EFFECTS OF PH AND CHOLESTEROL ON DMPA MEMBRANES - A SOLID-STATE H-2-NMR AND P-31-NMR STUDY

The effect of pH and cholesterol on the dimyristoylphosphatidic acid ( DMPA) model membrane system has been investigated by solid state H-2- and P-31-NMR. It has been shown that each of the three protonation sta tes of the DMPA molecule corresponds to a P-31-NMR powder pattern with characteristic aa values; this implies additionally that the proton e xchange on the membrane surface is slow on the NMR time scale (millise cond range). Under these conditions, the H-2-labeled lipid chains sens e only one magnetic environment, indicating that the three spectra det ected by P-31-NMR are related to charge-dependent local dynamics or or ientations of the phosphate headgroup or both. Chain ordering in the f luid phase is also found to depend weakly on the charge at the interfa ce. In addition, it has also been found that the first pK of the DMPA membrane is modified by changes in the lipid lateral packing (gel or f luid phases or in the presence of cholesterol) in contrast to the seco nd pK. The incorporation of 30 mol % cholesterol affects the phosphati dic acid bilayer in a way similar to what has been reported for phosph atidylcholine/cholesterol membranes, but to an extent comparable to 10 -20 mol % sterol in phosphatidylcholines. However, the orientation and molecular order parameter of cholesterol in DMPA are similar to those found in dimyristoylphosphatidylcholine.