HIGH-PRESSURE H-2-NMR STUDY OF THE ORDER AND DYNAMICS OF SELECTIVELY DEUTERATED DIPALMITOYL PHOSPHATIDYLCHOLINE IN MULTILAMELLAR AQUEOUS DISPERSIONS

High pressure H-2 multipulse NMR techniques were used to investigate t he effects of pressure on the structure and dynamics of selectively de uterated 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) multi lamellar aqueous dispersions. The samples were deuterated on both chai ns at positions 2, 9, or 13. The deuterium lineshapes, the spin-lattic e relaxation times, T-1, and the spin-spin relaxation times, T-2, were measured as a function of pressure from 1 bar to 5 kbar at 50 degrees C for the three deuterated DPPC samples. This pressure range permitte d us to explore the phase behavior of DPPC from the liquid-crystalline (LC) phase through various gel phases such as the Gel I (P-beta), Gel II (L(beta)), GeI III, Gel X, and the interdigitated, Gel i, gel phas e. Pressure had an ordering effect on all chain segments both in the L C phase and various high pressure gel phases as indicated by the incre ase in S-CD bond order parameter and the first moment, M1, with pressu re. Compared with the adjacent gel phases, the Gel i phase had the hig hest order. Also, in all gel phases the carbon-9 segment of the chains had the most restricted motions in contrast to the LC phase, where th e carbon-2 segment was the most restricted. In the LC phase, T-1 and T 2 values for all segments decreased with pressure, indicative of the f ast correlation time regime. Similarly, T-1 decreased with pressure in the Gel I and the interdigitated Gel i gel phases but changed to the slow correlation time regime at the Gel i/Gel II phase transition. For T-2, which reflects slow motions, the transition to the slow correlat ion time regime occurred already at LC/Gel I phase transition. Conside ring the various motions which contribute to relaxation, the behavior of T-1 and T-2 in the Gel II through Gel X phases showing discontinuit ies and slope changes at the phase transitions was, as expected, quite complex. In addition we found a straight line relationship for T-1(-1 ) vs. S-2(-1), and T-2(-1) vs. S-CD(2), for the deuterons in the 9 and 13 positions in the LC phase in the pressure range investigated.