Optical properties of gas-phase tryptophan-silver cations: charge transfer from the indole ring to the silver atom.

We present a joint experimental and theoretical investigation of the electronic excitation spectra of the tryptophan-silver complex. The photodissociation spectrum of gas-phase [Trp-Ag]+ was measured from 215 to 330 nm using a quadrupole ion trap coupled to an optical parametric-oscillator laser. The calculated time-dependent density functional theory (TD-DFT) absorption spectra for different prototypes of structures are presented. Low-energy transitions that are experimentally observed are only calculated for the charge-solvation (CS) structures. These transitions are a signature of the metal-pi interaction in [Trp-Ag]+. The recorded spectrum is compared to a Boltzmann average of the absorption spectrum obtained from direct molecular dynamics (MD) simulations involving simultaneous transitions to excited states based on semiempirical configuration interaction (CI) calculations. The results demonstrate that charge transfer can be photoinduced from the indole ring to the silver atom.     

Photochemicalsyn-anti isomerization of a Schiff base: A two-dimensional description of a conical intersection in formaldimine

The shape of theS ₀,S ₁, andT ₁ potential energy surfaces of formaldimine, CH₂=NH, is explored in the two-dimensional subspace defined by the twisting and linear inversion motions which correspond to the geometricalsyn-anti isomerization, using anab initio large-scale CI method. Minima in theS ₁ andT ₁ surfaces as well as aS ₀-S ₁ conical intersection are identified and the photoisomerization mechanism is discussed.     

Simultaneous determination of the novel thiosemicarbazone anti‐cancer agent,Bp4eT,and its main phase I metabolites in plasma: Application to a pilot pharmacokinetic study in rats

Novel thiosemicarbazone metal chelators are extensively studied anti‐cancer agents with marked and selective activity against a wide variety of cancer cells, as well as human tumor xenografts in mice. This study describes the first validated LC‐MS/MS method for the simultaneous quantification of 2‐benzoylpyridine 4‐ethyl‐3‐thiosemicarbazone (Bp4eT) and its main metabolites (E/Z isomers of the semicarbazone structure, M1‐E and M1‐Z, and the amidrazone metabolite, M2) in plasma. Separation was achieved using a C₁₈ column with ammonium formate/acetonitrile mixture as the mobile phase. Plasma samples were treated using solid‐phase extraction on 96‐well plates. This method was validated over the concentration range of 0.18–2.80 μM for Bp4eT, 0.02–0.37 μM for both M1‐E and M1‐Z, and 0.10–1.60 μM for M2. This methodology was applied to the analysis of samples from in vivo experiments, allowing for the concentration–time profile to be simultaneously assessed for the parent drug and its metabolites. The current study addresses the lack of knowledge regarding the quantitative analysis of thiosemicarbazone anti‐cancer drugs and their metabolites in plasma and provides the first pharmacokinetic data on a lead compound of this class. Copyright © 2013 John Wiley & Sons, Ltd.     

Spectroscopy of isolated, mass-selected tryptophan-Ag3 complexes: a model for photoabsorption enhancement in nanoparticle-biomolecule hybrid systems

We wish to show that gas-phase studies of small metal cluster-biomolecule complexes provide fundamental insights into the mechanism leading to enhanced optical absorption in nanoparticle-biomolecular hybrid systems. Here we present, for the first time, a joint experimental and theoretical study of photoabsorption and photofragmentation of the silver trimer-tryptophan cation complex ([Trp.Ag3]+). We demonstrate that binding the metal cluster to a biomolecule leads to a remarkably high optical absorption as compared to the bare tryptophan or the [Trp.Ag]+ complex. As calculations show this arises due to coupling between the excitations in the metallic subunit with a charge transfer excitation to the tryptophan molecule.     

Reactivity of atomic gold anions toward oxygen and the oxidation of CO: experiment and theory

Results for the binding of carbon monoxide and oxygen along with the oxidation of CO in the presence of atomic Au(-) have been obtained utilizing a fast-flow reactor mass spectrometer. In addition, density functional calculations have been performed to explain the experimental findings. It was observed that upon oxygen addition to the metal plasma, gold oxide species of the form AuO(n)(-), where n = 1-3, were produced. The addition of carbon monoxide to the preoxidized gold atom revealed that AuO(-) and AuO(3)(-) promote the oxidation of CO. Density functional calculations on structures and their energetics confirmed the experimental findings and allowed us to propose mechanisms for the oxidation of carbon monoxide. The reactions of CO with AuO(1,3)(-) proceed via complex formation with CO bound to the oxygen atom, followed by either cleavage of the Au-O bond or complex rearrangement to form a weakly bound CO(2) unit, leading in both cases to the emanation of CO(2).     

The effects of saturation and velocity selective population in two-step 6S1/2 → 6P3/2 → 6D5/2 laser excitation in cesium

Excited states population distributions created by two-step 6S_1/2 → 6P_3/2 → 6D_5/2 laser excitation in room temperature cesium vapor were quantitatively analyzed applying absorption and saturation spectroscopy. A simple method for the determination of the excited state population in a single excitation step that is based on the measurements of the saturated and unsaturated absorption coefficients was proposed and tested. It was shown that only ≈ 2% of the ground state population could be transferred to the first excited state by pumping the Doppler broadened line with a single-mode narrow-line laser. With complete saturation of the second excitation step, the population amounting to only ≈ 1% of the ground state can be eventually created in the 6D_5/2 state. The fluorescence intensity emerging at 7P_3/2 → 6S_1/2 transition, subsequent to the radiative decay of 6D_5/2 population to the 7P_3/2 state, was used to assess the efficiency of the population transfer in the chosen two-step excitation scheme. The limitations imposed on the sensitivity of such resonance fluorescence detector caused by velocity-selective excitation in the first excitation step were pointed out and the way to overcome this obstacle is proposed.     

Response dynamics to pulsed perturbations of the hydrogen peroxide-thiosulfate-Cu2+ reaction

The hydrogen peroxide-thiosulfate-Cu2+ reaction operated in a continuous flow stirred tank reactor is a pH-oscillator known to provide three different steady states, hysteresis, and oscillations. In addition to the various dynamical regimes established earlier, a question arises whether the reaction can be also excitable, whereby the system strongly responds to a small but supercritical external addition of certain chemical species. We carried out experiments aimed at finding excitability and studying response dynamics to single and repeated pulsed perturbations of varying amplitude and period. We found that the reaction displays a remarkable excitatory dynamics when forced. The available mechanism of the reaction involves several adjustable parameters, which need to be tuned so that the model corresponds to experimentally observed bifurcation diagrams. Our experimental findings are compared with numerical calculations, suggesting that the model is far from complete.