We then ran implicit solvent Poisson-Boltzmann calculation for the PKA-benzoic acid complex. Given such a calculation, the pKa values of the titratable groups of PKA and the benzoic acid are estimated. The ΔpK a of the benzoic acid carboxyl group was 1.2, thus shifting from 4.2 (experimental value) to 5.4 (protein-bound). No titratable group of PKA show a significant pKa shift (more than one log unit) which could give rise to a proton transfer (i.e. a protein residue which becomes protonated or de-protonated upon complexation with benzoic acid). This, together with the pH shift due to the composition of the solution, makes a protonated benzoic acid molecule very likely.