Optimized Biocatalytic Synthesis of 2-Selenopyrimidine Nucleosides by Transglycosylation**

Selenium-modified nucleosides are powerful tools to study the structure and function of nucleic acids and their protein interactions. The widespread application of 2-selenopyrimidine nucleosides is currently limited by low yields in established synthetic routes. Herein, we describe the optimization of the synthesis of 2-Se-uridine and 2-Se-thymidine derivatives by thermostable nucleoside phosphorylases in transglycosylation reactions using natural uridine or thymidine as sugar donors. Reactions were performed at 60 or 80 °C and at pH 9 under hypoxic conditions to improve the solubility and stability of the 2-Se-nucleobases in aqueous media. To optimize the conversion, the reaction equilibria in analytical transglycosylation reactions were studied. The equilibrium constants of phosphorolysis of the 2-Se-pyrimidines were between 5 and 10, and therefore differ by an order of magnitude from the equilibrium constants of any other known case. Hence, the thermodynamic properties of the target nucleosides are inherently unfavorable, and this complicates their synthesis significantly. A tenfold excess of sugar donor was needed to achieve 40−48 % conversion to the target nucleoside. Scale-up of the optimized conditions provided four Se-containing nucleosides in 6–40 % isolated yield, which compares favorably to established chemical routes.     

An Expedient Synthesis of Flexible Nucleosides through Enzymatic Glycosylation of Proximal and Distal Fleximer Bases

The structurally unique “fleximer” nucleosides were originally designed to investigate how flexibility in a nucleobase could potentially affect receptor–ligand recognition and function. Recently they have been shown to have low-to-sub-micromolar levels of activity against a number of viruses, including coronaviruses, filoviruses, and flaviviruses. However, the synthesis of distal fleximers in particular has thus far been quite tedious and low yielding. As a potential solution to this issue, a series of proximal fleximer bases (flex-bases) has been successfully coupled to both ribose and 2′-deoxyribose sugars by using the N-deoxyribosyltransferase II of Lactobacillus leichmannii (LlNDT) and Escherichia coli purine nucleoside phosphorylase (PNP). To explore the range of this facile approach, transglycosylation experiments on a thieno-expanded tricyclic heterocyclic base, as well as several distal and proximal flex-bases were performed to determine whether the corresponding fleximer nucleosides could be obtained in this fashion, thus potentially significantly shortening the route to these biologically significant compounds. The results of those studies are reported herein.     

核苷合成技术的研究进展

介绍了核苷的类别、用途。叙述了核苷合成技术进展，重点为酶法生物合成法；评述了各种方法的优缺点。认为微生物酶（细胞）合成核苷的前景乐观，该法反应简单、污染小、成本低。但合成核苷时要注意保持菌体的高活力和稳定性．并解决合成产物的分离纯化问题。     

Flow‐Synthesis of Nucleosides Catalyzed by an Immobilized Purine Nucleoside Phosphorylase from Aeromonas hydrophila: Integrated Systems of Reaction Control and Product Purification

A purine nucleoside phosphorylase from Aeromonas hydrophyla (AhPNP) was covalently immobilized in a pre‐packed stainless steel column containing aminopropylsilica particles via Schiff base chemistry upon glutaraldehyde activation. The resulting AhPNP‐IMER (Immobilized Enzyme Reactor, immobilization yield ≈50%) was coupled on‐line through a 6‐way switching valve to an HPLC apparatus containing an analytical or a semi‐preparative chromatographic column. The synthesis of five 6‐modified purine ribonucleosides was carried out by continuously pumping the reaction mixture through the AhPNP‐IMER until the highest conversion was reached, and then directing the reaction mixture to chromatographic separation. The conditions of the AhPNP‐catalyzed transglycosylations (2:1 ratio sugar donor:base acceptor; 10 mM phosphate buffer; pH 7.5; temperature 37 °C, flow rate 0.5 mL min⁻¹) were optimized by a fractional factorial experimental design. Coupling the bioconversion step with the product purification in such an integrated platform resulted in a fast and efficient synthetic process (yield=52–89%; <10 mg) where sample handling was minimized. To date, AhPNP‐IMER has retained completely its activity upon 50 reactions in 10 months.

