SPECIATION STUDIES OF BIOLOGICAL MOLECULES WITH METALS

The study of chemical equilibria, chemical speciation and the knowledge of thermodynamic properties in aqueous solutions play a role of primary importance in many scientific environments (natural waters and biological fluids, soils, air, etc.), since through them it is possible to model the behavior of the components present in multicomponent solutions and predict phenomena such as their biovailability, release, transport and possible degree of absorption in the organism1-2. Furthermore, for many molecules, important from a biological or environmental point of view and commonly used in various fields of science, there is a lack of information regarding their behavior in aqueous solutions and in different experimental conditions (ionic strength, ionic medium, temperature, pH), as well as information regarding their ability to interact with metals. In this context, we investigated three important biological molecules: Dopamine, Tryptophan, Histidine and Ofloxacin. The first three ligands are naturally present or synthetized in the organism, whilst ofloxacin is a fluoroquinolone antibiotic.3 The investigations were manly carried out by means of potentiometric measurements at different ionic strengths and temperature (0.15 < I/mol dm-3 < 1.00 and 288.15 < T/K < 310.15) in aqueous solutions containing NaCl as supporting electrolyte. Some checks were also performed by means of spectrophotometric titrations. The ligand interaction towards metal ions were studied considering the following ions: Ca2+, Cd2+, Cu2+, Mg2+, Mn2+, Sn2+, UO22+, Zn2+, CH3Hg+ and (CH3CH2)2Sn2+. The processing of the experimental data allowed to obtain different speciation models containing mono and polynuclear species, with species stability depending on the chemical nature of the metals and ligands and on the respective acid-base properties. In many cases, the formation of a sparingly soluble species at the end of the measurements was observed; the precipitate were analyzed by means of thermogravimetry.The dependence of the stability constants on I/mol dm-3 and T/K, was modelled by means of a modified Debye Hückel type equation, allowing the calculation of the stability constants at infinite dilution, of the parameters for the dependence on the ionic strength and the standard enthalpy change values of formation of the complex species. By using the empirical pL0.5 parameter, the sequestering ability of the different ligands towards the different metals was quantified at different ionic strengths, temperatures and pHs. References: [1] Kot, A., Namiesnik, J., Trends in Analytical Chemistry 2000, 19, 69-79. [2] Templeton, D. M., Ariese, F., Cornelis, R., Danielsson, L. G., Muntau, H., Van Leeuwen, H. P., Łobinsky, R., Pure and Applied Chemistry 2000, 72:8, 1453-1470. [3] Lentner, C., Geigy Scientific Tables, 8th edition. CIBA-Geigy: Basilea, Switzerland, 1981.