Visualization and measurement of the local absorption coefficients of single bilayer phospholipid membranes using scanning near-field optical microscopy.

Here we report the results of shear-mode thicknesses and absorption coefficient measurements made on neat membranes using scanning near-field optical microscopy (SNOM). Biomimic neat membranes composed of two different types of phoshpholipid molecules: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2- dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were found to exhibit different absorption coefficients under the SNOM. The localization of the lipids could be identified and correlated to the morphology of the membrane domains indicating that SNOM can be an effective and accurate approach for the label-free characterization of the structure-function relationships in cell membranes.

Analytical approaches to study domain formation in biomimetic membranes.

Domains in biological membranes are linked to a range of biochemical life functions and thus understanding the fundamental physico-chemical drivers of domain formation is one of the key problems of biophysics and chemical biology. The phospholipid bilayer that is the structural basis of the biomembrane is a complex multicomponent mixture, and hence domain formation may be the result of thermodynamic phase equilibria, or specific sequestration of certain lipids; possibly both. There are several obstacles in the way of studying domains and thermodynamic phases in biomembranes: the complexity of the lipid mixture, the two dimensional nature of the membrane and the variety of superstructures the lipid membrane can fold into.

Nanoviscosity Measurements Revealing Domain Formation in Biomimetic Membranes.

Partitioning of lipid molecules in biomimetic membranes is a model system for the study of naturally occurring domains, such as rafts, in biological membranes. The existence of nanometer scale membrane domains in binary lipid mixtures has been shown with microscopy methods; however, the nature of these domains has not been established unequivocally. A common notion is to ascribe domain separation to thermodynamic phase equilibria.

Formation of planar unilamellar phospholipid membranes on oxidized gold substrate.

Supported planar phospholipid membranes are used in a range of biophysical measurements, typically for characterizing protein-membrane interactions. Liposome deposition is the most common method to create such membranes. The ability of liposomes to fuse into a lamellar membrane during deposition is strongly dependent on the surface chemistry; some important substrate materials such as oxidized gold do not promote liposome fusion.

Cholesterol Rich Domains Identified in Unilamellar Supported Biomimetic Membranes via Nano-Viscosity Measurements.

Understanding the distribution of cholesterol in phospholipid membranes is of key importance in membrane biophysics, primarily since cholesterol enriched regions, rafts, are known to play a special role in protein function. In this work, quartz crystal microbalance with dissipation (QCM)-based viscosity measurements were used to study cholesterol-induced domain formation in partially suspended single bilayer membranes. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and its mixtures with different amounts of cholesterol were studied.

Viscoelastic changes measured in partially suspended single bilayer membranes.

For studies involving biomimetic phospholipid membrane systems, such as membrane-protein interactions, it is crucial that the supported membrane is biomimetic in its physical properties as well as in its composition. Two often overlooked aspects of biomimicry are the need for unrestrained lipid mobility, reflected in the viscoelastic properties of the membrane, and sufficient space between the membrane and the support for the insertion of transmembrane proteins. Here we show for a series of DMPC-based membranes that a partially suspended single bilayer membrane can be formed on functionalized gold surface without tethering.