Shape of fluid vesicles anchored by rigid rod

A method combined the self-consistent field theory (SCFT) for the rigid rod with the Helfrich curvature elasticity theory for the vesicle has been developed for studying the shape of vesicles anchored by rigid rod. Both the deformation of the vesicle and the density distribution of rod segments can be obtained. Because of the vesicle's impenetrability for the rod segments and the decrease of the available space for the rod orientational configurations, the anchored rod segments exert the inhomogeneous entropic pressure on the vesicle and induce the change of vesicle shape. The interaction between the rod segments and the vesicle membrane exerts an extra tension to the membrane. Thus the interaction between the vesicle membrane and the rod segments, the rod length, and the bending rigidity of vesicle are investigated as the important factors to the shape transformation of the vesicle and the density distribution of rod segments. This method can be extended to more complicated and real biological systems, such as polymers with different topological architectures/vesicle, multiple chains/vesicle, protein inclusions, etc.     

Mechanical stability of micropipet-aspirated giant vesicles with fluid phase coexistence

Micropipet aspiration of phase-separated lipid bilayer vesicles can elucidate physicochemical aspects of membrane fluid phase coexistence. Recently, we investigated the composition dependence of line tension at the boundary between liquid-ordered and liquid-disordered phases of giant unilamellar vesicles obtained from ternary lipid mixtures using this approach. Here we examine mechanical equilibria and stability of dumbbell-shaped vesicles deformed by line tension. We present a relationship between the pipet aspiration pressure and the aspiration length in vesicles with two coexisting phases. Using a strikingly simple mechanical model for the free energy of the vesicle, we predict a relation that is in almost quantitative agreement with experiment. The model considers the vesicle free energy to be proportional to line tension and assumes that the vesicle volume, domain area fraction, and total area are conserved during aspiration. We also examine a mechanical instability encountered when releasing a vesicle from the pipet. We find that this releasing instability is observed within the framework of our model that predicts a change of the compressibility of a pipet-aspirated membrane cylinder from positive (i.e., stable) to negative (unstable) values, at the experimental instability. The model furthermore includes an aspiration instability that has also previously been experimentally described. Our method of studying micropipet-induced shape transitions in giant vesicles with fluid domains could be useful for investigating vesicle shape transitions modulated by bending stiffness and line tension.     

Floret-shaped solid domains on giant fluid lipid vesicles induced by pH

Lateral lipid phase separation of titratable PS or PA lipids and their assembly in domains induced by changes in pH are significant in liposome-based drug delivery: environmentally responsive lipid heterogeneities can be tuned to alter collective membrane properties such as permeability (altering drug release) and surface topography (altering drug carrier reactivity) impacting, therefore, the therapeutic outcomes. At the micrometer scale fluorescence microscopy on giant unilamellar fluid vesicles (GUVs) shows that lowering pH (from 7.0 to 5.0) promotes condensation of titratable PS or PA lipids into beautiful floret-shaped domains in which lipids are tightly packed via hydrogen-bonding and van der Waals interactions. The order of lipid packing within domains increases radially toward the domain center. Lowering pH enhances the lipid packing order, and at pH 5.0 domains appear to be entirely in the solid (gel) phase. Domains phenomenologically comprise a circular "core" cap beyond which interfacial instabilities emerge resembling leaf-like stripes. At pH 5.0 stripes are of almost vanishing Gaussian curvature independent of GUVs' preparation path and in agreement with a general condensation mechanism. Increasing incompressibility of domains is strongly correlated with a larger number of thinner stripes per domain and increasing relative rigidity of domains with smaller core cap areas. Line tension drives domain ripening; however, the final domain shape is a result of enhanced incompressibility and rigidity maximized by domain coupling across the bilayer. Introduction of a transmembrane osmotic gradient (hyperosmotic on the outer lipid leaflet) allows the domain condensation process to reach its maximum extent which, however, is limited by the minimal expansivity of the continuous fluid membrane.     

