Abstract
Archaeal membranes have unique mechanical properties that enable these organisms to survive under extremely aggressive environmental conditions. The so-called bolalipids contribute to this exceptional stability. They have two polar heads joined by two hydrocarbon chains. The two headgroups can face different sides of the membrane (O-shape conformation) or the same side (U-shape conformation). We have developed an elasticity theory for bolalipid membranes and show that the energetic contributions of (i) tilt deformations, (ii) area compression/stretching deformations, (iii) as well as those of Gaussian splay from the two membrane surfaces are additive, while splay deformations yield a cross-term. The presence of a small fraction of U-shaped molecules resulted in spontaneous membrane curvature. We estimated the tilt modulus to be approximately equal to that of membranes in eukaryotic cells. In contrast to conventional lipids, the bolalipid membrane possesses two splay moduli, one of which is estimated to be an order of magnitude larger than that of conventional lipids. The projected values of elastic moduli act to hamper pore formation and to decelerate membrane fusion and fission.
Highlights
Theoretical investigations of conventional lipids’ mechanics have been carried out in the framework of microscopic and macroscopic models
The first elasticity theory for conventional lipid membranes was developed by Helfrich.[15]
Another big advance towards complete elasticity theory was work accomplished by Hamm and Kozlov,[20] in which the authors accounted for the bilayer’s intrinsic structure within the framework of so-called tilt deformation
Summary
Theoretical investigations of conventional lipids’ mechanics have been carried out in the framework of microscopic and macroscopic models. We aim for the development of an elasticity theory for bolalipid membranes and assume that all deformations are small, so we calculate their energy up to the second order. The reason is that the cross-terms would be linked to contributions from the average curvature of the lipid hydrocarbon tails They are negligible in comparison to F, as indicated by the observation of a substantial population of U-shapes in bolalipid membranes.[10,13] That is, the energy of such a significantly curved hydrocarbon chain is comparable with the characteristic energy of thermal fluctuations, kBT B 4 Â 10À21 J. Assuming that the headgroups of the U-shapes localize toward the external surface of the bolalipid membrane (Fig. 1b), we must account for different areas a1 and a2 of the two head-group regions. Bd can be obtained from experiments with GUVs, while Bs and Jss are attainable by measuring nanotube radii in and out of equilibrium
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