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1. Reduction of dynatomic curves

   https://doi.org/10.1017/etds.2017.140  

   DOYLE, JOHN R. ; KRIEGER, HOLLY ; OBUS, ANDREW ; PRIES, RACHEL ; RUBINSTEIN-SALZEDO, SIMON ; WEST, LLOYD ( October 2019 , Ergodic Theory and Dynamical Systems)

   In this paper, we make partial progress on a function field version of the dynamical uniform boundedness conjecture for certain one-dimensional families ${\mathcal{F}}$ of polynomial maps, such as the family $f_{c}(x)=x^{m}+c$ , where $m\geq 2$ . We do this by making use of the dynatomic modular curves $Y_{1}(n)$ (respectively $Y_{0}(n)$ ) which parametrize maps $f$ in ${\mathcal{F}}$ together with a point (respectively orbit) of period $n$ for $f$ . The key point in our strategy is to study the set of primes $p$ for which the reduction of $Y_{1}(n)$ modulo $p$ fails to be smooth or irreducible. Morton gave an algorithm to construct, for each $n$ , a discriminant $D_{n}$ whose list of prime factors contains all the primes of bad reduction for $Y_{1}(n)$ . In this paper, we refine and strengthen Morton’s results. Specifically, we exhibit two criteria on a prime $p$ dividing $D_{n}$ : one guarantees that $p$ is in fact a prime of bad reduction for $Y_{1}(n)$ , yet this same criterion implies that $Y_{0}(n)$ is geometrically irreducible. The other guarantees that the reduction of $Y_{1}(n)$ modulo $p$ is actually smooth. As an application of the second criterion, we extend results of Morton, Flynn, Poonen, Schaefer, andmore »Stoll by giving new examples of good reduction of $Y_{1}(n)$ for several primes dividing $D_{n}$ when $n=7,8,11$ , and $f_{c}(x)=x^{2}+c$ . The proofs involve a blend of arithmetic and complex dynamics, reduction theory for curves, ramification theory, and the combinatorics of the Mandelbrot set.« less
2. Biocatalytic removal of perchlorate and nitrate in ion-exchange waste brine

   https://doi.org/10.1039/C8EW00178B  

   Hutchison, Justin M. ; Zilles, Julie L. ( January 2018 , Environmental Science: Water Research & Technology)

   Biocatalytic technologies are characterized by targeted, rapid degradation of contaminants over a range of environmentally relevant conditions representative of groundwater, but have not yet been integrated into drinking water treatment processes. This work investigated the potential for a hybrid ion-exchange/biocatalytic process, where biocatalysis is used to treat ion-exchange waste brine, allowing reuse of the brine. The reduction rates and the fate of the regulated anions perchlorate and nitrate were tested in synthetic brines and a real-world waste brine. Biocatalysts were applied as soluble protein fractions from Azospira oryzae for perchlorate reduction and Paracoccus denitrificans and Haloferax denitrificans for nitrate reduction. In synthetic 12% brine, the biocatalysts retained activity, with rates of 32.3 ± 6.1 U (μg Mo) −1 for perchlorate ( A. oryzae ) and 16.1 ± 7.1 U (μg Mo) −1 for nitrate ( P. denitrificans ). In real-world waste brine, activities were slightly lower (20.3 ± 6.5 U (μg Mo) −1 for perchlorate and 14.3 ± 3.8 U (μg Mo) −1 for nitrate). The difference in perchlorate reduction was due to higher concentrations of nitrate, bicarbonate, and sulfate in the waste brine. The predominant end products of nitrate reduction were nitrous oxide or dinitrogen gas, depending on themore »source of the biocatalysts and the salt concentration. These results demonstrate biocatalytic reduction of regulated anions in a real-world waste brine, which could facilitate brine reuse for the regeneration of ion-exchange technologies and prevent reintroduction of these anions and their intermediates into the environment.« less
3. Reduction-induced CO dissociation by a [Mn(bpy)(CO) ₄ ][SbF ₆ ] complex and its relevance in electrocatalytic CO ₂ reduction

   https://doi.org/10.1039/c9dt04150h  

   Kuo, Hsin-Ya ; Tignor, Steven E. ; Lee, Tia S. ; Ni, Danrui ; Park, James Eujin ; Scholes, Gregory D. ; Bocarsly, Andrew B. ( January 2020 , Dalton Transactions)

   [Mn(bpy)(CO) 3 Br] is recognized as a benchmark electrocatalyst for CO 2 reduction to CO, with the doubly reduced [Mn(bpy)(CO) 3 ] − proposed to be the active species in the catalytic mechanism. The reaction of this intermediate with CO 2 and two protons is expected to produce the tetracarbonyl cation, [Mn(bpy)(CO) 4 ] + , thereby closing the catalytic cycle. However, this species has not been experimentally observed. In this study, [Mn(bpy)(CO) 4 ][SbF 6 ] ( 1 ) was directly synthesized and found to be an efficient electrocatalyst for the reduction of CO 2 to CO in the presence of H 2 O. Complex 1 was characterized using X-ray crystallography as well as IR and UV-Vis spectroscopy. The redox activity of 1 was determined using cyclic voltammetry and compared with that of benchmark manganese complexes, e.g. , [Mn(bpy)(CO) 3 Br] ( 2 ) and [Mn(bpy)(CO) 3 (MeCN)][PF 6 ] ( 3 ). Infrared spectroscopic analyses indicated that CO dissociation occurs after a single-electron reduction of complex 1 , producing a [Mn(bpy)(CO) 3 (MeCN)] + species. Complex 1 was experimentally verified as both a precatalyst and an on-cycle intermediate in homogeneous Mn-based electrocatalytic CO 2 reduction.
4. NfoR: Chromate Reductase or Flavin Mononucleotide Reductase?

   https://doi.org/10.1128/AEM.01758-20  

   O’Neill, Audrey G. ; Beaupre, Brett A. ; Zheng, Yuanzhang ; Liu, Dali ; Moran, Graham R. ( October 2020 , Applied and Environmental Microbiology)

   Pettinari, M. Julia (Ed.)

