Comparisons of Phosphorothioate and Phosphate Monoester Transfer Reactions: Activation Parameters, Solvent Effects, and the Effect of Metal Ions

Abstract

The thermodynamics, pH dependency, solvent effects, and the effect of divalent metal ions have been examined for the hydrolysis reactions of the phosphorothioate monoester p-nitrophenyl phosphorothioate (pNPPT) and compared with those of the corresponding phosphate monoester, p-nitrophenyl phosphate (pNPP). The pH dependency for pNPPT hydrolysis mirrors that for typical phosphate monoesters, with the monoanion being the most reactive species. The ratio of the rate constants for pNPPT hydrolysis to that for pNPP is 1380 for the monoanion reactions and 12.6 for the dianion reactions at 39 °C. The free energy of activation for hydrolysis of the pNPPT dianion in water is 27.9 kcal/mol versus 29.5 kcal/mol for pNPP; for the monoanion reactions the values are 22.2 kcal/mol for pNPPT and 26.8 kcal/mol for pNPP. The free energies of solvation for pNPPT and pNPP are within 0.1 kcal/mol of each other despite the poorer hydrogen bonding ability of sulfur versus oxygen. This minimal difference in ground-state energie...