The Contribution of Reactive Oxygen Species to the Photobleaching of Organic Fluorophores

Photoexcitation of fluorophores commonly used for biological imaging applications generates reactive oxygen species (ROS) which can cause bleaching of the fluorophore and damage to the biological system under investigation. In this study, we show that singlet oxygen contributes relatively little to Cy5 and ATTO 647N photobleaching at low concentrations in aqueous solution. We also show that Cy5 generates significantly less ROS when covalently linked to the protective agents, cyclooctatetraene (COT), nitrobenzyl alcohol (NBA) or Trolox. Such fluorophores exhibit enhanced photostability both in bulk solutions and in single‐molecule fluorescence measurements. While the fluorophores ATTO 647N and ATTO 655 showed greater photostability than Cy5 and the protective–agent‐linked Cy5 derivatives investigated here, both of ATTO 647N and ATTO 655 generated singlet oxygen and hydroxyl radicals at relatively rapid rates, suggesting that they may be substantially more phototoxic than Cy5 and its derivatives.     

A small molecule discrimination map of the antibiotic resistance kinome

Kinase-mediated resistance to antibiotics is a significant clinical challenge. These enzymes share a common protein fold characteristic of Ser/Thr/Tyr protein kinases. We screened 14 antibiotic resistance kinases against 80 chemically diverse protein kinase inhibitors to map resistance kinase chemical space. The screens identified molecules with both broad and narrow inhibition profiles, proving that protein kinase inhibitors offer privileged chemical matter with the potential to block antibiotic resistance. One example is the flavonol quercetin, which inhibited a number of resistance kinases in vitro and in vivo. This activity was rationalized by determination of the crystal structure of the aminoglycoside kinase APH(2″)-IVa in complex with quercetin and its antibiotic substrate kanamycin. Our data demonstrate that protein kinase inhibitors offer chemical scaffolds that can block antibiotic resistance, providing leads for co-drug design.     

Addition of N-heterocyclic carbenes to a ruthenium(VI) nitrido polyoxometalate: a new route to cyclic guanidines

The scope of N-atom transfer from the electrophilic ruthenium(VI) nitrido containing polyoxometalate [PW(11)O(39)Ru(VI)N](4-) has been extended to the N-heterocyclic carbene {CH(2)(Mes)N}(2)C and the coupling product {CH(2)(Mes)N}(2)CNH(2)(+) characterized by (1)H NMR and high-resolution mass spectrometry. Because guanidines display many fields of applications ranging from biology to supramolecular chemistry, this could afford an original route to the synthesis of cyclic guanidines. This also enlarges the potential of nitrido complexes in the synthesis of heterocycles, mainly illustrated in the literature through the formation of aziridines through N-atom transfer to alkenes. In the course of the reaction, the ruthenium(III)-containing polyoxometallic intermediate [PW(11)O(39)Ru(III){NC{N(Mes)CH(2)}(2)}](5-) has been thoroughly characterized by continuous-wave and pulsed electron paramagnetic resonance, which nicely confirms the presence of the organic moiety on the polyoxometallic framework, Ru K-edge X-ray absorption near-edge structure, and electrochemistry.     

Synthesis of spiroketals under neutral conditions via a type III ring-rearrangement metathesis strategy

A conceptually novel approach to spiro- and dispiroketals of various ring-sizes under neutral conditions has been designed which complements the classical thermodynamically driven tactic. Key steps involved the formation of α-alkoxyfurans, their [4+2] or [4+3] cycloaddition reactions and the ring-rearrangement metathesis of the resulting oxabicycles.     

Synthesis,Photophysical, and Two‐Photon Absorption Properties of Elongated Phosphane Oxide and Sulfide Derivatives