Enumerating DNA polyhedral links

With the fast-growing of DNA self-assembly techniques, a large number of DNA catenanes or more specifically, DNA polyhedral links were synthesized in the past decade. As a sequel of a recent paper (J. Math. Chem. 50, p. 1693, 2012) by the present authors on the enumeration problem of polyhedral links, this paper considers the enumeration problem of DNA polyhedral links. In contrast to a general molecular link, a DNA polyhedral link has four notable features: 1. the memory of DNA chain direction; 2. the accurate DNA complementary base pairing; 3. the twist patterns of the double-helical strands; 4. the migration in branched junction. These features put forward particular requests for treating the enumeration problem of DNA polyhedral links. In addition to using the standard Pólya’s counting theory, we here introduce the generating function and edge direction retentivity analysis to the enumeration problem, by which we establish explicit expressions of the numbers of DNA polyhedral links for three typical models. These models have been used as strategies or are potential strategies in the synthesis of DNA polyhedral links.     

Hypercarbons in polyhedral structures

Though carbon is mostly tetravalent and tetracoordinated, there are several examples where the coordination number exceeds four. Structural varieties that exhibit hypercarbons in polyhedral structures such as polyhedral carboranes, sandwich complexes, encapsulated polyhedral structures and novel planar aromatic systems with atoms embedded in the middle are reviewed here. The structural variety anticipated with hypercoordinate carbon among carboranes is large as there are many modes of condensation that could lead to large number of new patterns. The relative stabilities of positional isomers of polyhedral carboranes, sandwich structures, and endohedral carboranes are briefly described. The mno rule accounts for the variety of structural patterns. Wheel-shaped and planar hypercoordinated molecules are recent theoretical developments in this area.     

Diffusive dynamics of vesicles tethered to a fluid supported bilayer by single-particle tracking

We recently introduced a method to tether intact phospholipid vesicles onto a fluid supported lipid bilayer using DNA hybridization (Yoshina-Ishii, C.; Miller, G. P.; Kraft, M. L; Kool, E. T.; Boxer, S. G. J. Am. Chem. Soc. 2005, 127, 1356-1357). Once tethered, the vesicles can diffuse in two dimensions parallel to the supported membrane surface. The average diffusion coefficient, D, is typically 0.2 microm(2)/s; this is 3-5 times smaller than for individual lipid or DNA-lipid conjugate diffusion in supported bilayers. In this article, we investigate the origin of this difference in the diffusive dynamics of tethered vesicles by single-particle tracking under collision-free conditions. D is insensitive to tethered vesicle size from 30 to 200 nm, as well as a 3-fold change in the viscosity of the bulk medium. The addition of macromolecules such as poly(ethylene glycol) reversibly stops the motion of tethered vesicles without causing the exchange of lipids between the tethered vesicle and supported bilayer. This is explained as a depletion effect at the interface between tethered vesicles and the supported bilayer. Ca ions lead to transient vesicle-vesicle interactions when tethered vesicles contain negatively charged lipids, and vesicle diffusion is greatly reduced upon Ca ion addition when negatively charged lipids are present both in the supported bilayer and tethered vesicles. Both effects are interesting in their own right, and they also suggest that tethered vesicle-supported bilayer interactions are possible; this may be the origin of the reduction in D for tethered vesicles. In addition, the effects of surface defects that reversibly trap diffusing vesicles are modeled by Monte Carlo simulations. This shows that a significant reduction in D can be observed while maintaining normal diffusion behavior on the time scale of our experiments.     

