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. Complexity is addressed by the introduction of biomimetic membranes, simplified mixtures of synthetic lipids. Most studies of lipid phase equilibria have been conducted using a biomimetic membrane. This review is intended to address the challenges posed to analytical methodology by the membrane dimensions, while also discussing the question of the reference state. Four key methods are assessed for their strengths and weaknesses in identifying domains and thermodynamic phases in membranes: differential scanning calorimetry, fluorescence microscopy, atomic force microscopy and quartz crystal microbalance. It is demonstrated that, while these methods provide complementary information and thus should be used in tandem, quartz crystal microbalance based nano-viscosimetry measurements offer a breakthrough in measuring phase transition temperatures, and allow the compilation of phase diagrams, of single bilayers of lipid mixtures. By comparing the structural phases of the lipids used for the different methods, it is also shown that the membrane curvature in vesicular lipid samples inhibits the formation of domains which are only observed in flat lamellar membranes, or giant unilamellar liposomes where the role of curvature is negligible.

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. However, characterizing thermodynamic phases of single bilayer membranes has not been possible due to their extreme dimensions: the size of the domains falls to the order of tens to hundreds of nanometers whereas the membrane thickness is only a few nanometers. Here, we present direct measurements of phase transitions in single bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) phospholipid mixtures using quartz crystal microbalance-based nanoviscosity measurements. Coexisting thermodynamic phases have been successfully identified, and a phase diagram was constructed for the single bilayer binary lipid system. It was demonstrated that domain separation only takes place in planar membranes, and thus, it is absent in liposomes and not detectable in calorimetric measurements on liposome suspensions. On the basis of energetic analysis, the main transition was identified as the breaking of van der Waals interactions between the acyl chains.

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. Circumventing this determinism poses an enduring challenge to membrane biophysics. Here, the authors show that the effect of surface chemistry can be overcome by using osmotic stress. Reproducible single bilayer coverage was achieved on oxidized gold surface from liposomes of a variety of lipid compositions, as demonstrated by quartz crystal microbalance measurements and confirmed via fluorescence microscopy imaging. The continuity of the deposit was confirmed by fluorescence recovery after photobleaching. Using mixtures of di-myristoyl and di-palmitoyl lipids, it was also demonstrated that the formation of fused lamellar membranes upon osmotic stress is a sensitive function of the thermodynamic phase of the membrane.

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. QCM temperature ramping experiments identified domains of different phase transition temperatures in the mixed membranes. The phase transition of DMPC shifted from 23.4 °C toward lower temperatures with increasing cholesterol content. A second, continuous but much broader, transition peak has been observed for the DMPC: cholesterol mixtures suggest that a separate cholesterol rich domain coexists with the DMPC rich domain. Importantly, the sharp DMC phase transition peak gradually diminished and eventually disappeared over 15% cholesterol content, suggesting that the cholesterol rich domain has a definite stoichiometry and once this cholesterol concentration is reached the DMPC-rich domain disappears. DSC control experiments do not show the second domain, suggesting that the phase separation only happens in nontensioned (flat) membranes.

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. These membranes exhibit sufficient freedom of motion to represent the viscoelastic properties of a free lamellar bilayer membrane as demonstrated by determining the phase transition temperatures of these single bilayer membranes from the viscosity change upon chain melting using the dissipation signal of a quartz crystal microbalance (QCM-D). Atomic force microscopy imaging confirmed confluent, smooth membrane coverage of the QCM-D sensor that completely obscured the roughness of the sputtered gold surface. High-force AFM imaging was able to push membrane patches into the valleys of the gold morphology, confirming the inherently suspended nature of the MPA supported membrane. We show that the correlation between frequency and dissipation changes in the QCM-D sensograms is a sensitive indicator of the morphology of the membrane.