Theoretical pKa estimates for solvated P(OH)5 from coordination constrained Car-Parrinello molecular dynamics

Car-Parrinello molecular dynamics simulations have been performed on P(OH)(5) and its monanionic and dianionic derivatives in liquid water, representing minimal models of the transition complex occurring in the hydrolysis of RNA. P(OH)(5) and PO5H4- have been found to be stable in aqueous solution, while PO5H32- is reduced to PO5H4- within femtoseconds by solvent-solute proton transfer. The deprotonated hydroxyl group of the monoanion exhibits pronounced equatoriphilicity. This observation has been underlined by theoretical pK(a) determination of axial and equatorial hydroxyl groups of P(OH)(5), respectively, by means of coordination constrained ab initio molecular dynamics, which has shown the equatorial site to be more acidic by 4.4 units, the absolute values being 14.2 and 9.8.