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Observation of Water Separated Ion-Pairs between Cations and Phospholipid Headgroups B

van der Post, Sietse T., Hunger, Johannes, Bonn, Mischa, Bakker, Huib J.
The Journal of physical chemistry 2014 v.118 no.16 pp. 4397-4403
ammonium, calcium, cesium, dielectric spectroscopy, phosphates, phospholipids, physical chemistry, potassium, sodium, solvents
In this work, we present evidence for ion pair formation of cations with a high surface charge density (Na⁺ and Ca²⁺) and phosphate groups of phospholipids. We used femto-second infrared pump–probe and dielectric spectroscopy to probe the dynamics of water molecules in solutions of phosphorylethanolamine and different types of cations. We find that sodium and calcium cooperatively retard the dynamics of water in solutions of phosphorylethanolamine, implying the formation of solvent separated ion pairs. This ion-specific interaction is absent for potassium, cesium and ammonium. We compare our results to dielectric spectroscopy experiments, which probes the rotation of all dipolar molecules and ions in solution. The rotation of the dipolar phosphorylethanolamine ion shows that long-lived ion-pairs are only formed with calcium and not with ammonium, cesium, potassium, and sodium. This finding implies that the association between calcium and the phosphate is strong with lifetimes exceeding 200 ps, while the interaction with sodium is relatively short-lived (∼20–100 ps).