Conformational equilibria and photoinduced tautomerization in 2-(2′-pyridyl)pyrrole

Depending on the polarity and protic abilities of the solvent, 2-(2′-pyridyl)pyrrole can exist in either syn or anti rotameric forms. In nonpolar solvents, intramolecular excited state single proton transfer is observed, manifested by the appearance of low-energy tautomeric emission. The solvent-assisted excited state double proton transfer reaction is also detected. DFT calculations confirm low barriers for both single and double proton transfer processes in the lowest excited singlet state and show different character of the tautomerization in both cases: in the intramolecular reaction, mutual approach of two nitrogen atoms plays an important role.     

Determination of oxytetracycline in bovine kidney and medicated milk replacer by liquid chromatography

Improvements and optimization of AOAC INTERNATIONAL Official Method 995.09 for the detection of oxytetracycline in bovine kidney at the new U.S. tolerance of 12 ppm are reported. Recoveries from kidney fortified at 4 concentrations over the range of 3-40 ppm averaged 84-98%. Results from the kidney of a calf fed medicated milk replacer containing oxytetracycline are also reported. Additionally, adaptation of this method to the detection of oxytetracycline in medicated milk replacer is discussed.     

Electrochemistry of Prussian Blue in silica sol-gel electrolytes doped with polyamidoamine dendrimers

The inclusion of a generation-4 polyamidoamine (G4-PAMAM) dendrimer in a silica sol-gel yielded a solid electrolyte that was used to encapsulate Prussian Blue (PB), iron(III) hexacyanoferrate(II), and cobalt hexacyanoferrate. The PB was synthesized in the doped silica by sequential immersion of a monolith in 0.1 M K₄Fe(CN)₆, water, and 0.1 M FeCl₃. Inclusion of G4-PAMAM resulted in a nanoporous anion-exchange material with a capacity of 10.1 mmol g^–1, which is about four times greater than the capacity of silica alone. Relative to its G0 counterpart, the G4-PAMAM doped silica increased the rate of formation of PB by a factor of ca. 20. The solid state voltammetry of PB in the doped silica had the usual features for this compound. At 0.1 V vs. a Ag quasi-reference electrode, a reversible reduction was seen; the relationship between current and scan rate was that for a surface-confined redox couple. The quasi-reversible oxidation of PB was observed at 0.85 V. Inclusion of G4-PAMAM increased the lifetime of silica as a solid electrolyte from a few days to at least three months. Raman microprobe mapping analysis demonstrated that PB was homogeneously distributed across the entire width (ca. 1 mm) of the G4-doped monolith with 20-h immersions. Electronic Publication     

Excited-state intramolecular proton transfer reaction modulated by low-frequency vibrations: an effect of an electron-donating substituent on the dually fluorescent bis-benzoxazole

Excited-state intramolecular proton transfer (ESIPT) reaction has been studied in a molecule showing dual fluorescence, the 2,5-bis(2-benzoxazolyl)-4-methoxyphenol (BBMP), and its isotopomers, where the methoxy, and alternatively, the OH group has been deuterated. Attention is focused on the influence of electron donating OCH(3) substituent on fast excited state reaction. Comparison between the resonance-enhanced multiphoton ionization spectrum and the laser-induced excitation of the primary and phototautomeric emissions has been done. The geometry, electron density distribution, vibrational structure as well as the potential energy profiles in the S(0) and S(1) states of four possible rotameric forms of BBMP were calculated with application of the density functional theory (DFT). It allowed identifying the most probable conformer and assessing the role of low-frequency motions for the ESIPT efficiency.     

Development and validation of a gas chromatography/mass spectrometry procedure for confirmation of para-toluenesulfonamide in edible fish fillet tissue

Chloramine-T is a disinfectant being developed as a treatment for bacterial gill disease in cultured fish. As part of the drug approval process, a method is required for the confirmation of chloramine-T residues in edible fish tissue. The marker residue that will be used to determine the depletion of chloramine-T residues from the edible tissue of treated fish is para-toluenesulfonamide (p-TSA), a metabolite of chloramine-T. The development and validation of a procedure for the confirmation of p-TSA is described. Homogenized fish tissue is dried by mixing with anhydrous sodium sulfate, and the mixture is extracted with methylene chloride. The extract is passed through a silica gel solid-phase extraction column, from which p-TSA is subsequently eluted with acetonitrile. The acetonitrile extract is evaporated, and the oily residue is dissolved in hexane. The hexane solution is shaken with fresh acetonitrile. The acetonitrile solution is evaporated and the residue is redissolved in dilute potassium hydroxide solution. The aqueous solution is extracted with methylene chloride to further remove more of the fat co-extractive. The aqueous solution is reacted with pentafluorobenzyl bromide in presence of tetrabutylammonium hydrogensulfate. The resulting di-(pentafluorobenzyl) derivative of p-TSA is analyzed by gas chromatography/mass spectrometry. This method permits the confirmation of p-TSA in edible fish tissue at 20 ppb.     

