HYDROSTATIC PRESSURE-INDUCED CONFORMATIONAL-CHANGES IN PHOSPHATIDYLCHOLINE HEADGROUPS - A H-2 MMR STUDY

The effects of pressure and temperature on 1,2-dipalmitoyl-sn-glycero- 3-phosphocholine and 1,2-dimyristoyl-sn-glycero-3-phosphocholine headg roup conformations were examined using deuterium nuclear magnetic reso nance. Isothermal compression was found to produce a decrease in the c holine alpha deuteron quadrupole splitting and increases in the cholin e beta and gamma deuteron quadrupole splittings. A similar counterdire ctional change, seen in the presence of positive surface charge, has b een attributed to tilting of the headgroup away from the bilayer surfa ce in response to the torque exerted on the phosphocholine dipole by p ositive surface charges. The direction of the change in headgroup deut eron quadrupole splitting is consistent with the pressure-induced redu ction in area per lipid in the liquid crystalline phase, which can be inferred from the ordering of phospholipid acyl chains under comparabl e conditions. The temperature dependences of the headgroup deuteron qu adrupole splittings were also examined. It was found that at elevated pressure, the alpha splitting was insensitive to temperature, whereas the beta and gamma splittings decreased. The response of the beta deut eron splitting to temperature was found to be weaker at elevated press ure than at ambient pressure.