Comparison between implicit and hybrid solvation methods for the determination of pKa of mono-protonated form of 13 2-(demethoxycarbonyl) pheophytin a in methanol

Abstract

Both implicit solvation method (dielectric polarizable continuum model, DPCM) and hybrid solvation method (cluster-continuum model) were adopted to calculate the pKa of mono-protonated form of 132- (demethoxycarbonyl) pheophytin a (Pheo) in methanol. In the cluster-continuum model calculations, we considered only 1 solvent molecule attached explicitly and others treated implicitly whereas in the DPCM calculations all the solvent molecules were treated implicitly. DPCM calculations were carried out on Pheo, PheoH+, Pheo-CHaOH and PheoH+-CH3OH in methanol solution. The aim of these calculations was to determine the free energy changes involved in the deprotonation of PheoH+ (AGsol) and finally to obtain the corresponding pKa value. DPCM calculations were carried out employing the restricted open-shell density functional treatment (ROB3LYP) using the 6-31G(d) basis set to determine the free energy of solvation of bare Pheo and PheoH+ and of the clusters, Pheo-CH3OH and PheoH+- CH3OH in methanol. In-vacuo geometries of all the species were obtained by performing optimizations at ROB3LYP level using the 6-31G(d) basis. Electronic energies of all the species were then obtained by carrying out single point DFT calculations using 6-311+G(2d, 2p) basis set on the respective optimized geometries. Differences in thermal energy and molecular entropy were calculated by carrying out frequency calculations at ROB3LYP/STO-3G level on the optimized geometries of the truncated models. The optimized geometries of the clusters display intermolecular hydrogen bonding interactions. The pKa values of PheoH+ calculated by DFT-DPCM and cluster-continuum methods are 6-12 and 4-70 respectively while the observed value is 4-14. The hydrogen bonding interaction between the solute and the solvent can be attributed for the good performance of the cluster-continuum model over pure continuum model. © Indian Academy of Sciences.