Liposome fusion rates depend upon the conformation of polycation catalysts

Cryo-TEM and NaCl-leakage experiments demonstrated that the cationic polymer polylysine induces fusion of anionic liposomes but that the cationic polymer poly(N-ethyl-4-vinylpyridinium bromide) (PEVP) does not, although both polymers bind strongly to the liposomes. The difference was traced to the thickness of the coatings at constant charge coverage. Polylysine is believed to form planar β-sheets that are sufficiently thin to allow membrane fusion. In contrast, looping and disorganization among adsorbed PEVP molecules physically prevent fusion. A similar effect is likely to be applicable to important polycation-induced fusion of cell membranes.     

Comparison of oleyl and elaidyl isomer surfactant-counterion systems in drag reduction, rheological properties and nanostructure

Compared with quaternary ammonium cationic surfactants with saturated alkyl chains, quaternary ammonium cationic surfactants with one double-bond in their alkyl chains, when mixed with appropriate counterions (in certain molar concentration ratios, ξ), can reach much lower effective drag-reduction temperatures, while maintaining the upper drag-reduction temperature limit of the corresponding saturated drag reducing surfactant solutions. No previous study has compared the effects of cis- vs. trans-unsaturated alkyl hydrocarbon tail configurations (oleyl vs. elaidyl) trimethyl ammonium chloride cationic surfactants at different counterion/surfactant concentration ratios on micellar nanostructures, (1)H NMR spectra and on rheological and drag-reduction behavior of their solutions. Since neither pure oleyl (cis-) nor elaidyl (trans-) trimethyl ammonium chloride surfactants are commercially available, they were synthesized and their 5mM solutions with NaSal counterion at concentrations of 5mM, 7.5mM and 12.5mM were studied.     

Complexation of anionic liposomes with spherical polycationic brushes

Spherical polycationic brushes, consisting of polystyrene particles with linear cationic macromolecules grafted onto their surfaces, were electrostatically complexed with small unilamellar anionic liposomes. Complexation was monitored using a multimethod approach that included laser electrophoresis, dynamic light scattering, fluorescence, cryogenic transmission electron microscopy, and conductivity. Liposomes adsorb onto the outer edges of the brushes rather than penetrate into their dense polycationic layer. The integrity of the liposomes remains unaltered when the liposomes reside on the polycationic brushes. The resulting complexes (roughly 40 liposomes per brush) do not dissociate into their components upon exposure to physiological solutions. The system is potentially useful in that liposomes are gathered into well-defined clusters with a high encapsulating potential. Multicomponent constructs can be easily prepared if polycationic brushes are allowed to bind to a mixture of liposomes that encapsulate different guests. This work provides an example of "systems chemistry" whereby as many as eight components, each with its own particular location and function (i.e., polystyrene core, polycationic graft, egg lecithin, cardiolipin, two fluorescent dyes, water, and buffer), collectively self-assemble.     

Unusual temperature gap in turbulent drag reduction of cationic surfactants with mixed counterions

Unusual temperature gaps have previously been observed in the turbulent drag reduction effectiveness of Dobon and Habon cationic surfactant systems containing two anionic counterions of different binding strengths. Here, we report drag reduction data for a cationic surfactant with a mixture of dodecyl sulfate and tosylate counterions showing a temperature gap. Cryo-TEM images of nanostructures at different temperatures for this system support an explanation for this gap, based on the counterions’ relative binding strengths.     

Achieving fully proportional representation by clustering voters

Both the Chamberlin–Courant and Monroe rules are voting rules that solve the problem of fully proportional representation: given a set of candidates and a set of voters, they select committees of candidates whose members represent the voters so that the voters’ total dissatisfaction is minimized. These two rules suffer from a common disadvantage, namely being computationally intractable. As both the Chamberlin–Courant and Monroe rules, explicitly or implicitly, partition voters so that the voters in each part share the same representative, they can be seen as clustering algorithms. This suggests studying approximation algorithms for these voting rules by means of cluster analysis, which is the subject of this paper. Using ideas from cluster analysis we develop several approximation algorithms for the Chamberlin–Courant and Monroe rules and experimentally analyze their performance. We find that our algorithms are computationally efficient and, in many cases, are able to provide solutions which are very close to optimal.     

