Binding ability of l-carnosine towards Cu2+, Mn2+ and Zn2+ in aqueous solution

Abstract

A thermodynamic study on the interactions of a naturally occurring dipeptide, l-carnosine, and three transition metal cations, Cu2+, Mn2+ and Zn2+, is reported. Their characterization in solution evidences the chelating properties of carnosine that give rise to different types of metal complexes. Potentiometric data allowed to obtain a speciation model for each system, which includes species with 1:1 metal to ligand ratio for both Mn2+- and Zn2+- carnosine systems and 1:1 and 1:2 metal to ligand ratios for Cu2+. The dependence of formation constants of the complex species on ionic strength (in the range 0.15 ≤ I/mol/L ≤ 0.98) and temperature (288.15 ≤ T/K ≤ 310.15) was defined, as well as changes in enthalpy and entropy. 1H NMR titrations were also performed in the study of the Zn2+- carnosine system, and the results of the analysis of experimental data confirmed potentiometric ones. The integration with further investigation by High Resolution (HR) Matrix Assisted Laser Desorption Ionization (MALDI) Mass Spectrometry (MS) allowed to obtain the formation of species with 1:2 metal to ligand ratios for also Mn2+- and Zn2+- carnosine systems and to clarify the chelating mode. Finally, sequestering ability of carnosine towards Cu2+, Mn2+ and Zn2+ was quantified in physiological conditions (pH = 7.4, T = 310.15 K and I = 0.15 mol/L), and a sequestration profile of the dipeptide clearly describes its highest sequestering ability with respect to the Cu2+ ion, compared to that towards Mn2+ and Zn2+ cations.