Topology of polyhedral clathrate frameworks. 2. Frameworks constructed of polyhedral stories

The main features of the most widespread class of polyhedra clathrate frameworks formed of polyhedral stories are investigated. An algorithm for constructing such frameworks is suggested. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 1, pp. 123–130, January–February, 1996. Translated by L. Smolina     

Surface activity of lysozyme and dipalmitoyl phosphatidylcholine vesicles at compressed and supercritical fluid interfaces

The surface activities of lysozyme and dipalmitoyl phosphatidylcholine (DPPC) vesicles at aqueous/compressed fluid interfaces are examined via high-pressure interfacial tension measurements using the pendant drop technique. The density and interfacial tension in compressible fluid systems vary significantly with pressure, providing a versatile medium for elucidating interactions between biomolecules and fluid interfaces and a method to elicit pressure-dependent interfacial morphological responses. The effects of lysozyme concentration (0.0008, 0.01, and 1 mg/mL) and pressure (> or = 7 MPa) on the dynamic surface response in the presence of ethane, propane, N2, and CO2 at 298 K were examined. Interfacial lysozyme adsorption reduced the induction phase and quickly led to interfacial tensions consistent with protein conformational changes and monolayer saturation at the compressed fluid interfaces. Protein adsorption, as indicated by surface pressure, correlated with calculated Hamaker constants for the compressed gases, denoting the importance of dispersion interactions. For DPPC at aqueous/compressed or aqueous/supercritical CO2 interfaces (1.8-20.7 MPa, 308 K), 2-3-fold reductions in interfacial tension were observed relative to the pure binary fluid system. The resulting surface pressures infer pressure-dependent morphological changes within the DPPC monolayer.     

F-structure of polyhedral water clusters

Energy and entropy characteristics of the F-structures of polyhedral water clusters containing tetragonal, pentagonal, and hexagonal faces with the intermolecular potential ST2 are studied.Institute of Thermal Physics, Ural Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 77–82, July–August, 1993.Translated by L. Smolina     

The braid index of polyhedral links

Polyhedral links have been used to model DNA polyhedra and protein catenanes. Some topological characteristics of a type of polyhedral links fabricated from a polyheron by the method of ‘n-branched curve and X-tangled covering’ have been elucidated. In this paper, we pay close attention to their braid index considered significant in view of DNA nanotechnology, and proved that the MFW inequality is sharp for the polyhedral links. Our results demonstrate that the braid index of the links is directed by their crossing numbers. In addition, the studies of the polyhedral links can facilitate the research of the properties of DNA molecules, and can characterize their structural complexity.     

Supercrystal structures of polyhedral PbS nanocrystals

We analyzed the interaction between chemically grafted polysaccharide layers in aqueous solutions. To fabricate such layers, an end-terminated dextran silane coupling agent was synthesized and the polydextran was grafted to oxidized silicon wafers and to silica particles. This resulted in the formation of a 28 nm thick layer (in air) and a grafted amount of 40 mg/m(2) as determined by ellipsometry. The physical properties of the grafted layer were investigated in aqueous solutions by atomic force microscope imaging and colloidal probe force measurements. Surface and friction forces were measured between one bare and one polydextran coated silica surface. A notable feature was a bridging attraction due to affinity between dextran and the silica surface. Surface interactions and friction forces were also investigated between two surfaces coated with grafted polydextran. Repulsive forces were predominant, but nevertheless a high friction force was observed. The repulsive forces were enhanced by addition of sodium dodecyl sulfate (SDS) that associates with the tethered polydextran layers. SDS also decreased the friction force. Our data suggests that energy dissipation due to shear-induced structural changes within the grafted layer is of prime importance for the high friction forces observed, in particular deformation of protrusions in the surface layer.     

The architecture of Platonic polyhedral links

A new methodology for understanding the construction of polyhedral links has been developed on the basis of the Platonic solids by using our method of the ‘n-branched curves and m-twisted double-lines covering’. There are five classes of platonic polyhedral links we can construct: the tetrahedral links; the hexahedral links; the octahedral links; the dodecahedral links; the icosahedral links. The tetrahedral links, hexahedral links, and dodecahedral links are, respectively, assembled by using the method of the ‘3-branched curves and m-twisted double-lines covering’, whereas the octahedral links and dodecahedral links are, respectively, made by using the method of the ‘4-branched curves’ and ‘5-branched curves’, as well as ‘m-twisted double-lines covering’. Moreover, the analysis relating topological properties and link invariants is of considerable importance. Link invariants are powerful tools to classify and measure the complexity of polyhedral catenanes. This study provides further insight into the molecular design, as well as theoretical characterization, of the DNA polyhedral catenanes.     

