Double-Bond Character of Phosphates in Solid and Liquid Phases Probed by Oxygen K-Edge X-ray Absorption Spectroscopy

Double-bond character of phosphate groups has important roles for biological reactions and was investigated using the P═O π* peaks in the oxygen K-edge X-ray absorption spectroscopy (XAS). The O K-edge XAS spectra of phosphates in the solid phase found that the double-bond character of phosphates is enhanced as the negative charge increases, where the weak P═O π* peak was observed in NaH₂PO₄, but distinct P═O π* peaks were obtained in Na₂HPO₄ and Na₃PO₄. Conversely, the P═O π* peaks were not observed in the XAS spectra of any phosphates in the liquid phase, indicating a substantial reduction in the double-bond character of the phosphate groups owing to the interaction of Na⁺ ions and the formation of hydration structures. The XAS spectra of diphosphates validated that change in the double-bond character of the phosphate groups is a general phenomenon also observed in high-energy phosphate compounds, which is closely related to the reactivity of adenosine triphosphate hydrolysis.