     

Calibration and Uncertainty Analysis of an Optical Emission Spectrometer Measuring the Absolute Spectral Radiant Exitance of UV Signatures

An optical emission spectrometer (OES) was designed and constructed to detect the absolute spectral radiant exitance of UV signatures for a wavelength range of 240 nm to 440 nm. Using a directly heated graphite black body furnace operating at a temperature of 3000 K, the spectral response of the OES in the UV range was calibrated. To evaluate the performance of the OES system, the emission spectra of molecules were measured in a butane flame and the absolute emissions of the UV signatures were calculated. The uncertainty of the absolute measurement of the UV signature was analyzed by considering various uncertainty budgets, and the total uncertainty of the OES was found to be 1.66 %.     

Synthesis and Electronic Spectra of Substituted p‐Distyrylbenzenes for the Use in Light‐Emitting Diodes

The influence of substitution on the absorption and Luminescence spectra of oligo(phenylenevinylene)s has been studied using distyrylbenzene (DSB) as a model compound. The degree, character, and pattern of substitution was varied systematically, altering the electronic properties of the DSB, the wavelength of the emitted light could be tuned over a range of 100 nm. The syntheses of 6b—h were performed by twofold Wittig Horner‐olefinations of bisphoshonates 1a, b with substituted benzaldehydes 2a—i, 6i via Heck‐reaction of the dibromosulfonylbenzene 3, 6k by Siegrist‐reaction of 4 with N‐phenylbenzaldimine and the Knoevenagel‐reaction of benzyl cyanide with 5 led to 6l .     

Metal Fluorocarbon Pyrolants,XVI : Theoretical and Experimental Investigation of Poly[bis(2,2,2‐trifluoroethoxy)phosphazene] (PTFEP) as Oxidizer in Magnesium Based Pyrolants

Consolidated magnesium/poly[bis(2,2,2‐trifluoroethoxy)phosphazene] (MPZ) pyrolants display a stable combustion regime between ξ(Mg) 30–83 wt‐%. The combustion rate increases with increasing magnesium content. Mixtures containing between 15–29 wt‐% Mg do ignite with a butane torch but extinguish rapidly thereafter. The combustion flames for each stoichiometric range exhibit particular features such as luminous carbon particles, after burn luminescence of Mg and burning Mg‐rich ejecta. UV/Vis spectra of the combustion flames show a distinct continuum superimposed from signals for transient CF₂, Mg, MgF, MgO, P₂⁺, PH, PN, and PO. The thermochemical calculation using NASA‐CEA indicate the highest adiabatic temperatures 1889 K at ξ(Mg)=35 wt‐%. The aerobic combustion of MPZ yields similar temperatures as common magnesium/polytetrafluoroethylene pyrolants.     

Synthesis of Reactive Condensation Products of Acetylacetone and Their Transformation into Deeply Coloured Methine Dyes

Starting from acetylacetone 1 , boric acid and a diol, or a 2‐aminothiazole 16 and perchloric acid several dimethyl‐substituted 1,3,2‐dioxaborines 5 and thiazolo[3,2‐a]pyrimidinium salts 17 , resp., have been prepared and transformed by raction with 4‐N,N‐dimethylamino‐benzaldehyde 3a or 3‐methyl‐2‐methylmercapto‐benzthiazolium perchlorate 13 into deeply colored polymethine dyes 11, 12, 14, 15, and 18—21 , and 23—24 their spectroscopic data were recorded.     

