Vladimir Lorman

Dielectric permittivity of antiferroelectric liquid crystals

Abstract The complex dielectric permittivity has been measured as a function of the frequency, the temperature and the biasing field for the antiferroelectric liquid crystal tolan C8 and C10. At the transition from the ferrielectric phase to the antiferroelectric one the remarkable maximum has been observed for low frequencies on the curves e(T). This contribution to e is discussed as an antiphase azimuthal mode in the frame of the model assuming that in the SmCγ* phase the tilt angle is practically constant, and the difference in the azimuthal angle in the adjacent layers is different from 180°.

How curvature-generating proteins build scaffolds on membrane nanotubes

Significance Lipid membranes are dynamic assemblies, changing shape on nano- to micron-sized scales. Some proteins can sculpt membranes by organizing into a molecular scaffold, dictating the membrane’s shape and properties. We combine microscopy, mathematical modeling, and simulations to explore how Bin/Amphiphysin/Rvs proteins assemble to form scaffolds on nanotubes. We show that the way protein locally deforms the membrane affects where it will nucleate before making a scaffold. In this process, the protein’s amphipathic helices—which shallowly insert into the membrane—seem dispensable. Surprisingly, the scaffold forms at low protein density on the nanotube. We simulate a structure of protein scaffolds at molecular resolution, shedding light on how these proteins may sculpt the membrane to facilitate important dynamic events in cells. Bin/Amphiphysin/Rvs (BAR) domain proteins control the curvature of lipid membranes in endocytosis, trafficking, cell motility, the formation of complex subcellular structures, and many other cellular phenomena. They form 3D assemblies that act as molecular scaffolds to reshape the membrane and alter its mechanical properties. It is unknown, however, how a protein scaffold forms and how BAR domains interact in these assemblies at protein densities relevant for a cell. In this work, we use various experimental, theoretical, and simulation approaches to explore how BAR proteins organize to form a scaffold on a membrane nanotube. By combining quantitative microscopy with analytical modeling, we demonstrate that a highly curving BAR protein endophilin nucleates its scaffolds at the ends of a membrane tube, contrary to a weaker curving protein centaurin, which binds evenly along the tube’s length. Our work implies that the nature of local protein–membrane interactions can affect the specific localization of proteins on membrane-remodeling sites. Furthermore, we show that amphipathic helices are dispensable in forming protein scaffolds. Finally, we explore a possible molecular structure of a BAR-domain scaffold using coarse-grained molecular dynamics simulations. Together with fluorescence microscopy, the simulations show that proteins need only to cover 30–40% of a tube’s surface to form a rigid assembly. Our work provides mechanical and structural insights into the way BAR proteins may sculpt the membrane as a high-order cooperative assembly in important biological processes.

Positional, Reorientational, and Bond Orientational Order in DNA Mesophases

We investigate the orientational order of transverse polarization vectors of long, stiff polymer molecules and their coupling to bond orientational and positional order in high density mesophases. Homogeneous ordering of transverse polarization vector promotes distortions in the hexatic phase, whereas inhomogeneous ordering precipitates crystallization of the 2D sections with different orientations of the transverse polarization vector on each molecule in the unit cell. We propose possible scenarios for going from the hexatic phase, through the distorted hexatic phase, to the crystalline phase with an orthorhombic unit cell observed experimentally for the case of DNA.

On the existence of two ferrielectric phases in antiferroelectric liquid crystals

Abstract The optical rotatory power (ORP) has been measured as a function of the temperature for the antiferroelectric liquid crystal tolan C10 and C8, which respectively show in DSC measurements one and two ferrielectric phases. At the transition from the ferroelectric phase to the antiferroelectric one, it has been observed a change in the sign of the ORP for three different wavelenghts. These observations occur inside the ferrielectric phases, where the variations of the ORP as a function of the temperature are discussed as the consequence of the existence of different isostructural helicoidal ferrielectric phases.

Chiral quasicrystalline order and dodecahedral geometry in exceptional families of viruses.

On the example of exceptional families of viruses we (i) show the existence of a completely new type of matter organization in nanoparticles, in which the regions with a chiral pentagonal quasicrystalline order of protein positions are arranged in a structure commensurate with the spherical topology and dodecahedral geometry, (ii) generalize the classical theory of quasicrystals (QCs) to explain this organization, and (iii) establish the relation between local chiral QC order and nonzero curvature of the dodecahedral capsid faces.

