Hydration scheme of the purine and pyrimidine bases and base-pairs of the nucleic acids

The hydration scheme of the bases of the nucleic acids, leading to a representation of their first hydration shell, was computed using the overlap multipole procedure. The shell involves five, four, four and three bound water molecules in G, A, C and T respectively. The formation of the base pairs displaces one water molecule in the A-T pair and four water molecules in the G-C pair from the hydration shell. The hydration produces a destabilization of the pairing energy in comparison to the binding in vacuo, greater for the G-C pair than for the A-T pair. There remains nevertheless an appreciable residual affinity for inter-base hydrogen bonding in water.