IR AND RAMAN-SPECTRA, CONFORMATIONAL FLEXIBILITY, AND SCALED QUANTUM-MECHANICAL FORCE-FIELDS OF SODIUM DIMETHYL-PHOSPHATE AND DIMETHYL-PHOSPHATE ANION

Quantum chemical calculations, involving Hartree-Fock (HF), perturbati on (MP2), and density functional (DFT) theories, are carried out for t he dimethyl phosphate anion (DMP) and sodium dimethyl phosphate (NaDMP ), model systems for the DNA phosphate group. Energies, geometries, an d harmonic force fields of different conformations of DMP and NaDMP ar e compared. In addition, atomic charges derived from the HF/6-31G and MP216-31+G electrostatic potential of DMP and NaDMP are calculated i n order to determine the effects of counterions upon the charge distri bution. Finally, IR and Raman spectra of solid and aqueous NaDMP, reco rded here in the 80-4000 cm(-1) (IR) and 150-3100 cm(-1) (Raman) spect ral regions, are assigned using differentially scaled Hartree-Fock, MP 2, and B3-LYP force fields of DMP and NaDMP. Our interpretation of the individual vibrational bands confirms the results of the previous emp irical normal-coordinate analysis of DMP of Thomas et al. (Biophys. J. 1994, 66, 225). Also, the predicted frequencies, IR and Raman intensi ties, depolarization ratios, and C-13 iSotopic frequency shifts agree well with the experimental data: Among the computational methods, the best results are obtained using the B3-LYP gradient corrected density functional. The proposed scale factors for the HF/3-21G(), HF/6-31G*, MP2/6-31+G, and B3-LYP/6-31G* force fields of DMP or NaDMP are trans ferable to larger systems involving phosphodiester moiety, for example to nucleotides or phospholipids.