MOLECULAR-DYNAMICS CT WATER IN ORIENTED DPPC MULTILAYERS STUDIED BY QUASI-ELASTIC NEUTRON-SCATTERING AND DEUTERIUM-NUCLEAR MAGNETIC-RESONANCE RELAXATION

The dynamics of water between highly oriented multilayers of 1,2-dipal mitoyl-sn-glycero-3-phosphocholine (DPPC) has been studied in two time domains at different hydration levels. Incoherent quasielastic neutro n scattering (QENS) and deuterium-nuclear magnetic resonance (NMR) lon gitudinal (T-1) relaxation were employed to investigate both the high- frequency motions of water (10(-9)-10(-11) s time scale) and their ani sotropy, while 2H-NMR transverse (T-2) relaxation was used for obtaini ng information on low frequency dynamical processes (microsecond time scale). Our results show that high frequency dynamics (picosecond-time scale) at low hydration (three to four water molecules per lipid) can be understood solely as a uniaxial rotation of the water molecules ti ghtly bound to DPPC head groups with a correlation time tau(rot) appro ximate to 62 ps at 55 degrees C and a rotational radius of 1 +/- 0.1 A ngstrom, but with no detectable translational degrees of freedom. The 2H-NMR T-1 data (nanosecond-time scale) can be explained satisfactoril y on the basis of fast rotations with the correlation time above and a slower reorientation of the rotational axis (correlation time tau 1 a pproximate to 6 ns). Both QENS and 2H-NMR T-1 measurements provide an apparent activation energy of E(a) = 32 +/- 1.0 kJ/mol for this proces s. Increasing the hydration level of the multilayers leaves the rotati onal motion essentially unchanged, but enables additional translationa l motion which can be considered as a jump diffusion process (diffusio n coefficient D = 16 +/- 1X10(-10) m(2)/s at 44 degrees C and a mean r esidence time of tau(o) = 2.0 +/- 0.5 ps) of nonbound water. It is int eresting to note that this diffusion is completely isotropic on the ch aracteristic length scale of this QENS experiment (equal to or less th an 10 Angstrom). Temperature variation shows that the phase state of t he lipids has no significant effect on the high frequency dynamics of the water molecules. Measurements of the 2H-NMR quadrupolar splitting of water (D2O) at temperatures around the phase transition temperature T-m of the oriented DPPC multilayers clearly show a coexistence of th e crystalline L(beta') phase and of the fluid L(alpha) phase over a ra nge of up to 4 degrees C at both sides of T-m. The intermediate P-beta ' (''ripple'') phase is suppressed as we worked at hydration levels be low saturation. In the coexistence range, exchange of water takes plac e between crystalline and fluid lipid domains due to water diffusion. This exchange causes a pronounced minimum of the 2H-NMR transverse rel axation time T-2 at T-m since this low frequency process satisfies app roximately a critical damping condition for a two-site chemical exchan ge process.