Enthalpy-entropy compensation for some drugs dissociation in aqueous solutions

The observed enthalpy-entropy compensation seems to be strongly indicative of a common mechanism of drugs dissociation. The regression triplet (data, model, method) is used for the estimation of dependencies between thermodynamic parameters Delta H-0, Delta S-0, Delta G(0) derived from dissociation constant measurements at two different temperatures and Van't Hoff equation. The orthogonal regression analysis involves 87 experimental pK(a) values performed on 44 drugs to give unbiased parameter estimates for a linear dependence Delta H-0 = 40.43 + 0.966 T Delta S-0 with the Pearson's correlation coefficient r = 0.8953. The standard free energy change Delta G(0) for the dissociation reaction may be decomposed on Delta H-0 vs. pK(a) dependence and T Delta S-0 vs. pK(a) dependence. The Delta H-0 is sensitive to changes in pK(a) because of electrostatic effects. Increasing the Bronsted basicity of the drug causes an increase of the Delta H-0 term and decreases the T Delta S-0 term. When Delta H-0 is negative, then the value T Delta S-0 is the dominant factor, which determines that Delta G(0) is positive. The ionization of a neutral acid involves formation of two ions so that the entropy decreases. On the second ionization of the same acid, there are now three ions and the anion has a charge, so the entropy again only decreases. (C) 2012 Elsevier B.V. All rights reserved.