Alexander G. Petrov

Flexoelectricity of model and living membranes

The theory and experiments on model and biomembrane flexoelectricity are reviewed. Biological implications of flexoelectricity are underlined. Molecular machinery and molecular electronics applications are pointed out.

Mechanosensitivity of cell membranes

Physical and biophysical mechanisms of mechano-sensitivity of cell membranes are reviewed. The possible roles of the lipid matrix and of the cytoskeleton in membrane mechanoreception are discussed. Techniques for generation of static strains and dynamic curvatures of membrane patches are considered. A unified model for stress-activated and stress-inactivated ion channels under static strains is described. A review of work on stress-sensitive pores in lipid-peptide model membranes is presented. The possible role of flexoelectricity in mechano-electric transduction, e.g. in auditory receptors is discussed. Studies of flexoelectricity in model lipid membranes, lipid-peptide membranes and natural membranes containing ion channels are reviewed. Finally, possible applications in molecular electronics of mechanosensors employing some of the recognized principles of mechano-electric transduction in natural membranes are discussed.

Curvature-electric effect in black lipid membranes

Upon periodical bending of a BLM, by means of oscillating hydrostatic pressure with sound frequency, the generation of an a.c. electric current with the same frequency can be observed under short circuit conditions. Previously, this phenomenon was attributed by us to a displacement current due to the oscillating flexoelectric polarization. The latter is proportional to the membrane curvature and depends on the lipid dipole moment and surface charge.The theory of this effect is outlined here. Earlier results concerning dipolar and quadrupolar contributions to the total current are presented and new expressions about charge contributions are derived for the two basic regimes of free and blocked lateral lipid exchange.Further, a systematic study of the frequency dependence of the amplitude and phase of the curvature-electric signal from a bacterial phosphatidylethanolamine/n-decane BLM is reported. Constant membrane curvature at each vibration frequency was assured by a calibration of the capacitance current observed with a small transmembrane voltage.The frequency dependence of the curvature-electric current amplitude was characterized by two regions: low frequency plateau and high frequency slope, the boundary between them being about 160 Hz. Such behaviour suggested a switching of the mechanism of membrane polarization from free to blocked lateral lipid exchange. Frequency dependence of the phase shift was characterized by low frequency and high frequency plateaus and a gradual transition between them. From phase measurements on initially curved membranes the sign of the membrane flexo-coefficient was found to be negative.The influence of some modifiers of the surface charge and surface dipole, as well as of the membrane conductivity, upon the value of the effect was studied. Surface charge was separately measured by the internal field compensation method under an ionic strength gradient. The membrane flexoelectric coefficient was evaluated and compared to the theoretical predictions. A conclusion was drawn that under the present experimental conditions the main contribution to the effect comes from the curvatureinduced shift of the surface charge equilibrium.

Flexoelectric Model for Active Transport

A general model of active transport is proposed, based on the flexoelectric membrane properties, with regard to liquid crystalline membrane structure. The physical principles of the flexoelectric effect are briefly reviewed and it is admitted that the spherical deformation of planar membrane produces a flexoelectric polarization which results in depolarizing electric field. The orientational state of integral protein globuli in this field is considered and it is found to be similar to that of a trigger. It is shown that a change of the globula dipole moment by adhering to the negative end of a proton may cause its reorientation accompanied with translocation of the proton to the outer membrane side. The theoretical formulae describing such a mechanism are derived and both the value and the sign of transmembrane potential are calculated as a function of flexoelectric membrane coefficients. Two chanal feedback control leading to stable self-regulation both of the membrane curvature and transmembrane potential, is involved in this model. In view of such mechanism a natural explanation may be given for a number of active transport features: curved membrane sectors are found to be metabolitically active; transport energy is secured by membrane itself; assymmetry and vector character of the transport are clearly demonstrated. There exist at least two experiments which are consistent with the notion of flexoelectricity and permit in principle to measure the value of flexoelectric membrane coefficient.

