Diffusion and molecular dynamics of lipo-fullerenes in phospholipid membranes studied by NMR and quasi-elastic neutron scattering

Lipo-fullerenes are lipophilic C-60 derivatives (six pairs of alkyl chains symmetrically grafted to the C-60 cage) that intercalate in phospholipid bi layers by the formation of rodlike structures of nanoscopic dimensions. Pro ton NMR measurements in the fringe field of a superconducting magnet (SFF-N MR) were employed to measure the long-range self-diffusion of lipo-fulleren es intercalated in oriented multilayers of dipalmitoylphosphatidylcholine ( DPPC) between 27 and 70 degrees C, The lipo-fullerene diffusion was found t o be an order of magnitude slower than that of the DPPC in the host bilayer . The lipo-fullerene diffusion continued largely unaffected even under cond itions when the host bilayer assumed a solidlike gel state, indicating a de coupling of lipo-fullerene and DPPC motion. Quasi-elastic neutron scatterin g (QENS) was used to study the molecular dynamics of the lipo-fullerenes wi thin the bilayer at four energy resolutions of the spectrometer (1, 19, 62, and 500 mu eV), covering selectively the gighertz to terahertz frequency r ange of molecular motion. We find that the molecular dynamics in this frequ ency range is dominated by the motion of the 12 alkyl chains attached to ea ch C-60 At 25 degrees C where both the DPPC and the lipo-fullerenes are in a solidlike state, the dynamics of the latter is dominated by kink defects of the alkyl chains at high frequency (terahertz range) and alkyl chain jum ps at lower frequency gighertz range). At 75 degrees C when both components are fluid, the lipo-fullerene dynamics can be described by a superposition of segmental rotational motion of chains and a spatially restricted diffus ion of the chain inside a spherical volume. The radii of the volumes were f ound to scale linearly with the distance of the chain segment from the full erene cage.