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Lipid bilayer mechanics is the study of the physical material properties of lipid bilayers, classifying bilayer behavior with stress and strain rather than biochemical interactions. Local point deformations such as membrane protein interactions are typically modelled with the complex theory of biological liquid crystals but the mechanical properties of a homogeneous bilayer are often characterized in terms of only three mechanical elastic modulus: the area expansion modulus Ka, a bending modulus Kb and an edge energy . For fluid bilayers the shear modulus is by definition zero, as the free rearrangement of molecules within plane means that the structure will not support shear stresses. These mechanical properties affect several membrane-mediated biological processes. In particular, the values of Ka and Kb affect the ability of proteins and small molecules to insert into the bilayer.〔M. L. Garcia."Ion channels: Gate expectations." Nature. 430. (2004) 153-155.〕〔T. J. McIntosh and S. A. Simon."Roles of Bilayer Material Properties in Function and Distribution of Membrane Proteins." Annu. Rev. Biophys. Biomol. Struct. 35. (2006) 177–198.〕 Bilayer mechanical properties have also been shown to alter the function of mechanically activated ion channels.〔T. M. Suchyna, S. E. Tape, R. E. Koeppe, O. S. Andersen, F. Sachs and P. A. Gottlieb."Bilayer-dependent inhibition of mechanosensitive channels by neuroactive peptide enantiomers." Nature. 6996. (2004) 235-240.〕 ==Area expansion Modulus== Since lipid bilayers are essentially a two dimensional structure, Ka is typically defined only within the plane. Intuitively, one might expect that this modulus would vary linearly with bilayer thickness as it would for a thin plate of isotropic material. In fact this is not the case and Ka is only weakly dependent on bilayer thickness. The reason for this is that the lipids in a fluid bilayer rearrange easily so, unlike a bulk material where the resistance to expansion comes from intermolecular bonds, the resistance to expansion in a bilayer is a result of the extra hydrophobic area exposed to water upon pulling the lipids apart.〔D. Boal, "Mechanics of the Cell". 2002, Cambridge, UK: Cambridge University Press.〕 Based on this understanding, a good first approximation of Ka for a monolayer is 2γ, where gamma is the surface tension of the water-lipid interface. Typically gamma is in the range of 20-50mJ/m2.〔Israelachvili, J., ''Intermolecular and Surface Forces.'' 2nd ed. 2002: Academic Press.〕 To calculate Ka for a bilayer it is necessary to multiply the monolayer value by two, since a bilayer is composed of two monolayer leaflets. Based on this calculation, the estimate of Ka for a lipid bilayer should be 80-200 mN/m (note: N/m is equivalent to J/m2). It is not surprising given this understanding of the forces involved that studies have shown that Ka varies strongly with solution conditions〔C. A. Rutkowski, L. M. Williams, T. H. Haines and H. Z. Cummins."The elasticity of synthetic phospholipid vesicles obtained by photon correlation spectroscopy." Biochemistry. 30. (1991) 5688-5696.〕 but only weakly with tail length and unsaturation.〔W. Rawicz, K. C. Olbrich, T. McIntosh, D. Needham and E. Evans."Effect of chain length and unsaturation on elasticity of lipid bilayers." Biophysical Journal. 79. (2000) 328-39.〕 The compression modulus is difficult to measure experimentally because of the thin, fragile nature of bilayers and the consequently low forces involved. One method utilized has been to study how vesicles swell in response to osmotic stress. This method is, however, indirect and measurements can be perturbed by polydispersity in vesicle size.〔Rutkowski, C.A., L.M. Williams, T.H. Haines, and H.Z. Cummins. "The elasticity of synthetic phospholipid vesicles obtained by photon correlation spectroscopy." Biochemistry. (1991). 30. 5688-5696.〕 A more direct method of measuring Ka is the pipette aspiration method, in which a single giant unilamellar vesicle (GUV) is held and stretched with a micropipette.〔E. Evans, V. Heinrich, F. Ludwig and W. Rawicz."Dynamic tension spectroscopy and strength of biomembranes." Biophysical Journal. 85. (2003) 2342-2350.〕 More recently, atomic force microscopy (AFM) has been used to probe the mechanical properties of suspended bilayer membranes,〔S. Steltenkamp, M. M. Muller, M. Deserno, C. Hennesthal, C. Steinem and A. Janshoff."Mechanical properties of pore-spanning lipid bilayers probed by atomic force microscopy." Biophysical Journal. 91. (2006) 217-226.〕 but this method is still under development. One concern with all of these methods is that, since the bilayer is such a flexible structure, there exist considerable thermal fluctuations in the membrane at many length scales down to sub-microscopic. Thus, forces initially applied to an unstressed membrane are not actually changing the lipid packing but are rather “smoothing out” these undulations, resulting in erroneous values for mechanical properties.〔Rawicz, W., K.C. Olbrich, T. McIntosh, et al. "Effect of chain length and unsaturation on elasticity of lipid bilayers." Biophysical Journal. (2000). 79. 328-39.〕 This can be a significant source of error. Without the thermal correction typical values for Ka are 100-150 mN/m and with the thermal correction this would change to 220-270 mN/m. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lipid bilayer mechanics」の詳細全文を読む スポンサード リンク
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