Polyaniline doped by a new class of dopants,benzoic acid and substituted benzoic acid: synthesis and characterization

This work reports on a new class of dopants, benzoic acid and substituted benzoic acids such as 2‐hydroxybenzoic acid, 2‐chlorobenzoic acid, 4‐nitrobenzoic acid, 2‐methoxybenzoic acid, 3‐methylbenzoic acid, 4‐methylbenzoic acid, 3‐aminobenzoic acid and 4‐aminobenzoic acid, for polyaniline. Benzoic acids can be used to dope polyaniline by mixing benzoic acid (or a substituted benzoic acid) with polyaniline in the common solvent 1‐methyl‐2‐pyrrolidone. Properties of benzoic acid doped polyaniline salts are studied using Fourier transform infra‐red, X‐ray diffraction spectroscopy, scanning electron microscopy, thermogravimetric analysis and conductivity measurements. The conductivity of polyaniline‐benzoic acid salt was found to be high (10⁻² S/cm) when compared to polyaniline‐substituted benzoic acid salts (10⁻³–10⁻⁵ S/cm). Copyright © 2005 John Wiley & Sons, Ltd.     

Solid‐state NMR spectroscopic studies on the interaction of sorbic acid with phospholipid membranes at different pH levels

²H, ³¹P, and ¹H‐magic‐angle‐spinning (MAS) solid‐state NMR spectroscopic methods were used to elucidate the interaction between sorbic acid, a widely used weak acid food preservative, and 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphocholine (DMPC) bilayers under both acidic and neutral pH conditions. The linewidth broadening observed in the ³¹P NMR powder pattern spectra and the changes in the ³¹P longitudinal relaxation time (T₁) indicate interaction with the phospholipid headgroup upon titration of sorbic acid or decanoic acid into DMPC bilayers over the pH range from 3.0 to 7.4. The peak intensities of sorbic acid decrease upon addition of paramagnetic Mn²⁺ ions in DMPC bilayers as recorded in the ¹H MAS NMR spectra, suggesting that sorbic acid molecules are in close proximity with the membrane/aqueous surface. No significant ²H quadrupolar splitting (Δν_Q) changes are observed in the ²H NMR spectra of DMPC‐d₅₄ upon titration of sorbic acid, and the change of pH has a slight effect on Δν_Q, indicating that sorbic acid has weak influence on the orientation order of the DMPC acyl chains in the fluid phase over the pH range from 3.0 to 7.4. This finding is in contrast to the results of the decanoic acid/DMPC‐d₅₄ systems, where Δν_Q increases as the concentration of decanoic acid increases. Thus, in the membrane association process, sorbic acids are most likely interacting with the headgroups and shallowly embedded near the top of the phospholipid headgroups, rather than inserting deep into the acyl chains. Thus, antimicrobial mode of action for sorbic acid may be different from that of long‐chain fatty acids. Copyright © 2009 John Wiley & Sons, Ltd.     

Control of Biocatalytic Transformations by Programmed DNA Assemblies

This study demonstrates the self‐assembly of inhibitor/enzyme‐tethered nucleic acid fragments or enzyme I‐, enzyme II‐modified nucleic acids into functional nanostructures that lead to the controlled inhibition of the enzyme or the activation of an enzyme cascade. In one system, the anti‐cocaine aptamer subunits are modified with monocarboxy methylene blue (MB⁺) as the inhibitor and with choline oxidase (ChOx). The cocaine‐induced self‐assembly of the aptamer subunits complex results in the inhibition of ChOx by MB⁺. In a further configuration, two nucleic acids of limited complementarity are functionalized at their 3′ and 5′ ends with glucose oxidase (GOx) and horseradish peroxidase (HRP), respectively, or with MB⁺ and ChOx. In the presence of a target DNA sequence, synergistic complementary base‐pairing occurs, thus leading to stable supramolecular Y‐shaped nanostructures of the nucleic acid units. A GOx/HRP bienzyme cascade or the programmed inhibition of ChOx by MB⁺ is demonstrated in the resulting nucleic acid nanostructures. A quantitative theoretical model that describes the nucleic acid assemblies and that results in the inhibition of ChOx by MB⁺ or in the activation of the GOx/HRP cascade, respectively, is provided.     

