Congresses,conferences, symposia,meetings, and seminars in the field of chemical sciences held in 1995–1996

Information

Congresses, conferences, symposia, meetings, and seminars in the field of chemical sciences held in 1995–1996     

Congresses,Conferences, Symposia,Meetings, and Seminars in the Field of Chemical Sciences held in 1995–1996

Information

Congresses, Conferences, Symposia, Meetings, and Seminars in the Field of Chemical Sciences held in 1995–1996     

Lipid bilayer disruption by polycationic polymers: the roles of size and chemical functional group

Polycationic polymers are used extensively in biology to disrupt cell membranes and thus enhance the transport of materials into the cell. The highly polydisperse nature of many of these materials makes obtaining a mechanistic understanding of the disruption processes difficult. To design an effective mechanistic study, a monodisperse class of polycationic polymers, poly(amidoamine) (PAMAM) dendrimers, has been studied in the context of supported dimyristoylphosphatidylcholine (DMPC) lipid bilayers using atomic force microscopy (AFM). Aqueous solutions of amine-terminated generation 7 (G7) PAMAM dendrimers caused the formation of 15-40-nm-diameter holes in lipid bilayers. This effect was significantly reduced for smaller G5 dendrimers. For G3, no hole formation was observed. In addition to dendrimer size, surface chemistry had a strong influence on dendrimer-lipid bilayer interactions. In particular, acetamide-terminated G5 did not cause hole formation in bilayers. In all instances, the edges of bilayer defects proved to be points of highest dendrimer activity. A proposed mechanism for the removal of lipids by dendrimers involves the formation of dendrimer-filled lipid vesicles. By considering the thermodynamics, interaction free energy, and geometry of these self-assembled vesicles, a model that explains the influence of polymer particle size and surface chemistry on the interactions with lipid membranes was developed. These results are of general significance for understanding the physical and chemical properties of polycationic polymer interactions with membranes that lead to the transport of materials across cell membranes.     

Atomic force microscopy study of early morphological changes during apoptosis

Apoptosis is defined by a distinct set of morphological changes observed during cell death including loss of focal adhesions, the formation of cell membrane buds or blebs, and a decrease in total cell volume. Recent studies suggest that these dramatic morphological changes, particularly apoptotic volume decrease (AVD), are an early prerequisite to apoptosis and precede key biochemical time-points. Here we use atomic force microscopy to observe early stage AVD of KB cells undergoing staurosporine-induced apoptosis. After a 3-h exposure to 1 microM staurosporine, a 32% decrease in total cell height and a 50% loss of total cell volume is observed accompanied by only a 15% change in cell diameter. The observed AVD precedes key biochemical hallmarks of apoptosis such as loss of mitochondrial membrane potential, phosphatidyl serine translocation, nuclear fragmentation, and measurable caspase-3 activity. This suggests that morphological volume changes occur very early in the induction of apoptosis.     

Bridging between short-range and long-range dependence with mixed spatio-temporal Ornstein–Uhlenbeck processes

While short-range dependence is widely assumed in the literature for its simplicity, long-range dependence is a feature that has been observed in data from finance, hydrology, geophysics and economics. In this paper, we extend a Lévy-driven spatio-temporal Ornstein–Uhlenbeck process by randomly varying its rate parameter to model both short-range and long-range dependence. This particular set-up allows for non-separable spatio-temporal correlations which are desirable for real applications, as well as flexible spatial covariances which arise from the shapes of influence regions. Theoretical properties such as spatio-temporal stationarity and second-order moments are established. An isotropic g-class is also used to illustrate how the memory of the process is related to the probability distribution of the rate parameter. We develop a simulation algorithm for the compound Poisson case which can be used to approximate other Lévy bases. The generalized method of moments is used for inference and simulation experiments are conducted with a view towards asymptotic properties.     

Random polarizations

We derive conditions under which random sequences of polarizations (two-point symmetrizations) on S^d${\mathbb{S}}^d$, R^d${\mathbb{R}}^d$, or H^d${\mathbb{H}}^d$ converge almost surely to the symmetric decreasing rearrangement. The parameters for the polarizations are independent random variables whose distributions need not be uniform. The proof of convergence hinges on an estimate for the expected distance from the limit that yields a bound on the rate of convergence. In the special case of i.i.d. sequences, almost sure convergence holds even for polarizations chosen at random from suitable small sets. As corollaries, we find bounds on the rate of convergence of Steiner symmetrizations that require no convexity assumptions, and show that full rotational symmetry can be achieved by randomly alternating Steiner symmetrizations in a finite number of directions that satisfy an explicit non-degeneracy condition. We also present some negative results on the rate of convergence and give examples where convergence fails.     

Series resonance circuit with ferroelectric capacitor

A series resonance circuit under sinuousoidal driving is investigated experimentally. The inductance consists of an air coil. The capacitance is made up of a ferroelectric material that introduces its nonlinear dielectric properties into the circuit. The dynamical system linear coil-nonlinear capacaitor shows an interesting behaviour. The phase portrait differs in general from the ellipse of the harmonic oscillator. For appropriate external conditions period doubling sequences, chaos and therein enclosed periodic windows might occur. Starting from a cubic nonlinearity of the dielectric properties a Duffing equation is proposed as a model for periodic behaviour of the series resonance circuit. Simulations of experimentally recorded phase portraits yield good agreement between experiment and model.     

Electrochemistry and homogeneous self-exchange kinetics of the aqueous 12-tungstoaluminate(5-/6-) couple

The effect of alkali metal (M) chloride or triflate supporting electrolytes (0.1-1.0 mol L(-1)) on the midpoint potential E(m) of the aqueous AlW12O40(5-/6-) couple in cyclic voltammetry, after correction (E(corr)) for liquid junction potentials, can be represented in terms of ionic strength according to the extended Debye-Hückel equation. However, unrealistically short AlW12O40(5-/6-)-cation closest-approach distances are required to accommodate the specific effects of M+, and the infinite-dilution potential E(corr)(0) values are not quite consistent from one M+ to another. The pressure dependence of Em is qualitatively consistent with expectations based on the Born-Drude-Nernst theory. The strong accelerating effects of supporting electrolytes on the standard electrode reaction rate constant k(el) at pH 3 as measured by alternating current voltammetry (ACV), and on the homogeneous self-exchange rate constant k(ex) at pH 3-7 as measured by 27Al line broadening, depend specifically on the identity and concentration of M+ (Li+ < Na+ < K+ < Rb+) rather than on the ionic strength, whereas the effect of the nature of the supporting anion (Cl- or CF3SO3-) is negligible. Extrapolation of k(el) and k(ex) to zero [M+] indicates that the uncatalyzed electron transfer rate is negligibly small relative to the M+ catalyzed rates. The kinetic effects of M+ show no evidence of the saturation expected had they been due primarily to ion pairing with AlW12O40(5-/6-). The catalytic effect of M+ operates primarily through lowering the enthalpy of activation, which is partially offset by a strongly negative entropy of activation and, for the homogeneous exchange catalyzed by K+ or Rb+, becomes mildly negative; thus, the catalytic effect of M(+) is enthalpy-driven but entropy-limited. For the electrode reaction, the volume of activation averages +4.5 +/- 0.2 cm(3) mol(-1) for all M+ and [M+], in contrast to the negative value predicted theoretically for the uncatalyzed reaction. These results are consistent with a reaction mechanism, previously proposed for other anion-anion electron-transfer reactions, in which anion-anion electron transfer is facilitated by partially dehydrated M+.