Enzyme assays with boronic acid appended bipyridinium salts

In-vitro fluorescent enzyme assays have been developed for sucrose phosphorylase (SPO) and phosphoglucomutase (PGM). These assays make use of a selective carbohydrate sensing system that detects the unlabeled enzymatic products fructose and glucose-6-phosphate. The system comprises 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt as the reporter unit and boronic acid appended viologens as selective receptors with working ranges from 70 μM to 1.0 mM for fructose (SPO) and 190 μM to 2.0 mM for glucose-6-phosphate (PGM). The change in fluorescence can be converted into product concentration, allowing initial reaction velocities and Michaelis-Menten kinetics to be calculated. The assays are also carried out in multiwell plate formats, making them suitable for high-throughput screening of enzyme inhibitors. Rapid PGM inhibition screening is demonstrated with EDTA and LiCl. The PGM assay can also be used for enzyme quantification with a detection limit of 50 ng mL⁻¹.     

Concerted and stepwise proton-coupled electron transfers in aquo/hydroxo complex couples in water: oxidative electrochemistry of [Os(II)(bpy)(2)(py)(OH(2))](2+)

Successive oxidation of transition metal(II) aqua complexes (M(II)OH(2) to M(III)OH) is a domain in which proton-coupled electron transfer reactions are extremely common. The mechanism of these PCET reactions-concerted or stepwise-is an important issue in the understanding and design of natural or artificial systems catalyzing the formation of dioxygen by four-electron oxidation of water. Concerted proton-coupled electron transfer from an aqua metal(II) to a hydroxo metal(III) complex requires the close proximity of a proton-accepting group with a pK value between those of the aqua complexes. Otherwise, stepwise electron-proton or proton-electron pathways involving high-energy intermediates are followed. Concerted proton-electron pathways involving water as proton-acceptor or proton-donor group are inefficient. Cyclic voltammetry of the title complex in buffered aqueous solution and re-examination of previous results for the same complex attached to an electrode surface are used to establish these conclusions, which provide a starting point on the route to higher degrees of oxidation, such as those involved in the catalysis of water oxidation.     

Noise-immune cavity-enhanced optical heterodyne detection of HO2 in the near-infrared range

Accurate measurements of the absolute concentrations of radical species present in the atmosphere are invaluable for better understanding atmospheric processes and their impact on Earth systems. One of the most interesting species is HO(2), the hydroperoxyl radical, whose atmospheric daytime levels are on the order of 10 ppt and whose observation therefore requires very sensitive detection techniques. In this work, we demonstrate the first steps toward the application of external-cavity diode-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) to the detection of the HO(2) radical in the near-infrared range. Measurements of stable species and of HO(2) were made in a laboratory setting, and the possibilities of extending the sensitivity of the technique to atmospheric conditions are discussed.     

Eta' photoproduction on the proton for photon energies from 1.527 to 2.227 GeV

Differential cross sections for the reaction gamma p --> eta' p have been measured with the CLAS spectrometer and a tagged photon beam with energies from 1.527 to 2.227 GeV. The results reported here possess much greater accuracy than previous measurements. Analyses of these data suggest for the first time the coupling of the eta'N channel to both the S11(1535) and P11(1710) resonances, known to couple strongly to the etaN channel in photoproduction on the proton, and the importance of J = 3/2 resonances in the process.     

Exponential Rates of Convergence in the Ergodic Theorem: A Constructive Approach

We prove that, once an algorithm of perfect simulation for a stationary and ergodic random field F taking values in S^\mathbbZdS^{\mathbb{Z}^{d}}, S a bounded subset of R ⁿ , is provided, the speed of convergence in the mean ergodic theorem occurs exponentially fast for F. Applications from (non-equilibrium) statistical mechanics and interacting particle systems are presented.     

Time-resolved studies of single quantum dots in magnetic fields

We present time-resolved and time-integrated spectroscopy of single InAs quantum dots grown in a GaAs matrix. We observe a number of interesting features in the spectra, including the zero field splitting of exciton and biexciton lines due to quantum dot asymmetry. By the application of an in-plane magnetic field, the normally optically active and inactive exciton states become mixed, enabling us to optically probe the normally inaccessible ‘dark’ states. Time resolved measurements on the mixed states show decay times several times longer than the exciton lifetime at zero field, which we show to be consistent with a dark exciton lifetime orders of magnitude longer than that for bright exciton.