Derivatives of 5‐Oxy‐pyrido[2,3‐b]quinoxaline‐9‐carboxylic acid: A tricyclic system useful for the synthesis of potential intercalators

The synthesis of a new series of 5‐oxy‐pyrido[2,3‐b]quinoxaline‐9‐carboxamides 4a‐i and N¹,N²‐Bis(5‐oxy‐pyrido[2,3‐b]quinoxaline‐9‐benzoyl)ethylenediamine ( 5 ) is reported starting from 2‐chloro‐3‐nitropyri‐dine. Fundamental steps of the synthetic pathway are i) preparation of 2‐(3‐nitro‐pyridin‐2‐ylamino)benzoic acid ( 1 ) via copper‐catalyzed condensation of 2‐chloro‐3‐nitropyridine with o‐anthranilic acid, ii) intramolecular cyclization of the acid 1 to 5‐oxy‐pyrido[2,3‐b]quinoxaline‐9‐carboxylic acid ( 2b ) upon treatment with concentrated sulfuric acid and oleum and iii) conversion of the acid 2 to the desired amides 4a‐i and 5 . Compounds 4a‐i and 5 are oxygenated azaanalogs of phenazines, a wellknown series of intercalators with cytotoxic activity.     

Determination of phenoxy herbicides in water samples using phase transfer microextraction with simultaneous derivatization followed by GC‐MS analysis

A sensitive and accurate method for the determination of two model phenoxy herbicides, 4‐chloro‐2‐methylphenoxy acetic acid and 4‐chloro‐2‐methylphenoxy propanoic acid, in water is explained. This method utilizes a simple phase transfer catalyst‐assisted microextraction with simultaneous derivatization. Factors affecting the performance of this method including pH of the aqueous matrix, temperature, extraction duration, type and amount of derivatization reagents, and type and amount of the phase transfer catalyst are examined. Derivatization and the use of phase transfer catalyst have proven to be especially vital for the resolution of the analytes and their sensitive determination, with an enrichment factor of 288‐fold for catalyzed over noncatalyzed procedure. Good linearity ranging from 0.1 to 80 μg L^?1 with correlation of determination (r²) between 0.9890 and 0.9945 were obtained. Previous reported detection limits are compared with our new current method. The low LOD for the two analytes (0.80 ng L^?1 for 4‐chloro‐2‐methylphenoxy propanoic acid and 3.04 ng L^?1 for 4‐chloro‐2‐methylphenoxy acetic acid) allow for the determination of low concentrations of these analytes in real samples. The absence of matrix effect was confirmed through relative recovery calculations. Application of the method to seawater and tap water samples was tested, but only 4‐chloro‐2‐methylphenoxy propanoic acid at concentrations between 0.27 ± 0.01 and 0.84 ± 0.06 μg L^?1 was detected in seawater samples.     

A Facile Route to Boronic Acid Functional Polymeric Microspheres via Epoxide Ring Opening

Boronic acid‐functionalized microspheres are prepared for the first time via mild epoxide ring opening based on porous cross‐linked polymeric microspheres (diameter ≈ 10 μm, porosity ≈ 1000 Å). Quantitative chemical analysis by XPS and EA evidences that there is a greater functionalization with boronic acid when employing a sequential synthetic method [1.7 atom% boron (XPS); 1.12 wt% nitrogen (EA)] versus a one‐pot synthetic method [0.2 atom% boron (XPS); 0.60 wt% nitrogen (EA)] yielding grafting densities ranging from approximately 2.5 molecules of boronic acid per nm² to 1 molecule of boronic acid per nm², respectively. Furthermore, the boronic acid‐functionalized microspheres are conjugated with a novel fluorescent glucose molecule demonstrating a homogeneous spatial distribution of boronic acid.     

