Kinetic and Thermodynamic Aspects of Phosphate Diester Hydrolysis in Alkaline Solutions: The Role of Non-Leaving Groups

Alkaline hydrolysis of phosphate diester compounds with a common leaving group (LG) and varying non-leaving groups (NLG) was investigated. The nitrophenyl group was employed as a leaving group due to its favorable leaving group properties, significant absorbance for reaction monitoring, and selective cleavage of the phosphorus-oxygen bond, which is relevant for DNA and RNA degradation. Reactions were conducted under pseudo first-order conditions in NaOH solutions ۷۰ °C. In addition, the alkaline hydrolysis was studied using a ۱M NaOH solution at different temperatures (۵۰–۸۰ °C), pH (۱۲-۱۴) levels, and viscosities (with ۰% - ۴۰% glycerol). Reaction progress was continuously monitored by measuring the absorbance increase of the ۴-nitrophenolate ion at λ = ۴۰۰ nm. The rate constant (kobs) was determined by fitting the data to a first-order equation. The results demonstrate kobs increased with higher NaOH concentration, temperature, pH, and solution viscosity, except for NppNPP. Investigation of the effect of the non-leaving group's pKa revealed slight sensitivity of the hydrolysis reaction with βNLG = -۰.۱۲. Thermodynamic parameters were determined and the values were as follows: ∆H = ۴۸.۸ kJ.mol-۱, ∆S = -۱۷۱.۶ J.mol-۱.K-۱, ∆G = ۹۹.۹ kJ.mol-۱ for BpNPP; ∆H = ۶۴.۹ kJ.mol-۱, ∆S = -۱۲۷.۱ J.mol-۱.K-۱, ∆G = ۱۰۲.۸ kJ.mol-۱ for PypNPP; ∆H = ۸۶.۳ kJ.mol-۱, ∆S = -۷۶.۷ J.mol-۱.K-۱, ∆G = ۱۰۹.۲ kJ.mol-۱ for MpNPP, and ∆H = ۶۳.۷ kJ.mol-۱, ∆S = -۱۵۲ J.mol-۱.K-۱, and ∆G = ۱۰۹ k kJ.mol-۱ for NppNPP. All four hydrolysis processes have positive ∆G values, which means they are non-spontaneous and require external energy to proceed.