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The extent of contact and solvent shared ion pairs of phosphate groups withNa$^+$, Ca$^{2+}$ and Mg$^{2+}$ ions in aqueous environment and their relevancefor the stability of polyanionic DNA and RNA structures is highly debated.Employing the asymmetric phosphate stretching vibration of dimethyl phosphate(DMP), a model system of the sugar-phosphate backbone of DNA and RNA, wepresent linear infrared, femtosecond infrared pump-probe and absorptive 2D-IRspectra that report on contact ion pair formation via the presence of blueshifted spectral signatures. Compared to the linear infrared spectra, thenonlinear spectra reveal contact ion pairs with increased sensitivity becausethe spectra accentuate differences in peak frequency, transition dipole momentstrength, and excited state lifetime. The experimental results are corroboratedby long time scale MD simulations, benchmarked by density functionalsimulations on phosphate-ion-water clusters. The microscopic interpretationreveals subtle structural differences of ion pairs formed by the phosphategroup and the ions Na$^+$, Ca$^{2+}$ and Mg$^{2+}$. Intricate properties of thesolvation shell around the phosphate group and the ion are essential to explainthe experimental observations. The present work addresses a challenging toprobe topic with the help of a model system and establishes new experimentaldata of contact ion pair formation, thereby underlining the potential ofnonlinear 2D-IR spectroscopy as an analytical probe of phosphate-ioninteractions in complex biological systems.

Aqueous Contact Ion Pairs of Phosphate Groups with Na$^+$, Ca$^{2+}$ and  Mg$^{2+}$ -- Structural Discrimination by Femtosecond Infrared Spectroscopy  and Molecular Dynamics Simulations