Hydroxylation and Ring‐Opening Mechanism of an Unusual Flavoprotein Monooxygenase, 2‐Methyl‐3‐hydroxypyridine‐5‐carboxylic Acid Oxygenase: A Theoretical Study

Hybrid meta‐GGA density functional theory (the MPWB1K functional) was used to study the hydroxylation and ring‐opening mechanism of 2‐methyl‐3‐hydroxypyridine‐5‐carboxylic acid oxygenase (MHPCO). This enzyme catalyses the conversion of 2‐methyl‐3‐hydroxypyridine‐5‐carboxylic acid (MHPC) to α‐(N‐acetylaminomethylene)succinic acid (AAMS), which is the essential ring‐opening step in the bacterial degradation of vitamin B₆. MHPCO belongs to the flavin‐containing aromatic hydroxylases family. However, MHPCO is capable of catalysing a subsequent aromatic ring‐cleavage reaction to give acyclic products rather than hydroxylated aromatic ones. Our calculations show that the re‐aromatisation of the hydroxylated intermediate occurs spontaneously in aqueous solution; this implies that the ring‐opening process occurs inside the enzyme’s active site, in which limited water is available. The instability of the hydroxylated intermediate of MHPCO is the main reason why acyclic products are formed. Previously proposed mechanisms for the ring‐opening step were studied, and were shown to be less likely to occur (ΔΔG^≠298>35 kcal mol^?1). Two new pathways with reasonable barrier heights (ΔΔG^≠298<15 kcal mol^?1) are reported herein, which are in accordance with all experimental information present to date.     

Isolation of a furan fatty acid from Hevea brasiliensis latex employing the combined use of pH‐zone‐refining and conventional countercurrent chromatography

Furan fatty acids are valuable and bioactive minor fatty acids that usually contribute <0.1% to the fatty acid content of food samples. Their biological role still remains unclear as authentic furan fatty acid standards are not readily available and thorough experimental studies verifying the relevance of furan fatty acids are thus virtually impossible. An efficient protocol for the isolation of the furan fatty acid 9‐(3‐methyl‐5‐pentylfuran‐2‐yl)‐nonanoic acid from hydrolyzed and centrifuged latex of Hevea brasiliensis was developed using countercurrent chromatography. A first run using pH‐zone‐refining countercurrent chromatography provided 48.4 mg of 9‐(3‐methyl‐5‐pentylfuran‐2‐yl)‐nonanoic acid from 210 mg latex extract in a purity of 95%. The purity was increased to 99% by means of one second run in conventional countercurrent chromatography mode. The Structure and purity of 9‐(3‐methyl‐5‐pentylfuran‐2‐yl)‐nonanoic acid were determined by gas chromatography coupled to mass spectrometry and ¹H and ¹³C NMR spectroscopy.     

Synthesis and characterization of anhydrous conducting polyimide/ionic liquid complex membranes via a new route for high‐temperature fuel cells

The paper deals with the synthesis and characterization of a new series of anhydrous conducting acid‐doped complex membranes based on polyimide (PI) and ionic liquid (IL) for high‐temperature fuel cells via a new route. For this purpose, three imidazolium‐based ILs (RIm⁺BF₄^?) with different alkyl chain lengths (R=methyl, ethyl, and butyl) are added into polyamic acid (PAA) intermediate prepared from the reaction of benzophenonetetracarboxylic dianhydride and diaminodiphenylsulfone in different –COOH/imidazolium molar ratios (n = 0.5, 1, and 2). Then, the thermally imidized complex membrane was doped with H₂SO₄. The conductivities of acid‐doped PI/IL complex membranes prepared by taking n of 1 are found to be in the range of 10^?4?10^?5 S cm^?1 at 180°C, whereas the acid‐free PI/IL complex membranes show the conductivity at around 10^?9?10^?10 S cm^?1. Thermogravimetric analysis results reveal that the acid‐doped PI/IL complex membranes are thermally stable up to 250°C. Dynamic mechanical analysis results of the acid‐doped ionically interacted complex membrane show that the mechanical strengths of the PI/IL complex membranes including 1‐methyl imidazolium tetrafluoroborate (MeIm‐BF₄) and 1‐ethyl 3‐methyl imidazolium tetrafluoroborate (EtIm‐BF₄) are comparable with those of pristine PI until 200°C. Furthermore, it can be clearly emphasized that the ionic interaction between carboxylic acid groups of PAA's and IL's cations offers a positive role in long‐term conductivity stability by preventing the IL migration at high temperatures. On the other hand, preliminary methanol permeability tests of the acid‐doped membranes show that they can also be considered as an alternative for direct methanol fuel cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Polar‐Functionalized,Crosslinkable, Self‐Healing,and Photoresponsive Polyolefins

