Quantum-mechanical studies of environmental effects on biomolecules VI.Ab initio Studies on the hydration scheme of the phosphate group

Ab initio SCF computations using the STO 3G basis set have been performed on the hydration scheme of the dimethylphosphate anion (DMP^?) within the “supermolecule” approach. This model compound represents the phosphate group in nucleic acids, phospholipid components of membranes and a number of other important biological structures. In the first place the principal hydration sites for the fixation of a water molecule have been established and this was followed by the determination of the polyhydration scheme of the first solvation shell, which may involve up to six water molecules. The effect of hydration on the relative stabilities of thegg,gt andtt forms of (DMP^?) was evaluated. The study was extended to the computation of the energies of interaction in the second and third hydration shells and the conclusion was reached that organized hydration around (DMP^?) is essentially limited to two hydration shells with possible residual organization of a third shell around specific sites. The structure of the bound water was investigated. Charge transfer is observed from (DMP^?) to the bound water, the perturbation extending essentially to the first and with decreasing intensity, to the second hydration shell.