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Slow Reduction (slow + reduction)
Selected AbstractsAnaerobic transformation of compounds of technical toxaphene.ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2003Abstract Technical toxaphene (Melipax) and the single compounds of technical toxaphene (CTTs) 2,2,5- endo, 6- exo, 8,8,9,10- octachlorobornane (B8-806), 2,2,5- endo, 6- exo, 8,9,9,10-octachlorobornane (B8-809), 2,2,5,5,8,9,9,10,10-nonachlorobornane (B9- 1025), 2- endo, 3- exo, 5- endo, 6- exo, 8,8,9,10,10-nonochlorobornane (B9-1679), 2- endo, 3- exo, 5- endo, 6- exo, 8,9,10,10-octachlorobornane (B8-1414), 2- endo, 3- exo, 5- endo, 6- exo, 8,8,9,10-octachlorobornane (B8-1412), and 2- exo, 3- endo, 5- exo, 9,9,10,10-heptachlorobornane (B7-1453) were treated with suspensions of the anaerobic bacterium Dehalospirillum multivorans. After 7 d, more than 50% of technical toxaphene was transformed, and the relative amount of early eluting CTTs increased. After 16 d, only 2- exo, 3- endo, 6- exo, 8,9,10-hexachlorobornane (B6-923), 2- endo, 3- exo, 5- endo, 6- exo, 8,9,10-heptachlorobornane (B7-1001), and a few minor penta- and hexachloro-CTTs were detected in the samples. The result of the transformation was comparable with observations in naturally contaminated sediments and soil. However, the performance with D. multivorans was more simple and reproducible, as well as faster, than use of soil, sediment, or anaerobic sewage sludge. In agreement with reports in the literature, reductive dechlorination at geminal chlorine atoms (gem -Cls) was found to be the major CTT transformation pathway. Experiments conducted with CTTs and gem -Cls at both primary and secondary carbons clarified that the initial Cl -> H substitution takes place at the secondary carbon C2. Furthermore, the 2- endo -Cl position was preferably substituted with hydrogen. In the case of B8-806, the dechlorination at the secondary carbon C2 was approximately 20-fold faster than the subsequent, slow reduction at the primary carbon C8. The three different formerly unknown heptachloro-CTTs, 2- exo, 3- endo, 6- exo, 8,9,9,10-heptachlorobornane (B7-1473), 2- exo, 3- endo, 6- endo, 8,9,9,10-hepatchlorobornane (B7-1461), and 2- exo, 3- endo, 6- exo, 8,8,9,10-heptachlorobornane (B7-1470) were found as intermediates of the B8-806/809 transformation. Treatment of B9-1679 with D. multivorans indicated that gem -Cls on the bridge (C8 and C9) are dechlorinated faster than gem -Cls on the bridgehead (C10). [source] Characterization of a nif-regulated flavoprotein (FprA) from Rhodobacter capsulatusFEBS JOURNAL, Issue 3 20002S] ferredoxin, Redox properties, molecular interaction with a [2Fe A flavoprotein from Rhodobacter capsulatus was purified as a recombinant (His)6 -tag fusion from an Escherichia coli clone over-expressing the fprA structural gene. The FprA protein is a homodimer containing one molecule of FMN per 48-kDa monomer. Reduction of the flavoprotein by dithionite showed biphasic kinetics, starting with a fast step of semiquinone (SQ) formation, and followed by a slow reduction of the SQ. This SQ was in the anionic form as shown by EPR and optical spectroscopies. Spectrophotometric titration gave a midpoint redox potential for the oxidized/SQ couple of Em1 = +20 mV (pH 8.0), whereas the SQ/hydroquinone couple could not be titrated due to the thermodynamic instability of SQ associated with its slow reduction process. The inability to detect the intermediate form, SQ, upon oxidative titration confirmed this instability and led to an estimate of Em2 , Em1 of > 80 mV. The reduction of SQ by dithionite was significantly accelerated when the [2Fe,2S] ferredoxin FdIV was used as redox mediator. The midpoint redox potential of this ferredoxin was determined to be ,275 ± 2 mV at pH 7.5, consistent with FdIV serving as electron donor to FprA in vivo. FdIV and FprA were found to cross-react when incubated together with the 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, giving a covalent complex with an Mr of , 60 000. Formation of this complex was unaffected by the redox states of the two proteins. Other [2Fe,2S] ferredoxins, including FdV and FdVI from R. capsulatus, were ineffective as electron carriers to FprA, and cross-reacted poorly with the flavoprotein. The possible function of FprA with regard to nitrogen fixation was investigated using an fprA -deleted mutant. Although nitrogenase activity was significantly reduced in the mutant compared with the wild-type strain, nitrogen fixation was apparently unaffected by the fprA deletion even under iron limitation or microaerobic conditions. [source] Controlling the Edge Length of Gold Nanoprisms via a Seed-Mediated Approach,ADVANCED FUNCTIONAL MATERIALS, Issue 9 2006E. Millstone Abstract A straightforward method is investigated for controlling and reinitiating the growth of single-crystalline Au nanoprisms. This work is based on seeding methodology, and depends on the slow reduction of metal ions onto the surface of a growing nanoprism. In this manner, we can tailor the edge length of Au nanoprisms between 100 and 300,nm without changing their thickness or crystallinity. Each nanoprism size has been characterized by UV-vis-NIR (NIR: near-IR) spectroscopy, transmission electron microscopy (TEM) techniques, and statistical analysis. Based on this work and existing silver halide crystal-growth theories, a preliminary mechanism is proposed which comments on