A Monte Carlo simulation study of the aqueous hydration of r(GpC)2: comparison with crystallographic ordered water sites.

Monte Carlo computer simulation is described for the dinucleotide duplex rGpC together with 562 water molecules at an environmental density of 1 g/cc in a cubic cell under periodic boundary conditions. Water-water interactions were treated using the TIP4P potential and the solute water interactions by TIP4P spliced with the nonbonded interactions from the AMBER 3.0 force field. The simulation was subjected to proximity analysis to obtain solute coordinate numbers and pair interaction energies for each solute atom. Hydration density distributions partitioned into contributions from the major groove side, the minor groove side and the sugar-phosphate backbone were examined, and the probabilities of occurrence for one- and two-water bridges in the simulation were enumerated. The results were compared with observations of crystallographic ordered water sites from x-ray diffraction studies on G and C containing small molecules, and in crystal structure determinations of the sodium, calcium, and ammonium salts of rGpC. The calculated results are generally consistent with the observed sites, except for cytosine N4, where a hydration site is predicted yet none observed in rGpC salts, and for guanine N3, which appears in this calculation to compete unfavorably with the adjacent donor site at guanine N2. There is, however, a significant probability of finding a one-water G-N3-W-G-N2 bridge indicated in the simulation. An explanation for the guanine N3 discrepancy in terms of electrostatic potentials is also offered. The calculated one- and two-water bridges in the rGpC hydration complex coincide in a number of cases to those observed in the ordered water structure of the sodium rGpC crystal hydrate.