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

Classical molecular dynamics (MD) simulation of (center dot)OH in liquid water at 37 degrees 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 (center dot)OH-H(2)O interactions, including short-range interactions of hydrogen atoms. Scans of the potential energy surface of the (center dot)OH-H(2)O 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 angstrom, of (center dot)OH resulted in the dipole moment of 1.76 D. The radical-water radial distribution functions revealed that (center dot)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 degrees 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 degrees 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 (n) over bar = 0.85 of radical-water H-bonds has been calculated using geometric definition of H-bond and (n) over bar = 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) x 10(-9) m(2) s(-1), is the same as (3.1 +/- 0.5) x 10(-9) m(2) s(-1) calculated for water in solution and in pure solvent. To the best of our knowledge, this is the first study of the (center dot)OH(aq) properties at a biologically relevant body temperature.