Monte Carlo simulations and neutron diffraction studies of the peptide forming system 0.5 mol kg−1CuCl2–5mol kg−1NaCl–H2Oat 293 and 353 K

Abstract

Monte Carlo simulations at temperatures of 293 and 353 K have been performed for a 0.5 mol kg⁻¹ CuCl₂–5 mol kg⁻¹ NaCl aqueous solution, which has been found to induce condensation of amino acids to peptides. Because pair potentials are insufficient to describe Cu^II solvation phenomena correctly, three-body terms were incorporated, based on abinitio derived potentials for Cu²⁺–H₂O–H₂O and Cu²⁺–Cl⁻–H₂O interactions. The structure of the solvated ions is discussed in terms of radial density functions and coordination number, angular and energy distributions. A neutron diffraction study was also carried out on a solution of the same concentration at 298 K. The method of isotopic substitution was applied to the copper(II) ions (⁶³Cu and ⁶⁵Cu) and to the water molecules (H₂O and D₂O) and results obtained for the Cu²⁺ coordination and the water structure. Comparison of the former with those of the simulation shows that there is broad agreement for the Cu^II coordination; both techniques show evidence of direct contact between Cu²⁺ and Cl⁻ and indicate [Cu(H₂O)₅Cl₁]⁺ to be the prevalent copper species in solution. In addition, the neutron diffraction and isotopic substitution (NDIS) results show that the H-bond structure is strongly perturbed by the presence of a high concentration of ions.