the interplay between crystal growth and surface chemistry that ultimately dictates the morphology of the resulting nanostructure. [source] "Click" Polymer-Supported Palladium Nanoparticles as Highly Efficient Catalysts for Olefin Hydrogenation and Suzuki Coupling Reactions under Ambient ConditionsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2009Cátia Ornelas Abstract Complexation of palladium(II) acetate [Pd(OAc)2] or dipotassium tetrachloropalladate [K2PdCl4] to "click" polymers functionalized with phenyl, ferrocenyl and sodium sulfonate groups gave polymeric palladium(II)-triazolyl complexes that were reduced to "click" polymer-stabilized palladium nanoparticles (PdNPs). Transmission electron microscopy (TEM) showed that reduction using sodium borohydride (NaBH4) produced PdNPs in the 1,3,nm range of diameters depending on the nature of the functional group, whereas slow reduction using methanol yielded PdNPs in the 22,25,nm range. The most active of these PdNPs (0.01% mol Pd), stabilized by poly(ferrocenyltriazolylmethyl)styrene, catalyzed the hydrogenation of styrene at 25,°C and 1 atm hydrogen, with turnover numbers (TONs) of 200,000. When stabilized by the water-soluble poly(sodium sulfonate-triazolylmethyl)styrene, the PdNPs (0.01% mol Pd) catalyze the Suzuki,Miyaura coupling between iodobenzene (PhI) and phenylboronic acid [PhB(OH)2] in water/ethanol (H2O/EtOH) at 25,°C with TONs of 8,200. This high catalytic activity is comparable to that obtained with "click" dendrimer-stabilized PdNPs under ambient conditions. [source] PHOTOSYNTHETIC FUNCTION IN DUNALIELLA TERTIOLECTA (CHLOROPHYTA) DURING A NITROGEN STARVATION AND RECOVERY CYCLEJOURNAL OF PHYCOLOGY, Issue 5 2003Erica B. Young Phytoplankton can be exposed to periods of N starvation with episodic N resupply. N starvation in Dunaliella tertiolecta (Butcher) measured over 4 days was characterized by slow reduction in cell chl and protein content and chl/carotenoid ratio and a decline in photosynthetic capacity and maximum quantum yield of photosynthesis (Fv/Fm). In the early stages of N starvation, cell division was maintained despite reduction in cellular chl. Chl content was more sensitive than carotenoids to N deprivation, and cellular chl a was maintained preferentially over chl b under N starvation. NO3, resupply stimulated rapid and complete recovery of Fv/Fm (from 0.4 to 0.7) within 24 h and commencement of cell division after 10 h, although N-replete levels of cell chl and protein were not reestablished within 24 h. Recovery of Fv/Fm was correlated with increases in cell chl and protein and was more related to increases in Fm than to changes in F0. Recovery of Fv/Fm was biphasic with a second phase of recovery commencing 4,6 h after resupply of NO3,. Uptake of NO3, from the external medium and the recovery of Fv/Fm, cell chl, and protein were inhibited when either cytosolic or chloroplastic protein synthesis was inhibited by cycloheximide or lincomycin, respectively; a time lag observed before maximum NO3, uptake was consistent with synthesis of NO3, transporters and assimilation enzymes. When both chloroplastic and cytosolic translation was inhibited, Fv/Fm declined dramatically. Dunaliella tertiolecta demonstrated a capacity to rapidly reestablish photosynthetic function and initiate cell division after N resupply, an important strategy in competing for limiting inorganic N resources. [source] Measurements of Buccal Tissue Volumes at Single-Implant Restorations after Local Bone Grafting in Maxillas: A 3-Year Clinical Prospective Study Case SeriesCLINICAL IMPLANT DENTISTRY AND RELATED RESEARCH, Issue 2 2003Odont PhD, Torsten Jemt LDS ABSTRACT Purpose: The purpose of this study was to measure changes in buccal and proximal tissue volumes after local bone grafting and single-implant treatment. Materials and Methods: Ten patients were provided with buccal bone grafts 6 months prior to implant treatment in central upper incisor regions. Following a healing time of 6 months, abutments and single-implant crowns were installed and followed up for 2 years. Clinical photographs and impressions were taken prior to the surgical intervention as well as after crown placement and at first and second annual checkups. The photographs and study models were analyzed with regard to papilla regeneration and changes in buccal crest volume during the study period by means of a clinical papilla index and optical scanning of study models. Results: All bone grafts healed without problems. A significant reduction of the buccal crest volume (-50%, p <.01) was observed in the grafted area before abutment connection. However, a significant increase of tissue volume (+100%, p <.05) was noticed at the subsequent crown placement, followed by a second but slow reduction of the volume during the following 2 years of function. The interdental papillae increased significantly (p <.05) in volume during the first year, almost completely filling up the embrasure areas after 2 years. Conclusions: It may be concluded that local bone grafting seems to be a valuable protocol to create sufficient bone volume for implant placement. However, significant resorption of the graft may be present, which reduces the impact of grafting on the esthetic outcome. Instead, placement of the abutment cylinder and the crown seems to play a more important role for reestablishing the tissue volume at the implant-supported single crowns. [source] | |||||||||||||