Microvesicles (or native vesicles) are membrane fragments comprised of various lipids and other plasma membrane-associated molecules that are shed from cell surfaces. Microvesicles are believed to play a role in mediating intercellular communication by facilitating transfer of bioactive molecules between cells. It has also recently been reported that an elevated level of microvesicle shedding can be associated with cellular dysfunction and thus molecular analysis of microvesicles has the potential to reveal biological information pertaining to cells that shed microvesicles. Accompanying the growing recognition of the significance of microvesicles is the need for a better understanding of their behavior at interfaces. Towards this goal, this presentation will describe the capture of model phospholipid vesicles on surfaces via specific binding events, and the influence of the captured vesicles (or, more precisely, as revealed in this presentation, phospholipid assemblies that are formed from the captured vesicles) on ordering transitions induced in nematic liquid crystals (LCs). The results provide fundamental insights into the interactions of phospholipid-decorated interfaces with LCs, and thereby provide guidance for the design of surfaces on which phospholipid assemblies captured through ligand-receptor recognition can be reported via ordering transitions in LCs.
Specifically, we have found that phospholipid vesicles incorporating ligands, when captured from solution onto surfaces presenting receptors for these ligands, can trigger surface-induced orientational ordering transitions in nematic phases of 4’-pentyl-4-cyanobiphenyl (5CB). Whereas avidin-functionalized surfaces incubated against vesicles comprised of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) were observed to cause the LC to adopt a parallel orientation at the surfaces, the same surfaces incubated against biotinylated vesicles (DOPC and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(biotinyl) (biotin-DOPE)) caused homeotropic (perpendicular) ordering of the LC. The use of a combination of atomic force microscopy (AFM), ellipsometry and quantitative fluorimetry, performed as a function of vesicle composition and vesicle concentration in solution, revealed the capture of intact vesicles containing 1% biotin-DOPE from buffer at the avidin-functionalized surfaces; subsequent exposure to water prior to contact with the LC, however, resulted in rupture of the majority of vesicles into interfacial multilayer assemblies with a maximum phospholipid loading set by random close-packing of the intact vesicles captured initially on the surface (5.1 ± 0.2 phospholipid molecules/nm2). At high concentrations of biotinylated lipid (> 10% biotin-DOPE) in the vesicles, the limiting lipid loading was measured to be 4.0 ± 0.3 phospholipid molecules/nm2, consistent with the maximum phospholipid loading set by spontaneous formation of a bilayer during incubation with the biotinylated vesicles. Independent of the initial morphology of the phospholipid assembly captured on the surface (intact vesicle, planar multilayer), we measured homeotropic ordering of the LC on the surfaces. We interpret this result to infer reorganization of the phospholipid bilayers either prior to or upon contact with the LCs such that interactions of the acyl chains of the phospholipid and the LC dominate the ordering of the LC, a conclusion that is further supported by quantitative measurements of the orientation of the LC as a function of surface density of phospholipid (>1.8 molecules/nm2 is required to cause homeotropic ordering of the LC).