en>Yobot: /* References */WP:CHECKWIKI error fixes using AWB (10093)

2014-05-05T12:25:18Z

References: WP:CHECKWIKI error fixes using AWB (10093)

en>Helpful Pixie Bot: Dated {{Clarify}}{{Notability}}. (Build KF)

2012-05-14T01:00:09Z

Dated {{Clarify}}{{Notability}}. (Build KF)

New page

'''Lipid bilayer mechanics''' is the study of the physical material properties of [[lipid bilayers]], classifying bilayer behavior with [[stress (mechanics)|stress]] and [[strain (materials science)|strain]] rather than biochemical interactions. Local point deformations such as membrane protein interactions are typically modelled with the complex theory of [[Liquid_Crystal#Biological_liquid_crystals|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 <math>\Lambda</math>. 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.<ref name="Garcia2004">M. L. Garcia."Ion channels: Gate expectations." Nature. 430. (2004) 153-155.</ref><ref name="McIntosh2006">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.</ref> Bilayer mechanical properties have also been shown to alter the function of mechanically activated [[ion channels]].<ref name="Suchyna2004">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.</ref>

==Area expansion Modulus==
{{further2|[[Elasticity of cell membranes]]}}
{{see also|Elastic 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 [[Covalent bond|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.<ref name="Boal2002">D. Boal, "Mechanics of the Cell". 2002, Cambridge, UK: Cambridge University Press.</ref>
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.<ref name= "Israelachvili2002">Israelachvili, J., ''Intermolecular and Surface Forces.'' 2nd ed. 2002: Academic Press.</ref> 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<ref name="Rutkowski1991">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.</ref> but only weakly with tail length and unsaturation.<ref name="Rawicz2000">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.</ref>

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.<ref name= "Rutkowski1991">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.</ref> 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]].<ref name="Evans2003">E. Evans, V. Heinrich, F. Ludwig and W. Rawicz."Dynamic tension spectroscopy and strength of biomembranes." Biophysical Journal. 85. (2003) 2342-2350.</ref> More recently, atomic force microscopy (AFM) has been used to probe the mechanical properties of suspended bilayer membranes,<ref name="Steltenkamp2006">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.</ref> 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.<ref name="Rawicz2000">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.</ref> 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.

==Bending Modulus==
{{see also|Beam (structure)}}
Bending modulus is defined as the energy required to deform a membrane from its intrinsic curvature to some other curvature. For an ideal bilayer the intrinsic curvature is zero, so this expression is somewhat simplified. The bending modulus, compression modulus and bilayer thickness are related by <math> K_b= K_a *t</math> such that if two of these parameters are known the other can be calculated. This relationship derives from the fact that to bend the inner face must be compressed and the outer face must be stretched.<ref name="Boal2002" /> The thicker the membrane, the more each face must deform to accommodate a given curvature (see [[bending moment]]). Many of the values for Ka in literature have actually been calculated from experimentally measured values of Kb and t. This relation holds only for small deformations, but this is generally a good approximation as most lipid bilayers can support only a few percent strain before rupturing.<ref name="Hallett1993">F. R. Hallett, J. Marsh, B. G. Nickel and J. M. Wood."Mechanical properties of vesicles: II A model for osmotic swelling and lysis." Biophysical Journal. 64. (1993) 435-442.</ref>

[[Image:Phospholipids aqueous solution structures.svg|thumb|170px|left|Cross section view of the structures that can be formed by phospholipids in aqueous solutions]]

==Curvature==
Only certain classes of lipids can form bilayers. Two factors primarily govern whether a lipid will form a bilayer or not: solubility and shape. For a self assembled structure such as a bilayer to form, the lipid should have a low solubility in water, which can also be described as a low [[critical micelle concentration]] (CMC).<ref name= "Israelachvili2002"/> Above the CMC, molecules will aggregate and form larger structures such as bilayers, [[micelles]] or inverted micelles.

The primary factor governing which structure a given lipid forms is its shape (i.e.- its intrinsic curvature).<ref name="Boal2002" /> Intrinsic curvature is defined by the ratio of the diameter of the head group to that of the tail group. For two-tailed [[phosphatidylcholine|PC]] lipids, this ratio is nearly one so the intrinsic curvature is nearly zero. Other headgroups such as [[Phosphatidylserine|PS]] and [[Phosphatidylethanolamine|PE]] are smaller and the resulting diacyl (two-tailed) lipids thus have a negative intrinsic curvature. Lysolipids tend to have positive spontaneous curvature because they have one rather than two [[alkyl]] chains in the tail region. If a particular lipid has too large a deviation from zero intrinsic curvature it will not form a bilayer.

==Edge Energy==
[[Image:Pore schematic.svg|right|thumb|300px|Schematic showing two possible conformations of the lipids at the edge of a pore. In the top image the lipids have not rearranged, so the pore wall is hydrophobic. In the bottom image some of the lipid heads have bent over, so the pore wall is hydrophilic.]]
Edge energy is the energy per unit length of a free edge contacting water. This can be thought of as the work needed to create a hole in the bilayer of unit length L. The origin of this energy is the fact that creating such an interface exposes some of the lipid tails to water, which is unfavorable. <math>\Lambda</math> is also an important parameter in biological phenomena as it regulates the self-healing properties of the bilayer following electroporation or mechanical perforation of the cell membrane.<ref name="Evans2003" /> Unfortunately, this property is both difficult to measure experimentally and to calculate. One of the major difficulties in calculation is that the structural properties of this edge are not known. The simplest model would be no change in bilayer orientation, such that the full length of the tail is exposed. This is a high energy conformation and, to stabilize this edge, it is likely that the some of the lipids rearrange their head groups to point out in a curved boundary.{{citation needed|date=November 2011}} The extent to which this occurs is currently unknown and there is some evidence that both hydrophobic (tails straight) and hydrophilic (heads curved around) pores can coexist.<ref name="Weaver1996">J. C. Weaver and Y. A. Chizmadzhev."Theory of electroporation: A review " Biochemistry and Bioenergetics. 41. (1996) 135-160.</ref>

==References==
{{reflist}}

{{DEFAULTSORT:Lipid Bilayer Mechanics}}
[[Category:Biotechnology]]
[[Category:Membrane biology]]

Concentration dimension - Revision history

en>Yobot: /* References */WP:CHECKWIKI error fixes using AWB (10093)

en>Helpful Pixie Bot: Dated {{Clarify}}{{Notability}}. (Build KF)