Theoretical study of the Cu2+-glycine interaction in ammonia and temperature effects

Abstract

In this study, we have investigated the potential energy of surfaces of the various glycine tautomers, also the interaction between Cu2+ cation with these glycine tautomers in both the gas and solvent phases and for the first time, the glycine-Cu2+(NH3)n=3−6 clusters in the gas-phase. For the studied clusters, we reported their temperature-dependent conformer distributions, along with the binding energies per ammonia molecule and clustering energies. After comparing the binding energies obtained computationally by using the DLPNO-CCSD(T)/CBS level of theory to those obtained from ten DFT functionals (M06, M06HF, M06-2X, M08HX, MN15, MN12SX, ωB97XD, PW6B95D3, TPSSH, APFD), as well as from the ab initio MP2 method, we selected the DFT functional, M08HX, in conjunction with the aug-cc-pVDZ basis set, as our method of choice. Evidence emerged from our study that the solvation medium and the presence of the Cu2+ cation influence the stability of glycine tautomers. The relative stability of Gly-Cu2+(NH3)n clusters in the gas phase is influenced by the coordination of the Cu2+ cation. The study of the relative population in the gas phase of the Gly-Cu2+(NH3)n=3−6 clusters, predicts that for cluster size n=4, the structure of the more compact penta-coordinated Cu2+ ion dominates at low temperatures. Conversely, the conformers for the less compact tetra-coordinated Cu2+ cation are prevalent at high temperatures. For cluster size n=6 with more ammonia molecules per cluster, the conformers containing the compacted is penta-coordinated Cu2+ cation, are predominant at all temperatures. Binding energies computed for ionic interactions of the Gly-Cu2+ complexes, resulted in the following values at room temperature: the binding electronic energy (ΔEn), binding enthalpy (ΔHn) and binding free energy (ΔGn) of ionic interaction are respectively -218.3, -218.4, -211.6 kcal/mol in the gas phase and -93.8, -94.9, -85.2 kcal/mol in the solvent phase. In order to extrapolate this also to the Gly-Cu2+ ammonia clusters, a suitable function of best fit was applied. The computational prediction of binding electronic energy, binding enthalpy and binding free energy, per ammonia molecule in the gas phase, at saturation, produced the values -25.9, -26.1 and -17.0 kcal/mol, respectively. We therefore recommend this function fit for further study of such systems.