Molecular dynamics of water in oriented DPPC multilayers studied by quasielastic neutron scattering and deuterium-nuclear magnetic resonance relaxation

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Abstract
The dynamics of water between highly oriented multilayers of 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC) has been studied in two time domains at different hydration levels. Incoherent quasielastic neutron scattering (QENS) and deuterium‐nuclear magnetic resonance (NMR) longitudinal (T1) relaxation were employed to investigate both the high‐frequency motions of water (10−9–10−11 s time scale) and their anisotropy, while 2H‐NMR transverse (T2) relaxation was used for obtaining 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 tightly bound to DPPC head groups with a correlation time τrot≊62 ps at 55 °C and a rotational radius of 1±0.1 Å, but with no detectable translational degrees of freedom. The 2H‐NMR T1 data (nanosecond‐time scale) can be explained satisfactorily on the basis of fast rotations with the correlation time above and a slower reorientation of the rotational axis (correlation time τ1≊6 ns). Both QENS and 2H‐NMR T1 measurements provide an apparent activation energy of Ea=32±1.0 kJ/mol for this process. Increasing the hydration level of the multilayers leaves the rotational motion essentially unchanged, but enables additional translational motion which can be considered as a jump diffusion process (diffusion coefficient D=16±1×10−10 m2/s at 44 °C and a mean residence time of τ0=2.0±0.5 ps) of nonbound water. It is interesting to note that this diffusion is completely isotropic on the characteristic length scale of this QENS experiment (≤10 Å). Temperature variation shows that the phase state of the 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 Tm of the oriented DPPC multilayers clearly show a coexistence of the crystalline Lβ’ phase and of the fluid Lα phase over a range of up to 4 °C at both sides of Tm. The intermediate Pβ’ (‘‘ripple’’) phase is suppressed as we worked at hydration levels below saturation. In the coexistence range, exchange of water takes place between crystalline and fluid lipid domains due to water diffusion. This exchange causes a pronounced minimum of the 2H‐NMR transverse relaxation time T2 at Tm since this low frequency process satisfies approximately a critical damping condition for a two‐site chemical exchange process.