fac-[Re(H2O)(3)(CO)(3)](+) Complexed with Histidine and Imidazole in Aqueous Solution: Speciation, Affinity and Binding Features

The use of isotopes of transition metal has opened many perspectives in the development of novel compounds utilizable for therapeutic or diagnostic purposes. In order to be exploited, though, they must be stabilized by chelating systems in coordination complexes. The tricarbonyl complexes of Technetium(I) and Rhenium(I) have assumed important roles in these fields as they are readily conjugated to biomolecules to form stable probe useful in the targeting approach. In this context the amino acid histidine represents a valuable ligand for bioconjugation. Since little is reported on the chemistry and speciation of Re(I) and Tc(I) in aqueous solution, here, we analyze the interaction of fac-[Re(H2O)(3)(CO)(3)](+) with histidine or with the simple imidazole ring by means of potentiometric and spectroscopic techniques. We describe in details the equilibria that arise in solution, calculate the constants of each equilibrium reaction and report coordination properties and atomic details of the formed complexes. Our data represent a valuable contribute towards the exploitation in nuclear medicine of histidine containing biomolecules complexing Tc (I) and Re(I).