Rumiana Dimova

Boron Carbon Nitride Nanostructures from Salt Melts: Tunable Water-Soluble Phosphors

A simple, high yield, chemical process is developed to fabricate layered h-BN nanosheets and BCNO nanoparticles with a diameter of ca. 5 nm at 700 °C. The use of the eutectic LiCl/KCl salt melt medium enhances the kinetics of the reaction between sodium borohydride and urea or guanidine as well as the dispersion of the nanoparticles in water. The carbon content can be tuned from 0 to 50 mol % by adjusting the reactant ratio, thus providing precise control of the light emission of the particles in the range 440-528 nm while reaching a quantum yield of 26%. Because of their green synthesis, low toxicity, small size, and stability against aggregation in water, the as-obtained photoluminescent BCNO nanoparticles show promise for diagnostics and optoelectronics.

Time scales of membrane fusion revealed by direct imaging of vesicle fusion with high temporal resolution

Membrane fusion is a vital process of life involved, for example, in cellular secretion via exocytosis, signaling between nerve cells, and virus infection. In both the life sciences and bioengineering, controlled membrane fusion has many possible applications, such as drug delivery, gene transfer, chemical microreactors, or synthesis of nanomaterials. Until now, the fusion dynamics has been elusive because direct observations have been limited to time scales that exceed several milliseconds. Here, the fusion of giant lipid vesicles is induced in a controlled manner and monitored with a temporal resolution of 50 μs. Two different fusion protocols are used that are based on synthetic fusogenic molecules and electroporation. For both protocols, the opening of the fusion necks is very fast, with an average expansion velocity of centimeters per second. This velocity indicates that the initial formation of a single fusion neck can be completed in a few hundred nanoseconds.

Transport of Beads by Several Kinesin Motors

The movements of beads pulled by several kinesin-1 (conventional kinesin) motors are studied both theoretically and experimentally. While the velocity is approximately independent of the number of motors pulling the beads, the walking distance or run-length is strongly increased when more motors are involved. Run-length distributions are measured for a wide range of motor concentrations and matched to theoretically calculated distributions using only two global fit parameters. In this way, the maximal number of motors pulling the beads is estimated to vary between two and seven motors for total kinesin concentrations between 0.1 and 2.5 μg/ml or between 0.27 and 6.7 nM. In the same concentration regime, the average number of pulling motors is found to lie between 1.1 and 3.2 motors.

Domains in membranes and vesicles

Biomimetic membranes such as lipid bilayers with several molecular components exhibit intramembrane domains. These domains are formed after the membranes are quenched into a two-phase coexistence region. Solid domains tend to form facets or cylindrical segments whereas liquid domains tend to form spherical buds. In the latter case, one can distinguish three tension regimes; budding always occurs in the low tension regime for which we present preliminary experimental results for vesicle membranes composed of phospholipid, sphingomyelin and cholesterol. A multicomponent vesicle which adheres to a substrate surface consists of a bound and an unbound membrane segment which differ in their compositions. This shift in composition can induce domains within the bound segment. Such domains are also formed when membranes adhere via sticker molecules and can be driven (i) by the interplay of sticker adhesion and shape fluctuations and (ii) by the competition between sticker and repeller molecules.

Giant vesicles in electric fields

This review is dedicated to electric field effects on giant unilamellar vesicles, a cell-size membrane system. We summarize various types of behavior observed when vesicles are subjected either to weak AC fields at various frequency, or to strong DC pulses. Different processes such as electro-deformation, -poration and -fusion of giant vesicles are considered. We describe some recent developments, which allowed us to detect the dynamics of the vesicle response with a resolution below milliseconds for all of these processes. Novel aspects on electric field effects on vesicles in the gel phase are introduced.

Membrane nanotubes induced by aqueous phase separation and stabilized by spontaneous curvature

Tubular membrane structures are widespread in eukaryotic cells, but the mechanisms underlying their formation and stability are not well understood. Previous work has focused on tube extrusion from cells and model membranes under the application of external forces. Here, we present novel membrane/polymer systems, where stable tubes form in the absence of externally applied forces. Solutions of two water-soluble polymers, polyethylene glycol and dextran, were encapsulated in giant lipid vesicles, cell-size model systems. Hypertonic deflation induced phase separation of the enclosed solution. The excess membrane area created during the deflation process was stored in a large number of membrane nanotubes inside the vesicle. The tubes had a diameter below optical resolution and became visible only when fluorescently labeled. The tubes were rather stable: In the absence of external forces, they existed for several days. A theoretical analysis of the shapes of the deflated vesicles reveals that these shapes would be unstable if the membranes had no spontaneous curvature. Using the large separation of length scales between the tube diameter and the overall size of the vesicles, the spontaneous curvature can be calculated and is found to be about -1/(240 nm) for a certain range of polymer concentrations. The nanotubes could also be retracted back into the mother vesicle by increasing the membrane tension via micropipette aspiration of the vesicle. Membrane tubes, which can form and be retracted easily, should be relevant for lipid storage in cells.

