Solvation Thermodynamics of Amino Acids with Hydrophobic and Hydrophilic Side Chains in Aqueous Mixtures of Protic Ethylene Glycol

Abstract

The zwitterionic as well as solvophilic and solvophobic solvation behavior of DL-tyrosine, DL-leucine, DL-isoleucine and DL-threonine in aqueous and aqueous ethylene glycol systems have been studied at 298.15 K. For this study the solubilities of these amino acids in different solvent mixtures were measured at nine equidistant temperatures in the range of 278.15–318.15 K under 0.1 MPa pressure using the ‘formol titrimetric’ method. The order of experimentally found solubilities is DL-Thr > DL-Ile > DL-Leu > DL-Tyr. The standard transfer Gibbs energies, $$\Delta G_{{\text{t}}}^{{0}} (i)$$ , and entropies, $$T\Delta S_{{\text{t}}}^{{0}} (i)$$ of transfer were computed using the solubility data and are discussed. The observed $$\Delta G_{{\text{t}}}^{{0}} (i)$$ and $$T\Delta S_{{\text{t}}}^{{0}} (i)$$ versus composition profiles are in confusing order because of various interaction effects. The chemical effects of the transfer Gibbs energies $$\Delta G_{{\text{t,ch}}}^{{0}} (i)$$ and entropies of transfer values $$T\Delta S_{{\text{t,ch}}}^{{0}} (i)$$ have been obtained after removal of the cavity forming effect, dipole–dipole and dipole-induced-dipole interaction effects. The chemical contribution of these DL-amino acids are guided by the united effects of gradual change in the dispersion interaction, basicity and acidity, hydrogen bonding effects, solvophobic and solvophilic solvation of aqueous ethylene glycol as compared to those in the reference solvent, pure water. The solvophilic and solvophobic solvation effects of the transfer Gibbs energies, $$\Delta G_{{\text{t,H1HbH}}}^{{0}} (i)$$ , and entropies of transfer, $$T\Delta S_{{\text{t,H1HbH}}}^{{0}} (i)$$ , for the amino acids have been obtained after subtraction of the dispersion effect from the chemical transfer energies, which are guided by the side chains with amino acids.