Molecular dynamics of lipid bilayers studied by incoherent quasi-elastic neutron scattering

Abstract

Molecular motions in highly oriented multilayers of dipalmitoylphosphatidylcholine were studied as a function of temperature and hydration using incoherent quasi-elastic neutron scattering (QENS). The short range diffusive motions of the lipid molecules and the chain/headgroup dynamics were evaluated : 1) by measurement of the dependence of the elastic incoherent structure factor (EISF), the line-width Γ and the dynamic structure factors on the scattering vector Q for two orientations of the sample. The orientations were chosen such that the scattering vecto Q was either predominantly perpendicular or parallel to the membrane normal ; 2) by comparing data from protonated and chain deuterated lipids and 3) by the use of instruments of different energy resolution (i.e. time-of-flight and backscattering spectrometers exploring time regimes of 10-13 s to 10-11 s and 10-11 s to 10-9 s respectively). In the fluid phase the time-of-flight spectra revealed a restricted isotropic in-plane and out-of-plane diffusion of the hydrocarbon chain and headgroup protons. The mean displacements range from ≈0.6 A for methylene protons near the glycerol backbone to 7 A for protons near the chain ends. These values are obtained for a water content of 23 wt%. The values are somewhat increased at 30wt% of water. Measurements of the temperature variation of the EISF and the line-width Γ revealed a remarkably high degree of chain dynamics in the gel (Lβ')-phase. The total elastic intensity as observed with the backscattering instrument showed that Lα-Lβ'-phase transition is only well expressed at Q-values around 1 A-1, while the number and mobility of the chain defects characterized at Q≈2 A-1 (possibly gtg-kinks) increase continuously between 2 °C and 70 °C. In the time regime explored by the backscattering instrument, motions of the whole lipid molecules are also seen. It was interpreted in terms of a superposition of local in-plane and out-of-plane diffusion and lateral diffusional jumps between adjacent sites as predicted by the free volume model. For a sample containing 12 wt% of water at 60 °C the diffusion coefficient for the out-of-plane motion is D∥=6×10-6 cm2/s with an amplitude of 2.25 A. In-plane the diffusion coefficients range from Dmin ⊥=1.5×10-7 cm2/s to Dmax ⊥=6×10-6 cm2/s. The lateral diffusion coefficient is Dlat=9.7×10-8 cm2/s in reasonable agreement with FRAP measurements. The strong increase of the lateral mobility with increasing water content yielded an exponential law for the variation of the diffusion coefficient with excess area per lipid (i.e. hydration) in agreement with the free volume model. The out-of-plane motion is characterized by an amplitude of about 0.5 A in the time-of-flight time regime and of 2-3 A in the backscattering time regime. The origin of this discrepancy could be the thermally excited membrane undulations since their relaxation times of ≈3×10-9 s (obtained in a separate spin-echo study) agree roughly with the reciprocal line-width of 2.5×10-9 s for the backscattering instrument at Q↦0. The time-of-flight result of 0.5 A can be attributed to a dynamic surface roughness.