pKa Calculation of Some Biologically Important Carbon Acids - An Assessment of Contemporary Theoretical Procedures

In this study, the aqueous pK(a) values for 13 neutral, 10 cationic, and 5 anionic carbon acids, including amino acids, peptides, and related species have been calculated using the high level ab initio composite procedure, G3MP2+//BMK, combined with solvation energies that were calculated using the CPCM-(UAKS/UAHF), COSMO-RS, and SM6 continuum models. The pK(a)s were further calculated using three schemes, namely the direct method and the proton exchange method as well as the inclusion of an explicit solvent water molecule. The results of this study indicate that the direct method is unsuitable for computing the pKa of carbon acids, whereas the other two schemes perform significantly better with varying degrees of success, depending on the charge of the carbon acid. Specifically, the combination of the proton exchange scheme and CPCM-UAKS model performed particularly well for neutral species, with mean absolute deviations (MADs) of similar to 1 pK(a) unit. The ionic species were more problematic, though the combination of the proton exchange scheme and the SM6 and CPCM-UAKS models performed reasonably well for the cationic and anionic acids, respectively. The inclusion an explicit water molecule generally improved the calculated values for anionic carbon acids.