Potentiometric and Electrochemical Behavior of Eu–Glycylhistidine Complexes with Biologically Important Zwitterionic Buffers

Potentiometric equilibrium measurements have been performed at (25.0 ± 0.1) °C and ionic strength I = 0.1 mol·L−1 KNO3 for the interaction of Eu–Glycylhistidine (Glyhis) with five biologically important zwitterionic buffers (Z) (HEPES, EPPS, PIPES, POPSO and HEPPSO). The formation constants for the binary Eu(III) + Glyhis or Eu(III) + Z systems and ternary Eu(III) + Glyhis + Z systems have been evaluated, and the lowest value for formation constant is assigned to the system Eu(III) + Glyhis + PIPES. The electrochemical behavior of the Eu–Glyhis complex with selected zwitterionic buffers was studied on the surface of a glassy carbon electrode using cyclic, differential pulse and square wave voltammetric techniques. The values of the standard heterogeneous rate constant (K s), diffusion coefficients of the reduction (D red) and oxidation (D ox) processes were calculated for all the studied systems. The data reveal that PIPES is the most suitable of these zwitterionic buffers for study of the biological behavior of Glycylhistidine in its europium complexes.