Apparate zur Gasanalyse und zum Arbeiten mit Gasen

Ohne Zusammenfassung     

Secondary organic aerosol formation from limonene ozonolysis: homogeneous and heterogeneous influences as a function of NO(x)

Limonene has a high emission rate both from biogenic sources and from household solvents. Here we examine the limonene + ozone reaction as a source for secondary organic aerosol (SOA). Our data show that limonene has very high potential to form SOA and that NO(x) levels, O(3) levels, and UV radiation all influence SOA formation. High SOA formation is observed under conditions where both double bonds in limonene are oxidized, but those conditions depend strongly on NO(x). At low NO(x), heterogeneous oxidation of the terminal double bond follows the initial limonene ozonolysis (at the endocyclic double bond) almost immediately, making the initial reaction rate limiting. This requires a high uptake coefficient between ozone and the first-generation, unsaturated organic particles. However, at high NO(x), this heterogeneous processing is inhibited and gas-phase oxidation of the terminal double bond dominates. Although this chemistry is slower, it also yields products with low volatility. UV light suppresses production of the lowest volatility products, as we have shown in earlier studies of the alpha-pinene + ozone reaction.     

Catalytic Mechanism of a Heme and Tyrosyl Radical-Containing Fatty Acid α-(Di)oxygenase

The steady-state catalytic mechanism of a fatty acid α-(di)oxygenase is examined, revealing that a persistent tyrosyl radical (Tyr379(?)) effects O(2) insertion into C(α)-H bonds of fatty acids. The initiating C(α)-H homolysis step is characterized by apparent rate constants and deuterium kinetic isotope effects (KIEs) that increase hyperbolically upon raising the concentration of O(2). These results are consistent with H(?) tunneling, transitioning from a reversible to an irreversible regime. The limiting deuterium KIEs increase from ～30 to 120 as the fatty acid chain is shortened from that of the native substrate. In addition, activation barriers increase in a manner that reflects decreased fatty acid binding affinities. Anaerobic isotope exchange experiments provide compelling evidence that Tyr379(?) initiates catalysis by H(?) abstraction. C(α)-H homolysis is kinetically driven by O(2) trapping of the α-carbon radical and reduction of a putative peroxyl radical intermediate to a 2(R)-hydroperoxide product. These findings add to a body of work which establishes large-scale hydrogen tunneling in proteins. This particular example is novel because it involves a protein-derived amino acid radical.     

Über die Beeinflussung des dielektrischen Relaxationsspektrums durch Molekülorientierung

Ohne Zusammenfassung     

Stereochemistry of C-6 nucleophilic displacements on 1,1-difluorocyclopropyldibenzosuberanyl substrates. An improved synthesis of multidrug resistance modulator LY335979 trihydrochloride

Studies of the displacement chemistry of 1,1-difluorocyclopropyldibenzosuberanyl alcohol 4 and its activated bromide derivative 6 have led to an improved approach to anti-2, a key precursor to LY335979 3HCl (1). Bromination of either syn-4 or anti-4 gave anti-oriented 6, indicating thermodynamically controlled product stereochemistry via a stabilized 1,1-difluorohomotropylium ion intermediate. Reaction of 6 with piperazine proceeded irreversibly to provide an isomeric mixture of piperazine products, with the syn:anti product ratio increased by solvent effects. Reaction of 6 with pyridine and pyrazine, on the other hand, gave anti-pyridinium and pyrazinium salts, respectively, apparently via equilibration of initially formed syn products. Reduction of pyrazinium salt 11 with lithium borohydride/TFA provided anti-2 unaccompanied by its syn isomer. A practical and expeditious approach to 1 was derived from these new results.     

The preparation of selected benzopyrano[3,4-b]pyrazoles by the condensation of c(α), n-dilithiocarboalkoxyhydrazones and salicylate esters

Selected C(α),N-dilithiocarboalkoxyhydrazones were prepared in an excess of lithium diisopropylamide (LDA) and condensed with a variety of salicylate esters to give intermediates that were acid-cyclized to benzopyrano[3,4-b]pyrazoles (coumarin-pyrazoles).     

Increasing the imaging depth of coherent anti-Stokes Raman scattering microscopy with a miniature microscope objective

A miniature objective lens with a tip diameter of 1.3 mm was used for extending the penetration depth of coherent anti-Stokes Raman scattering (CARS) microscopy. Its axial and lateral focal widths were determined to be 11.4 and 0.86 microm, respectively, by two-photon excitation fluorescence imaging of 200 nm beads at a 735 nm excitation wavelength. By inserting the lens tip into a soft gel sample, CARS images of 2 microm polystyrene beads 5 mm deep from the surface were acquired. The miniature objective was applied to CARS imaging of rat spinal cord white matter with a minimal requirement for surgery.