Capillary electrophoresis of microbial aggregates

Uncontrolled aggregation of bacterial cells is a significant disadvantage of electrophoretic separations. Various aspects of the electrophoretic behavior of different strains of Gram‐positive Bacillus cereus, Bacillus subtilis, Sarcina lutea, Staphylococcus aureus(1), and Micrococcus luteus bacteria and Gram‐negative Escherichia coli bacteria were investigated in this study. Our findings indicate that bacteria can be rapidly analyzed by CZE with surface charge modification by calcium ions (Ca²⁺). Bound Ca²⁺ ions increase zeta potential to more than 2.0 mV and significantly reduce repulsive forces. Under the above conditions, bacterial cells create compact aggregates, and fewer high‐intensity signals are observed in electropherograms. The above can be attributed to the bridging effect of Ca²⁺ between bacterial cells. CE was performed to analyze bacterial aggregates in an isotachophoretic mode. A single peak was observed in the electropherogram.     

Mass spectrometry of surfactant aggregates

In contrast with the enormous amount of literature produced during many decades in the field of surfactant aggregation in liquid, liquid crystalline and solid phases, only a few investigations concerning surfactant self- assembling in the gas phase as charged aggregates have been carried out until now. This lack of interest is disappointing in view of the remarkable theoretical and practical importance of the inherent knowledge. The absence of surfactant-solvent interactions makes it easier to study the role of surfactant-surfactant forces in determining their peculiar self-assembling features as well as the ability of these assemblies to incorporate selected solubilizate molecules. Thus, the study of gas-phase surfactant and surfactant-solubilizate aggregates is a research subject which has exciting potential, including mass and energy transport in the atmosphere, origin of life and simulation of supramolecular aggregation in interstellar space. On the other hand, the structural and dynamic properties of surfactant aggregates in the gas phase could be exploited in a number of interesting applications such as atmospheric cleaning agents, transport and protection of pulmonary drugs or biomolecules and as nanoreactors for specialized chemical reactions in confined space. Spectrometric techniques, together with molecular dynamics simulations, have been the principal investigative tools in this field and appearto be particularly suited to gaining fundamental information on the structure and stability of surfactant-based supramolecular aggregates, charge state effects, entrapment of solubilizate molecules, preferential solubilization sites and chemical reactions localized in a single organized aggregate. The main aim of this review is to present the actual state of the art in this novel and exciting research field underlining the knowledge acquired up to now as well as the aspects needing a more deep understanding. Moreover, intriguing departures of the behavior of surfactant solutions under electrospray ionization conditions from that of ionic, polar and apolar analytes will be discussed.     

Structures of DNA-linked nanoparticle aggregates

The room-temperature structure of DNA-linked gold nanoparticle aggregates is investigated using a combination of experiment and theory. The experiments involve extinction spectroscopy measurements and dynamic light scattering measurements of aggregates made using 60 and 80 nm gold particles and 30 base-pair DNA. The theoretical studies use calculated spectra for models of the aggregate structures to determine which structure matches the observations. These models include diffusion-limited cluster-cluster aggregation (DLCA), reaction-limited cluster-cluster aggregation (RLCA), and compact (nonfractal) cluster aggregation. The diameter of the nanoparticles used in the experiments is larger than has been considered previously, and this provides greater sensitivity of spectra to aggregate structure. We show that the best match between experiment and theory occurs for the RLCA fractal structures. This indicates that DNA hybridization takes place under irreversible conditions in the room-temperature aggregation. Some possible structural variations which might influence the result are considered, including the edge-to-edge distance between nanoparticles, variation in the diameter of the nanoparticles, underlying lattice structures of on-lattice compact clusters, and positional disorders in the lattice structures. We find that these variations do not change the conclusion that the room-temperature structure of the aggregates is fractal. We also examine the variation in extinction at 260 nm as temperature is increased, showing that the decrease in extinction at temperatures below the melting temperature is related to a morphological change from fractal toward compact structures.     

