Light-enhanced catalysis by pyridoxal phosphate-dependent aspartate aminotransferase

The mechanisms of pyridoxal 5'-phosphate (PLP)-dependent enzymes require substrates to form covalent "external aldimine" intermediates, which absorb light strongly between 410 and 430 nm. Aspartate aminotransferase (AAT) is a prototypical PLP-dependent enzyme that catalyzes the reversible interconversion of aspartate and α-ketoglutarate with oxalacetate and glutamate. From kinetic isotope effects studies, it is known that deprotonation of the aspartate external aldimine C(α)-H bond to give a carbanionic quinonoid intermediate is partially rate limiting in the thermal AAT reaction. We show that excitation of the 430-nm external aldimine absorption band increases the steady-state catalytic activity of AAT, which is attributed to the photoenhancement of C(α)-H deprotonation on the basis of studies with Schiff bases in solution. Blue light (250 mW) illumination gives an observed 2.3-fold rate enhancement for WT AAT activity, a 530-fold enhancement for the inactive K258A mutant, and a 58600-fold enhancement for the PLP-Asp Schiff base in water. These different levels of enhancement correlate with the intrinsic reactivities of the C(α)-H bond in the different environments, with the less reactive Schiff bases exhibiting greater enhancement. Time-resolved spectroscopy, ranging from femtoseconds to minutes, was used to investigate the nature of the photoactivation of C(α)-H bond cleavage in PLP-amino acid Schiff bases both in water and bound to AAT. Unlike the thermal pathway, the photoactivation pathway involves a triplet state with a C(α)-H pK(a) that is estimated to be between 11 and 19 units lower than the ground state for the PLP-Val Schiff base in water.     

Generation and noise analysis of a wide-band optical -frequency comb based on recirculating frequency shifter

By optimizing the gain configuration and length of the loop, a 90-tone optical frequency comb （OFC） is successfully generated based on recirculating frequency shifter structure. The peak-to-peak power fluctuation of the 90-tone OFC is 4.26 dB and the tone-to-noise ratio is higher than 19.17 dB. To further analyze the noise accumulation feature of the tones when travelling around the loop, linewidth of the tones is measured by delayed self-heterodyne interferometer structure. The result shows the linewidth of the tones deteriorates little during the recirculating process, indicating that the generated OFC is an ideal multi-wavelength source for high-speed communication svstems.     

Compact and high-energy diode-side-pumped Q-switched Nd:YAG slab laser system for space application

We develop a compact and high-energy Nd：YAG slab laser system consisting of an oscillator and an amplifier for space applications. The oscillator is a diode-side-pumped electro-optically Q-switched slab laser with a cross-Porro resonator. The KD＊P Pockels cell with a low driving voltage of 950 V is used to polarization output coupling. The amplifier is a Nd：YAG zigzag slab pumped at bounces. The maximum output pulse energy of 341 m3 with 13 ns pulse duration is obtained from the system at the repetition rate of 20 Hz and the beam quality factors are M2=3.1 and M2=3.5. The beam pointing stabilities of the laser system are 3.05μrad in the X-direction and 3.99 grad in the Y-direction, respectively.     

Optimization of Direct Aromatic 18F-Labeling of Tetrazines

Radiolabeling of tetrazines has gained increasing attention due to their important role in pretargeted imaging or therapy. The most commonly used radionuclide in PET imaging is fluorine-18. For this reason, we have recently developed a method which enables the direct aromatic ¹⁸F-fluorination of tetrazines using stannane precursors through copper-mediated fluorinations. Herein, we further optimized this labeling procedure. 3-(3-fluorophenyl)-1,2,4,5-tetrazine was chosen for this purpose because of its high reactivity and respective limited stability during the labeling process. By optimizing parameters such as elution conditions, precursor amount, catalyst, time or temperature, the radiochemical yield (RCY) could be increased by approximately 30%. These conditions were then applied to optimize the RCY of a recently successfully developed and promising pretargeting imaging agent. This agent could be isolated in a decay corrected RCY of 14 ± 3% and Am of 201 ± 30 GBq/µmol in a synthesis time of 70 min. Consequently, the RCY increased by 27%.     

