Predominance of short range Coulomb forces in phosphate-water interactions—a theoretical analysis

Abstract

Electric forces play a key role in the interaction of negatively charged phosphate groups with the dipolar water molecules of an aqueous environment. Both fluctuation amplitudes and effective spatial range of the electric fields that fluctuate on a multitude of time scales have remained highly controversial. We employ the dimethylphosphate anion (DMP) as a fundamental building block of the phosphodiester backbone in DNA to model electric fields at the phosphate-water interface. DMP is considered to be solvated in bulk water and the fluctuating electric forces exerted on the (PO2)− moiety are calculated by combining the ab initio based effective fragment potential approach that accounts for electric fields due to static multipoles and polarization contributions due to induced dipoles, with molecular dynamics. We demonstrate that the total time-averaged electric field generated by water molecules arises to a large extent from the first water layer. The second layer contributes some 18% with noticeable contributions from induction. We further show that the solvent electric field experienced by the phosphate group is the dominant contribution to the pronounced solvatochromism of the asymmetric (PO2)− stretch vibration. Accounting for a field expansion up to quadrupoles and polarization due to induced dipoles allows us to simulate solvent induced frequency shifts and lineshapes in almost quantitative agreement to experiment. Our theoretical model strongly supports the picture of short-range electric forces that arise locally from the first and second hydration shell.