In-plane homogeneity and lipid dynamics in tethered bilayer lipid membranes (tBLMs)
Tethered bilayer lipid membranes (tBLMs) were prepared by the self-assembly of thiolated lipidic anchor molecules on gold, followed by phospholipid precipitation via rapid solvent exchange. They were characterized by their in-plane structure, dynamics and dielectric properties. We find that the in-plane homogeneity and resistivity of the tBLMs depend critically on a well-controlled sample environment during the rapid solvent-exchange procedure. The in-plane dynamics of the systems, assessed by fluorescence correlation spectroscopy (FCS) as the diffusivity of free, labeled phospholipid dissolved in the membrane, depend on the density of the lipidic anchors in the bilayer leaflet proximal to the substrate as well as on details of the molecular structure of the anchor lipid. In DOPC tBLMs in which tethers are laterally dilute (sparsely tethered bilayer lipid membranes, stBLMs), measured diffusivities, D ≈ 4 µm2 s−1, are only slightly greater than those reported in physisorbed bilayers (M. Przybylo, J. Sýkora, J. Humpolíckova, A. Benda, A. Zan and M. Hof, Langmuir, 2006, 22, 9096–9099). However, when we distinguish label diffusion in the proximal and in the distal bilayer leaflets, we observe distinct diffusivities, D ≈ 2 µm2 s−1 and 7 µm2 s−1, respectively. The value observed in the distal leaflet is identical to that in free membranes. stBLMs completed with phytanoyl lipids (DPhyPC) show consistently lower label diffusivity than those completed with unsaturated chains (DOPC). As the length of the tether chain increases, a reduction in the apparent diffusivity is observed, which we interpret as an increased propensity of the proximal bilayer leaflet to host free lipid. We also investigated preparation conditions that control whether the tBLMs are laterally homogeneous, as assessed by optical microscopy. In laterally heterogeneous bilayers, the label diffusivity varies only by a factor of 2 to 4, indicating that the regions in the bilayers with different label solubilities do not correspond to distinct phases, such as a fluid phase coexisting with a gel phase.