Competition between intersystem crossing and internal conversion in isolated large molecules

The excess energy and deuteration dependence of the radiationless decay rate in “isolated” aromatic hydrocarbons (anthracene, 9,10-dimethylanthracene, phenanthrene and fluorene) suggest that S₁→S₀ internal conversion dominates over S₁→T intersystem crossing for molecules with very large excess vibrational energies.     

Preparation of highly efficient manganese catalase mimics

The series of compounds [Mn(bpia)(mu-OAc)](2)(ClO(4))(2) (1), [Mn(2)(bpia)(2)(muO)(mu-OAc)](ClO(4))(3).CH(3)CN (2), [Mn(bpia)(mu-O)](2)(ClO(4))(2)(PF(6)).2CH(3)CN (3), [Mn(bpia)(Cl)(2)](ClO)(4) (4), and [(Mn(bpia)(Cl))(2)(mu-O)](ClO(4))(2).2CH(3)CN (5) (bpia = bis(picolyl)(N-methylimidazol-2-yl)amine) represents a structural, spectroscopic, and functional model system for manganese catalases. Compounds 3 and 5 have been synthesized from 2 via bulk electrolysis and ligand exchange, respectively. All complexes have been structurally characterized by X-ray crystallography and by UV-vis and EPR spectroscopies. The different bridging ligands including the rare mono-mu-oxo and mono-mu-oxo-mono-mu-carboxylato motifs lead to a variation of the Mn-Mn separation across the four binuclear compounds of 1.50 A (Mn(2)(II,II) = 4.128 A, Mn(2)(III,III) = 3.5326 and 3.2533 A, Mn(2)(III,IV) = 2.624 A). Complexes 1, 2, and 3 are mimics for the Mn(2)(II,II), the Mn(2)(III,III), and the Mn(2)(III,IV) oxidation states of the native enzyme. UV-vis spectra of these compounds show similarities to those of the corresponding oxidation states of manganese catalase from Thermus thermophilus and Lactobacillus plantarum. Compound 2 exhibits a rare example of a Jahn-Teller compression. While complexes 1 and 3 are efficient catalysts for the disproportionation of hydrogen peroxide and contain an N(4)O(2) donor set, 4 and 5 show no catalase activity. These complexes have an N(4)Cl(2) and N(4)OCl donor set, respectively, and serve as mimics for halide inhibited manganese catalases. Cyclovoltammetric data show that the substitution of oxygen donor atoms with chloride causes a shift of redox potentials to more positive values. To our knowledge, complex 1 is the most efficient binuclear functional manganese catalase mimic exhibiting saturation kinetics to date.     

On-column preconcentration and separation of DNA fragments using polymer solutions in the presence of electroosmotic flow

We demonstrated DNA preconcentration and separation in the presence of electroosmotic flow (EOF) using poly(ethylene oxide) (PEO) solutions. After injecting large volumes of DNA samples into a capillary filled with free tris(hydroxymethyl)aminomethane (Tris)-borate (TB) buffers, PEO solutions entered the capillary by EOF and acted as sieving matrices. In contrast to conventional methods (in the absence of EOF), controlling the EOF was also useful for resolution optimization. We have found that PEO adsorption on the capillary wall was more pronounced when low ionic strength buffers were used. Thus, the EOF decreased with increasing injection length, which led to longer migration times and changes in resolution and stacking efficiency. All resolution values were higher than 1.5 when 1.0 microg/mL DNA samples were injected at 240 V/cm for 60 s (0.67 microL). In addition, as low as 0.015 microg/mL DNA samples (an about 66-fold increase in sensitivity) were detected when the injection was performed at 250 V/cm for 60 s.     