Red Blood Cell Membrane Dynamics during Malaria Parasite Egress

Precisely how malaria parasites exit from infected red blood cells to further spread the disease remains poorly understood. It has been shown recently, however, that these parasites exploit the elasticity of the cell membrane to enable their egress. Based on this work, showing that parasites modify the membranes spontaneous curvature, initiating pore opening and outward membrane curling, we develop a model of the dynamics of the red blood cell membrane leading to complete parasite egress. As a result of the three-dimensional, axisymmetric nature of the problem, we find that the membrane dynamics involve two modes of elastic-energy release: 1), at short times after pore opening, the free edge of the membrane curls into a toroidal rim attached to a membrane cap of roughly fixed radius; and 2), at longer times, the rim radius is fixed, and lipids in the cap flow into the rim. We compare our model with the experimental data of Abkarian and co-workers and obtain an estimate of the induced spontaneous curvature and the membrane viscosity, which control the timescale of parasite release. Finally, eversion of the membrane cap, which liberates the remaining parasites, is driven by the spontaneous curvature and is found to be associated with a breaking of the axisymmetry of the membrane.

Field-Induced Behavior in a Liquid Formed by Achiral Banana-Shaped Molecules in the Vicinity of the Phase Transition Isotropic - B 2 Phase

We have experimentally investigated the electric-field effect on the Isotropic - B 2 phase transition of an achiral banana-shaped mesogen 1,3-phenyllene bis[4-(4-10-alkoxy-benoyloxy)phenyliminomethyl] ( C 11 compound) [1]. From microscopic observations under electric fields and measurements of the D-E hysteresis loops, the T (temperature) - E (electric field) phase diagram was established, in which a critical field is observed in the isotropic liquid. This field characterizes the first nucleation of a ferroelectric state noted B F induced by the electric field. The results are interpreted in the framework of a phenomenological model that describes the Isotropic - B 2 phase transition by the condensation of a vector wave directly from the isotropic liquid [2].

Structure and Properties of New de Vries SmA* Liquid Crystals

We synthesized a chiral mesogen AC11 and two different siloxanes MSi2-AC11 and DSi3-AC11 derived from it by a hydrosilylation reaction. The effects of varying the molecular architecture were investigated with special attention being devoted to the occurrence of the de Vries SmA* phase. Strong hints that all the three molecules follow the scenario suggested by de Vries were found in the behavior of the susceptibility by studies of their electrooptical response in the SmA* phase. In particular, DSi3-AC11 containing two mesogens interconnected by a siloxane spacer presented a critical exponent γ as high as 1.97, indicating a practically pure de Vries behavior. Temperature-dependent X-ray analysis revealed that the three materials underwent a small layer shrinkage at the SmA*–SmC* transition.

Topological Control of Life and Death in Non-Proliferative Epithelia

Programmed cell death is one of the most fascinating demonstrations of the plasticity of biological systems. It is classically described to act upstream of and govern major developmental patterning processes (e.g. inter-digitations in vertebrates, ommatidia in Drosophila). We show here the first evidence that massive apoptosis can also be controlled and coordinated by a pre-established pattern of a specific ‘master cell’ population. This new concept is supported by the development and validation of an original model of cell patterning. Ciona intestinalis eggs are surrounded by a three-layered follicular organization composed of 60 elongated floating extensions made of as many outer and inner cells, and indirectly spread through an extracellular matrix over 1200 test cells. Experimental and selective ablation of outer and inner cells results in the abrogation of apoptosis in respective remaining neighbouring test cells. In addition incubation of outer/inner follicular cell-depleted eggs with a soluble extract of apoptotic outer/inner cells partially restores apoptosis to apoptotic-defective test cells. The 60 inner follicular cells were thus identified as ‘apoptotic master’ cells which collectively are induction sites for programmed cell death of the underlying test cells. The position of apoptotic master cells is controlled by topological constraints exhibiting a tetrahedral symmetry, and each cell spreads over and can control the destiny of 20 smaller test cells, which leads to optimized apoptosis signalling.

Local mechanism for crystal-quasicrystal transformations

Abstract It is shown that the transformation mechanism which takes place at a crystal-quasicrystal transformation preserves a lattice of nodes in whose vicinity the crystal and quasicrystal structures are similar. Between the nodes fusion of neighbouring positions and creation of new positions occur, resulting in a disordering of the quasicrystal state.