Controlling Emissive Properties by Intramolecular Hydrogen Bonds: Alkyl and Aryl meso-Substituted Porphycenes

Porphycene, a porphyrin isomer, is an efficient fluorophore. However, four-fold meso substitution with alkyl groups decreases the fluorescence quantum yield by orders of magnitude. For aryl substituents, this effect is small. To explain this difference, we have synthesized and studied a mixed aryl-alkyl-substituted compound, 9,20-diphenyl-10,19-dimethylporphycene, as well as the 9,20-diphenyl and 9,20-dimethyl derivatives. Analysis of the structural, spectroscopic, and photophysical data of the six porphycenes, combined with quantum chemical calculations, shows a clear correlation between the strength of the intramolecular NH⋅⋅⋅N hydrogen bonds and the efficiency of the radiationless depopulation of the lowest-excited singlet state. This result led us to propose a model in which the delocalization of the inner protons in the cavity of the macrocycle is responsible for the nonradiative deactivation channel. The applicability of the model is confirmed by the literature data for other alkyl- or aryl-substituted porphycenes. The finding of a correlation between structural and emissive characteristics enables a rational design of porphycenes with desired photophysical properties.     

Parent,Unsubstituted Hemiporphycene: Synthesis and Properties

Among seven possible nitrogen‐in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene ( HPc ), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner‐cavity dimensions, orbital‐energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH‐stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol^?1. Tautomerization from the higher‐ to the lower‐energy form is detected in the lowest‐excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H‐bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N ??? N distance and the N?H?N angle.     

Strengthening Silica Prepared by Sol-Gel Chemistry by Inclusion of a Polyamidoamine Dendrimer

Silica prepared from tetramethyl orthosilicate by an acid-catalyzed sol-gel process is strengthened by inclusion of a generation-zero polyamidoamine (G0-PAMAM) dendrimer in the precursor sol. Evidence is provided by impact test and rupture strength measurements. Impact mean failure energies are 17 and 30 mJ for undoped and doped silica, respectively. The corresponding rupture strengths are 32.9 and 41.6 MPa. Also measured is the Knoop Hardness Number (KHN) for the undoped and PAMAM-doped silica. The KHN values are indistinguishable among the undoped and doped sol-gels and are within 15% of that reported for borosilicate glass. Doping with generation-4 PAMAM does not strength the gels. A model is proposed whereby the G0-PAMAM crosslinks the silica and G4-PAMAM serves as a templating agent that leads to larger colloidal particles. Atomic force microscopy of the surface features of the silica supports the hypothesis.     

Development of a quantitative multiclass/multiresidue method for 21 veterinary drugs in shrimp

A multiclass/multiresidue method has been developed and validated for the determination of 21 veterinary drug residues in shrimp, including sulfonamides (sulfadiazine, sulfamerazine, sulfamethazine, sulfachloropyridazine, sulfadimethoxine, and sulfaquinoxaline); tetracyclines (oxytetracycline, tetracycline, and chlortetracycline); (fluoro)quinolones (norfloxacin, ciprofloxacin, enrofloxacin, sarafloxacin, difloxacin, flumequine, oxolinic acid, and nalidixic acid); and cationic dyes (malachite green, gentian violet, leucomalachite green, and leucogentian violet), using HPLC/MS/MS. All drugs were quantifiable over a no less than 10-fold range with matrix-matched standards for linear external calibration, except for oxytetracycline, tetracycline, norfloxacin, and ciprofloxacin, for which norfloxacin-d5 was used as an internal standard. Two grams of preground shrimp sample was extracted twice with extractant at two different pH values. The combined supernatant was further diluted with an aqueous internal standard solution, and 50 microL extract was injected into the HPLC instrument. An online SPE system was set up for automated sample cleanup. A triple quadrupole mass spectrometer equipped with an electrospray ionization source was operated in the multiple-reaction-monitoring mode to acquire data. The method has been validated at three levels within the designated linear ranges for each drug, with accuracies between 77 and 115%, and most CV values below 15%.