Stability and state of aggregation of aqueous fibrinogen and dipalmitoylphosphatidylcholine lipid vesicles

The stability and state of aggregation of aqueous fibrinogen (FB) and dipalmitoylphosphatidylcholine (DPPC) vesicles in water or buffer at 25 degrees C were studied with dynamic light scattering (DLS), UV-vis spectroturbidimetry (ST), and cryo-transmission electron microscopy (cryo-TEM). In water, when 1000 ppm (0.10 wt %) DPPC dispersions were prepared with a protocol including extensive sonication, they contained mostly vesicles and were quite clear, transparent, and stable for at least 30 days. FB mixtures with water (0.075 wt %) were quite unstable and biphasic. They formed large aggregates which eventually precipitated. The addition of DPPC vesicles into these unstable FB dispersions reversed FB aggregation and precipitation and produced stable translucent microdispersions. The inferred lipid/protein aggregates were limited in size, with average diameters ranging from 200 to 300 nm. In buffer, DPPC dispersions were also clear and quite stable, with average dispersed particles diameter of ca. 90 nm. FB dissolved in aqueous buffer and formed transparent and stable solutions. Adding salt to an aggregated FB dispersion in water reversed the aggregation. FB aggregated and redissolved in the presence of the citrate and after the citrate was removed. There was no effect of citrate (present in FB initially) in the FB aggregation or redissolution. FB molecules in buffer form dimers or higher aggregates. Their average aggregation number is 2, determined with Rayleigh scattering analysis of turbidity data. The average hydrodynamic diameter of FB solutions from DLS was 30 nm. Mixing a stable FB solution in buffer and a stable DPPC dispersion in buffer produced highly unstable mixtures, in which large aggregates precipitated. These results have implications in understanding the interactions of lipids and proteins in many biological applications and food processing applications.     

Sensitivity of Staphylococcus aureus strains to broadband visible light

The phototoxic effect of illumination with broadband visible light on the viability of two Staphylococcus aureus strains was examined in the present study. A difference in the light sensitivity of the two strains was found. Illumination of the tested strains with a fluence rate of 180 J cm⁻² caused a reduction of up to 99.8% in the colony count of one of the strains (the "sensitive" strain). Illumination of the other strain (the "resistant" strain) resulted in a 55.5% reduction in viability. Proliferation of both strains was observed at low fluence rates of light. The phototoxic effect was found to be dependent on oxy radical production. The light-sensitive strain produced higher amounts of hydroxyl and superoxide radicals than the "resistant" strain. Adaptation to oxidative stress was exhibited only by the "resistant" strain. The "sensitive" strain produced ten times more endogenous porphyrins and secreted almost nine times more porphyrins than the resistant strain. Furthermore, the "resistant" strain produced twice as many carotenoids that protect the strain from illumination than the "sensitive" strain. These results indicate that high intensities of visible light cause bacterial photoeradication, a reaction which may assist wound healing by killing the infecting bacteria. On the other hand, low intensities of white light were found to enhance bacterial proliferation and thus prolong wound infection.     

Optical properties of current carrying molecular wires

We consider several fundamental optical phenomena involving single molecules in biased metal-molecule-metal junctions. The molecule is represented by its highest occupied and lowest unoccupied molecular orbitals, and the analysis involves the simultaneous consideration of three coupled fluxes: the electronic current through the molecule, energy flow between the molecule and electron-hole excitations in the leads, and the incident and/or emitted photon flux. Using a unified theoretical approach based on the nonequilibrium Green's function method we derive expressions for the absorption line shape (not an observable but a useful reference for considering yields of other optical processes) and for the current induced molecular emission in such junctions. We also consider conditions under which resonance radiation can induce electronic current in an unbiased junction. We find that current driven molecular emission and resonant light induced electronic currents in single molecule junctions can be of observable magnitude under appropriate realizable conditions. In particular, light induced current should be observed in junctions involving molecular bridges that are characterized by strong charge-transfer optical transitions. For observing current induced molecular emission we find that in addition to the familiar need to control the damping of molecular excitations into the metal substrate the phenomenon is also sensitive to the way in which the potential bias is distributed on the junction.     

Resolving intermediate solution structures during the formation of mesoporous SBA-15

The evolution of the solution microstructures during the formation of the hexagonal mesoporous material SBA-15 was studied by direct imaging and freeze-fracture replication cryo-TEM. A reaction mixture was sampled at different times after the addition of tetramethoxyorthosilane (TMOS) to an acidic solution of Pluronic P123 held at 50 degrees C. Solution microstructures were detected by direct imaging cryo-TEM in the time window of 6.5-40 min after the addition of the TMOS (t = 0). The micrographs revealed that the initial spheroidal micelles evolve into threadlike micelles, which become longer and straighter with time. Then bundles with the dimensions similar to those found in the final material appeared, although there was no sign of a hexagonal arrangement up to 40 min. Due to the appearance of a precipitate at 40 min the sample became too viscous, preventing clear observation of its content. To observe the structures present after 40 min, freeze-fracture replication was carried out as well. Such samples were collected also at 22 min and showed the presence of threadlike micelles in agreement with the direct imaging cryo-TEM micrographs. The 2 h samples showed some areas of hexagonal ordered structures, which become very clear at 2 h 50 min. The cryo-TEM measurements were carried out under the same reaction conditions used in earlier in situ EPR experiments, thus allowing us to correlate molecular level events with the microstructure shape evolutions. This showed that the elongation of the micelles is a consequence of a reduction of the polarity and the water content within the micelles due to silicate adsorption and polymerization. Similar experiments were carried out also on SBA-15 prepared with HCl and TMOS at 35 degrees C. The appearance of threadlike micelles, followed by clustering of the TLMs, was observed under these conditions as well, but the reaction rate was faster. This suggests that the observed mechanism for the formation of SBA-15 is general.     

