Measurements of Lipid Vesicle Charge in Solutions of Zwitterions

Abstract

Binding of water soluble molecules to biological membranes is an important step in cellular signaling. We investigate the physical nature of such interactions using model lipid membranes. In particular, neutral lipid vesicles can acquire electrostatic charge by binding free ions from solution. Interestingly, they can also acquire charge from zwitterionic molecules that have dipole character. Examples are amino acids, pH buffers such as MOPS and HEPES, as well as adenosine triphosphate (ATP) and its hydrolysis products. By using x-ray scattering we have determined that the presence of zwitterions affects the lattice spacing of multilamellar phosphatidylcholine vesicles in a manner that is consistent with the presence of electrostatic repulsion between neighboring membranes. The remaining question is how to quantify the surface electric charge and, equally interesting, how to determine the sign of the charge conferred by zwitterions on membranes. Here we report measurements by dynamic light scattering (DLS) and by diffusion of phosphatidylcholine lipid vesicles in constant electric fields for a class of zwitterionic solutes. We find that the measured electrostatic charging can qualitatively account for the swelling of multilamellar structures and is consistent with NMR measurements of molecular binding. However, the results also show that more theoretical work is needed for quantitative analysis of electrostatic and van der Waals forces in these systems and for explanation of zwitterion affinities to biological membranes.