Hydrogen Bonding Versus Electrostatic Driving Forces of Phosphate Binding at the Air - Water Interface
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Date
2019-05
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The Ohio State University
Abstract
There is an increasing need to understand the principles of phosphate recognition. Phosphate is in high demand due to fertilizer and biofuel production but supplies are limited because of depleting phosphorus rock mines. Eutrophication caused by agricultural runoff leaves the human phosphate cycle open and devastates aquatic ecosystems. Aqueous phosphate recognition and recycling could play an important role in energy conservation, food security, and water management. Phosphate recognition also has biological applications in adenosine triphosphate binding. However the principles of aqueous phosphate capture are not well understood. Langmuir monolayers at the air – water interface provide a unique environment to study the physical properties and chemical driving forces of phosphate binding. An amphiphilic receptor with an ammonium headgroup (U-Ammo+) and a receptor with a guanidinium headgroup (U-Guan+) were employed in this study. U-Ammo+ provides pure electrostatic binding interactions through the charged dimethyl ammonium headgroup, and U-Guan+ provides both hydrogen bonding and electrostatic interactions through the charged guanidinium headgroup. The binding constants were determined for both molecules using surface sensitive infrared analysis at 5.5 °C and 31.5 °C via a Langmuir-type fit. The binding constants were used with temperature in Van't Hoff equations to obtain enthalpy, entropy, and free energy of phosphate binding. Overall U-Guan+ had larger binding constants and free energy driving forces than U-Ammo+, suggesting U-Guan+ is a better phosphate receptor. Both receptor-phosphate binding showed enthalpy as the main driving force. U-Guan+ showed less entropic hindrance to binding suggesting preorganization. U-Guan+ has previously shown selectivity up to 1:1000 phosphate-chloride while in this study, U-Ammo+ showed minimal phosphate selectivity at 1:1 phosphate-chloride concentration.