Sonochemical coating of cotton and polyester fabrics with "antibacterial" BSA and casein spheres

A novel antibacterial coating for cotton and polyester fabrics has been developed by using drug-loaded proteinaceous microspheres made of bovine serum albumin and casein proteins. The microbubbles were created and anchored onto the fabrics (see figure) in a one-step reaction that lasts 3 min. The sonochemically produced "antibacterial fabrics" have been characterized. The efficiency of the sonochemical process in converting the native proteins into microspheres, encapsulating the drug, and coating the fabric has also been studied.     

Mechanistic aspects of photoinactivation of Candida albicans by exogenous porphyrins

The mechanism of photoinactivation of Candida albicans by 3.5 μM uncharged, cationic or anionic porphyrins under blue light (407-420 nm) was found to be dependent on the uptake of porphyrins into yeast cells, and was also dependent on the presence or absence of proteins in the photosensitization medium. In a very protein-rich medium, a decrease in viability was observed only with the uncharged porphyrin. Photoinactivation by uncharged or cationic porphyrins in a protein-poorer medium resulted in total eradication, whereas no significant decrease was observed with the anionic porphyrin. Phototreatment in PBS resulted in eradication with all three porphyrins. X-ray microanalysis after phototreatment by the uncharged or cationic porphyrins in the protein-poor medium exhibited ion loss, indicating cell-membrane damage. Transmission electron microscopy indicated cellular and chromosomal damage. No ion loss or cell damage was observed in this medium with the anionic porphyrin. The efficiency of photoeradication of C. albicans is dependent on porphyrin uptake, which might lead (upon illumination) to processes that facilitate the formation of reactive oxygen species that damage the cells. Uptake of charged porphyrins is dependent on protein quantity and quality in the photosensitization microenvironment. This fact must be taken into account when using charged photosensitizers.     

Dynamics of tip-substrate interactions in atomic force microscopy

Dynamical interactions between a scanning tip and a silicon substrate are investigated using molecular dynamics simulations of both the constant-height and constant-force scan modes. Localized temporary and permanent modifications of the substrate occur, depending on tip-substrate-strate separation and scan geometry. Implications for resolving structural and force characteristics in scanning tip spectroscopies, employing atomically sharp as well as large ordered of disordered tips are discussed.     

Accelerated energy transfer between molecules near a solid particle

The Forster-Dexter theory of energy transfer between molecules is generalized to include the effects of a nearby solid state particle. It is found that the energy transfer rate between a donor and acceptor molecule may be enhanced by many orders of magnitude when the molecular transition frequencies lie in the vicinity of the resonance frequency of the particle and when the particle possesses sharp features. Due to increased damping near the particle, however, this may or may not lead to increased acceptor molecule radiation.     

Local‐Curvature‐Controlled Non‐Epitaxial Growth of Hierarchical Nanostructures

Site‐selective growth on non‐spherical seeds provides an indispensable route to hierarchical complex nanostructures that are interesting for diverse applications. However, this has only been achieved through epitaxial growth, which is restricted to crystalline materials with similar crystal structures and physicochemical properties. A non‐epitaxial growth strategy is reported for hierarchical nanostructures, where site‐selective growth is controlled by the curvature of non‐spherical seeds. This strategy is effective for site‐selective growth of silica nanorods from non‐spherical seeds of different shapes and materials, such as α‐Fe₂O₃, NaYF₄, and ZnO. This growth strategy is not limited by the stringent requirements of epitaxy and is thus a versatile general method suitable for the preparation of hierarchical nanostructures with controlled morphologies and compositions to open up a verity of applications in self‐assembly, nanorobotics, catalysis, electronics, and biotechnology.     

STRUCTURE-ACTIVITY RELATIONSHIP OF PORPHINES FOR PHOTOINACTIVATION OF BACTERIA

Abstract— The antibacterial photodynamic effects of uncharged ( o -tetrahydroxyphenyl porphine [THPP], m -THPP and p -THPP), cationic (5,10,15,20-tetra[4- N -methylpyridyllporphine [TMPyP]) and anionic (5,10,15,20-tetra[4-sulfonatophenyl porphine] [TPPS₄]) porphines on Staphylococcus aureus and Escherichia coli bacteria inactivation were examined. The results show that uncharged porphines provoked antibacterial photodynamic activity on S. aureus, and also on E. coli in the presence of the membrane-disorganizing peptide polymixin B nonapeptide (PMNP). The TMPyP compound was highly photoactive toward gram-positive bacteria but only marginally effective on gram-negative cells, whereas TPPS₄ showed no activity on either gram-positive or gram-negative bacteria. The photoactivity of TMPyP is due to the electrostatic attraction between the positively charged sensitizer molecule and the negatively charged membrane of the gram-positive target cells. For TPPS₄, the inactivity toward gram-positive bacteria is due to electrostatic repulsion between the charged sensitizer molecule and the cell membrane. For gram-negative bacteria, the inactivity is conceivably due to preferential (electrostatic) binding to the positively charged PMNP, which is an adjuvant for membrane disorganization, but has no effect on cell viability. For hydrophobic sensitizers, the photoactivity depends on the state of aggregation. The extent of deaggregation of the different THPP isomers was determined by fluorescence measurements of bound sensitizers and could be positively correlated with their photoinactivation capacity. We conclude that the structure-activity relationships of these porphines are affected by their net charge and by aggregation.