumulation of deleterious metals from polluted environments. Fungi, in particular, often exhibit a remarkable ability to accumulate a large variety of elements, ranging from the heaviest of the transition metals such as lead, to the alkali metals, including radioisotopes like 137Cs. Previous studies of my host group with norbadione A, the pigment of an edible mushroom Bay boletus, revealed that norbadione A was able to accumulate caesium due to allosteric properties. During my postdoctoral training, electrospray mass spectrometry allowed us to demonstrate that the natural dipotassic norbadione A complex exchanges its potassium cations with caesium. Furthermore, a fruitful combination of analytical methods, namely electrospray mass spectometry (ES-MS), potentiometry and absorption spectrophotometry, clearly showed that norbadione A forms mainly mononuclear complexes with heavy metals (Cd(II), Ni(II) and Pb(II)) and lanthanides (La(III), Eu(III) and Lu(III)), taken as models for actinides. By contrast with alkali cations, significant negatively cooperative effects between the two heavy metals could be measured. These data suggest that the divalent cations could be stabilised in the central cavity of norbadione A and that electrostatic repulsions between the two metals would not favour accumulation. Moreover, the decrease of the stability of the corresponding atromentic and norbadione A complexes with the electronic density of the cations strongly suggests the formation of outer-sphere complexes. The chemical structure of pulvinone-type pigments could be dimeric as for norbadione A, but monomeric analogues are also present, sometimes in large amounts. Norbadiona A forms with divalent cations complexes which are of about two orders of magnitude more stable then that formed with atromentic acid. For lanthanide ions this difference is even more pronounced. The stability of norbadione A complexes with lanthanides is about five orders of magnitude large than that of atromentic acid complexes. Our results also demonstrated that norbadione A might be a major complexing molecule in mushrooms. Low molecular weight organic acids, like malic, succinic, citric or oxalic acids, cannot indeed compete with dimeric pulvinic acids derivatives such as norbadione A.

Due to the presence of the catechol moiety in xerocomic acid, a dimeric self-assembled structure could be also obtained via the chelation of either Fe(III) or Al(III) transferred from the soil. Our data showed that Al(III) could act as an 'anchor' and gather two xerocomic acids. Such an edifice may then 'mimicanorbadione A offering the pulvinone-type sites for coordination of deleterious cations. Moreover, we have examined a fluorescent probe, which will be able to visualise the mobilisation of iron(III) by bacteria in soils and its accumulation in plants and mushrooms. We considered linear ligands based on three hydroxamates units and took into account the effect of the spacers, the localisation of the fluorophore and its bulkiness on the iron(III) coordination properties (J. Am. Chem. Soc., 2005).