Emergence of compositionally tunable nanoscale dynamical heterogeneity in model binary lipid biomembranes.

While the existence of nanoscale dynamical heterogeneity in biological membranes has been suggested to act as an active functional platform for enabling various cellular processes like signal transduction and viral or bacterial entry, it has been extremely difficult to detect the existence of such domains. Model lipid bilayer membranes have been widely used to detect such dynamical heterogeneity in order to avoid complications arising from the compositional heterogeneity of cellular membranes. However, even in model biological membranes the issue of nanoscale lipid dynamics has remained controversial and unresolved due to the difficulty of detecting the existence of such dynamical heterogeneity on the scale of 10-300 nm. Here we report direct evidence of nanoscale lipid dynamical heterogeneity in model binary lipid bilayer membranes using a combination of super-resolution stimulated emission depletion (STED) microscopy and fluorescence correlation spectroscopy (FCS). We control the phase behavior of the lipid bilayers by varying their composition and discuss how this leads to the emergence of dynamical lipid domains on the scale of 80-150 nm, which is also dependent on the lipid phase in which such dynamics are observed. Notably, our work shows that the presence of cholesterol is not required for the existence of such domains even in fluid like bilayers, as has been widely believed, and specifies the minimal conditions required for the emergence of such dynamical heterogeneity in cellular membranes. Our work will thus not only be of great significance towards understanding the nanoscale dynamic organizing principles of cellular membranes but could also be useful in understanding the dynamics of related soft matter systems and nanoparticle-cell membrane interactions.

A new microscopic insight into membrane penetration and reorganization by PETIM dendrimers.

Dendrimers are highly branched polymeric nanoparticles whose structure and topology, largely, have determined their efficacy in a wide range of studies performed so far. An area of immense interest is their potential as drug and gene delivery vectors. Realizing this potential, depending on the nature of cell surface-dendrimer interactions, here we report controlled model membrane penetration and reorganization, using a model supported lipid bilayer and poly(ether imine) (PETIM) dendrimers of two generations. By systematically varying the areal density of the lipid bilayers, we provide a microscopic insight, through a combination of high resolution scattering, atomic force microscopy and atomistic molecular dynamics simulations, into the mechanism of PETIM dendrimer membrane penetration, pore formation and membrane re-organization induced by such interactions. Our work represents the first systematic observation of a regular barrel-like membrane spanning pore formation by dendrimers, tunable through lipid bilayer packing, without membrane disruption.