Preparation and UV/Vis Spectroscopic Characterization of N,N‐Disubstituted 2‐Amino‐5‐arylazoselenazoles and Some of Their Carbocyclic and Heterocyclic Analogues

By coupling of aryldiazonium salts 5 with N,N‐disubstituted 2‐amino‐5H‐selenazoles 4 deeply coloured 5‐arylazo‐substituted 2‐amino‐selenazoles 6a—6l have been prepared and their solvatochromic properties determined by means of UV/Vis spectroscopy, and compared with the ones of several other arylazosubstituted N,N‐dialkylanilines 7a—7d , 2‐(N,N‐dialkylamino)‐thiophenes 8a—8e , and 2‐(N,N‐dialkylamino)‐thiazoles 9a—9c .     

Study of Photoluminescence Excitation Spectra of Tris(8-hydroxyquinoline)aluminum(III) (Alq3) in Solutions and Films

Absorption, photoluminescence, and photoluminescence excitation (PLE) spectra have been investigated for tris(8-hydroxyquinoline)aluminum(III) (Alq₃) in solution at various Alq₃ concentrations and in doped and neat films. The redshifted PLE spectra, with respect to the absorption spectra, are obtained, together with a dip located near the absorption band peak. The dip becomes deeper with increasing concentration. It is found that the unusual PLE spectra are caused by the penetration depth effect, not by previously suggested microcrystallites of Alq₃.     

Efficient aerobic oxidation of hydrocarbons with O2 catalyzed by DDQ/NHPI

BACKGROUND: Organocatalysis, a promising strategy for the oxidation of organic compounds, does not involve the use of a catalytic metal. In this work, an efficient organocatalyst system consisting of 2,3‐dichloro‐5,6‐dicyano‐benzoquinone (DDQ) and N‐hydroxyphthalimide (NHPI) was studied. RESULTS: 72.2% conversion with 92.3% selectivity for acetophenone was obtained in ethylbenzene oxygenation catalyzed by DDQ/NHPI under 0.3 MPa of molecular oxygen at 80 °C for 10 h. In addition, other hydrocarbons were also oxidized with high efficiency using this catalyst system. UV/Vis spectroscopy of the catalytic system indicated that DDQ accelerated the generation of free radical phthalimido‐N‐oxyl (PINO) by abstracting a hydrogen atom from NHPI. CONCLUSION: An efficient organocatalyst system consisting of DDQ and NHPI for selective oxidation of hydrocarbons to corresponding ketones with molecular oxygen as oxidant is reported. DDQ promoted the generation of PINO from NHPI, and the oxidation reaction was accelerated via PINO. This organocatalyst system should be useful for the design of highly selective catalysts for hydrocarbon oxidation. Copyright © 2010 Society of Chemical Industry     

Effect of Methyl Group on Acyclic Serinol Scaffold for Tethering Dyes on the DNA Duplex Stability

Acyclic serinol derivatives are useful scaffolds for tethering dyes within DNA duplexes. Here we synthesised an inverse l ‐threoninol (il ‐threoninol) scaffold and compared its effect on DNA duplex stability to other acyclic artificial nucleic acid scaffolds that are based on d ‐threoninol, l ‐threoninol, and serinol. When planar trans‐azobenzene was incorporated into the DNA duplex through a single bulge‐like motif (the wedge), the il ‐threoninol scaffold stabilised the duplex most efficiently. When scaffolds were incorporated in complementary positions (dimer motif) or in three adjacent positions (cluster motif), d ‐threoninol was the most stabilising. CD spectra indicated that the effect of scaffold on the duplex stability was closely related to the winding induced by each scaffold. When trans‐azobenzene was photo‐isomerised to non‐planar cis‐azobenzene, il ‐threoninol destabilised the duplex most strongly, irrespective of the number of artificial residues incorporated. The properties of the il ‐threoninol scaffold make it a useful tether for dyes or other functionalities.     

