The Influence of Electric Fields on the Ordering of Lipid Monolayers and Lipid-Protein Binding

This research is devoted to studies of the influence of an externally controlled electric potential difference on Gibbs monolayers of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), as well as its influence on the binding of the C2 domain of cytosolic phospholipase-A2 (cPLA2α-C2) protein to the SOPC monolayer. X-ray reflectivity, molecular dynamics (MD) simulations and electrochemical methods are used to gather microscopic and macroscopic information about the ordering, orientation, and binding configuration of the SOPC. SOPC monolayer assembles at the interface between an aqueous electrolyte solution and an organic electrolyte solution with 1,2-dichloroethane (DCE) as the solvent. The area per lipid of SOPC for different potential differences between the interface is determined from interfacial tension measurements. Cyclic voltammetry confirms the result and provides evidence that the interfacial behavior of SOPC monolayers is reversible. X-ray reflectivity measurements are analyzed to determine the interfacial electron density profile, including the thicknesses of the phosphocholine (PC) head group region and the SOPC hydrocarbon chain regions of the monolayer. MD simulation at fixed area per lipid shows small differences in the orientation of SOPC molecules under electric field. This indicates that the primary effect in the experiments is due to the changing area per lipid as a function of electric potential difference, and the subsequent re-orientation of the lipid to accommodate the change in interfacial density. X-ray reflectivity analysis on cPLA2α-C2 domains bound to the SOPC monolayer on the interface between water and DCE provides information on the angular orientation and penetration depth of the domains. The best-fit orientations and penetration of X-ray reflectivity curves for different potential difference are provided, and the result suggests an increase in the electron density of tailgroup layer due to both protein and DCE penetration into this region. Under very high potential difference (0.38 V), a significant change in reflectivity curve is observed, which could be caused by a decrease of the density of cPLA2α-C2 and lipid on the interface.