Formation Constants and Coordination Thermodynamics for Binary and Ternary Complexes of Copper(II), l-Hydroxyproline, and an Amino Acid Enantiomer

Abstract

While a rich database is available at or near room temperature (288.15 to 298.15 K) defining the chemical equilibria of solutions containing the copper(II) ion and an amino acid, data describing the temperature dependence of reaction thermodynamics for these systems remain scarce. In addition, data defining enthalpy, entropy, and heat capacity changes for the formation of mixed amino acid chelate complexes are extremely limited, hindering our understanding of the driving forces for complex formation and stabilization. Here, protonation constants and concentration-based equilibrium constants for Cu(II)−amino acid complexes are reported from 288.15 to 348.15 K for leucine, valine, proline, phenylalanine, and hydroxyproline in aqueous solutions containing 0.1 M KNO3. The logarithmic (ln) values of the protonation and binary stepwise concentration equilibrium constants (Ki) for each amino acid ligand are found to be linearly dependent on the inverse of temperature, indicating negligible change in heat capacity for each of these protonation and complexation reactions. However, ln(Ki) data for ternary complexes formed between Cu(II), l-hydroxyproline, and any one of the other amino acid enantiomers show a nonlinear dependence on inverse temperature, indicating a negative change in heat capacity. Enthalpy and entropy changes for ternary complex formation are therefore temperature-dependent quantities. Our thermodynamic data, when combined with statistical analysis of reaction stoichiometry, reveal that ternary Cu(II)(d‘ or l‘)(l-hydroxyproline) complexes are consistently hyperstable as compared to their parent bis-binary complexes at all solution temperatures studied.