On the feasibility of N2 fixation via a single-site FeI/FeIV cycle: Spectroscopic studies of FeI(N2)FeI, FeIV=N, and related species

The electronic properties of an unusually redox-rich iron system, [PhBPR 3]FeNx (where [PhBPR 3] is [PhB(CH2PR2)3]−), are explored by Mössbauer, EPR, magnetization, and density-functional methods to gain a detailed picture regarding their oxidation states and electronic structures. The complexes of primary interest in this article are the two terminal iron(IV) nitride species, [PhBPiPr 3]FeN (3a) and [PhBPCH2Cy 3]FeN (3b), and the formally diiron(I) bridged-Fe(μ-N2)Fe species, {[PhBPiPr 3]Fe}2(μ-N2) (4). Complex 4 is chemically related to 3a via a spontaneous nitride coupling reaction. The diamagnetic iron(IV) nitrides 3a and 3b exhibit unique electronic environments that are reflected in their unusual Mössbauer parameters, including quadrupole-splitting values of 6.01(1) mm/s and isomer shift values of −0.34(1) mm/s. The data for 4 suggest that this complex can be described by a weak ferromagnetic interaction (J/D < 1) between two iron(I) centers. For comparison, four other relevant complexes also are characterized: a diamagnetic iron(IV) trihydride [PhBPiPr 3]Fe(H)3(PMe3) (5), an S = 3/2 iron(I) phosphine adduct [PhBPiPr 3]FePMe3 (6), and the S = 2 iron(II) precursors to 3a, [PhBPiPr 3]FeCl and [PhBPiPr 3]Fe-2,3:5,6-dibenzo-7-aza bicyclo[2.2.1]hepta-2,5-diene (dbabh). The electronic properties of these respective complexes also have been explored by density-functional methods to help corroborate our spectral assignments and to probe their electronic structures further

Performance Characteristics of qPCR Assays Targeting Human- and Ruminant-Associated Bacteroidetes for Microbial Source Tracking across Sixteen Countries on Six Continents

Numerous quantitative PCR assays for microbial fecal source tracking (MST) have been developed and evaluated in recent years. Widespread application has been hindered by a lack of knowledge regarding the geographical stability and hence applicability of such methods beyond the regional level. This study assessed the performance of five previously reported quantitative PCR assays targeting human-, cattle-, or ruminant-associated Bacteroidetes populations on 280 human and animal fecal samples from 16 countries across six continents. The tested cattle-associated markers were shown to be ruminant-associated. The quantitative distributions of marker concentrations in target and nontarget samples proved to be essential for the assessment of assay performance and were used to establish a new metric for quantitative source-specificity. In general, this study demonstrates that stable target populations required for marker-based MST occur around the globe. Ruminant-associated marker concentrations were strongly correlated with total intestinal Bacteroidetes populations and with each other, indicating that the detected ruminant-associated populations seem to be part of the intestinal core microbiome of ruminants worldwide. Consequently tested ruminant-targeted assays appear to be suitable quantitative MST tools beyond the regional level while the targeted human-associated populations seem to be less prevalent and stable, suggesting potential for improvements in human-targeted methods

Evaluation of commercial qPCR kits for detection of SARS-CoV-2 in pooled samples

AbstractDue to the current pandemic, global shortage of reagents has drawn interest in developing alternatives to increase the number coronavirus tests. One such alternative is sample pooling. Here we compared commercial kits that are used in COVID-19 diagnostics, in terms of sensitivity and feasibility for use in pooling. We showed that pooling of up to 60 samples did not affect the efficiency of the kits. Also, the RNA dependent RNA polymerase (RdRp) is a more suitable target in pooled samples than the Envelope (E) protein. This approach could provide an easy method of screening large number of samples and help adjust different government regulations.</jats:p

Density Functional Study of the Electric Hyperfine Interactions and the Redox-Structural Correlations in the Cofactor of Nitrogenase. Analysis of General Trends in ⁵⁷Fe Isomer Shifts

