Softening of DMPG Lipid Membranes along the Anomalous Gel-Fluid Transition

Abstract

Phosphatidylglycerols are an anionic phospholipid found in mitochondrial membranes as a precursor to cardiolipin and are a major component of the cytoplasmic membrane of bacteria. The phase behavior of these lipids not only depends on the hydrocarbon tail length, but also is highly sensitive to the pH and ionic strength of the surrounding solution. In particular, the phase behavior of dimyristoylphosphatidylglycerol (DMPG) with saturated 14 carbon tails is highly unusual. The calorimetric profile shows several broad peaks that span 20 degrees and sharpen with increasing buffer ionic strength. Here we use neutron spin echo spectroscopy (NSE) to study the nanoscale membrane fluctuations along the anomalous phase transition in DMPG vesicle solutions as a function of buffer ionic strength. Our results show that the membrane dynamics are enhanced during the melting transition, which we quantify as a decrease in the bending modulus. The results provide direct experimental evidence for the increase in fluctuations needed to form the holey or perforated bilayer structures proposed in other works. Our results provide new insights into the nanoscale membrane dynamics during the melting transition and demonstrate how these dynamics are necessary for changes in the bilayer structure.