Converse Flexoelectric Effect in Bent-Core Nematic Liquid Crystals

We report on the converse flexoelectric effect in two bent-core nematic liquid crystals with opposite dielectric anisotropies. The results are based on electro-optic investigations of inplane field-driven distortions in homeotropic samples (the Helfrich method). They are interpreted by an extension of the Helfrich theory that takes into account the higher order distortions. The bend flexocoefficient for both the compounds is of the usual order of magnitude as in calamitics, unlike in a previously investigated bent-core nematic for which giant values of the bend flexocoefficient are reported. In order to resolve this discrepancy, we propose a molecular model with nonpolar clusters showing quadrupolar flexoelectricity. The study also includes measurements on surface polarization instabilities in the dielectrically positive material; the splay flexocoefficient thereby deduced is also of the conventional order.

Flexoelectricity and Surface Polarization

Abstract The comparison between experimental condition in two experiments measuring MBBA bend flexo-coefficient and an analogy with lyotropic liquid crystals lead to the concept that the reason for the big difference obtained is the opposite direction of the surface polarization due to the hydro-phobic or hydrophilic surfaces used and weak amphiphilic character of MBBA molecule,

Generalized Asymmetry of Thermotropic and Lyotropic Mesogens

Abstract The multipole model of molecular asymmetry of rod-like and disc-like mesogens was developed by us earlier (1981). It included the electrical, sterical and biphilic asymmetry. That model was generalized later on (1985) by including the flexibility asymmetry as well. Accordingly, mesogens were classified as generalized multipoles: monopoles, dipoles, quadrupoles etc. Furthermore, the generalized dipoles were considered as functions of the generalized fields by introducing generalized polarizabilities. These new lines of development of the model are reviewed here. They are especially important for lyotropics and membranes. Since 1981 a number of new mesogens corresponding to the theoretical predictions of the model were synthesized. We are applying our general classification to the following substances: cone-shaped discotics, flexible core discotics, discoidal amphiphilics, phasmids, biforked mesogens etc. Peculiarities in the structural organization of the tilted compounds are qualitatively discuss...

Order Parameter and Molecular Polarizabilities of Liquid Crystals with Nematic and Smectic Phases

Abstract In four substances exhibiting nematic and smectic phases the refractive indices ne, no , the density and the order parameter have been measured as a function of the temperature. From the experimental data the molecular polarizability anisotropy was calculated using different internal field models. The results were compared with the data obtained by an extrapolation procedure and by the addition of tabulated bond polarizabilities.

Multipole Model of the Molecular Asymmetry in Thermotropic and Lyotropic Liquid Crystals. Volume and Surface Effects

Abstract The molecular asymmetry of mesogenic molecules is defined as a deviation of the corresponding molecular property from spherical and axial symmetry. Three types of asymmetry-electrical, steric and biphilic, are considered using a multipole representation. Two basic molecular shapes are included-rod-like and plate-like. The first three multipoles-monopole, dipole, and quadrupole, are involved. In the steric case a monopole does not exist. The effective shape of a molecule includes not only steric, but also electrical intermolecular repulsive interactions. The biphilic asymmetry is determined by the distribution of the hydrophilic and hydrophobic groups along the molecule. Three types of generalized fields—electrical, deformational, and biphilic, are also defined and their volume and surface sources are discussed. The effects of the molecular asymmetry on the intermolecular interactions in thermotropic (rod-like and disc-like) and lyotropic liquid crystals are qualitatively discussed in an analogy t...

Pure and mixed lipid black foam films as models of membrane fusion

Black foam films (BFF) from water solutions of the phospholipid dilauroyl lecithin (DLL) with admixtures of palmitoyl lysolecithin (Lyso) were formed. Microscopic BFF were studied by the method of Scheludko and Exerowa. The formation probability for BFF and the BFF lifetime in a black state before film rupture were measured as functions of the film composition. At a fixed overal lipid concentration it was shown that an increased percentage of Lyso exponentially increased the lifetime of the film up to the CMC of Lyso. This stabilizing Lyso effect nicely corresponds with its stabilizing action on the waiting time for fusion of two contacting black lipid membranes (BLM), as found by Chernomordik et al. In contrast, Lyso is known to destabilize a single BLM. In this way we have found experimental proof of our earlier prediction that Lyso should have opposite effects on the lifetimes of BLM and BFF. In addition, we have shown for the first time that foam films made of lipids are a convenient model for monlayer membrane fusion studies. This model is characterized by its simplicity and experimental reliability and provides a means for quick screening of the fusogenic capacity of various amphiphilic and hydrophilic admixtures.