dc.description.abstracteng | In this work we investigate, via computer simulations with an implicit-solvent
coarse-grained model, the interplay between curvature and composition in self-
assembled lipid structures comprised by two lipid species with different spontaneous curvatures. The different structures considered in this study are: i)
planar bilayers with thermally induced shape fluctuations, which allow the investigation of low curvature regimes, ii) cylindrical bilayers with different mid-
plane radius of curvature and iii) inverted-monolayer tubes immersed in an
hydrophobic solvent. Beside these different self-assembled morphologies and
curvature regimes, this study also considers different degrees of segregation
between unlike lipid species. To analyze the results from simulations with planar and cylindrical bilayers, a phenomenological model describing the thermodynamics of mixing in
the two monolayers of lipid membranes was proposed. The novelty of this
model is that beside incorporating contributions from the free energy of mixing on the two membrane leaflets, it also accounts for their difference in curvature. The later contribution becomes particularly important for membranes
whose local curvature modulations are comparable to the inverse bilayer thickness. A salient prediction of the model is the existence of a saturation curvature
for which the composition difference between the apposing leaflets becomes
maximal. The occurrence of this saturation curvature will depend on system
segregation. In particular, for low incompatibility between lipid species, the
saturation curvature may lie beyond the inverse bilayer thickness, which sets
a limit on the curvatures that are physically accessible to the system. In contrast, for high incompatibility the saturation curvature may be comparable to
the thermally-induced curvature fluctuations observed in planar bilayers.
Another advantage of the proposed model is that many of its parameters can
be directly measured from easily accessible experimental quantities, such as the
power spectra of shape and composition fluctuations. Furthermore, the two
parameters to fit from measurements of the local curvature and composition
asymmetry across the bilayer, are directly related to the intrinsic curvature of
the constituting lipids.Finally, spontaneous curvatures from the analysis with bilayer membranes
were in good agreement with those obtained from a common experimental
technique, where lipids with unknown curvature are inserted into an inverted
structure whose self-assembled and spontaneous curvatures coincide. It has
been seen, however, that this technique may only be used under low segregation conditions and that the anisotropy of this inverted structures together with
the change of lipid conformations due to the interdigitation of the hydrophobic
solvent inside of the tail region may induce errors in the determination of the
spontaneous curvature of the guest lipids. | de |