Determination of the thermodynamic quantities for complex formation of U(VI) with amino acids in aqueous solution

Abstract

The thermodynamic quantities (ΔG, ΔH, and ΔS) for the formation of U(VI) complexes with three amino acids, L-serine, L-phenylalanine, and L-cysteine were determined, where the stability constants were obtained by potentiometric titration and the reaction enthalpies were directly measured by calorimetric titration. Prior to the U(VI) complexation study, the ΔG, ΔH, and ΔS of the protonation of the three amino acids were determined using the same techniques. The obtained thermodynamic quantities are compared to discuss the complex formation's driving forces and mechanism. The obtained thermodynamic data indicates that U(VI)-L-serine and U(VI)-L-phenylalanine complexes are entropy-driven reactions, whereas a significant enthalpy benefit drives L-cysteine complexation. From the comparison between the obtained thermodynamic data in this study and the spectroscopic and theoretical analyses in previous research, it was suggested that the complex formation of the amino acid with U(VI) proceed without significant involvement of the amino group while the thiol group in L-cysteine significantly contributes to the complexation with U(VI).