Synthesis and transformations of polyhedral compounds

Derivatives of 1,3-diaza- and 1,3,5-triazaadamantanes react with 1,2-dibromoethane in the presence of base to give derivatives of 1,4-diaza- and 1,3,6-triazahomoadamantanes, respectively. Schiff bases were obtained containing the 1,3,6-triazahomoadamantane ring.For Communication 14 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1098–1100, August, 1992.     

Minimal distortion pathways in polyhedral rearrangements

A definition of minimum distortion paths between two polyhedra in terms of continuous shape measures (CShM) is presented. A general analytical expression deduced for such pathways makes use of one parameter, the minimum distortion constant, that can be easily obtained through the CShM methodology and is herein tabulated for pairs of polyhedra having four to eight vertexes. The work presented here also allows us to obtain representative model molecular structures along the interconversion pathways. Several commonly used polytopal rearrangement pathways are shown to be in fact minimum distortion pathways: the spread path leading from the tetrahedron to the square, the Berry pseudorotation that interconverts a square pyramid and a trigonal bipyramid, and the Bailar twist for the interconversion of the octahedron and the trigonal prism. Examples of applications to the analysis of the stereochemistries of several families of metal complexes are presented.     

Mie plasmon in polyhedral metal clusters

We study the dependence of the classical plasmon frequency on the symmetry of the metal cluster and show that all clusters with at least two three-fold axes have the same plasmon frequency as the spherical cluster, ω _p /√3. In these cases the effect of the geometry will only appear in the spill-out correction and in other quantum mechanical corrections.     

Span 80 vesicles have a more fluid, flexible and "wet" surface than phospholipid liposomes

The surface properties of Span 80 vesicles at various cholesterol contents, together with those of various liposomes, were characterized by using fluorescence probes. The membrane fluidity of the Span 80 vesicles was measured by 1,6-diphenyl-1.3.5-hexatriene (DPH) and trimethlyammonium-DPH (TMA-DPH), and the results suggested that the surface of the Span 80 vesicles was fluid due to the lateral diffusion of Span 80 molecules. The depolarization measured by TMA-DPH and the headgroup mobility measured by dielectric dispersion analysis indicated the high mobility of the head group of Span 80 vesicles. This suggested that the surface of Span 80 vesicles was flexible due to the head group structure of Span 80, sorbitol. In addition, spectrophotometric analysis with 6-dodecanoyl-N, N-dimethyl-2-naphthylamine and 8-anilino-1-naphthalenesulfonic acid indicated that the water molecules could easily invade into the interior of the Span 80 vesicle membrane, suggesting that the membrane surface was more wet than the liposome surface. These surface properties indicated that the protein could interact with the interior of vesicle membranes, which was similar to the case of cholesterol. Thus the present results confirmed that the Span 80 vesicle surfaces showed the unique characteristics of fluidity, flexibility, and "wetness", whereas the liposome surfaces did not.     

Photoswitchable vesicles

Vesicles are self-assembled nanosized structures that result from the auto-organization of amphiphilic compounds into closed bilayers. These systems are highly attractive subjects of fundamental research owing to their role as biomimetic models of cell membranes and compartmentalization systems. Vesicles also display very interesting features and properties for practical applications, namely as superior nanocontainers for encapsulation of functional molecules and materials. Exerting control over the formation, dissociation and permeability of these containers is therefore of paramount importance for many of their potentials applications. Being light the stimulus of choice for remote spatiotemporal control, we have compiled and revised selected published works on photoresponsive vesicles with the aim of providing an overview of the main strategies, advantages and limitations of such systems.