The nonpolar nature of polyolefins is one of their biggest limitations. Now, an efficient route to generate polar‐functionalized, crosslinkable, self‐healing, photoresponsive polyolefins with thermoplastic, elastomeric, and thermosetting properties is reported. Tunable amounts of carboxylic acid and a cyclic comonomer are installed onto polyolefins by palladium‐catalyzed terpolymerization reactions. The incorporated carboxylic acid unit can alter the surface properties of polyolefins. The subsequently introduced Fe³⁺/citric acid combination induces dynamic crosslinking and enables self‐healing. Under UV light irradiation, citric acid reduces Fe³⁺ to Fe²⁺ and decreases the crosslinking density. The Fe²⁺ moiety can be easily oxidized back to Fe³⁺, making the process reversible at the expense of citric acid. The incorporated cyclic comonomer modulates the crystallinity of polyolefins, provides elastic properties, and installs carbon–carbon double bonds for sulfur‐induced vulcanization.     

Extraordinary Radical Scavengers: 4‐Mercaptostilbenes

In the past decade, there was a great deal of interest and excitement in developing more active antioxidants and cancer chemoprevention agents than resveratrol, a naturally occurring stilbene. In this work, eight resveratrol‐directed 4‐mercaptostilbenes were constructed based on the inspiration that thiophenol should be a stronger radical scavenger than phenol, and their reaction rates with galvinoxyl (GO^.) and 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH^.) radicals in methanol and ethyl acetate were measured by using stopped‐flow UV/Vis spectroscopy at 25 °C. Kinetic analysis demonstrates that 4‐mercaptostilbenes are extraordinary radical scavengers, and the substitution of the 4‐SH group for the 4‐OH group in the stilbene scaffold is an important strategy to improve the radical‐scavenging activity of resveratrol. Surprisingly, in methanol, some of the 4‐mercaptostilbenes are 10⁴‐times more active than resveratrol, dozens of times to hundreds of times more effective than known antioxidants (α‐tocopherol, ascorbic acid, quercetin, and trolox). The detailed radical‐scavenging mechanisms were discussed based on acidified‐kinetic analysis. Addition of acetic acid remarkably reduced the GO^. and DPPH^. radical‐scavenging rates of the 4‐mercaptostilbenes in methanol, a solvent that supports ionization, suggesting that the reactions proceed mainly through a sequential proton loss electron transfer mechanism. In contrast, an interesting acid‐promoted kinetics was observed for the reactions of the 4‐mercaptostilbenes with DPPH^. in ethyl acetate, a solvent that weakly supports ionization. The increased ratio in rates is closely correlated with the electron‐rich environment in the molecules, suggesting that the acceleration could benefit from the contribution of the electron transfer from the 4‐mercaptostilbenes and DPPH^.. However, the addition of acetic acid had no influence on the GO^.‐scavenging rates of the 4‐mercaptostilbenes in ethyl acetate, due to the occurrence of the direct hydrogen atom transfer. Our results show that the radical‐scavenging activity and mechanisms of 4‐mercaptostilbenes depends significantly on the molecular structure and acidity, the nature of the attacking radical, and the ionizing capacity of the solvent.     

Molecular Vise Approach to Create Metal‐Binding Sites in MOFs and Detection of Biomarkers

We report a new approach to create metal‐binding site in a series of metal–organic frameworks (MOFs), where tetratopic carboxylate linker, 4′,4′′,4′′′,4′′′′‐methanetetrayltetrabiphenyl‐4‐carboxylic acid, is partially replaced by a tritopic carboxylate linker, tris(4‐carboxybiphenyl)amine, in combination with monotopic linkers, formic acid, trifluoroacetic acid, benzoic acid, isonicotinic acid, 4‐chlorobenzoic acid, and 4‐nitrobenzoic acid, respectively. The distance between these paired‐up linkers can be precisely controlled, ranging from 5.4 to 10.8 Å, where a variety of metals, Mg²⁺, Al³⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺ and Pb²⁺, can be placed in. The distribution of these metal‐binding sites across a single crystal is visualized by 3D tomography of laser scanning confocal microscopy with a resolution of 10 nm. The binding affinity between the metal and its binding‐site in MOF can be varied in a large range (observed binding constants, K_obs from 1.56×10² to 1.70×10⁴ L mol^?1), in aqueous solution. The fluorescence of these crystals can be used to detect biomarkers, such as cysteine, homocysteine and glutathione, with ultrahigh sensitivity and without the interference of urine, through the dissociation of metal ions from their binding sites.     

