The Influence of Lipid Dipole Moment and Interfacial Water Structure on Protein Adsorption to Mixed Lipid Monolayers

Abstract

AbstractProtein interactions with a heterogeneous surface, such as a two‐component lipid film, often deviate from simple additivity of the adsorption to the respective single component, “homogenous” films. This behavior was highlighted in previous research where adsorption of the negatively charged protein ferritin to mixed cationic / non‐ionic lipid films was greater than adsorption to either of the single component lipid films [Britt et al., Phys. Chem. Chem. Phys., 2, 4594‐4599, 2000]. Here we investigate mixing induced changes in lipid dipole moments and local water structure as factors leading to amplification of monolayer charge and subsequent enhanced protein adsorption to the mixed lipid films. Surface potential and Fourier transform infrared spectroscopy (FTIR) revealed a reduction in both lipid tilt angle and gauche defects in the mixtures that would lead to a greater contribution of lipid alkyl‐tail dipoles to the measured (normal) surface potential; however, as the orientation of the alkyl‐tail dipole is from water (negative) to air (positive) the greater dipolar contribution for reduced tilt angles actually opposes adsorption of a negatively charged protein from the subphase. FTIR also revealed a mixing induced disordering of water solvating the cationic lipid head group, suggesting water dipoles were less effective in screening charge, which in turn would enhance the electrostatic interactions between anionic protein and the mixed cationic / nonionic lipid film.