Kinetic study of dichlorocyclopropanation of 4-vinyl-1-cyclohexene by a novel multisite phase transfer catalyst

In this work, the dichlorocyclopropanation of 4-vinyl-1-cyclohexene catalyzed by a new novel phase transfer catalyst was carried out in an alkaline solution/chloroform two-phase medium. This new synthesized phase transfer catalyst, 1,4-bis(triethylmethylammonium)benzene dichloride (DC-X), which possesses two-site activity, was obtained from the reaction of ,′-dichloro-p-xylene and triethylamine. This new novel phase transfer catalyst exhibits higher reactivity than those of the other quaternary ammonium salts. The reaction of chloroform and alkali was carried out at the interface to generate dichlorocarbene which can further react with 4-vinyl-1-cyclohexene to produce mono-dichlorocyclopropane and bis-dichlorocyclopropane products. Rational mechanism of the dichlorocyclopropanation is proposed according to the experimental evidence. The reaction follows a pseudo-first-order rate law. Kinetics of the reactions such as: effect of the catalysts, agitation speed, temperature, alkaline concentration, amount of DC-X catalyst, amount of 4-vinyl-1-cyclohexene (reactant) and volume of chloroform (organic solvent) on the reaction rate were investigated in detail. Peculiar phenomenon for the dependence of the reaction rate on the amount of DC-X catalyst is explained satisfactorily.     

Trace Analysis of Heavy Metals with Two New Disodium Bisdithiocarbamates

Two chelating reagents, disodium N,N′-dibenzylethylenebisdithiocarbamate 1 and disodium piperazinebisdithiocarbamate 2, were synthesized and used to preconcentrate trace metals in aqueous samples. For analysis of Cu(II) using a UV-vis spectrometer, Beer's law was obeyed from 5.0 μg L^?1 to 6.0 mg L^?1 for reagent 1, and from 0.2 mg L^?1 to 6.0mg L^?1 for reagent. 2. The chelation ratio for reagent 1 to Cu(II) was determined to be 1:1, with a formation constant 1.0 × 10⁹ M^?l. The dependence of extraction and extraction efficiency of reagent 1 on pH was also studied with an atomic absorption spectrometer for nine heavy-metal ions-Cu(II), Fe(III), Pb(II), Co(II), Cr(VI), Ni(II), Zn(II), Mn(II) and Cd(II). Except Cr(VI) and Mn(II), the recovery yields of the other seven metal ions were almost quantitative at pH = 4 ? 6. The recovery was 82% for Cr(VI) at pH = 4 ? 5, and 52% for Mn(II) at pH = 6 ? 7.     

Selective Inhibition of Collagen‐Induced Platelet Aggregation by a Cyclic Peptide from Drymaria diandra

Four cyclic peptides, diandrine A–D ( 1 – 4 ), were isolated from the MeOH extract of Formosan Drymaria diandra. Their structures were elucidated by chemical and spectroscopic analyses as cyclo(‐Gly¹‐Pro²‐Trp³‐Pro⁴‐Tyr⁵‐Phe⁶‐), cyclo(‐Gly¹‐Pro²‐Leu³‐Pro⁴‐Leu⁵‐Trp⁶‐Ser⁷‐Ser⁸‐), cyclo(Gly¹‐Gly²‐Pro³‐Tyr⁴‐Trp⁵‐Pro⁶‐), and cyclo(Gly¹‐Gly²‐Pro³‐Tyr⁴‐Trp⁵‐Pro⁶‐), respectively. Compounds 3 and 4 were stable conformational isomers. Cyclopeptide 1 showed a selective inhibitory effect on collagen‐induced platelet aggregation with an IC₅₀ value of 44.2 μM .     

