Structural Changes in DMPG upon changes of ionic strength and pH - What to learn from SANS, DSC, FCS, Flourescence Microscopy, FTIR and Viscosity Measurements

Commonly pure phospholipid membranes are used as models for the more complex real biological membranes. Self-assembling the phospholipids can exhibit a number of different lamellar and nonlamellar phases. They are undergoing a cooperative melting reaction, which is linked to loss in conformational order of the lipid alkyl chains. In certain cases, like for aqueous dispersions of dimyristoyl phosphatidylglycerol (DMPG) a negatively charged phospholipid, this is resulting in an extended network system. The transitions associated are depending on temperature, pressure, lipid concentration and sample environment, such as ionic strength and the pH value (Schneider, M. F. et al., PNAS, 96 (1999) 14312; M.T. Lamy-Freund, K.A. Riske, Chem. Phys. Lipids, 122 (2003) 19).We studied DMPG dispersions under different pH and ionic strength conditions using methods gaining complementary information about the changes in material properties. In particular we studied the structural changes using small angle neutron scattering (SANS) as well as fluorescence microscopy and the thermodynamical by differential scanning calorimetry (DSC). In addition rheology, Fluorescence Correlation Spectoscopy (FCS) and Fourier-Transform Infrared Spectroscopy (FTIR) was performed. This results in a comprehensive model, taking into account the thermodynamic and structural changes below, in and above the region of the phase transition.