NMR Studies of the Order and Dynamics of Dipalmitoylphosphatidylchoiine Bilayers as a Function of Pressure

Abstract

We have used 2H NMR methods to examine the order and dynamics of dipalmitoylphosphatidylcholine (DPPC) in multilamellar and small unilamellar vesicles in water as a function of pressure. Multipulse 2H NMR techniques were used with selectively deuterated DPPC on both chains at positions C-2, C-9, or C-13, to obtain lineshapes, spin-lattice relaxation times (T1), and spin-spin relaxation times (T2) at 50 °C from 1 bar to 5.2 kbar pressure. This pressure range allowed us to explore the phase behavior of DPPC from the liquid crystalline (LC) phase through various gel phases (Gl, Gll, Glll, GX), including the interdigited Gi phase. Pressure has an ordering effect: on all chain segments in all the phases. In the LC phase, the order parameter (SCD) decreases from C-2 > C-9 > C-13, while in the gel phases SCD decreases from C-9 > C-13 > C-2, indicating that in the gel phases the middle segments of the chains are more restricted in their motions than the ends. In the LC phase, T1 and T2 values for all segments decrease with pressure and have an order from C-13 > C-9 > C-2. These results suggest that similar conformational motions and molecular rotational motions occur in the LC state in all segments, but have increased amplitudes and frequencies toward the methyl ends. At the phase transitions, discontinuities and abrupt reversal of the slopes for the T1 or T2 dependences on pressure indicate major changes in motional modes and rates for DPPC molecules in the different structures. In the second part of this study, we have measured the lateral diffusion of DPPC in sonicated vesicles in D2O as a function of pressure. The spin-lattice relaxation rate in the rotating frame T−11p was plotted as a function of the square root of the spin-locking field angular frequency (ω1)1/2, and the lateral diffusion coefficient (D) was calculated from the slope. Pressure effects are observed on lateral diffusion in the LC phase (D = 5.4 − 2 × 10−9 cm2 seconds, from 1 to 300 bar) but are negligible in the GI phase (D ≈ 1.0 × 10−9 cm2 seconds, from 400 to 800 bar).