Phosphate-Magnesium Ion Interactions in Water Probed by Ultrafast Two-Dimensional Infrared Spectroscopy

Electric interactions between ions and ionic molecular groups in aqueous solution play a fundamental role in chemistry and biology. While Mg²⁺ ions are known to strongly affect the structure and folding dynamics of biomolecules, the relevance of different solvation geometries and the underlying interactions are mainly unresolved. We study dynamics and couplings between the hydrated Mg²⁺ and the dimethylphosphate anion, an established model system for the DNA and RNA backbone. The asymmetric (PO₂^-) stretching vibration serves as a sensitive noninvasive probe of phosphate-ion interactions. Femtosecond two-dimensional infrared (2D-IR) spectroscopy directly maps Mg²⁺ ions in contact with the phosphate groups via a distinct blue-shifted signature in the 2D spectrum. Data for different Mg²⁺ concentrations are analyzed by microscopic density functional theory modeling of cluster geometries and associated spectroscopic features, providing spatial assignments of the observed 2D-IR signatures. Phosphate-ion interactions arising from electrostatic Coulomb forces and exchange repulsion are the predominant origin of the observed frequency shifts.