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Mononuclear non-haem iron (NHFe) enzymes catalyse a broad range of oxidative reactions, including halogenation, hydroxylation, ring closure, desaturation and aromatic ring cleavage reactions. They are involved in a number of biological processes, including phenylalanine metabolism, the production of neurotransmitters, the hypoxic response and the biosynthesis of secondary metabolites (1-3). The reactive intermediate in the catalytic cycles of these enzymes is a high-spin S = 2 Fe(IV)=O species, which has been trapped for a number of NHFe enzymes (4-8), including the halogenase SyrB2 (syringomycin biosynthesis enzyme 2). Computational studies aimed at understanding the reactivity of this Fe(IV)=O intermediate (9-13) are limited in applicability owing to the paucity of experimental knowledge about its geometric and electronic structure. Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) is a sensitive and effective method that defines the dependence of the vibrational modes involving Fe on the nature of the Fe(IV)=O active site (14-16). Here we present NRVS structural characterization of the reactive Fe(IV)5O intermediate of a NHFe enzyme, namely the halogenase SyrB2 from the bacterium Pseudomonas syringae pv. syringae. This intermediate reacts via an initial hydrogen-atom abstraction step, performing subsequent halogenation of the native substrate or hydroxylation of non-native substrates (17). A correlation of the experimental NRVS data to electronic structure calculations indicates that the substrate directs the orientation of the Fe(IV)=O intermediate, presenting specific frontier molecular orbitals that can activate either selective halogenation or hydroxylation., SyrB2, an α-ketoglutarate (αKG)-dependent NHFe enzyme found in Pseudomonas syringae pv. syringae, halogenates the methyl group of L-threonine (L-Thr) using non-ribosomal peptide synthetase machinery (18). The Fe(II) active site is [...]

The electronic spin state of [Fe.sup.2+] in ferropericlase, ([Mg.sub.0.75][Fe.sub.0.25])O, transitions from a high-spin (spin unpaired) to low-spin (spin paired) state within the Earth's mid-lower mantle region. To better understand the local electronic environment of high-spin [Fe.sup.2+] ions in ferropericlase near the transition, we obtained synchrotron Mossbauer spectra (SMS) of ([Mg.sub.0.75],[Fe.sub.0.25])O in externally heated and laser-heated diamond anvil cells at relevant high pressures and temperatures. Results show that the quadrupole splitting (QS) of the dominant high-spin [Fe.sup.2+] site decreases with increasing temperature at static high pressure. The QS values at constant pressure are fitted to a temperature-dependent Boltzmann distribution model, which permits estimation of the crystal-field splitting energy ([[DELTA].sub.3]) between the [d.sub.xy] and [d.sub.xz] or [d.sub.zy] orbitals of the [t.sub.2g] states in a distorted octahedral [Fe.sup.2+] site. The derived [[DELTA].sub.3] increases from approximately 36 meV at 1 GPa to 95 meV at 40 GPa, revealing that both high pressure and high temperature have significant effects on the 3d electronic shells of [Fe.sup.2+] in ferroperielase. The SMS spectra collected from the laser-heated diamond cells within the time window of 146 ns also indicate that QS significantly decreases at very high temperatures. A larger splitting of the energy levels at high temperatures and pressures should broaden the spin crossover in ferropericlase because the degeneracy of energy levels is partially lifted. Our results provide information on the hyperfine parameters and crystal-field splitting energy of high-spin [Fe.sup.2+] in ferropericlase at high pressures and temperatures, relevant to the electronic structure of iron in oxides in the deep lower mantle. Keywords: Ferroperielase, diamond-anvil cell, spin transition, Mossbauer spectroscopy, high pressures, high temperatures

High pressure is an effective tool to induce exotic quantum phenomena in magnetic topological insulators by controlling the interplay of magnetic order and topological state. This work presents a comprehensive high-pressure study of the crystal structure and magnetic ground state up to 62 GPa in an intrinsic topological magnet EuSn2P2. With a combination of high resolution X-ray diffraction, 151Eu synchrotron Mössbauer spectroscopy, X-ray absorption spectroscopy, molecular orbital calculations, and electronic band structure calculations, it has been revealed that pressure drives EuSn2P2 from a rhombohedral crystal to an amorphous phase at 36 GPa accompanied by a fourfold enhancement of magnetic ordering temperature. In the pressure-induced amorphous phase, Eu ions take an intermediate valence state. The drastic enhancement of magnetic ordering temperature from 30 K at ambient pressure to 130 K at 41.2 GPa resulting from Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions likely attributes to the stronger Eu–Sn interaction at high pressure. These rich results demonstrate that EuSn2P2 is an ideal platform to study the correlation of the enhanced RKKY interactions, disordered lattice, intermediate valence, and topological state. [ABSTRACT FROM AUTHOR]

The electronic environment of the Fe nuclei in two silicate perovskite samples, [Fe.sub.0.05][Mg.sub.0.95]Si[O.sub.3] (Pv05) and [Fe.sub.0.1][Mg.sub.0.9]Si[O.sub.3] (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mossbauer spectroscopy (SMS). Such investigations of extremely small and dilute [sup.57]Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the 'three-doublet' model, which assumes two [Fe.sup.2+]-like sites and one [Fe.sup.3+]-like site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, [Fe.sup.3+]/[SIGMA]Fe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of [Fe.sup.3+] remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)Si[O.sub.3] perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of [Fe.sup.3+] from a high-spin to a low-spin state.

NPAT, a substrate of the cyclin E-Cdk2 complex, has been found to link cyclin E-Cdk2 to the regulation of replication-dependent histone gene transcription. NPAT was shown to activate histone gene transcription, and the activation depends on promoter elements already thought to mediate cell cycle-dependent transcription.

Xenopus oocyte cell extracts administered to Xenopus egg extracts block the Xenopus sperm nuclei and double-stranded plasmid DNA template replication. The immature oocytes contain molecules that negatively control the replication of chromosomal DNA. Negative factors decrease during the maturation of the oocytes. The inhibitory process is not caused due to template DNA degradation or suppression of chain elongation by the oocytes.

Studies have revealed that four chromosomal replication origin regions in higher eukaryotes commonly share modular sequence elements such as DNA unwinding elements, pyrimidine tracts, scaffold-associated regions and transcriptional regulatory sequences. Initiator protein binding sites and inherently destabilized regions might also possibly be shared. A DNA replication initiation model which postulates uncoupled DNA duplex unwinding, transient single stranded intermediates and DNA synthesis at multiple sites is proposed.