Investigation of the Substrate Range of CYP199A4: Modification of the Partition between Hydroxylation and Desaturation Activities by Substrate and Protein Engineering

The cytochrome P450 enzyme CYP199A4, from Rhodopseudomonas palustris HaA2, can efficiently demethylate 4‐methoxybenzoic acid. It is also capable of oxidising a range of other related substrates. By investigating substrates with different substituents and ring systems we have been able to show that the carboxylate group and the nature of the ring system and the substituent are all important for optimal substrate binding and activity. The structures of the veratric acid, 2‐naphthoic acid and indole‐6‐carboxylic acid substrate‐bound CYP199A4 complexes reveal the substrate binding modes and the side‐chain conformational changes of the active site residues to accommodate these larger substrates. They also provide a rationale for the selectivity of product oxidation. The oxidation of alkyl substituted benzoic acids by CYP199A4 is more complex, with desaturation reactions competing with hydroxylation activity. The structure of 4‐ethylbenzoic acid‐bound CYP199A4 revealed that the substrate is held in a similar position to 4‐methoxybenzoic acid, and that the C^β C? H bonds of the ethyl group are closer to the heme iron than those of the C^α (3.5 vs. 4.8 Å). This observation, when coupled to the relative energies of the reaction intermediates, indicates that the positioning of the alkyl group relative to the heme iron may be critical in determining the amount of desaturation that is observed. By mutating a single residue in the active site of CYP199A4 (Phe185) we were able to convert the enzyme into a 4‐ethylbenzoic acid desaturase.     

Three 3‐Amino‐1, 2, 4‐Triazole‐Based Magnetic Complexes Incorporated with Different Carboxylate‐Containing Coligands: Synthesis,Structures, and Magnetic Properties

Three 3‐amino‐1, 2, 4‐triazole (atz)‐based paramagnetic complexes, [Mn(atz)(pa)]_n ( 1 ), {[Mn(atz)_1.5(hip)] · H₂O}_n ( 2 ), and [Mn(H₂O)₂(atz)₂(nb)₂] ( 3 ) (H₂pa = o‐phthalic acid, H₂hip = 5‐hydroxylisophthalic acid, and Hnb = p‐nitrobenzoic acid) were prepared by introducing different carboxylate‐containing aromatic coligands, and structurally and magnetically characterized. Helical Mn^II‐atz and bent Mn^II‐pa^2– chains are crosslinked by sharing the same metal sites to generate a honeycomb‐shaped framework of 1 . The undulated Mn^II‐atz layers constructed from 22‐member metallomacrocycles are periodically supported by ditopic hip^2– ligands to lead to a pillared‐layer structure of 2 . In contrast, complex 3 is a centrosymmetric mononuclear entity, which is assembled into a three‐dimensional supramolecular network by abundant hydrogen‐bonding interactions. The structural difference of 1 – 3 is significantly due to the combinations of the flexible coordination modes adopted by the mixed atz and carboxylate groups. Weak and comparable antiferromagnetic couplings are observed in the nearest neighbors of 1 – 3 , which are cooperatively transmitted either by short carboxylate and/or atz heterobridges or by weak non‐covalent interactions.     