Fractal structure of asphaltene aggregates

A photographic technique coupled with image analysis was used to measure the size and fractal dimension of asphaltene aggregates formed in toluene-heptane solvent mixtures. First, asphaltene aggregates were examined in a Couette device and the fractal-like aggregate structures were quantified using boundary fractal dimension. The evolution of the floc structure with time was monitored. The relative rates of shear-induced aggregation and fragmentation/restructuring determine the steady-state floc structure. The average floc structure became more compact or more organized as the floc size distribution attained steady state. Moreover, the higher the shear rate is, the more compact the floc structure is at steady state. Second, the fractal dimensions of asphaltene aggregates were also determined in a free-settling test. The experimentally determined terminal settling velocities and characteristic lengths of the aggregates were utilized to estimate the 2D and 3D fractal dimensions. The size-density fractal dimension (D(3)) of the asphaltene aggregates was estimated to be in the range from 1.06 to 1.41. This relatively low fractal dimension suggests that the asphaltene aggregates are highly porous and very tenuous. The aggregates have a structure with extremely low space-filling capacity.     

Optical rotation of noncovalent aggregates

Dilute solutions of (R)-(-)-pantolactone in CCl4 were studied by polarimetry in conjunction with theoretical calculations of [alpha]D. Our data demonstrate that the self-association of a chiral solute results in a change in [alpha]D that can be accounted for by the presence of hydrogen-bonded dimeric species. The theoretical analysis predicts a concentration-dependent specific rotation in good agreement with experiment. Further exploration of monomer and dimer [alpha]D differences, through atomic map analysis, reveals large contributions to [alpha]D from the hydrogen-bonded hydroxyl groups in the tightly-coupled dimer. This study extends the computation of chiroptical properties to the accurate concentration-dependent prediction of [alpha]D for noncovalently interacting self-associating species.     

From molecules to aggregates

The preparation of organometallic oxides, imides and nitrides is described. The molecular structures of these compounds resemble those found in binary systems. However, due to the organic envelope of the molecular solids, they are soluble in organic solvents, easy to crystallize and unambiguously characterizable by single X-ray structural analysis and NMR investigations. Moreover, inorganic oxides can be incorporated in organometallic phosphonates or organoalumoxanes. Herein we describe the organometallic phosphonates as hosts with a hydrophobic exterior.     

Hierarchical self-assembly of columnar aggregates

Biology often uses hierarchical self-assembly to produce complex functional structures from smaller components. At each level of this stepwise process, non-covalent interactions bring together the subunits of a lower level of complexity, using the information encoded in their structures. Applying this approach to synthetic systems represents a formidable challenge, because it requires a high degree of command of non-covalent interactions. In this tutorial review, recent developments in the hierarchical self-assembly of discrete columnar aggregates are discussed.     

Computer simulation studies of aggregates of associating polymers: Influence of low-molecular-weight additives solubilizing the aggregates

The structure of aggregates in solutions of chain molecules with associating groups at one of the ends is studied by Monte Carlo computer simulations using the bond fluctuation model. The main attention is paid to the influence of additives of low-molecular-weight solvent solubilizing the aggregates. It is shown that upon the addition of solvent the aggregates adopt a three-layer structure with the ‘lake’ of the solvent molecules in the central region surrounded by the layer of associating end-groups of polymer chains, which in turn is surrounded by the outer corona formed by the chain tails. The equilibrium form of the aggregates becomes close to that of a droplet of low-molecular-weight liquids. The regimes are found when the addition of the low-molecular-weight solvent stabilizes the multiplets and even induces the aggregate formation.     

Exciton dynamics of GaSe nanoparticle aggregates

Time-resolved and static spectroscopic results on GaSe nanoparticle aggregates are presented to elucidate the exciton relaxation and diffusion dynamics. These results are obtained in room-temperature TOP/TOPO solutions at various concentrations. The aggregate absorption spectra are interpreted in terms of electrostatic coupling and covalent interactions between particles. The spectra at various concentrations may then be interpreted in terms of aggregate distributions calculated from a simple equilibrium model. These distributions are used to interpret concentration-dependent emission anisotropy kinetics and time-dependent emission spectral shifts. The emission spectra are reconstructed from the static emission spectra and decay kinetics obtained at a range of wavelengths. The results indicate that the aggregate z axis persistence length is about 9 particles. The results also show that the one-dimensional exciton diffusion coefficient is excitation wavelength dependent and has a value of about 2 x 10(-5) cm(2)/s following 406 nm excitation. Although exciton diffusion results in very little energy relaxation, subsequent hopping of trapped electron/hole pairs occurs by a Forster mechanism and strongly red shifts the emission spectrum.     

