MOLECULAR ORDER AND DYNAMICS OF PHOSPHATIDYLCHOLINE BILAYER-MEMBRANES IN THE PRESENCE OF CHOLESTEROL, ERGOSTEROL AND LANOSTEROL - A COMPARATIVE-STUDY USING H-2-NMR, C-13-NMR AND P-31-NMR SPECTROSCOPY

We report the results of a comparative study of the molecular order and dynamics of phosphatidylcholine (PC) bilayer membranes in the absence and presence of cholesterol, ergosterol and lanosterol, using deuterium (H-2) nuclear magnetic resonance (NMR) of deuterated phospholipid molecules, in addition to solid state C-13 and P-31-NMR. Using dimyristoylphosphatidylcholines (DMPCs) specifically labeled at positions 2', 3', 4', 6', 8', 10' and 12' of the sn-2 chain together with the perdeuterated 2-[H-2(27)]DMPC derivative, the order profile for 9 of the 13 methylene groups of the sn-2 chain was established at 25 degrees C for DMPC, DMPC/cholesterol, DMPC/ergosterol and DMPC/lanosterol membranes, at a fixed sterol/phospholipid mol ratio of 30%, and in the presence of excess water. The overall ordering effects were found to be ergosterol > cholesterol >> lanosterol. Transverse relaxation (T-2e) studies of these systems indicated that while for DMPC, DMPC/cholesterol and DMPC/ergosterol the relative relaxation rates were in qualitative agreement with models which assume cooperative motions of the bilayer molecules as the main relaxation mechanism, those in DMPC/lanosterol were anomalously high, suggesting alterations of lipid packing. Using dipalmitoylphosphatidylcholine (DPPC) deuterated at the trimethylammonium group of the choline moiety, we found that the differential ordering and motional effects induced by the sterols in the acyl chains were also reflected in the headgroup, both in the gel (L(beta)) and liquid-crystalline phases. C-13 and H-1 spin dynamics studies of these systems, including cross-polarization, rotating frame longitudinal relaxation and dipolar echo relaxation rates showed that the mobility of the different regions of the phospholipid molecules in the binary lipid systems were inversely correlated with the ordering effects induced by the sterols. A novel combination of C-D bond order parameters (obtained by H-2-NMR) and C-13-H-1 cross polarization rates confirmed these results, The effects of the same sterols at the same molar proportion on the unsaturated lipid 1-[H-2(31)]palmitoyl-2-oleoyl-Sn-glycero-3-phosphatidylcholine (H-2(31)-POPC) at 25 and 35 degrees C were different from those observed on DMPC and showed ordering effects which are largest for cholesterol, while ergosterol and lanosterol produced significantly smaller effects. Transverse relaxation studies indicate that while cholesterol does not perturb cooperative motions in POPC, both ergosterol and lanosterol do. Again, high-resolution solid state C-13-NMR studies support the conclusions of the H-2-NMR experiments. Titration experiments using both H-2 and C-13-NMR show that ergosterol affects POPC bilayer structure up to 50 mol% but it increases the order of the phospholipid acyl chains only up to about 25 mol%. Beyond that level, it has a smaller ordering effect, possibly indicating aggregation or other more complex phase behavior. At > 30 mol% ergosterol,C-13 spectra reveal the presence of a second form of the sterol. However, P-31-NMR spectra show that all POPC/sterol systems retain a bilayer configuration up to 30 mol% sterol. The concentration of ergosterol which induces maximum order in the POPC membranes coincides with that present in the plasma membranes of the protozoan parasite Trypanosoma cruzi. Taken together, our results indicate that the effects of sterols on PC bilayers are very complex, and depend on both sterol structure and on the fatty acids esterified to the phospholipid.