Thermodynamic Signatures of the Origin of Anti-Hofmeister Selectivity for Phosphate at Aqueous Interfaces

The selectivities and driving forces governing phosphate recognition by charged receptors at prevalent aqueous interfaces is unexplored relative to the many studies in homogeneous solutions. Here we report on electrostatic binding versus hydrogen-bond-assisted electrostatic binding of phosphate (H2PO4-) for two important receptor classes in the unique microenvironment of the air-water interface. We find that the methylated ammonium receptor (U-Ammo+) is dominated by electrostatic binding to phosphate anions and fails to be selective for phosphate binding over chloride, whereas the highly phosphate-selective guanidinium receptor (U-Guan+) provides synergistic hydrogen-bonding and electrostatic interactions. Apparent binding constants were evaluated in situ for U-Ammo+ and U-Guan+ using temperature-controlled infrared reflection-absorption spectroscopy with Langmuir-type fitting. Thermodynamic quantities showed enthalpically driven binding affinities of U-Guan+ and U-Ammo+ receptors (ΔH°b = -71 (±9) kJ/mol and ΔH°b = -155 (±13) kJ/mol, respectively). U-Guan+ revealed a nearly fourfold smaller entropic barrier to binding (ΔS°b = -132 (±34) J/mol K) than the U-Ammo+ receptor (ΔS°b = -440 (±45) J/mol K), attributed to hydration differences. The larger entropic penalty for the U-Ammo+ receptor is correlated with a molecular expansion shown in surface pressure-area isotherms, whereas the smaller entropic penalty of the U-Guan+ receptor conversely correlated with no expansion. The U-Guan+ receptor also revealed anti-Hofmeister selectivity for phosphate over chloride, while the non-hydrogen-bonding U-Ammo+ receptor followed Hofmeister selectivity. Our results indicate that hydrogen bonding is an integral chemical design element for achieving anti-Hofmeister selectivity for phosphate.