Comparison of Ethylation at External Surface and Internal Cavity of H-MCM-22 Zeolite from Theoretical Calculations

Summaryof main observation and conclusion Understanding and optimizing structure of active sites is of significance in zeolite catalysis.Benzene ethylation is an industrially important process catalyzed by H-MCM-22 zeolite;while the active sites still remain elusive.In this work,density functional theory(DFT)calculations were employed to investigate the benzene ethylation at two different types of Br?nsted acid sites(BAS)in H-MCM-22 zeolite,namely the internal cavity(IC)acid site and the external surface(ES)acid site.Both the stepwise and concerted pathways were addressed.The compari-son of the calculated energetics of two pathways indicates that the benzene ethylation reaction primarily proceeds via the concerted pathway at both the IC and ES acid sites of H-MCM-22.The calculated overall Gibbs free energies at reaction condition(473 K and 3.5 MPa)on the IC and ES acid sites are 90 and 86 kJ/mol,with the rate constants of 1.20×10^3and 2.92×10^3s^-1,respectively.It indicates that benzene ethylation could occur both on the IC and ES acid sites,with the catalytic activity of IC acid site being slightly lower than that of ES acid site.Furthermore,we theoretically reveal that the acid strength at the ES site is slightly weaker than that at the IC site via the frequency shift after the adsorption of CO.The differences in dispersion interaction between ES and IC sites are also quantified by the adsorption of base molecules with different sizes.The calculated results in this work demonstrate that the acid sites at the external surface of H-MCM-22 zeolites are suitable for benzene ethylation to produce ethylbenzene,providing theoretical implications for tailoring the distribution of active sites in H-MCM-22 zeolite.     

The Synthesis of Methylated,Phosphorylated, and Phosphonated 3′‐Aminoacyl‐tRNASec Mimics

The twenty first amino acid, selenocysteine (Sec), is the only amino acid that is synthesized on its cognate transfer RNA (tRNA^Sec) in all domains of life. The multistep pathway involves O‐phosphoseryl‐tRNA:selenocysteinyl‐tRNA synthase (SepSecS), an enzyme that catalyzes the terminal chemical reaction during which the phosphoseryl–tRNA^Sec intermediate is converted into selenocysteinyl‐tRNA^Sec. The SepSecS architecture and the mode of tRNA^Sec recognition have been recently determined at atomic resolution. The crystal structure provided valuable insights that gave rise to mechanistic proposals that could not be validated because of the lack of appropriate molecular probes. To further improve our understanding of the mechanism of the biosynthesis of selenocysteine in general and the mechanism of SepSecS in particular, stable tRNA^Sec substrates carrying aminoacyl moieties that mimic particular reaction intermediates are needed. Here, we report on the accurate synthesis of methylated, phosphorylated, and phosphonated serinyl‐derived tRNA^Sec mimics that contain a hydrolysis‐resistant ribose 3′‐amide linkage instead of the natural ester bond. The procedures introduced allow for efficient site‐specific methylation and/or phosphorylation directly on the solid support utilized in the automated RNA synthesis. For the preparation of (S)‐2‐amino‐4‐phosphonobutyric acid–oligoribonucleotide conjugates, a separate solid support was generated. Furthermore, we developed a three‐strand enzymatic ligation protocol to obtain the corresponding full‐length tRNA^Sec derivatives. Finally, we developed an electrophoretic mobility shift assay (EMSA) for rapid, qualitative characterization of the SepSecS‐tRNA interactions. The novel tRNA^Sec mimics are promising candidates for further elucidation of the biosynthesis of selenocysteine by X‐ray crystallography and other biochemical approaches, and could be attractive for similar studies on other tRNA‐dependent enzymes.     

New Methylene Blue (NMB) Encapsulated in Mesoporous AlMCM‐41 Material and Its Application for Amperometric Determination of Ascorbic Acid in Real Samples

New methylene blue (NMB) dye incorporated into AlMCM‐41 surfactant‐free and hybrid surfactant‐AlMCM‐41 mesophase. UV‐vis evidence shows that new methylene blue dye protonated in both cases of zeolites. New methylene blue is electroactive in zeolites and their electrochemical activity has been studied by cyclic voltammetry and compared to that of NMB in aqueous solutions. New methylene blue molecules are not released to the solution during CV measurements and are accessible to H₃O⁺ ions. The presence of surfactant affects the kinetics of the redox process through proton ions diffusion. The midpoint potentials (E_m) values show that new methylene blue dye incorporated into AlMCM‐41 can be reduced easily with respect to solution new methylene blue. New methylene blue interacting with surfactant polar heads and residual Br^? ions as a results, it shows a couple of peaks in high potential with respect to new methylene blue solution. The electrode made with methylene blue‐AlMCM‐41 without surfactant was used for the mediated oxidation of ascorbic acid. The anodic peak current observed in cyclic voltammetry was linearly dependent on the ascorbic acid concentration. The calibration plot was linear over the ascorbic acid concentration range 1.0×10^?5 to 5.0×10^?4 M. The detection limit of the method is 1.0×10^?5 M, low enough for trace ascorbic acid determination in various real samples.     