Exploring the Protein Folding Dynamics of Beta3s with Two-Dimensional Ultraviolet (2DUV) Spectroscopy

Ultraviolet (UV) spectra of proteins originate from electronic excitations of their backbone chromophore and aromatic side chains and provide a sensitive probe of the secondary structure. Recently developed femtosecond lasers allow multidimensional spectroscopy to be extended into the UV regime. Two-dimensional UV (2DUV) techniques, with short pulses, provide a promising tool to study the structures and dynamics of proteins. We combined 2DUV spectroscopy and molecular dynamics generated free energy profiles to simulate the protein electronic transitions and UV photon echo signals to monitor the protein folding process of the small protein Beta3s. Near-ultraviolet (NUV) and far-ultraviolet (FUV) signals illustrate the variation of the 2D correlation plots when the protein evolves along the underlying free energy landscape. Chiral polarization configurations of the NUV and FUV pulses are sensitive to protein structural evolution. This work provides a protocol for applying multidimensional UV spectroscopy to study protein folding.     

New Halogenated Water‐Soluble Chlorin and Bacteriochlorin as Photostable PDT Sensitizers: Synthesis,Spectroscopy, Photophysics,and in vitro Photosensitizing Efficacy

Chlorin and bacteriochlorin derivatives of 5,10,15,20‐tetrakis(2‐chloro‐5‐sulfophenyl)porphyrin have intense absorptions in the phototherapeutic window, high water solubility, high photostability, low fluorescence quantum yield, long triplet lifetimes, and high singlet oxygen quantum yields. Biological studies revealed their negligible dark cytotoxicity, yet significant photodynamic effect against A549 (human lung adenocarcinoma), MCF7 (human breast carcinoma) and SK‐MEL‐188 (human melanoma) cell lines upon red light irradiation (cutoff λ<600 nm) at low light doses. Time‐dependent cellular accumulation of the chlorinated sulfonated chlorin reached a plateau at 2 h, as previously observed for the related porphyrin. However, the optimal incubation time for the bacteriochlorin derivative was significantly longer (12 h). The spectroscopic, photophysical, and biological properties of the compounds are discussed in relevance to their PDT activity, leading to the conclusion that the bacteriochlorin derivative is a promising candidate for future in vivo experiments.     

Combustion of Ytterbium Metal

Ytterbium metal powder burns with a luminous vapor phase diffusion flame in oxygen. Consolidated stoichiometric mixtures with both polytetrafluoroethylene and hexachloroethane upon ignition yield intense luminous flames. With UV/Vis spectroscopy of the combustion flames Yb, YbO, YbCl, and YbF species have been identified contributing to the selective emission in the green spectral range. The flame temperatures of ytterbium combustion in oxygen and with halocarbon based oxidizers are in the same range as for comparable magnesium based systems.     

Photovoltaic performance and impedance spectroscopy of ZnS-Cu-Go nanocomposites

In this work, novel, non-toxic and cost effective ZnS-Cu-GO nanocomposite is synthesized via wet chemical route to study its photovoltaic properties. Three samples including ZnS,ZnS-Cu and ZnS-Cu-GO were prepared and deposited as sensitizers on ZnO coated FTO substrates to assemble PV devices.The samples were characterized using UV–Vis NIR spectroscopy, Atomic force microscopy (AFM).Electrochemical impedance spectroscopy (EIS) and AM 1.5 Sun Simulator. It was observed that ZnS-Cu-GO exhibitedsuperiorchargetransport, remarkably high open circuit voltage(0.8 V) and Fill factor (0.806).The current density significantly enhanced and maximum solar cell efficiency was observed for ZnS-Cu-GO based PV device. A pronounced red shift of 360 nm in the absorption spectra was observed in the ZnS-Cu-GO due to fine dispersion of GO sheets.The AFM analysis showed that incorporation of GO and Cu maximized grain density and trench like grain boundaries in ZnS-Cu-GO which facilitated charge transport mechanism.A detailed electrochemical impedance study to probe charge dynamics in the prepared PV devices is presented herein.