Influence of Charge on the Elastic Properties of Lipid Membranes

Lipid membrane elastic properties play an important role in the membrane deformations and dynamic morphological transitions necessary for cell function. A key elastic property underlying these dynamics is the bending rigidity, motivating significant research efforts to quantify the effects of lipid structure and additives on the membrane bending modulus. To date, the majority of experimental research has focused on the dynamics of model membrane systems composed of zwitterionic lipids; however, most biomembranes are negatively charged at physiological conditions due to the presence of charged lipid headgroups. Here we study the bending dynamics of negatively-charged phosphatidylglycerol (PG) bilayers using neutron spin echo spectroscopy. Our results show that the charged lipid bilayers are softer than analogous zwitterionic phosphatidylcholine (PC) bilayers in both the gel and fluid phases at low ionic strength conditions. Interestingly, theoretical predictions indicate that the opposite should be true and that the bending rigidity should increase with increasing surface charge. We propose that this discrepancy is due to charged-related differences in the area per headgroup and lipid hydration between PG and PC bilayers that are not considered by existing theoretical models. Our results provide new insights into the influence of charge on the membrane elastic properties and demonstrate how these dynamics are coupled to charge effects on the membrane structure.