Electrostatics for exploring hydration patterns of molecules. 3. Uracil

A systematic investigation of the stepwise hydration of uracil has been car ried out. The rich molecular electrostatic potential (MESP) topographical f eatures of uracil provide clues on the probable water binding sites. The mo st noteworthy among these are the MESP minima due to carbonyl oxygens where the MESP value is negative as well as those over the uracil ring at which the MESP value is positive. Hydrated structures of uracil have been obtaine d employing an electrostatics-based model, EPIC (Electrostatic potential fo r Intermolecular Complexation) followed by ab initio optimization at the HF /6-31G(d,p) levels. Further geometry optimizations carried out at the HF/6- 31+G(2d,p) and B3LYP/6-31+G(2d,p) levels, for smaller clusters, lead to tre nds in interaction energies which an in general agreement with those observ ed at the HF/6-31G(d,p) level. These structures incorporate squares and cub es of water as the dominant building blocks. An analysis of the difference in interaction energies (Delta E-ret) of U...a(H2O) and (H2O)(n) clusters i s seen to provide valuable information regarding hydration shells. This Del ta E-ret stabilizes to -7 to -8 kcal mol(-1) for n > 8 and this value of it may correspond to the number of water molecules that make up the first sol vation shell of uracil, based on a criterion of the interaction energy.