Nanocage aggregates composed of bilayer sheets

A partially fluorinated carboxylate-type anionic gemini surfactant, N,N'-di(3-perfluorohexyl-2-hydroxypropyl)- N,N'-diacetic acid-ethylenediamine (2C6Fedda), formed aggregates with a cagelike structure in an aqueous alkali solution. These aggregates were composed of several bilayer sheets. The TEM micrograph showed that the bilayer sheets were produced from a condensed self-assembly core. The leaflike bilayer sheets can form folds and link to each other at their edges. The typical size of the spherical cage ranged from ca. 200 to 1500 nm.     

Structural aggregates of rod-coil copolymer solutions

The optoelectronic properties of rod-coil diblock copolymers with π-conjugation are greatly affected by molecular packing, which is closely related to their micellar morphology. Self-assembly of rod-coil block copolymer B(y)A(x) in a selective solvent for its coil block is studied by using dissipative particle dynamics, where B(y)A(x) denotes the polymer comprising of y rodlike B beads and x coil-like A beads. The influences of polymer concentration, component compatibility, solvent quality for coil block, rod-block length, and π-π interaction on the resulting aggregate conformations are examined. It was found that distinctly different from coil-coil copolymers, the aggregates of rod-coil copolymers exhibit morphological and structural diversity induced by the intrinsically rigid nature of the rod blocks. In general, the aggregate adopts the overall shape of sphere, cylinder, perforated sheet, or network. The morphology of the rod-block domain within aggregate is even richer and the interesting structures such as porous sphere, spherical spiral, helical bundles, discrete chunks, and nematic cylinder are observed. The short-range order parameter indicates that as rod length is long enough, neighboring rods begin to orient parallel to one another and nematic domains appear. Moreover, in the presence of π-π interactions, the neighboring rods within the B domains become more coherently oriented and smectic domains can thus be formed.     

Sulfide aggregates and clusters of platinum

The homometallic and lteterometallic chemistry of platinum sulfide polynuclear complexes with or without metal-metal bonds is reviewed. The synthetic strategies, structural and bonding characteristics, and chemical properties of these materials are emphasized. Other dinuclear platinum sulfide complexes which are potential precursors to larger aggregates (non-M-M bonded) or clusters (M-M bonded) are also identified.     

Modelling of polymer aggregates in solution

The self-consistent mean field model of Scheutjens and Fleer is used to model spherical aggregates of homopolymers and monomer—polymer particles in solution. For homopolymer aggregates we found that the chain ends are preferentially located at the exterior side of the polymer/solvent interface. The distribution of the end segments may be an important parameter in latex film formation. For monomer—polymer particles a “core-shell” structure is found with an extended core containing a monomer—polymer mixture and a thin shell a few nanometres thick strongly enriched with monomer. The monomer-enriched shell seems to function as a solvating envelope for the dangling chain ends. These results are compared with other simulations based on a single chain in a spherical-cavity model.     

Photoinduced morphism of gemini surfactant aggregates

The photochemical behaviour of an azobenzene chromophore inserted in a gemini surfactant imparts photocontrol to the resulting amphiphile assemblies, including the collapse, upon irradiation, of the multi lamellar vesicles formed in aqueous solution.     

Nanosized silver/thiocholesterol aggregates

Linear aggregates including nanosized silver particles (d = 2.5 ± 0.5 nm) stabilized in a cholesteric mesophase of thiocholesterol forming by cooling from the isotropic state are obtained. The composition and the structure of the aggregates are determined by FTIR, UV-Vis spectroscopy, transmission electron microscopy, and electronic diffraction. The optical properties of the silver-thiocholesterol and silver-thiocholesterol-toluene systems are studied.     

Bonding properties in organolithium aggregates

Results are given of the application of the CNDO/2 method to bonding in alkyllithium aggregates. Calculations relative to monomers and dim ers of methyl-, ethyl-, and vinyllithium, plus methyllithium tetramer are described.