Site‐Selective and Metal‐Free Aliphatic CH Oxidation Enabled Synthesis of [5,24,25‐D3]‐(25S)‐Δ7‐Dafachronic acid

Steroid hormones play significant roles in both worms and mammalians. (25S)‐Δ⁷‐Dafachronic acid (Δ⁷‐DA, 1 ) is a member of the dafachronic acid hormonal series that regulates both development and lifespan of C. elegans. Despite its importance, effective tools for the illumination of its mode of action are lacking. Herein, we report an efficient synthesis of trideuterated Δ⁷‐DA, [5,24,25‐D3]‐(25S)‐Δ⁷‐dafachronic acid ([D₃]‐Δ⁷‐DA, 2 ), as a useful chemical tool for subsequent biological studies. Key steps for this bioinspired synthesis approach include site‐selective aliphatic C?H oxidation mediated by methyl(trifluoromethyl)dioxirane (TFDO), and the iridium/phosphine‐oxazoline‐catalyzed late‐stage asymmetric deuterium reduction.     

Exploring Reversible Quenching of Fluorescence from a Pyrazolo[3,4‐b]quinoline Derivative by Protonation

Pyrazolo[3,4‐b]quinoline derivatives are reported to be highly efficient organic fluorescent materials suitable for applications in light‐emitting devices. Although their fluorescence remains stable in organic solvents or in aqueous solution even in the presence of H₂O, halide salts (LiCl), alkali (NaOH) and weak acid (acetic acid), it suffers an efficient quenching process in the presence of protic acid (HCl) in aqueous or ethanolic solution. This quenching process is accompanied by a change in the UV spectrum, but it is reversible and can be fully recovered. Both steady‐state and transient fluorescence spectra of 1‐phenyl‐3,4‐dimethyl‐1H‐pyrazolo‐[3,4‐b]quinoline (PAQ5) during quenching are measured and analyzed. It is found that a combined dynamic and static quenching mechanism is responsible for the quenching processes. The ground‐state proton‐transfer complex [PAQ5 ??? H⁺] is responsible for static quenching. It changes linearly with proton concentration [H⁺] with a bimolecular association constant K_S=1.95 M ^?1 controlled by the equilibrium dissociation of HCl in ethanol. A dynamic quenching constant K_D=22.4 M ^?1 is obtained by fitting to the Stern–Volmer equation, with a bimolecular dynamic quenching rate constant k_d=1.03×10⁹ s^?1 M ^?1 under ambient conditions. A change in electron distribution is simulated and explains the experiment results.     

Proton‐conducting polymers based on benzimidazoles and sulfonated benzimidazoles

A sulfonated derivative of polybenzimidazole is reported, and its properties are analyzed in comparison with related polybenzimidazole proton‐conducting materials. Poly(2,5‐benzimidazole), poly(m‐phenylenebenzobisimidazole), and poly[m‐(5‐sulfo)‐phenylenebenzobisimidazole] were prepared by condensation of the corresponding monomers in polyphosphoric acid. Several adducts of these polymers with phosphoric acid were prepared. The resulting materials were characterized by chemical analysis, Fourier transform infrared spectroscopy, and thermogravimetric analysis; also, the dc conductivity of doped and undoped derivatives was measured. Similar to what has been observed for the commercial polybenzimidazole polymer (also examined here for comparison), the title polymers exhibit high thermal stability. Furthermore, their doping with phosphoric acid leads to a significant increase in conductivity from less than 10^?11 Scm^?1 for the undoped polymers to 10^?4 Scm^?1 (both at room temperature) for their acid‐loaded derivatives. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3703–3710, 2002