Protonation/deprotonation process of Emodin in aqueous solution and pKa determination: UV/Visible spectrophotometric titration and quantum/molecular mechanics calculations

We combined theoretical and experimental studies to elucidate the important deprotonation process of Emodin in water. We used the UV/Visible spectrophotometric titration curves to obtain its pK(a) values, pK(a1) = 8.0 +/- 0.1 and pK(a2) = 10.9 +/- 0.2. Additionally, we obtained the pK(a) values of Emodin in the water-methanol mixture (1:3v/v). We give a new interpretation of the experimental data, obtaining apparent pK(a1) = 6.2 +/- 0.1, pK(a2) = 8.3 +/- 0.1 and pK(a3) > 12.7. Performing quantum mechanics calculations for all possible deprotonation sites and tautomeric isomers of Emodin in vacuum and in water, we identified the sites of the first and second deprotonation. We calculated the standard deprotonation free energy of Emodin in water and the pK(a1), using an explicit model of the solvent, with Free Energy Perturbation theory in Monte Carlo simulations obtaining, Delta G(aq) = 12.1 +/- 1.4 kcal/mol and pK(a1) = 8.7 +/- 0.9. With the polarizable continuum model for the solvent, we obtained Delta G(aq) = 11.6 +/- 1.0 kcal/mol and pK(a1) = 8.3 +/- 0.7. Both solvent models gave theoretical results in very good agreement with the experimental values. (C) 2014 Elsevier B.V. All rights reserved.