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On the role of anisotropy of membrane constituents on the elastic properties of highly curved lipid membranesŠarka Perutkova, 2017, doctoral dissertation
Abstract: The primary objective of the thesis covers in the theoretical study the role of
anisotropic membrane components in the elasticity of highly curved biological
membranes. To show the importance of anisotropy, we focused on one
type of non-lamellar membrane self-assembly - the inverted hexagonal phase
and the membrane tubular protrusions with attached proteins. These two
structures represent excellent examples of highly curved structures in which
the anisotropy of molecules or small domains plays an important role. In
the first part of the thesis, we developed a theoretical model describing the
stability of the inverted hexagonal phase, which considers lipid anisotropy
and deviations from the circularity of the pivotal plane cross-section. We applied
a wedge-like model of phospholipid molecules, in which the phospholipid
molecule is described as a wedge, with the angle of the wedge increasing with
temperature. However, we also took into account the average orientation of
lipids by including the deviatoric bending energy contribution derived from
statistical physics. Theoretical predictions of our model showed that a crosssection
of the inverted hexagonal phase is an intermediate between a circle
and a hexagon, and that it has lower energy than the circular cross-section.
The results were in agreement with observations gathered by the small angle
X-ray scattering. By comparing our results with the experiments, we
predicted some values of the mean intrinsic curvature and the phospholipid
chain stiffness. In the second part of the thesis, we developed a theoretical
model, which describes the stabilisation of membrane nanotubes containing
attached anisotropic flexible rod-like proteins. We derived the free energy
of a vesicle with nanotube taking into account the rotational averaging of
the anisotropic attached proteins. We also added the entropy contribution
due to the non-homogeneous lateral distribution of proteins. Our theoretical
results showed that rod-like attached proteins and membrane domains
can stabilise the membrane tubular protrusions if we consider the protein/
domain anisotropy. Our results were also in agreement with experimental results
in which isotropic membrane constituents were found on the tips of the
nanotube or on the mother vesicle; however, the anisotropic membrane constituents
were detected along the nanotubes. Our results showed that rod-like
attached proteins and membrane domains can stabilise the membrane tubular
protrusions if we consider the protein/domain anisotropy. The anisotropy of membrane constituents can lower the membrane free energy in regions of high
curvature. The main aim of the thesis was to show that the anisotropy of
membrane constituents can lower the membrane free energy in regions of high
curvature and that the rotational averaging of anisotropic membrane components
should be considered in the evaluation of the membrane free energy at
highly curved membrane structures.
Keywords: Biomembranes, Lipid anisotropy, Inverted hexagonal phase, Rotational averaging, Rod-like proteins, Membrane nanotubes, Membrane
protein sorting
Published in DKUM: 25.05.2017; Views: 2372; Downloads: 107
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