Arginine-Phosphate Recognition Enhanced in Phospholipid Monolayers at Aqueous Interfaces

Due to the growing world population, there is an ever-increasing need to develop better receptors to recover and recycle phosphate for use in agricultural processes. This need is driven by agricultural demand and environmental concerns because phosphate eutrophication has a damaging effect on fresh water supplies by fueling algal blooms. The air/water interface provides a unique region with a dielectric constant (Iμ ) that diminishes from high in bulk water (Iμ = 80) to significantly lower (e.g., Iμ < 40) near the monolayer surface to potentially enhance affinities during molecular recognition. The work presented here uses a model system of phosphate binding to an amino acid, arginine, and utilizes the interfacial properties of the phospholipid monolayer, 1,2-dipalmitoyl-sn-glycero-3-phosphatidic acid, as the phosphate source to quantify binding. Employing arginine as a probe molecule allows for the evaluation of its guanidinium moiety for phosphate chelation. Surface pressure-area isotherms from Langmuir monolayer studies, and the corresponding infrared reflection absorption spectroscopy, were used along with Brewster angle microscopy for in situ determination of molecular binding interactions and the surface binding constants of the phosphate-guanidinium complex, which are shown here to be greater than 10³ M^-1. The binding constant in bulk solution, determined by NMR titrations of phosphate and arginine, is determined to be on the order of 0.1 M^-1. The greater than 10 000-fold increase from the bulk aqueous solution to the air/water interface reveals that the interface provides a region of enhanced binding affinity.