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.     

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.     

Exploring long-range proton conduction,the conduction mechanism and inner hydration state of protein biopolymers

Proteins are the main proton mediators in various biological proton circuits. Using proteins for the formation of long-range proton conductors is offering a bioinspired approach for proton conductive polymers. One of the main challenges in the field of proton conductors is to explore the local environment within the polymers, along with deciphering the conduction mechanism. Here, we show that the protonic conductivity across a protein-based biopolymer can be hindered using straightforward chemical modifications, targeting carboxylate- or amine-terminated residues of the protein, as well as exploring the effect of surface hydrophobicity on proton conduction. We further use the natural tryptophan residue as a local fluorescent probe for the inner local hydration state of the protein surface and its tendency to form hydrogen bonds with nearby water molecules, along with the dynamicity of the process. Our electrical and spectroscopic measurements of the different chemically-modified protein materials as well as the material at different water–aprotic solvent mixtures result in our fundamental understanding of the proton mediators within the material and gaining important insights on the proton conduction mechanism. Our biopolymer can be used as an attractive platform for the study of bio-related protonic circuits as well as a proton conducting biopolymer for various applications, such as protonic transistors, ionic transducers and fuel cells.

Post formation modification of protein-based materials can attenuate the proton conduction efficiency resulting from change in conduction mechanism, charge carrier mobility, carrier concentrations and inner hydration layer.     

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.     

In vivo effects of porphyrins on bacterial DNA.

The DNA damage in intact Staphylococcus aureus and E. coli cells induced by photosensitized deuteroporphyrin or hemin is described. Treatment of S. aureus cultures with hemin or photosensitized deuteroporphyrin (Dp) caused time-dependent changes in the plasmidial DNA profiles. The major observation was the disappearance of the plasmid supercoiled fraction. The chromosomal DNA was also affected by hemin and by photosensitized Dp, since its degradation products were detected after exposing the bacterial cells to the porphyrin drugs. Photosensitization of E. coli cells, pretreated with Dp and polymyxin B nonapeptide (PMBNP), also resulted in plasmidial damage. No such damage occurred when E. coli cultures were treated with hemin and PMBNP. The above results can be tightly correlated with the antimicrobial action of porphyrins. Their damage to the bacterial DNA seems to reflect one of the in vivo effects of these porphyrins.     

Real‐Time In‐Situ Monitoring of a Tunable Pentapeptide Gel–Crystal Transition

Supramolecular gels often become destabilized by the transition of the gelator into a more stable crystalline phase, but often the long timescale and sporadic localization of the crystalline phase preclude a persistent observation of this process. We present a pentapeptide gel–crystal phase transition amenable for continuous visualization and quantification by common microscopic methods, allowing the extraction of kinetics and visualization of the dynamics of the transition. Using optical microscopy and microrheology, we show that the transition is a sporadic event in which gel dissolution is associated with microcrystalline growth that follows a sigmoidal rate profile. The two phases are based on β‐sheets of similar yet distinct configuration. We also demonstrate that the transition kinetics and crystal morphology can be modulated by extrinsic factors, including temperature, solvent composition, and mechanical perturbation. This work introduces an accessible model system and methodology for studying phase transitions in supramolecular gels.