HIGH-PRESSURE P-31 NMR-STUDY OF DIPALMITOYLPHOSPHATIDYLCHOLINE BILAYERS

High-pressure P-31 NMR was used for the first time to investigate the effects of pressure on the structure and dynamics of the phosphocholine headgroup in pure 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar aqueous dispersions and in DPPC bilayers containing the positively charged form of the local anesthetic tetracaine (TTC). The P-31 chemical shift anisotropies, DELTA-sigma, and the P-31 spin-lattice relaxation times, T1, were measured as a function of pressure from 1 bar to 5 kbar at 50-degrees-C for both pure DPPC and DPPC/TTC bilayers. This pressure range permitted us to explore the rich phase behavior of DPPC from the liquid-crystalline (LC) phase through various gel phases such as gel I (P(beta)'), gel II (L(beta)'), gel III, gel IV, gel X, and the interdigitated, Gi, gel phase. For pure DPPC bilayers, pressure had an ordering effect on the phospholipid headgroup within the same phase and induced an interdigitated Gi gel phase which was formed between the gel I (P(beta)') and gel II (L(beta)') phases. The P-31 spin-lattice relaxation time measurements showed that the main phase transition (LC to gel I) was accompanied by the transition between the fast and slow correlation time regimes. Axially symmetric P-31 NMR lineshapes were observed at pressures up to approximately 3 kbar but changed to characteristic axially asymmetric rigid lattice lineshapes at higher pressures (3.1-5.1 kbar). As expected, the addition of the positively charged form of TTC increased the absolute value of the P-31 chemical shift anisotropy, \DELTA-sigma\, and brought about a change in the conformation of the headgroup which swung toward the bilayer normal, from its usual orientation parallel to the membrane surface. The addition of TTC also raised the critical pressure of the main phase transition and induced the formation of an interdigitated Gi gel phase directly from the LC phase. Interestingly the expressions for P-31, DELTA-sigma, introduced by Scherer and Seelig (1989) to determine the effect of electric surface charge on the polar headgroup of phospholipid bilayers were found applicable for the LC phase at high pressures and also for the gel phases at pressures in excess of 3.5 kbar.