Interactions of Alkali Metal Chlorides with Phosphatidylcholine Vesicles

We study the interaction of alkali metal chlorides with lipid vesicles made of palmitoyloleoylphosphatidylcholine (POPC). An elaborate set of techniques is used to investigate the binding process at physiological conditions. The alkali cation binding to POPC is characterized thermodynamically using isothermal titration calorimetry. The isotherms show that for all ions in the alkali group the binding process is endothermic, counterintuitively to what is expected for Coulomb interactions between the slightly negatively charged POPC liposomes and the cations. The process is entropy driven and presumably related to the liberation of water molecules from the hydration shells of the ions and the lipid headgroups. The measured molar enthalpies of the binding of the ions follows the Hofmeister series. The binding constants were also estimated, whereby lithium shows the strongest affinity to POPC membranes, followed by the rest of the ions according to the Hofmeister series. Cation adsorption increases the net surface potential of the vesicles as observed from electrophoretic mobility and zeta potential measurements. While lithium adsorption leads to slightly positive zeta potentials above a concentration of 100 mM, the adsorption of the rest of the ions mainly causes neutralization of the membrane. This is the first study characterizing the binding equilibrium of alkali metal chlorides to phosphatidylcholine membranes at physiological salt concentrations.

Vesicles in electric fields: Some novel aspects of membrane behavior†

This review focuses on the effects of electric fields on giant unilamellar vesicles, a cell-size membrane system. We describe various types of behavior of vesicles subjected to either alternating fields or strong direct current pulses, such as electrodeformation, -poration and -fusion. The vesicle response to alternating fields in various medium conditions is introduced and the underlying physical mechanisms are highlighted, supported by theoretical modeling. New aspects of the response of vesicles with charged or neutral membranes, in fluid or gel-phase, and embedded in different solutions, to strong direct current pulses are described including novel applications of vesicle electrofusion for nanoparticle synthesis.

Electrohydrodynamic Model of Vesicle Deformation in Alternating Electric Fields

We develop an analytical theory to explain the experimentally observed morphological transitions of quasispherical giant vesicles induced by alternating electric fields. The model treats the inner and suspending media as lossy dielectrics, and the membrane as an impermeable flexible incompressible-fluid sheet. The vesicle shape is obtained by balancing electric, hydrodynamic, bending, and tension stresses exerted on the membrane. Our approach, which is based on force balance, also allows us to describe the time evolution of the vesicle deformation, in contrast to earlier works based on energy minimization, which are able to predict only stationary shapes. Our theoretical predictions for vesicle deformation are consistent with experiment. If the inner fluid is more conducting than the suspending medium, the vesicle always adopts a prolate shape. In the opposite case, the vesicle undergoes a transition from a prolate to oblate ellipsoid at a critical frequency, which the theory identifies with the inverse membrane charging time. At frequencies higher than the inverse Maxwell-Wagner polarization time, the electrohydrodynamic stresses become too small to alter the vesicles quasispherical rest shape. The model can be used to rationalize the transient and steady deformation of biological cells in electric fields.

Calcium Binding and Head Group Dipole Angle in Phosphatidylserine−Phosphatidylcholine Bilayers

Manipulating the plasma membrane, the gateway to the cell interior, with chemical and physical agents for genetic and pharmacological therapy, and understanding the interactions of lipid membrane components with proteins and other structural and functional elements of the cell, require a detailed biomolecular membrane model. We report here progress along one path toward such a model: molecular dynamics simulations of mixed, zwitterionic-anionic, asymmetric phospholipid bilayers with monovalent and divalent cations. With phosphatidylcholine/phosphatidylserine systems, we identify temporal and concentration boundaries for equilibration of calcium with the bilayer and saturation of the calcium capacity of the membrane, we demonstrate the electrostatic- and entropic-driven associations of calcium and sodium ions with polar groups in the bilayer interface region, expressed in spatial distribution profiles and in changes in the orientation of the phospholipid head groups, and we describe for the first time simulations of dynamic, calcium-mediated adjustments in the conformation of mixed phospholipid species coresident in the same leaflet of the bilayer. The results are consistent with experimental observations and point the way to further refinement and increased realism of these molecular models.