Ab initio and ABEEM/MM fluctuating charge model studies of dimethyl phosphate anion in a microhydrated environment

Dimethyl phosphate (DMP) anion has been used extensively as a model compound to simulate the properties of phosphate group. A 35-point DMP anion potential model is constructed based on the atom-bond electronegativity equalization fluctuating charge molecular force field (ABEEM/MM), and it is employed to study the properties of gas-phase DMP anion and DMP-(H₂O) _n (n = 1–3) clusters. The ABEEM/MM model reproduces well the properties obtained by available experiments and QM calculations, including charge distributions, geometries, and conformational energies of gas-phase DMP-water complexes. Furthermore, molecular dynamics simulation on the DMP anion in aqueous solution based on the ABEEM/MM shows that a remarkable first hydration shell around the nonesterified oxygen atom of DMP anion is formed with a coordination number of 5.2. It is also found that two hydrogen atoms of one water molecule form two hydrogen bonds with two nonesterified oxygen atoms of DMP anion simultaneously. This work could be used as a starting point for us to establish the ABEEM/MM nucleic acid force field.