Optical Anisotropy of Supported Lipid Structures Probed by Waveguide Spectroscopy and Its Application to Study of Supported Lipid Bilayer Formation Kinetics

Abstract

Supramolecular conformation and molecular orientation was monitored during supported lipid bilayer (SLB) formation using dual polarization interferometry (DPI). DPI was shown to enable real time sensitive determination of birefringence of the lipid bilayer together with thickness or refractive index (with the other a fixed value). This approach removes differences in mass loading due to anisotropy, so the mass becomes solely a function of the lipid d n/d c value. DPI measurements show highly reproducible qualitative and quantitative results for adsorption of liposomes of different lipid compositions and in buffers with or without CaCl 2. The packing of solvent-free self-assembled SLBs is shown to differ from other preparation methods. Birefringence analysis accompanied by mass and thickness measurements shows characteristic features of vesicle adsorption and SLB formation kinetics previously not demonstrated by evanescent optical techniques, including indications of percolation-type rupture of clusters of liposomes on the surface and correlated adsorption kinetics induced by liposome charge repulsion. Our study demonstrates that understanding of mechanistic details for an adsorption process for which conformational changes and ordering occur can be elucidated using DPI and greatly enhanced by modeling of optical birefringence. The data is in some respects more detailed than what can be obtained with conventional biosensing techniques like surface plasmon resonance and complementary to methods such as the quartz crystal microbalance.

Rupture of zwitterionic lipid vesicles by an amphipathic, α-helical peptide: Indirect effects of sensor surface and implications for experimental analysis

“…These dn/dc values were chosen so that the resulting optical mass of the final lipid bilayers is ∼400 ng cm −2 [16]. The variation in the dn/dc values for planar bilayers on different substrates is physically reasonable within a certain range because membrane-substrate interactions, including hydrodynamic coupling through the hydration layer, may induce anisotropy of the membranes [30,31]. In all cases, the optical masses of the planar bilayers were slightly smaller than their respective acoustic masses, likely due to coupled solvent that is trapped between the solid supports and the membranes, as well as between the lipid headgroups in the distal leaflets [16].…”

Impact of the antimicrobial peptide Novicidin on membrane structure and integrity

“…For a thorough description on the implications of using DPI in the study of anisotropic films such as supported lipid bilayers see [25,26].…”

“…DPI experiments were made at 20°C. Data analysis was carried as described by [26]. In brief, the refractive index was fixed at 1.47 and we used a d n /d c value of 0.135 which corresponds to a constant layer density of $1.01 g/cm 3 .…”

Characterization of supported lipid bilayers incorporating and phosphoinositol-3,4,5-triphosphate by complementary techniques

“…The control over and characterization of assembly of surface supported lipid bilayers ͑SLBs͒ from liposomes for biosensing has been a hot topic in biointerface science for more than a decade thanks to its high potential for many applications. [13][14][15][16] Membrane conformation, 17 lipid order, 18 structure, 19 mechanical 20 and electrical 21,22 properties, and binding and insertion of peptides [23][24][25] have all been probed using SLBs. For these reasons, Steinem and co-workers made use of SLBs incorporating PIP 2 to study specific interactions with ezrin using quartz crystal microbalance, scanning force microscopy, epifluorescence, and colloidal probe microscopy.…”