Hydrogen-bonding between cytosine and water: Computational evidence for a ring-opened complex

Hydrogen-bonded complexes between water and cytosine are investigated. Particular emphasis is placed on a (O2–N3) complex involving two cytosine acceptors since previous work described difficulties characterizing this potential minimum. B3LYP calculations reveal that a ring-closed O2–N3 minimum exists within a shallow energy well when small (double-zeta) basis sets (without diffuse functions) are implemented. The depth of this well decreases to the point where only a plateau region is observed as the basis set is expanded to include diffuse functions. If well-balanced triple-zeta basis sets are implemented, then the O2–N3 complex can be fully optimized to a ring-opened structure with water coordinated to O2. This latter finding provides computational support for a ring-opened complex between water and 1-methylcytosine, which has been suggested based on experimental observations.