Hydration induced coordination transition and interaction mechanism of Na+ with adenine deoxyribonucleotide in microhydrated clusters.

The coordination behavior and non-covalent interactions in adenine deoxyribonucleotide (Adn)-Na+ clusters with up to five water molecules are systematically investigated. Three distinct conformations of the anhydrous Adn-Na+ cluster were identified. Detailed structural analysis revealed that the central conformation is the most stable, followed by the bridge conformation, and the sugar-phosphate backbone conformation. Upon the addition of water molecules, the Na+-Adn coordination transforms from a central geometry to a bridging configuration. At n = 4 water molecules, Na+ achieves a four-coordinate structure. With increasing hydration, Na+ gradually dissociates from Adn, cation-π interactions with the adenine ring are also weakened, as reflected by increased distances and altered angles. Energy decomposition analysis indicates that the Adn-Na+ interaction is predominantly governed by electrostatic forces, followed by orbital interactions. Multiple theoretical approaches consistently demonstrate that the Adn-Na+ interaction weakens progressively as the number of water molecules in the cluster increases.