Chemical Constituents from Annona Glabra

A new kaurane diterpenoid, annoglabasin G (16α‐hydro‐19‐acetoxy‐ent‐kauran‐17‐al) ( 1 ), along with 27 compounds including 18 kaurane diterpenoids, 16β‐hydro‐ent‐kauran‐17‐oic acid ( 2 ), 16α‐hydro‐ent‐kauran‐17‐oic acid ( 3 ), 19‐nor‐ent‐kauran‐4α‐ol‐17‐oic acid ( 4 ), 16α‐hydro‐19‐ol‐ent‐kauran‐17‐oic acid ( 5 ), ent‐kaur‐16‐en‐19‐oic acid ( 6 ), 16α‐hydroxy‐ent‐kauran‐19‐oic acid ( 7 ), 16α,17‐dihydroxy‐ent‐kauran‐19‐oic acid ( 8 ), 16β,17‐dihydroxy‐ent‐kauran‐19‐oic acid ( 9 ), 16α‐hydro‐ent‐kauran‐17,19‐dioic acid ( 10 ), 16β‐hydroxy‐17‐acetoxy‐ent‐kauran‐19‐oic acid ( 11 ), 16β‐hydro‐17‐hydroxy‐ent‐kauran‐19‐al ( 12 ), 16α‐hydro‐17‐hydroxy‐ent‐kauran‐19‐al ( 13 ), 16β,17‐dihydroxy‐ent‐kauran‐19‐al ( 14 ), 16α‐hydro‐19‐al‐ent‐kauran‐17‐oic acid ( 15 ), 16α‐hydro‐17‐acetoxy‐ent‐kauran‐19‐al ( 16 ), 16α‐hydro‐19‐acetoxy‐ent‐kauran‐17‐oic acid ( 17 ), ent‐kaur‐15‐en‐19‐oic acid ( 18 ) and ent‐kaur‐15‐en‐17‐ol‐19‐oic acid ( 19 ); four acetogenins, annomontacin ( 20 ), annonacin ( 21 ), isoannonacinone ( 22 ) and squamocin ( 23 ); four steroids, β‐sitosterol ( 24 ), stigmasterol ( 25 ), β‐sitosteryl‐D‐glucoside ( 26 ) and stigmasteryl‐D‐glucoside ( 27 ) and one oxoaporphine, liriodenine ( 28 ), were isolated from the fresh fruits of Annona glabra. These compounds were characterized and identified by physical and spectral evidence.     

Simultaneous determination of a drug candidate and its metabolite in rat plasma samples using ultrafast monolithic column high-performance liquid chromatography/tandem mass spectrometry

An ultrafast bioanalytical method using monolithic column high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) was evaluated for the simultaneous determination of a drug discovery compound and its metabolite in plasma. Baseline separation of the two compounds was achieved with run times of 24 or 30 s under isocratic or gradient conditions, respectively. The monolithic column HPLC/MS/MS system offers shorter chromatographic run times by increasing flow rate without sacrificing separation power for the drug candidate and its biotransformation product (metabolite). In this work, the necessity for adequate chromatographic resolution was demonstrated because the quantitative determination of the drug-related metabolism product was otherwise hampered by interference from the dosed drug compound. The chromatographic performance of a monolithic silica rod column as a function of HPLC flow rates was investigated with a mixture of the drug component and its synthetic metabolite. The assay reliability of the monolithic column HPLC/MS/MS system was checked for matrix ionization suppression using the post-column infusion technique. The proposed methods were successfully applied to the analysis of study rat plasma samples for the simultaneous quantitation of both the dosed drug and its metabolite. The analytical results obtained by the proposed monolithic column methods and the 'standard' silica particle-packed HPLC column method were in good agreement, within 10% error.     

Trace analysis of toxic metals. I

Summary A simple procedure for the amperometric determination of lead, mercury, cadmium and thallium in theg range by titration with EDTA is described. Using 0.5 ml of the test solution and 10^–5 M EDTA, the minimum quantity of these metal ions that can be determined is about 1 ppm. The accuracy is within ±5%.

---

Spurenanalyse toxischer Metalle. I.Bestimmung von Pb, Hg, Cd und Tl durch amperometrische Titration

Zusammenfassung Ein einfaches Verfahren zur amperometrischen Bestimmung von Mikrogrammengen Pb, Hg, Cd und Tl durch Titration mit ÄDTA wurde beschrieben. Bei Verwendung von 0,5 ml Probelösung und 10^–5 M ÄDTA-Lösung können die genannten Metallionen in der Größenordnung von 1 ppm bestimmt werden. Die Genauigkeit beträgt ±5%.

     

Integration of Separation Capillary with a Au Film Electrode and an Etched Decoupler in Amperometric Capillary Electrophoresis

Introduced in this article is fabrication of an integrated capillary for the applications of electrochemical detection in capillary electrophoresis. The separation section, voltage decoupler, and working electrode were composed into a single section of capillary. The porous decoupler was constructed by HF etching till the thickness of the capillary wall less than 20 μm. The working electrode was prepared by sputtering Au-films on the outlet of the capillary. The integration of the separation capillary with a complete detection assembly improves the convenience for the routine application of electrochemical measurements in capillary electrophoresis. For a 100-μm-i.d. capillary, the theoretical plate number of catechol and the migration time reproducibility can reach 120,000 and 1.9% RSD, respectively. The linear range exceeds 3 order of magnitude and the detection limit is lower than 0.65 μM.