We report a molecular dynamics (MD) simulation study of sodium dimethlyphosphate in aqueous solution. The dimethyl phosphate anion (DMP) is a model compound for the phosphodiester linkage in phospholipids and nucleic acids. We compare results from classical MD simulations based on an empirical force field to an ab initio MD simulation in which the forces are derived from the electronic structure in the framework of density functional theory with the Car-Parrinello approach. We have characterized the molecular and electronic structure of both the DMP anion and its first solvation shell in derail. To analyze the charge distribution in individual molecules, we have used maximally localized Wannier orbitals. The radial and orientational distributions of the water molecules revealed a softening of the structure of the first solvation shell of the DMP anion in the ab initio simulation compared to the force field simulation. The ab initio simulation reveals that both the geometry and the electronic structure of the DMP anion change significantly on moving from the gas phase to aqueous solution. In contrast, the dipole moments of the water molecules are comparable to bulk water, even in the vicinity of the DMP anion and the sodium counterion.