The development of remotely controlled nanoscopic sources of heat is essential for investigating and manipulating temperature sensitive processes at the nanoscale. Here, we use single gold nanoparticles to rapidly deposit controlled amounts of heat in nanoscopic regions of defined size. This allows us to induce and control nanoscale reversible gel-fluid phase transitions in phospholipid membranes. We exploit the optical control over the phase transition to determine the velocity of the fluid phase front into the gel phase membrane and to guide the nanoparticles to specific locations. These results illustrate how single gold nanoparticles enable local thermodynamic investigation and manipulation on nanoscale (bio-) systems.
In cell membranes, proteins and lipids diffuse in a highly crowded and
heterogeneous landscape, where aggregates and dense domains of proteins or
lipids obstruct the path of diffusing molecules. In general, hindered motion
gives rise to anomalous transport, though the nature of the onset of this
behavior is still under debate and difficult to investigate experimentally.
Here, we present a systematic study where proteins bound to supported lipid
membranes diffuse freely in two dimensions, but are increasingly hindered by
the presence of other like proteins. In our model system, the surface coverage
of the protein avidin on the lipid bilayer is well controlled by varying the
concentration of biotinylated lipid anchors. Using fluorescence correlation
spectroscopy (FCS), we measure the time correlation function over long times
and convert it to the mean-square displacement of the diffusing proteins. Our
approach allows for high precision data and a clear distinction between
anomalous and normal diffusion. It enables us to investigate the onset of
anomalous diffusion, which takes place when the area coverage of membrane
proteins increases beyond approximately 5%. This transition region exhibits
pronounced spatial heterogeneities. Increasing the packing fraction further,
transport becomes more and more anomalous, manifested in a decrease of the
exponent of subdiffusion.Comment: accepted for publication in Soft Matte
We study proteins at the surface of bilayer membranes using streptavidin and avidin bound to biotinylated lipids in a supported lipid bilayer (SLB) at the solid-liquid interface. Using X-ray reflectivity and simultaneous fluorescence microscopy, we characterize the structure and fluidity of protein layers with varied relative surface coverages of crystalline and noncrystalline protein. With continuous bleaching, we measure a 10-15% decrease in the fluidity of the SLB after the full protein layer is formed. We propose that this reduction in lipid mobility is due to a small fraction (0.04) of immobilized lipids bound to the protein layer that create obstacles to membrane diffusion. Our X-ray reflectivity data show a 40 A thick layer of protein, and we resolve an 8 A layer separating the protein layer from the bilayer. We suggest that the separation provided by this water layer allows the underlying lipid bilayer to retain its fluidity and stability.
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