Lateral compressibility, lipid bilayer

Figure 20. (a) The (dimensionless) lateral compressibility (dilatational modulus, elastic area expansion modulus) (left ordinate) and the dimensionless area per molecule (right ordinate) as a function of the tail length (t) of the PC lipids in equilibrium bilayer membranes. The conversion to real compressibilities and areas per molecule is discussed in the text, (b) The (dimensionless) surface tension and the (dimensionless) lateral compressibility as a function of the relative expansion for the C PC lipid... 

Furthermore, Brown (1994) listed the following properties as those that are important in determining the activation of rhodopsin average bilayer thickness see Dratz Subheading 6.2.) lateral compressibility see Litman and Mitchell, below) curvature stresses of the lipid-water and protein-lipid interfaces and electrostatic forces (determined by the charge of the headgroups). 

Under osmotic stress, the aqueous compartment shrinks and the mono-layer around the water cylinder bends. The osmotic work can be translated into monolayer- and bilayer-bending moduli (7) to yield a bilayer value of about 1.5 X 10 12 erg, which is in qualitative agreement with measurements from bilayer mechanics (38). It is possible to refine the analysis of Hn dehydration to include the change in area per lipid molecule and to extract both a bending modulus and a lateral compressibility moduli. This refinement gives moduli in close quantitative agreement with other measures of mechanical deformation (39). 

Nonlocal bending resistance A resistance to bending resulting from the differential expansion and compression of the two adjacent leaflets of a lipid bilayer. It is termed nonlocal because the leaflets can move laterally relative to one another to relieve local strains, such that the net resistance to bending depends on the integral of the change in curvature of the entire membrane capsule. 

We note that the elasticity discussed above is only for planar bilayers under compression or tension and does not extend to the bending of bilayers. On the contrary, our analysis has shown that the bending of a bilayer is favoured down to the critical packing radius (assuming that the lipids can freely rearrange by lateral movement and/or flip-flop), and that bending elasticity sets in only for radii smaller than this critical value. The elasticity of a fluid bilayer is therefore seen to be profoundly different from that of a classical elastic plate or shell. 

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