   ABSTRACT Soil bacteria can detoxify Cr(VI) ions by reduction. Within the last 2 decades, numerous reports of chromate reductase enzymes have been published. These reports describe catalytic reduction of chromate ions by specific enzymes. These enzymes each have sequence similarity to known redox-active flavoproteins. We investigated the enzyme NfoR from Staphylococcus aureus , which was reported to be upregulated in chromate-rich soils and to have chromate reductase activity (H. Han, Z. Ling, T. Zhou, R. Xu, et al., Sci Rep 7:15481, 2017, https://doi.org/10.1038/s41598-017-15588-y ). We show that NfoR has structural similarity to known flavin mononucleotide (FMN) reductases and reduces FMN as a substrate. NfoR binds FMN with a dissociation constant of 0.4 μM. The enzyme then binds NADPH with a dissociation constant of 140 μM and reduces the flavin at a rate of 1,350 s −1 . Turnover of the enzyme is apparently limited by the rate of product release that occurs, with a net rate constant of 0.45 s −1 . The rate of product release limits the rate of observed chromate reduction, so the net rate of chromate reduction by NfoR is orders of magnitude lower than when this process occurs in solution. We propose that NfoR is an FMNmore »reductase and that the criterion required to define chromate reduction as enzymatic has not been met. That NfoR expression is increased in the presence of chromate suggests that the survival adaption was to increase the net rate of chromate reduction by facile, adventitious redox processes. IMPORTANCE Chromate is a toxic by-product of multiple industrial processes. Chromate reduction is an important biological activity that ameliorates Cr(VI) toxicity. Numerous researchers have identified chromate reductase activity by observing chromate reduction. However, all identified chromate reductase enzymes have flavin as a cofactor or use a flavin as a substrate. We show here that NfoR, an enzyme claimed to be a chromate reductase, is in fact an FMN reductase. In addition, we show that reduction of a flavin is a viable way to transfer electrons to chromate but that it is unlikely to be the native function of enzymes. We propose that upregulation of a redox-active flavoprotein is a viable means to detoxify chromate that relies on adventitious reduction that is not catalyzed.« less
5. The importance of the counter-cation in reductive rare-earth metal chemistry: 18-crown-6 instead of 2,2,2-cryptand allows isolation of [Y ^II (NR ₂ ) ₃ ] ¹⁻ and ynediolate and enediolate complexes from CO reactions

   https://doi.org/10.1039/C9SC05794C  

   Ryan, Austin J. ; Ziller, Joseph W. ; Evans, William J. ( February 2020 , Chemical Science)

   The use of 18-crown-6 (18-c-6) in place of 2.2.2-cryptand (crypt) in rare earth amide reduction reactions involving potassium has proven to be crucial in the synthesis of Ln( ii ) complexes and isolation of their CO reduction products. The faster speed of crystallization with 18-c-6 appears to be important. Previous studies have shown that reduction of the trivalent amide complexes Ln(NR 2 ) 3 (R = SiMe 3 ) with potassium in the presence of 2.2.2-cryptand (crypt) forms the divalent [K(crypt)][Ln II (NR 2 ) 3 ] complexes for Ln = Gd, Tb, Dy, and Tm. However, for Ho and Er, the [Ln(NR 2 ) 3 ] 1− anions were only isolable with [Rb(crypt)] 1+ counter-cations and isolation of the [Y II (NR 2 ) 3 ] 1− anion was not possible under any of these conditions. We now report that by changing the potassium chelator from crypt to 18-crown-6 (18-c-6), the [Ln(NR 2 ) 3 ] 1− anions can be isolated not only for Ln = Gd, Tb, Dy, and Tm, but also for Ho, Er, and Y. Specifically, these anions are isolated as salts of a 1 : 2 potassium : crown sandwich cation, [K(18-c-6) 2 ] 1+ , i.e. [K(18-c-6) 2more »][Ln(NR 2 ) 3 ]. The [K(18-c-6) 2 ] 1+ counter-cation was superior not only in the synthesis, but it also allowed the isolation of crystallographically-characterizable products from reactions of CO with the [Ln(NR 2 ) 3 ] 1− anions that were not obtainable from the [K(crypt)] 1+ analogs. Reaction of CO with [K(18-c-6) 2 ][Ln(NR 2 ) 3 ], generated in situ , yielded crystals of the ynediolate products, {[(R 2 N) 3 Ln] 2 (μ-OCCO)} 2− , which crystallized with counter-cations possessing 2 : 3 potassium : crown ratios, i.e. {[K 2 (18-c-6) 3 ]} 2+ , for Gd, Dy, Ho. In contrast, reaction of CO with a solution of isolated [K(18-c-6) 2 ][Gd(NR 2 ) 3 ], produced crystals of an enediolate complex isolated with a counter-cation with a 2 : 2 potassium : crown ratio namely [K(18-c-6)] 2 2+ in the complex [K(18-c-6)] 2 {[(R 2 N) 2 Gd 2 (μ-OCHCHO) 2 ]}.« less