The influence of the interstitial atom, X, discovered in a recent crystallographic study of the MoFe protein of nitrogenase, on the electric hyperfine interactions of 57Fe has been investigated with density functional theory. A semiempirical theory for the isomer shift, δ, is formulated and applied to the cofactor. The values of δ for the relevant redox states of the cofactor are predicted to be higher in the presence of X than in its absence. The analysis strongly suggests a [Mo4+4Fe2+3Fe3+] oxidation state for the S = 3/2 state MN. Among C4-, N3-, and O2-, oxide is found to be the least likely candidate for X. The analysis suggests that X should be present in the cofactor states MOX and MR as well as in the alternative nitrogenases. The calculations of the electric field gradients (EFGs) indicate that the small values for ΔEQ in MN result from an extensive cancellation between valence and ligand contributions. X emerges from the analysis of the hyperfine interactions as an ionically bonded species. Its major effect is on the asymmetry parameters for the EFGs at the six equatorial sites, FeEq. A spin-coupling scheme is proposed for the state [Mo4+4Fe2+3Fe3+] that is consistent with the measured 57Fe A-tensors and ΔEQ values for MN and identifies the unique site exhibiting the small A value with the terminal Fe site, FeT. The optimized structure of a cofactor model has been calculated for several oxidation states. The study reveals a contraction in the average Fe−Fe distance upon increasing the number of electrons stored in the cluster, in accord with extended X-ray absorption fine structure studies. The reliability of the adopted methodology for predicting redox-structural correlations is tested for cuboidal [4Fe−4S] clusters. The calculations reveal a systematic increase in the S···S sulfide distances, in quantitative agreement with the available data. These trends are rationalized by a simple electrostatic model

Theoretical Analysis of the Jahn−Teller Distortions in Tetrathiolato Iron(II) Complexes

Crystallographic studies of [Fe(SR)4]2- (R is an alkyl or aryl residue) have shown that the FeIIS4 cores of these complexes have (pseudo) D2d symmetry. Here we analyze the possibility that these structures result from a Jahn−Teller (JT) distortion that arises from the e{3z2 − r2, x2 − y2} orbital ground state of FeII in Td symmetry. Special attention is paid to the influence of the second-nearest neighbors of Fe, which lowers the symmetry and reduces the full JT effect to a smaller, pseudo JT effect (PJT). To estimate the size of the PJT distortion, we have determined the vibronic parameters and orbital state energies for a number of [Fe(SR)4]2- models using density functional theory (DFT). Subsequently, this information is used for evaluating the adiabatic potential surfaces in the space of the JT-active coordinates of the FeS4 moiety. The surfaces reveal that the JT effect of FeII is completely quenched by the tetrathiolate coordination

Theoretical Analysis of the Three-Dimensional Structure of Tetrathiolato Iron Complexes

The three-dimensional structures of a number of [M(SR)4]n- complexes, where M is a 3d transition metal and R is an alkyl or aryl group, have been analyzed using density functional theory (DFT). Special attention is paid to the FeII/FeIII mimics of rubredoxin. The FeII model complex [Fe(SCH3)4]2- has an equilibrium conformation with D2d symmetry. The DFT energy has been decomposed into contributions for ligand−ligand and metal−ligand interactions. The latter contribution is analyzed with the angular overlap model (AOM) and constitutes the dominant stereospecific interaction in the FeII complex. The sulfur lone-pair electrons exert anisotropic π interactions on the 3d6 shell of FeII, which are controlled by the torsion angles, ωi, for the rotations of the Si−Cβ bonds around the Fe−Si axes. In contrast, the π interactions acting on the high-spin 3d5 shell of FeIII are isotropic. As a consequence, the stereochemistry of the FeIII complexes is determined by the Coulomb repulsions between the ligands and has S4 symmetry. The electrostatic repulsions between the lone pairs of the sulfurs are an essential component of the ligand−ligand interaction. The lone-pair repulsions distort the ∠SFeS‘ angles (δ + δt) and give rise to a correlation between δ and ω, which is confirmed by crystallographic data. Both the FeII and FeIII complexes exhibit structural bistability due to the presence of low-lying equilibrium conformations with S4 symmetry in which the complex can be trapped by the crystalline host