Reactivity of Diarylnitrenium Ions

Hydride abstraction from diarylamines with the trityl ion is explored in an attempt to generate a stable diarylnitrenium ion, Ar₂N⁺. Sequential H-atom abstraction reactions ensue. The first H-atom abstraction leads to intensely colored aminium radical cations, Ar₂NH^.+, some of which are quite stable. However, most undergo a second H-atom abstraction leading to ammonium ions, Ar₂NH₂⁺. In the absence of a ready source of H-atoms, a unique self-abstraction reaction occurs when Ar=Me₅C₆, leading to a novel iminium radical cation, Ar=N^.+Ar, which decays via a second self H-atom abstraction reaction to give a stable iminium ion, Ar=N⁺HAr. These products differ substantially from those derived via photochemically produced diarylnitrenium ions.     

HCl and DCl: a case study of different approaches for determining photo fractionation constants

The photoabsorption cross sections of HCl and DCl are calculated using the reflection principle and time dependent wavepacket propagation methods. The absorption cross sections are compared to high precision experimental absorption cross sections from the literature and the different results given by the methods are discussed. The results of the calculations emphasize the important roles that photodissociation dynamics and the change in transition dipole moment with internuclear distance play in isotopic fractionation. The wave number dependent fractionation constants have been determined. The process fractionation constant has been calculated in the Venusian atmosphere where photo-fractionation leads to enrichment in deuterium through loss of hydrogen to space. At an altitude of 70 km the process fractionation constant was found to be epsilon(p) = -344 per thousand and epsilon(p) = -256 per thousand for the experimental and the reflection principle methods, respectively. At the top of the atmosphere the process fractionation constant was evaluated to be epsilon(p) = -32 per thousand, epsilon(p) = -20 per thousand and epsilon(p) = -40 per thousand using the experimental data, the wavepacket and the reflection principle methods, respectively. Using the Rayleigh distillation formula it is concluded that HCl at the top of the Venusian atmosphere is fractionated (enriched in D) relative to the bulk composition prior to photolysis.     

Spatial distribution of stabilizer-derived nitroxide radicals during thermal degradation of poly(acrylonitrile-butadiene-styrene) copolymers: a unified picture from pulsed ELDOR and ESR imaging

Double Electron-Electron Resonance (DEER) provides information on the spatial distribution of radicals on the length scale of a few nanometres, while Electron Spin Resonance Imaging (ESRI) provides information on a length scale of millimetres with a resolution of about 100 micrometres. Despite the gap between these length scales, results from the two techniques are found to complement and support each other in the characterization of the identity and distribution of nitroxide radicals derived from the Hindered Amine Stabilizer (HAS) Tinuvin 770 in poly(acrylonitrile-butadiene-styrene) (ABS) copolymers. DEER measurements demonstrate that there is no significant formation of biradicals from the bifunctional HAS, and provide the distributions of local radical concentrations. These distributions are poorly resolved for model-free analysis of the DEER data by the Tikhonov regularization; the resolution was significantly improved by utilizing information obtained by ESRI. DEER data can be fitted with only one adjustable parameter, namely the average radical concentration, if 1D and 2D spectral--spatial ESRI results on both the spatial distribution of nitroxides and their distribution between the acrylonitrile--styrene-rich (SAN) and butadiene-rich (B) microphases are considered.     

A designed branched three-helix bundle protein dimer

The ultimate goals of de novo protein design are the construction of novel tertiary structures and functions. Here is presented the design and synthesis of a uniquely branched three-helix bundle that folds into a well-folded dimeric protein. The branching of this protein was performed by the method of native chemical ligation, which provides a chemoselective and stable amide bond between the unprotected fragments. This ligation strategy was possible by the presented facile preparation of a peptide (43 amino acids) with a specific side chain thioester, which is synthesized by general Fmoc solid phase peptide synthesis. From the presented structural analysis, it is seen that the folded protein is present as a stable and highly helical dimer, thus forming a six-helix bundle. This unique tertiary structure, composed of a dimer of three individual alpha-helices branched together, offers different possibilities for protein engineering, such as metal and cofactor binding sites, as well as for the construction of novel functions.     

