Membrane Charging and Interfacial Hydration

Due to thermal motion and molecular polarizability, physical interactions at the membrane-water interface have a pronounced dynamic character. In particular, the interplay between molecular disorder (entropy) and molecular interactions can have unexpected consequences primarily with regard to membrane electrostatics [1,2]. We show experimentally that significant charging occurs for lipid membranes in the presence of highly polarizable solutes such as anions and zwitterionic pH buffers. To complicate matters, this charging process takes place while there is a net deficit of solutes in the immediate vicinity of membranes. The important consequence is that electrostatic forces between macromolecular surfaces are then less screened and can act over long distances. We quantify both membrane electrostatics and solute deficit by using a number of experimental methods including small-angle x-ray scattering, buoyancy measurements, and molecular drift in electric fields. It is also shown that there are solute mixtures in which electrostatic cancellation occurs. These aspects of membrane electrostatics are relevant to studies of membrane fusion and of protein-lipid membrane interactions.[1] Petrache et al. J. Am. Chem. Soc. 2005, [2] Koerner et al. Biophys. J. 2011.