Photoisomerization of dichlorocarbene adducts of 2,4-cyclohexadienones

Irradiation of the nπ* band of the dichlorocyclopropyl enones $\text{1}$ and $\text{7}$ resulted in ring expansion and 1,2-chlorine migration to give $\text{2}$ and $\text{8}$ respectively, whereas the dihydrogen analog $\text{11}$ gave the vinylcyclopropane-cyclopentene rearrangement product $\text{12}$.     

Spin labeling of Natronomonas pharaonis halorhodopsin: probing the cysteine residues environment

Halorhodopsin from Natronomonas pharaonis (pHR) is a light-driven chloride pump that transports a chloride anion across the plasma membrane following light absorption by a retinal chromophore which initiates a photocycle. Analysis of the amino acid sequence of pHR reveals three cysteine residues (Cys160, Cys184, and Cys186) in helices D and E. Here we have labeled the cysteine residues with nitroxide spin labels and studied using electron paramagnetic resonance (EPR) spectroscopy their mobility, accessibility to various reagents, and the distance between the labels. It was revealed by following the d(1)/d parameter that the distance between the spin labels is ca. 13-15 Angstrom. The EPR spectrum suggests that one label has a restricted mobility while the other two are more mobile. Only one label is accessible to hydrophilic paramagnetic broadening reagents leading to the conclusion that this label is exposed to the water phase. All three labels are reduced by ascorbic acid and reoxidized by molecular oxygen. The rate of the oxidation is accelerated following retinal irradiation indicating that the protein experiences conformation alterations in the vicinity of the labels during the pigment photocycle. It is suggested that Cys186 is exposed to the bulk medium while Cys184, located close to the retinal ionone ring, exhibits an immobilized EPR signal and is characterized by a hydrophobic environment.     

Generation of free radicals by emodic acid and its [D-Lys6]GnRH-conjugate

In an attempt to develop an efficient chemotherapeutic agent targeted at malignant cells that express receptors to gonadotropin releasing hormone (GnRH) we coupled [D-Lys6]GnRH covalently to an emodin derivative, i.e. emodic acid (Emo) to yield [D-Lys6(Emo)]GnRH. Emodin is a naturally occurring anthraquinone which is widely used as a laxative and has other versatile biological activities. Physico-chemical studies employing electron paramagnetic resonance and electrochemistry of the conjugate as well as the (Emo) moiety showed that these compounds could be easily reduced either chemically, photochemically or enzymatically to their corresponding semiquinones. In the presence of oxygen the semiquinones generated reactive oxygen species (ROS), mainly superoxide and hydroxyl radicals, which were detected by the spin trapping method. Moreover, upon irradiation with visible light these compounds produced ROS and a highly reactive excited triplet state of Emo, which by itself may cause the oxidation of certain electron acceptors such as amino acids and bases of nucleic acids. Thus, [D-Lys6]GnRH-photosensitizer conjugates may be potentially used for targeted photodynamic chemotherapy aimed at treating cancer cells that carry GnRH receptors. These conjugates may also induce cytotoxicity in the dark similar to common conventional chemotherapeutic agents. The peptidic moiety, [D-Lys6]GnRH, was found to be stable toward highly reactive ROS generated either from enzymatic reduction or upon photoirradiation. The physico-chemical properties of Emo were only marginally influenced by the peptidic [D-Lys6]GnRH carrier.     

Die ma?analytische Bestimmung des Molybd?ns

Zusammenfassung Die fällungsmaßanalytische Bestimmung von Molybdat kann durch potentiometrische Titration mit Silbernitrat in acetatgepufferter Lösung geschehen, wobei zweckmäßig noch ein Zusatz von Äthanol zur Lösung erfolgt. Die Analysenwerte streuen etwa ± 1%, sind demnach für technische, nicht aber exakte Bestimmungen brauchbar.Eine einfache und genaue Bestimmung des Molybdäns ist jedoch möglich durch Reduktion der salzsauren Lösung im Silberreduktor zu Mo⁵⁺, das anschließend nach Zusatz von Phosphorsäure als Reaktions-beschleuniger und Diphenylamin als Indicator mit Dichromat auf Farbumschlag nach Blau titriert wird.     

Visible cascade Higgs decays to four photons at hadron colliders

The presence of a new singlet scalar particle a can open up new decay channels for the Higgs boson, through cascades of the form h --> 2a --> X, possibly making discovery through standard model channels impossible. If a is CP odd, its decays are particularly sensitive to new physics. Quantum effects from heavy fields can naturally make h --> 4 g the dominant decay which is difficult to observe at hadron colliders, and is allowed by CERN LEP for m(h) > 82 GeV. However, there are usually associated decays, either h --> 2g2gamma or h --> 4gamma, which are more promising. The decay h-->4gamma is a clean channel that can discover both a and h. At the CERN LHC with 300 fb(-1) of luminosity, a branching ratio of order 10(-4) is sufficient for discovery for a large range of Higgs boson masses. With total luminosity of approximately 8 fb(-1), discovery at the Fermilab Tevatron requires more than 5 x 10(-3) in branching ratio.     

Double-passed arrayed waveguide grating for 500-GHz pulse burst generation

500-GHz repetition rate pulse bursts on multiple wavelength shifted channels are generated via femtosecond readout of an arrayed waveguide grating (AWG). These bursts are combined onto a single channel by double-passing the AWG, resulting in a substantially increased number of output pulses.