Probing the Reactivity of the Ce=O Multiple Bond in a Cerium(IV) Oxo Complex

The reactivity of the cerium­(IV) oxo complex [(L_OEt)₂Ce^IV(O)­(H₂O)]·MeC­(O)­NH₂ (<b>1</b>; L_OEt^– = [CoCp­{P­(O)­(OEt)₂}₃]⁻, where Cp = η⁵-C₅H₅) toward electrophiles and Brønsted acids has been investigated. The treatment of <b>1</b> with acetic anhydride afforded the diacetate complex [Ce^IV(L_OEt)₂(O₂CMe)₂] (<b>2</b>). The reaction of <b>1</b> with B­(C₆F₅)₃ yielded [Ce^IV(L_OEt)₂(Me₂CONH₂)₂]­[B­(C₆F₅)₃(OH)]₂ (<b>3</b>), in which the [B­(C₆F₅)₃(OH)]⁻ anions are H-bonded to the O-bound acetamide ligands. The treatment of <b>1</b> with HCl and HNO₃ afforded [Ce^IV(L_OEt)₂Cl₂] and [Ce^IV(L_OEt)₂(NO₃)₂], respectively. Protonation of <b>1</b> with triflic acid (HOTf) gave the diaqua complex [Ce^IV(L_OEt)₂(H₂O)₂]­(OTf)₂ (<b>4</b>), in which the triflate anions are H-bonded to the two aqua ligands. The treatment of <b>1</b> with phenol afforded the phenoxide complex [Ce^IV(L_OEt)₂(OPh)₂] (<b>5</b>). The oxo-bridged bimetallic complex [(L_OEt)₂(Me₂CONH₂)­Ce^IV(O)­NaL_OEt] (<b>6</b>) with the Ce–O_oxo and Na–O_oxo distances of 1.953(4) and 2.341(4) Å, respectively, was obtained from the reaction of <b>1</b> with [NaL_OEt]. Density functional theory calculations showed that the model complex [(L_OMe)₂Ce^IV(Me₂CONH₂)­(O)­NaL_OMe] (<b>6A</b>; L_OMe^– = [CoCp­{P­(O)­(OMe)₂}₃]⁻) contains a polarized CeO multiple bond. The energy for dissociation of the {NaL_OMe} fragment from <b>6A</b> in acetonitrile was calculated to be +33.7 kcal/mol, which is higher than that for dissociation of the H-bonded acetamide from [(L_OMe)₂Ce^IV(O)­(H₂O)]·MeC­(O)­NH₂ (<b>1A</b>) (calculated to be +17.4 kcal/mol). In hexanes containing trace water, complex <b>1</b> decomposed readily to a mixture of a tetranuclear cerium­(IV) oxo cluster, [Ce^IV₄(L_OEt)₄(μ₄-O)­(μ₂-O)₄(μ₂-OH)₂] (<b>7</b>), and a cerium­(III) complex, [Ce^III(L_OEt)₂(H₂O)₂]­[L_OEt] [<b>8</b>(L_OEt)], whereas the cerium/sodium oxo complex <b>6</b> is stable under the same conditions. The crystal structures of <b>3</b>, <b>4</b>·H₂O, <b>6</b>, and <b>8</b>(L_OEt) have been determined

Campylobacter jejuni transcriptome changes during loss of culturability in water

Background: Water serves as a potential reservoir for Campylobacter, the leading cause of bacterial gastroenteritis in humans. However, little is understood about the mechanisms underlying variations in survival characteristics between different strains of C. jejuni in natural environments, including water. Results: We identified three Campylobacter jejuni strains that exhibited variability in their ability to retain culturability after suspension in tap water at two different temperatures (4°C and 25°C). Of the three strains C. jejuni M1 exhibited the most rapid loss of culturability whilst retaining viability. Using RNAseq transcriptomics, we characterised C. jejuni M1 gene expression in response to suspension in water by analyzing bacterial suspensions recovered immediately after introduction into water (Time 0), and from two sampling time/temperature combinations where considerable loss of culturability was evident, namely (i) after 24 h at 25°C, and (ii) after 72 h at 4°C. Transcript data were compared with a culture-grown control. Some gene expression characteristics were shared amongst the three populations recovered from water, with more genes being up-regulated than down. Many of the up-regulated genes were identified in the Time 0 sample, whereas the majority of down-regulated genes occurred in the 25°C (24 h) sample. Conclusions: Variations in expression were found amongst genes associated with oxygen tolerance, starvation and osmotic stress. However, we also found upregulation of flagellar assembly genes, accompanied by down-regulation of genes involved in chemotaxis. Our data also suggested a switch from secretion via the sec system to via the tat system, and that the quorum sensing gene luxS may be implicated in the survival of strain M1 in water. Variations in gene expression also occurred in accessory genome regions. Our data suggest that despite the loss of culturability, C. jejuni M1 remains viable and adapts via specific changes in gene expression

Cyanobacterial lipopolysaccharides and human health – a review

Cyanobacterial lipopolysaccharide/s (LPS) are frequently cited in the cyanobacteria literature as toxins responsible for a variety of heath effects in humans, from skin rashes to gastrointestinal, respiratory and allergic reactions. The attribution of toxic properties to cyanobacterial LPS dates from the 1970s, when it was thought that lipid A, the toxic moiety of LPS, was structurally and functionally conserved across all Gram-negative bacteria. However, more recent research has shown that this is not the case, and lipid A structures are now known to be very different, expressing properties ranging from LPS agonists, through weak endotoxicity to LPS antagonists. Although cyanobacterial LPS is widely cited as a putative toxin, most of the small number of formal research reports describe cyanobacterial LPS as weakly toxic compared to LPS from the Enterobacteriaceae. We systematically reviewed the literature on cyanobacterial LPS, and also examined the much lager body of literature relating to heterotrophic bacterial LPS and the atypical lipid A structures of some photosynthetic bacteria. While the literature on the biological activity of heterotrophic bacterial LPS is overwhelmingly large and therefore difficult to review for the purposes of exclusion, we were unable to find a convincing body of evidence to suggest that heterotrophic bacterial LPS, in the absence of other virulence factors, is responsible for acute gastrointestinal, dermatological or allergic reactions via natural exposure routes in humans. There is a danger that initial speculation about cyanobacterial LPS may evolve into orthodoxy without basis in research findings. No cyanobacterial lipid A structures have been described and published to date, so a recommendation is made that cyanobacteriologists should not continue to attribute such a diverse range of clinical symptoms to cyanobacterial LPS without research confirmation

Search for lepton-flavor violating decays of the Higgs boson in the mu tau and e tau final states in proton-proton collisions at root s=13 TeV

A search is presented for lepton-flavor violating decays of the Higgs boson to mu t and et. The dataset corresponds to an integrated luminosity of 137 fb(-1) collected at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV. No significant excess has been found, and the results are interpreted in terms of upper limits on lepton-flavor violating branching fractions of the Higgs boson. The observed (expected) upper limits on the branching fractions are, respectively, B(H -> mu t) e tau) < 0.22(0.16)% at 95% confidence level.Peer reviewe