Investigation of multicomponent diffusion in cat brain using a combined MTC-DWI approach

In this study, multiple-component water diffusion in the cat brain is investigated using an approach that combines diffusion-weighted images using multiple b values with magnetization transfer contrast (MTC). The MTC allows filter of signal originating from water molecules that rapidly exchange with binding sites on large macromolecular structures, and in brain white matter, it is assumed that a significant portion of the MTC is due to the interaction of water with the extraaxonal myelin sheath. Henceforth, multicomponent analysis of diffusion curves with and without MTC may shed light on the contribution of the extraaxonal water to the diffusion signal and on the relationship between diffusion components and tissue compartments in the brain. When a biexponential model was applied to the data, the volume fractions of the two diffusion components changed significantly in white matter with the application of the MTC. These changes are then discussed in the frame of tissue components and the possible interaction with the myelin sheath.     

Effect of variations in counterion to surfactant ratio on rheology and microstructures of drag reducing cationic surfactant systems

Rheology, drag reduction and cryo-TEM experiments were performed on Arquad 16–50/NaSal and Ethoquad O/12/NaSal surfactant systems at different counterion-to-surfactant ratios and at constant low surfactant concentrations, 5 mM, appropriate for drag reduction. The molar ratio of counterion-to-surface was varied from 0.6 to 2.5. All the surfactant systems described here are viscoelastic and drag reducing. The viscoelasticity and drag reducing effectiveness increase with increase in counterion/surfactant ratio. Network are present in the solutions with high ratio, and they are viscoelastic. However, shear is needed to induce network formation for solutions at low ratio. Cryo-TEM images confirm the existence of thread-like micelles which form entanglement networks, and show that the micellar network becomes denser with increasing counterion/surfactant ratio in one surfactant series. Both increase in the counterion/surfactant ratio and increase in the shear rate result in shorter relaxation times. For some of these systems, abrupt increase in viscosity is observed at certain shear rates which are time effects affecting microstructure rearrangements rather than formation of shear induced structures.     

Probe particles alter dynamic heterogeneities in simple supercooled systems

The authors present results from molecular dynamics simulations on the effect of smooth and rough probes on the dynamics of a supercooled Lennard-Jones (LJ) mixture. The probe diameter was systematically varied from one to seven times the diameter of the large particles of the LJ mixture. Mean square displacements show that in the presence of a large smooth probe the supercooled liquid speeds up, while in the presence of a large rough probe, the supercooled liquid slows down. Non-Gaussian parameters indicate that with both smooth and rough probes, the heterogeneity of the supercooled system increases. From the analysis of local Debye-Waller factors, it is evident that the change in the dynamics of the LJ system is heterogeneous, with the largest perturbations close to the probes. Large smooth and rough probes appear to set up heterogeneities in these supercooled systems that would otherwise not occur, and these heterogeneities persist for long times.     

Selective immobilization of DNA and antibody probes on electrode arrays: simultaneous electrochemical detection of DNA and protein on a single platform

A proof of concept procedure for the electroaddressable covalent immobilization of DNA and protein on arrayed electrodes along with simultaneous detection of multiple bioagents in the same sample solution is described. Carboxyphenyldiazonium was selectively deposited onto five of nine individually addressable electrodes in an array via bias assisted assembly. Amine functionalized DNA probes were covalently coupled to the carboxyl surface via carbodiimide chemistry. This was followed by the covalent immobilization of diazonium-antibody conjugates into the remaining four electrodes via cyclic voltammetry. Simultaneous electrochemical detection of a DNA sequence related to the breast cancer BRCA1 gene and the human cytokine protein interleukin-12, which is a substantial component in the immune system response and attack of tumor cells, is reported. These results demonstrate the possibility of selective patterning of diverse biomolecules on a single device and may have significant implications for future development of microarrays and biosensors.