Review pubs.acs.org/ac
Instrumental Methods to Characterize Molecular Phospholipid Films on Solid Supports Irep Gözen and Aldo Jesorka* Department of Chemical and Biological Engineering, Chalmers University of Technology, Kemivägen 10, 41296 Göteborg, Sweden
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CONTENTS
Method Overview Force Microscopy and Spectroscopy Techniques Secondary Ion Mass Spectrometry and Imaging Optical Techniques X-ray and Neutron Techniques Electron Microscopy Other Methods Computational Techniques Microfluidic Technology Areas of Application Lipid Chemistry and Composition Membrane Formation Membrane Structure and Domain Formation Membrane Interactions with Other Molecules Membrane Dynamics/Kinetics, Miscellaneous Studies Summary and Conclusions Author Information Corresponding Author Biographies Acknowledgments References
ization, imaging, and analysis. Molecular phospholipid film technologies are a key to the reconstitution of transmembrane proteins2−4 for functional studies and have been evaluated for use in high-throughput screening.5 Supported membranes are now a highly valued platform, suitable for a large variety of studies on biological as well as synthetic materials, where the experimenter can chose from a rich toolbox providing functional elements from chemical synthesis, biology and biophysics, computation, and micro/ nanofabrication. The current rapid advancement of bionanoscience6 and technology is tightly coupled to progress in biomembrane methods,7,8 which naturally requires that analytical methodology for characterization and observation of membrane structures and associated molecules and architectures keeps pace. The stunning complexity of many of the contemporary studies,9 often involving membrane proteins, nanopatterend surfaces, and sophisticated interfacing strategies, would not be possible without the application of state of the art measurement and analysis instrumentation. Moreover, the two predominant distance scales in phospholipid films, their thickness on one hand and the dimensions of the covered area on the other hand, differ by several orders of magnitude, making combinations of techniques strictly necessary in order to effectively address short-range and long-range features and phenomena in the same sample. In this review we focus on modern instrumental methods which are commonly applied to study different aspects of molecular lipid films on surfaces, with emphasis on the major areas of membrane research: chemical composition, formation methods of molecular lipid films, phase separation/domain formation and structural studies, dynamic properties and lipid interaction with other molecules. We begin with a brief overview over suitable instrumental methods, their state-of-theart abilities and limitations, and then progress to the latest applications to molecular lipid film studies. The study of lipid membranes is a transdisciplinary field at the interface between nanotechnology, biochemistry, and materials science, where progress is particularly dependent on the technological development of instrumental methods, in particular techniques with single-molecule capabilities and atomic resolution.10 The review should provide researchers with the necessary background to identify a range of suitable methods in order to successfully
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upported phospholipid films closely resemble cell membranes in structure and composition and serve as model systems in investigations of transmembrane transport, membrane protein function, drug−receptor interactions, and others. They have been investigated for several decades, predominantly with the goals of understanding their properties and dynamic features as well as their interaction with biological molecules and particles.1 Typical model systems comprise surface-supported lipid bilayers, lipid monolayers, and tethered lipid-bilayers on a variety of substrates. Bilayers are formed on hydrophilic high energy surfaces, such as SiO2 or TiO2, while monolayers prefer strongly hydrophobic polymers, e.g., Teflon or photoresists. Tethered bilayers are molecular films chemically linked to a suitable surface, where the linking molecule acts as a spacer or forms a cushion between the surface and membrane. Such arrangements facilitate the incorporation of bulky membrane proteins, which occupy space on both sides of a phospholipid bilayer. For some applications, the two-dimensional arrangement in surface adhered molecular films offers practical advantages over liposome (vesicle) suspensions or giant unilamellar vesicles. The surface support provides greater stability, more versatile and controlled fabrication, and most importantly, allows for a broader spectrum of analytical instrumentation for character© 2011 American Chemical Society
Special Issue: Fundamental and Applied Reviews in Analytical Chemistry Published: December 6, 2011 822
dx.doi.org/10.1021/ac203126f | Anal. Chem. 2012, 84, 822−838
Analytical Chemistry
Review
of photodiodes transduces the deflection. Although of advantage to improve resolution,21 AFM also does not strictly require high vacuum conditions during measurement. This increases the number of suitable samples and opens the method to soft material and biological samples,22 among them phospholipid membranes. There are two main scanning modes of AFM based on tip dynamics. In static mode (contact mode), the cantilever tip is immobile and dragged continuously in physical contact (