Deep SiC etching with RIE

SiC is currently an important topic in power devices. This new technology leads to lower power losses, faster switching, and higher working temperature. The design of SiC power devices requires the integration of edge termination techniques to obtain a high blocking voltage. The mesa structure approach is one well-established method. It could be used alone or in combination with a Junction Termination Extension (JTE). The mesa consists of a structure that removes material around the pn-junction. Due to the strong Si–C bonds, conventional chemical–wet etching solutions are inefficient on SiC, so plasma methods are required to etch SiC.The presented work is based on the use of an RIE reactor with an SF₆/ O₂ plasma. Its geometry structure and parameters were optimized. An etch rate of 0.35 μm/min was obtained without any trenching phenomenon. Trenches deeper than 10 μm deep were realized with a nickel etching mask that shows a high selectivity. AFM analysis revealed an etched surface as smooth as the initial one.     

The Influence of Secondary Interactions on the [N−I−N]+ Halogen Bond

[Bis(pyridine)iodine(I)]⁺ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three-center, four-electron [N−I−N]⁺ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their ¹⁵N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N−I−N]⁺ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron-rich moiety close by the [N−I−N]⁺ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations.     

Reduction of FeO/Pt(1 1 1) thin films by exposure to atomic hydrogen

Using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS) and density functional theory (DFT) calculations we have studied the reduction of ultra-thin films of FeO(1 1 1) grown on Pt(1 1 1) after exposure to atomic hydrogen at room temperature. A number of new ordered, partly-reduced FeO_x structures are identified and as a general trend we reveal that all the reduced FeO_x structures incorporate 2-fold coordinated Fe atoms as opposed to the original 3-fold coordinated Fe atoms in the FeO film. We find that when all the Fe atoms are 2-fold O-coordinated the FeO_x surface structure is resistant to further reduction at room temperature. We observe that water easily dissociates on the most heavily reduced FeO_x, structure in contrast to the initially inert FeO film, and reveal that it is possible to partially re-oxidize the FeO_x film by heating the surface slightly in the presence of water.     

Adsorbate-induced alloy phase separation: a direct view by high-pressure scanning tunneling microscopy

The influence of high pressures of carbon monoxide (CO) on the stability of a Au/Ni(111) surface alloy has been studied by high-pressure scanning tunneling microscopy. We show that CO induces a phase separation of the surface alloy at high pressures, and by means of time-lapsed STM movies we find that Ni atoms are removed from the surface layer during the process. Density functional theory calculations reveal the thermodynamic driving force for the phase separation to be the Au-induced compression of the CO overlayer with a resulting CO-CO repulsion. Furthermore, the atomistic mechanism of the process is shown to be kink-site carbonyl formation and evaporation which is found to be enhanced by the presence of Au.     

Oxidation of Pt(110)

Using scanning tunneling microscopy and temperature programmed desorption we investigate the Pt(110) surface under strongly oxidizing conditions involving either high-pressure O2 or atomic oxygen exposure. At low temperatures, only disordered Pt oxide structures are observed. After annealing ordered surface oxide islands are observed to coexist with a highly stable reconstructed (12x2)-O chemisorption structure. From density functional theory calculations a model for the surface oxide phase is revealed. The phase is found to be metastable, and its presence is explained in terms of stabilizing defects in the chemisorption layer and reduced Pt mobility.     

Bioactivities and chemical constituents of a Vietnamese medicinal plant Che Vang, Jasminum subtriplinerve Blume (Oleaceae)

Five crude extracts were made from leaves and stems of Jasminum subtriplinerve Blume (Oleaceae) and investigated for antimicrobial, antioxidant and cytotoxic activities. The extractions were done with petroleum ether, ethyl acetate, ethanol, methanol or water. All extracts exhibited anti-bacterial activity except the water fraction. On the other hand, all extracts exhibit antioxidant activity except the petroleum ether fraction using the DPPH radical scavenging assay. Only the petroleum ether fraction showed a cytotoxicity activity against tested cell-lines, Hep-G2 and RD with IC(50) values of 19.2 and 20 microg mL(-1), respectively. From the petroleum ether and ethyl acetate extracts, two triterpenes namely 3beta-acetyl-oleanolic acid and lup-20-en-3beta-ol and a sterol, stigmast-5-en-3beta-ol were isolated. The structure of those compounds were elucidated by spectrometric methods IR, MS, 1D-NMR, 2D-NMR and simulated ACD/NMR spectra. The data presented here indicate that J. subtriplinerve do contain compounds with interesting biological activity.