Abstract
We have determined the molecular structures of commonly used phosphatidylglycerols
(PGs) in the commonly accepted biologically relevant fluid phase. This was done by simultaneously
analyzing small-angle neutron and X-ray scattering data, with the constraint of a measured volume per lipid. We report the temperature dependence of bilayer parameters obtained using the one dimensional scattering density profile model derived from molecular dynamics simulations, including
the area per lipid, the overall bilayer thickness, as well as other intrabilayer parameters (e.g., hydrocarbon thickness). Lipid areas are found to be larger than their phosphatidylcholine (PC)
counterparts, a result likely due to repulsive electrostatic interactions taking place between the
charged PG headgroups even in the presence of Na+ counterions. In general, PG and PC bilayers show a similar response to changes in temperature and chain length, but differ in their response to chain unsaturation. For example, compared to their PC counterparts, the inclusion of a double bond in PG lipids results in a smaller incremental change in lipid area and bilayer thickness. However, the
extrapolated lipid area of saturated PG lipids at infinite chain length is found to be similar to that of
PCs, an indication of the glycerol-carbonyl backbone's pivotal role in influencing the lipid-water interface.
