Abstract
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 pKa values, pKa1=8.0±0.1 and pKa2=10.9±0.2. Additionally, we obtained the pKa values of Emodin in the water–methanol mixture (1:3v/v). We give a new interpretation of the experimental data, obtaining apparent pKa1=6.2±0.1, pKa2=8.3±0.1 and pKa3>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 pKa1, using an explicit model of the solvent, with Free Energy Perturbation theory in Monte Carlo simulations obtaining, ΔGaq=12.1±1.4kcal/mol and pKa1=8.7±0.9. With the polarizable continuum model for the solvent, we obtained ΔGaq=11.6±1.0kcal/mol and pKa1=8.3±0.7. Both solvent models gave theoretical results in very good agreement with the experimental values.
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