Photo-CIDNP MAS NMR in intact cells of Rhodobacter sphaeroides R26: molecular and atomic resolution at nanomolar concentration

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed in photosynthetic reaction centers of the carotenoid-less strain R26 of the purple bacterium Rhodobacter sphaeroides by (13)C solid-state NMR at three different magnetic fields (4.7, 9.4, and 17.6 T). The signals of the donor appear enhanced absorptive (positive) and of the acceptor emissive (negative). This spectral feature is in contrast to photo-CIDNP data of reactions centers of Rhodobacter sphaeroides wildtype reported previously (Prakash, S.; Alia; Gast, P.; de Groot, H. J. M.; Jeschke, G.; Matysik, J. J. Am. Chem. Soc. 2005, 127, 14290-14298) in which all signals appear emissive. The difference is due to an additional mechanism occurring in RCs of R26 in the long-living triplet state of the donor, allowing for spectral editing by different enhancement mechanisms. The overall shape of the spectra remains independent of the magnetic field. The strongest enhancement is observed at 4.7 T, enabling the observation of photo-CIDNP enhanced NMR signals from reaction center cofactors in entire bacterial cells allowing for detection of subtle changes in the electronic structure at nanomolar concentration of the donor cofactor. Therefore, we establish in this paper photo-CIDNP MAS NMR as a method to study the electronic structure of photosynthetic cofactors at the molecular and atomic resolution as well as at cellular concentrations.     

Ionic-charge dependence of the intermolecular coulombic decay time scale for aqueous ions probed by the core-hole clock

Auger electron spectroscopy combined with theoretical calculations has been applied to investigate the decay of the Ca 2p core hole of aqueous Ca(2+). Beyond the localized two-hole final states on the calcium ion, originating from a normal Auger process, we have further identified the final states delocalized between the calcium ion and its water surroundings and produced by core level intermolecular Coulombic decay (ICD) processes. By applying the core-hole clock method, the time scale of the core level ICD was determined to be 33 ± 1 fs for the 2p core hole of the aqueous Ca(2+). The comparison of this time constant to those associated with the aqueous K(+), Na(+), Mg(2+), and Al(3+) ions reveals differences of 1 and up to 2 orders of magnitude. Such large variations in the characteristic time scales of the core level ICD processes is qualitatively explained by different internal decay mechanisms in different ions as well as by different ion-solvent distances and interactions.     

The effect of carbon dioxide on xylose fermentation byPichia stipitis

Carbon dioxide was found to reduce the xylose fermentation rate of two different strains ofPichia stipitis (CBS 5773 and CBS 5776) significantly in anaerobic batch fermentations. The maximum specific fermentation rate in a carbon dioxide atmosphere was about 45% lower than the fermentation rate in nitrogen atmosphere. Further-more, the fermentation rate was found to be correlated to the growth rate. It is probable that the carbon dioxide influences the fermentation rate partly through decreasing the growth rate. It was also found that the fermentation rate of CBS 5773 was higher than for CBS 5776 and that the specific fermentation rate was lower at a higher cell density.     

On the origins of core-electron chemical shifts of small biomolecules in aqueous solution: insights from photoemission and ab initio calculations of glycine(aq)

The local electronic structure of glycine in neutral, basic, and acidic aqueous solution is studied experimentally by X-ray photoelectron spectroscopy and theoretically by molecular dynamics simulations accompanied by first-principle electronic structure and spectrum calculations. Measured and computed nitrogen and carbon 1s binding energies are assigned to different local atomic environments, which are shown to be sensitive to the protonation/deprotonation of the amino and carboxyl functional groups at different pH values. We report the first accurate computation of core-level chemical shifts of an aqueous solute in various protonation states and explicitly show how the distributions of photoelectron binding energies (core-level peak widths) are related to the details of the hydrogen bond configurations, i.e. the geometries of the water solvation shell and the associated electronic screening. The comparison between the experiments and calculations further enables the separation of protonation-induced (covalent) and solvent-induced (electrostatic) screening contributions to the chemical shifts in the aqueous phase. The present core-level line shape analysis facilitates an accurate interpretation of photoelectron spectra from larger biomolecular solutes than glycine.     

Collection and Determination of Mercury in Air

A method has been developed for the determination of low concentrations of mercury in air (nanograms/m³), i.e. in the range of the believed natural levels of mercury in the atmosphere (20 ng/m³). Mercury vapour has been collected from up to 200 1 of air in glass tubes containing thin films of gold on sieved ceramic powder. In the laboratory the absorbed mercury was then released into a quartz-window cell by heating the tube in an oven at 500°C.

In this paper it is demonstrated that, by using extremely thin films of precipitated gold, quantitative recovery is obtained and memory effects, which result from the use of thicker films, are avoided.