Molecular dynamics study of the hydration of the hydroxyl radical at body temperature

Abstract

Classical molecular dynamics (MD) simulation of ˙OH in liquid water at 37 °C has been performed using flexible models of the solute and solvent molecules. We derived the Morse function describing the bond stretching of the radical and the potential for ˙OH–H₂O interactions, including short-range interactions of hydrogen atoms. Scans of the potential energy surface of the ˙OH–H₂O complex have been performed using the DFT method with the B3LYP functional and the 6-311G(d,p) basis set. The DFT-derived partial charges, ±0.375e, and the equilibrium bond-length, 0.975 Å, of ˙OH resulted in the dipole moment of 1.76 D. The radical–water radial distribution functions revealed that ˙OH is not built into the solvent structure but it rather occupies distortions or cavities in the hydrogen-bonded network. The solvent structure at 37 °C has been found to be the same as that of pure water. The hydration cage of the radical comprises 13–14 water molecules. The estimated hydration enthalpy −42 ± 5 kJ mol⁻¹ is comparable with the experimental value −39 ± 6 kJ mol⁻¹ for 25 °C. Inspection of hydrogen bonds showed the importance of short-range interaction of hydrogen atoms and indicated that neglect of the angular condition greatly overestimates the number of the H-acceptor radical-water bonds. The mean number = 0.85 of radical-water H-bonds has been calculated using geometric definition of H-bond and = 0.62 has been obtained when the energetic condition, E_da ≤ −8 kJ mol⁻¹, was additionally considered. The continuous lifetimes of 0.033 ps and 0.024 ps have been estimated for the radical H-donor and the H-acceptor bonds, respectively. Within statistical uncertainty the radical self-diffusion coefficient, (2.9 ± 0.6) × 10⁻⁹ m² s⁻¹, is the same as (3.1 ± 0.5) × 10⁻⁹ m² s⁻¹ calculated for water in solution and in pure solvent. To the best of our knowledge, this is the first study of the ˙OH_aq properties at a biologically relevant body temperature.