Effect of pressure on the fast motions in ordered phase phospholipid bilayers

Application of hydrostatic pressure to phospholipid bilayers increases acyl chain order and raises the main transition temperature. ²H NMR spectra and quadrupole echo decay times were obtained at ambient pressure and pressures of 85 MPa and 196.1 MPa for ordered phase bilayers of a zwitterionic phospholipid : 16:0-16:0 PC-d₆₂ (DPPC-d₆₂) and an anionic phospholipid : 16:0-16:0 PG-d₆₂ (DPPG-d₆₂). The extent to which deuterium magnetization following an RF pulse is refocused in the echo after a second pulse is limited by the motions that modulate the orientation-dependent quadrupole interaction. Quadrupole echo decay times were recorded for a temperature range of 60°C to -26°C for both lipids, at ambient pressure and pressures of 85 MPa and 196.1 MPa. The temperature dependences of the echo decay times at ambient pressure are similar for the two lipids. The echo decay times pass through a minimum at the main transition, as the correlation times of some motions change from short to intermediate on the NMR timescale. Upon further cooling in the gel phase, the echo decay time passes through a maximum before falling to a second minimum. This indicates the persistence of some fast motions into the ordered phases. At 196.1 MPa, both lipids undergo a transition to more ordered crystalline phases near 0°C. This is reflected as a plateau in the temperature dependence of quadrupole echo decay times at high pressure. The application of 85 MPa shifts the main transition but does not lower the low temperature minimum in the echo decay time. The behavior of DPPGd₆₂ at 85 MPa is qualitatively similar to that at higher pressure. The q-CPMG pulse sequence is used to separate the contribution of slow and fast motions to the echo decay rate. This work provides insight into how chain packing affects local motion