RESUMEN
The present study further explores the behavior of polyoxometalate-based hybrid compounds as catalysts for liquid-phase cyclooctene epoxidation with H2O2. Precisely, it unveils the nature of the relevant active species derived from the hybrid based on Keggin polyoxometalate (POM) and bipyridines (bpy) of formula (2,2'-Hbpy)3[PW12O40] (1). Whereas (i) it is generally accepted that the catalytic oxidation of organic substrates by H2O2 involving Keggin HPAs proceeds via an oxygen transfer route from a peroxo intermediate and (ii) the catalytically active peroxo species is commonly postulated to be the polyperoxotungstate {PO4[W(O)(O2)2]4}3- complex (PW4), we show that the studied epoxidation reaction seems to be more sophisticated than commonly reported. During the catalytic epoxidation, 1 underwent a partial transformation into two oxidized species, 2 and 3. Compound 3 corresponding to 2,2'-bipyridinium oxodiperoxotungstate of formula [WO(O2)2(2,2'-bpy)] was shown to be the main species responsible for the selective epoxidation of cyclooctene since 2 (in which the POM is associated with a protonated mono-N-oxide derivative of 2,2'-bpy of formula (2,2'-HbpyO)3[PW12O40]) exhibited no activity. The structures of 1, 2, and 3 were solved by single-crystal X-ray diffraction and were independently synthesized. The speciation of 1 was monitored under catalytic conditions by 1H and 1H DOSY NMR spectroscopies, where the formation in situ of 2 and 3 was revealed. A reaction mechanism is proposed that highlights the pivotal, yet often underestimated, role of H2O2 in the reached catalytic performances. The active species responsible for the oxygen transfer to cyclooctene is a hydroperoxide intermediate species that is formed by the interaction between the anionic structure of the catalyst and H2O2. The latter operates as a "conservative agent" whose presence in the catalytic system is required to prevent the catalysts from deactivating irreversibly.
RESUMEN
A thorough investigation of two novel hybrid materials, namely, (2,2'-Hbpy)3[PW12O40] and (4,4'-H2bpy)1.5[PW12O40]·1.5H2O built from Keggin phosphotungstic acid (PTA) and bipyridine, describes the impact of bipyridine isomers in their formation and physicochemical properties. The hybrids' formation was confirmed by powder X-ray diffraction, while infrared spectroscopy (IR) proved the polyoxometalate (POM) structural preservation. The stoichiometric composition and thermal stability of the hybrids were solved by thermogravimetric analysis-mass spectrometry, which also revealed newly acquired hydrophobic properties. Raman and IR spectroscopies demonstrated that the POM skeleton units in both hybrids were distorted compared to the POM in PTA, which induced a decrease of their reduction potentials as observed by diffuse reflectance ultraviolet-visible spectroscopy (DR-UV-vis). The hybrids' acidity was assessed by ammonia temperature-programmed desorption, which showed no remaining acid sites compared to the strong acidic character of the pristine PTA. The properties of the hybrids were tested in the epoxidation of cyclooctene in the presence of H2O2. The reaction was boosted when the hybrids were pre-activated with H2O2.
RESUMEN
Introducción. Las infecciones por Escherichia coli productoras de BLEE son cada vez mas frecuentes en la comunidad. Ertapenem (ERT) presenta muy buena actividad frente a estas cepas y es una excelente indicación en infecciones severas en etapa de manejo ambulatorio. El objetivo de este trabajo fue determinar la frecuencia de selección de mutantes resistentes a carbapenemas en una colección de aislamientos clínicos de E. coli productoras de BLEE. Material y métodos. Se buscaron mutantes resistentes en uno y dos pasos a ERT, imipenem (IMI) y meropenem (MER) por inoculación de 109 ufc/ml en placas de agar Mueller Hinton conteniendo las carbapenemas a diferentes concentraciones. La concentración inhibitoria mínima (CMI) en las cepas originales y mutantes se determinó con el método epsilometrico Etest. Resultados. No se pudieron seleccionar mutantes resistentes con IMI y MER. Al utilizar ERT se obtuvieron mutantes resistentes en 13 de 57 aislamientos clínicos (22,8 %). Todos los mutantes resistentes fueron resistentes a ERT con CMI ≥ 1 mg/L pero mantuvieron sensibilidad a IMI y MER. Se obtuvieron 6 MR segundo paso con ERT las cuales presentaron resistencia de alto nivel a ERT (CMI ≥ 8 mg/L). Se observó resistencia cruzada a MER en 3 de ellas y en 1 a IMI. Los cuatro mutantes resistentes de segundo paso obtenidos con MER fueron resistentes a ERT y MER y en 2 de ellas se observó resistencia cruzada a IMI. Conclusiones. La selección de mutantes resistentes a ERT es frecuente en cepas de E. coli productoras de BLEE. Para obtener mutantes resistentes a MER e IMI es necesario un segundo paso de selección. El uso de ERT en infecciones con inóculo alto, focos no drenados y con cepas productoras de BLEE debería ser vigilado para reducir el riesgo de selección de resistencia (AU)
Introduction. The occurrence of community-associated infections due to extended-spectrum β-lactamase (ESBL)-producing Escherichia coli is increasing worldwide. These organisms are frequently resistant to many of the antimicrobial agents but remain susceptible to carbapenems. We investigated the in vitro emergence of carbapenem resistance in a collection of clinical isolates of ESBL -producing E. coli Material and methods. First and second-step resistant mutants were obtained from E. coli with ESBL. Aliquots of 50μl containing > 109 CFU were applied to Mueller-Hinton plates containing meropenem, imipenem or ertapenem. MICs for native strains and mutants were determined using the epsilometric test (E-test). Results. Resistant mutants were not selected with imipenem or meropenem. E. coli growth was observed on ertapenem (0.5 mg/L)-containing plates in 13 of 57 clinical isolates (22.8 %).The ertapenem MIC for these first-step mutants were ≥ 1 mg/L, remaining susceptible to imipenem and meropenem. The first-step mutants were used as native strains. Six second-step resistant mutants were selected with ertapenem. All were fully resistant (CMI ≥ 8 mg/L) to ertapenem, three were resistant to meropenem and one to imipenem. Four second-step resistant mutants were selected with meropenem. All were resistant to ertapenem, meropenem, and two of them were resistant to imipenem. Conclusions. Stable resistant mutants were easy to select with ertapenem among ESBL-producing E. coli. Two steps were necessary to select resistant mutants to meropenem or imipenem. The use of ertapenem in high-inoculum infections or in undrained focus of infection should be monitored to reduce the risk on selection of resistance (AU)