Investigation of the effect of polarization between a solvated biomolecule and solvent on the biomolecule ionization energy using the SAC-CI method in the ONIOM scheme: Ascorbic acid in water

Abstract

The importance of considering mutual polarization between the solute and solvent in the calculation of the ionization energy of the solute is investigated in this work. The symmetry-adapted cluster/configuration interaction methodology employing the single and double excitation operators (SAC-CI SD-R) using the ONIOM model was used to investigate the impact of mutual polarization between ascorbic acid (ASA) as a solute and water as a solvent on the ASA’s first ionization energy. For this purpose, only the water molecules, interacting directly and indirectly with each functional group of the ASA via hydrogen bonding and located in the first and second coordination shells of solvent, were considered in the quantum mechanics (QM) layer of the ONIOM model as well as ASA. To account for the mutual polarization between the ASA and water molecules of the first coordination shell, only one water molecule in the direct interaction, via hydrogen bonding, with each functional group of ASA was considered in the QM layer. In this case, the increase of the ionization energy of ASA was observed compared to when all of the water molecules of the first coordination shell are considered in the molecular mechanics (MM) layer of the ONIOM model. In the presence of the second coordination shell, the QM layer was extended in two ways to increase the extent of mutual polarization between the ASA and water: (a) considering one water chain for each functional group of ASA while for the rest of the ASA’s functional groups, only one water molecule was considered in the QM layer. (b) Increasing the number of water chains interacting with the functional groups of ASA in the QM layer to increase the size of the QM layer. In case (a), the increase of the first ionization energy of ASA was seen except for when a water chain is considered for the C = O functional group which results in a decrease in the ionization energy. In case (b) the increase in the number of water chains in the QM layer was accompanied by an increase in the first ionization energy of ASA. Also, considering the effect of the water molecules after the second coordination shell as a non-equilibrium polarized continuum model (PCM) showed a decrease in the ionization energy. Natural bonding orbital (NBO) calculations showed that the variation of the ASA’s first ionization energy due to hydrogen bonding is due to a change in the charge delocalization from the πC=C of ASA, its high-occupied molecular orbital (HOMO), to its vacant non-Lewis orbitals.