Hydrogen bonding mediated by key orbital interactions determines hydration enthalpy differences of phosphate water clusters.

Electronic structure calculations have been carried out to provide a molecular interpretation for dihydrogen phosphate stability in water relative to that of metaphosphate. Specifically, hydration enthalpies of biologically important metaphosphate and dihydrogen phosphate with one to three waters have been computed with second-order Møller-Plesset perturbation and density functional theory (B3LYP) with up to the aug-cc-pvtz basis set and compared to experiment. The inclusion of basis set superposition error corrections and supplemental diffuse functions are necessary to predict hydration enthalpies within experimental uncertainty. Natural bond orbital analysis is used to rationalize underlying hydrogen bond configurations and key orbital interactions responsible for the experimentally reported difference in hydration enthalpies between metaphosphate and dihydrogen phosphate. In general, dihydrogen phosphate forms stronger hydrogen bonds compared to metaphosphate due to a greater charge transfer or enhanced orbital overlap between the phosphoryl oxygen lone pairs, n(O), and the antibonding O-H bond of water. Intramolecular distal lone pair repulsion with the donor n(O) orbital of dihydrogen phosphate distorts symmetric conformations, which improves n(O) and sigma*(O-H) overlap and ultimately the hydrogen bond strength. Unlike metaphosphate, water complexed to dihydrogen phosphate can serve as both a hydrogen bond donor and a hydrogen bond acceptor, which results in cooperative charge transfer and a reduction of the energy gap between n(O) and sigma*(O-H), leading to stronger hydrogen bonds. This study offers insight into how orbital interactions mediate hydrogen bond strengths with potential implications on the understanding of the kinetics and mechanism in enzymatic phosphoryl transfer reactions.