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 70 °C. In addition, the alkaline hydrolysis was studied using a 1M NaOH solution at different temperatures (50–80 °C), pH (12-14) levels, and viscosities (with 0% - 40% glycerol). Reaction progress was continuously monitored by measuring the absorbance increase of the 4-nitrophenolate ion at λ = 400 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 = -0.12. Thermodynamic parameters were determined and the values were as follows: ∆H = 48.8 kJ.mol-1, ∆S = -171.6 J.mol-1.K-1, ∆G = 99.9 kJ.mol-1 for BpNPP; ∆H = 64.9 kJ.mol-1, ∆S = -127.1 J.mol-1.K-1, ∆G = 102.8 kJ.mol-1 for PypNPP; ∆H = 86.3 kJ.mol-1, ∆S = -76.7 J.mol-1.K-1, ∆G = 109.2 kJ.mol-1 for MpNPP, and ∆H = 63.7 kJ.mol-1, ∆S = -152 J.mol-1.K-1, and ∆G = 109 k kJ.mol-1 for NppNPP. All four hydrolysis processes have positive ∆G values, which means they are non-spontaneous and require external energy to proceed.