Abstract
The mechanical properties of biological systems are emerging as fundamental in determining their functional activity. For example, cells continuously probe their environment by applying forces and, at the same time, are exposed to forces produced by the same environment. Also in biological membranes, the activity of membrane related proteins are affected by the overall mechanical properties of the hosting environment. Traditionally, the mesoscopic mechanical properties of lipid bilayers have been studied by micropipette aspiration techniques. In recent years, the possibility of probing mechanical properties of lipid bilayers at the nanoscale has been promoted by the force spectroscopy potentiality of Atomic Force Microscopes (AFM). By acquiring force-curves on supported lipid bilayers (SLBs) it is possible to probe the mechanical properties on a scale relevant to the interaction between membrane proteins and lipid bilayers and to monitor changes of these properties as a result of a changing environment. Here, we review a series of force spectroscopy experiments performed on SLBs with an emphasis on the functional consequences the measured mechanical properties can have on membrane proteins. We also discuss the force spectroscopy experiments on SLBs in the context of theories developed for dynamic force spectroscopy experiments with the aim to extract the kinetic and energetic description of the process of membrane rupture.
Highlights
Since the monumental work “On growth and forms” by D’Arcy Thompson (1917) the feeling that mechanical forces can determine the evolution and behavior of living systems informed more and more the approaches of physicists to the study of bio-systems
We showed how force spectroscopy experiments on supported lipid bilayers provide useful information about the mechanical properties of the bilayer at the nanoscale
In the second case care should be paid to the fact that the influence of the rigid substrate immediately below the very thin (∼5 nm) lipid bilayer can alter the quantitative evaluation of the mechanical moduli (Dimitriadis et al, 2002)
Summary
Since the monumental work “On growth and forms” by D’Arcy Thompson (1917) the feeling that mechanical forces can determine the evolution and behavior of living systems informed more and more the approaches of physicists to the study of bio-systems. AFM offers great potentialities in the study of elastic and viscous properties of lipid bilayers assembled to form membranes supported by a rigid substrate In this context, the microscopic imaging capabilities of this technique can be coupled to its force spectroscopy potentialities. The dependence of the threshold-force on the tip speed allows retrieve information on the energy landscape underlying the rupture of the bilayer by the AFM tip The relevance of this process in the biological context is connected to its analogy with pore formation events in pure lipid membranes. The present review provides an overview of the potentiality of AFM for the characterization of the mechanical properties of supported lipid bilayers especially in relation to the thermodynamic state of the membrane The relevance of these mechanical properties for the functional activities of integral membrane proteins will be highlighted and, when possible, some specific experimental evidences will be presented. The review will end with a section dedicated to conclusions and perspectives of these studies in relation to their biological relevance
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