A series of rod‐shaped and related three‐branched push–pull derivatives containing phosphane oxide or phosphane sulfide (PO or PS)—as an electron‐withdrawing group conjugated to electron‐donating groups, such as amino or ether groups, with a conjugated rod consisting of arylene–vinylene or arylene–ethynylene building blocks—were prepared. These compounds were efficiently synthesized by a Grignard reaction followed by Sonogashira coupling. Their photophysical properties including absorption, emission, time‐resolved fluorescence, and two‐photon absorption (TPA) were investigated with special attention to structure–property relationships. These fluorophores show high fluorescence quantum yields and solvent‐dependent experiments reveal that efficient intramolecular charge transfer occurs upon excitation, thereby leading to highly polar excited states, the polarity of which can be significantly enhanced by playing on the end groups and conjugated linker. Rod‐shaped and related three‐branched systems show similar fluorescence properties in agreement with excitation localization on one of the push–pull branches. By using stronger electron donors or replacing the arylene–ethynylene linkers with an arylene–vinylene one induces significant redshifts of both the low‐energy one‐photon absorption and TPA bands. Interestingly, a major enhancement in TPA responses is observed, whereas OPA intensities are only weakly affected. Similarly, phosphane oxide derivatives show similar OPA responses than the corresponding sulfides but their TPA responses are significantly larger. Finally, the electronic coupling between dipolar branches promoted by common PO or PS acceptor moieties induces either slight enhancement of the TPA responses or broadening of the TPA band in the near infrared (NIR) region. Such behavior markedly contrasts with triphenylamine‐core‐mediated coupling, which gives evidence for the different types of interactions between branches.     

Dynamical properties of expansive one-sided cellular automata

To every one-sided one-dimensional cellular automatonF with neighbourhood radiusr we associate its canonical factor defined by considering only the firstr coordinates of all the images of points under the powers ofF. Whenever the cellular automaton is surjective, this factor is a subshift which plays a primary role in its dynamics. In this article we study the class of positively expansive one-sided cellular automata, i.e. those that are conjugate to their canonical factors. This class is a natural generalisation of the toggle or permutative cellular automata introduced in [He]. We prove that the canonical factors of positively expansive one-sided cellular automata are mixing subshifts of finite type that are shift equivalent to full shifts. Moreover, the uniform Bernoulli measure is the unique measure of maximal entropy forF. Consequently, their natural extensions are Bernoulli. We also describe a family of non-permutative positively expansive cellular automata. This research was partially supported by action E9314 of ECOS-CONICYT.     

Micro-scale and millimeter-scale rotating disk couette flows, experiments and analysis

Experimental results are presented for rotating Couette flows with and without circumferential pressure gradients between a rotating disk and a stationary fluid chamber. The spinning disk and the top of the C-shaped fluid chamber are separated by a μ-scale gap that forms the fluid chamber passage with inner and outer radii of 1.19 and 2.38 mm, respectively. Ranges of experimental conditions are presented to demonstrate the fluid dynamics of the test arrangement, and for determination of fluid viscosity, and gas slip flow accommodation coefficients. As such, the test arrangement provides means to determine such fluid and flow properties using microliter sample sizes, with relatively low magnitudes of experimental uncertainty.     

Cationic copolymerization of tetrahydrofuran with propylene oxide. V. Effect of diol cocatalyst structure and concentration with antimony pentachloride as catalyst

The effects of the chemical nature and the concentration of diol cocatalysts in the copolymerization of propylene oxide (PO) and tetrahydrofuran (THF) catalyzed by antimony pentachloride were studied. The least bulky diol, 1,2-ethanediol, was found to be the most active species, judging from the highest rate of copolymerization. The variation of rate was much smaller with the other diols used. Furthermore, the position of the hydroxyl groups did not have much effect on the rate. The concentration of cocatalyst strongly influenced the rate of consumption of both monomers. This is explained by the effect of diol concentration on the equilibrium that exists between the catalyst and the cocatalyst on one side, and the active oxonium complex on the other. The mechanism of cocatalytic effects is discussed in terms of newer concepts of ionic polymerization. Specific solvation of ion-pairs is visualized as partly responsible for the overall effect of the cocatalysts. Water in quantities commeasurable with those of the cocatalyst was found to retard the reaction. It also gave rise to oligomeric products, as evidenced by gel-permeation chromatography.     

Saturation properties of TEA-CO2 amplifiers in the nanosecond pulse regime

Experimental data are presented for the energy extraction of a nanosecond pulse propagating with different energy flux densities in a grid-type double-discharge TEA-CO₂ amplifier. The results show that the amplifying medium behaves essentially as a two-level system with a saturation energy of 155 mJ/cm².