Computational Study on the Coordinate Systems of Proline with Cu, Cu+ and Cu2+

The geometric and electronic structures, energetics, and vibrational frequencies of different coordinate systems formed between 15 conformers of proline (Pro) and Cu, Cu+, and Cu2+ were investigated in detail, using the M06-2X and omega B97XD methods with 6-311++G(2d, p) and TZVP basis sets. A total of 20, 16, and 16 stable [Pro-Cu](0/1+/2+) complexes were obtained at four levels. These structures demonstrated that 12 conformers of Pro exist in the [Pro-Cu] and [Pro-Cu](+) systems, while 11 conformers are present in the [Pro-Cu](2+) complexes. The most stable complexes are evidently not formed by the lowest energy conformer of Pro with Cu, Cu+, and Cu2+. In the CI3, CI4, CII7, and CII8 complexes, the carboxyl group hydrogen of Pro was found to transfer to the imino nitrogen to forma zwitterion. Both the relative energy difference and the deformation energy of Pro gradually increase along with the charge number of the Cu. The binding energies of the [Pro-Cu](0/1+/2+) systems were determined to be in the ranges of -60.0 to -5.0, -340.0 to -170.0, and -1100.0 to -860.0 kJ.mol(-1), respectively. The stretching vibrational frequencies of the N-H and O-H bonds in Pro all exhibit a general red shift on complexation. Additionally, each system shows charge transfer from the Pro to the Cu, even in the case of [Pro-Cu](2+), some complexes that have more than one negative charge.