Ferropericlase (Mg,Fe)O is one of the most abundant minerals of the earth’s lower mantle. The high-spin (HS) to low-spin (LS) transition in the Fe ²⁺ ions may dramatically alter the physical and ...chemical properties of (Mg,Fe)O in the deep mantle. To understand the effects of compression on the ground electronic state of iron, electronic and magnetic states of Fe ²⁺ in (Mg ₀.₇₅Fe ₀.₂₅)O have been investigated using transmission and synchrotron Mössbauer spectroscopy at high pressures and low temperatures (down to 5 K). Our results show that the ground electronic state of Fe ²⁺ at the critical pressure P c of the spin transition close to T = 0 is governed by a quantum critical point (T = 0, P = P c) at which the energy required for the fluctuation between HS and LS states is zero. Analysis of the data gives P c = 55 GPa. Thermal excitation within the HS or LS states (T > 0 K) is expected to strongly influence the magnetic as well as physical properties of ferropericlase. Multielectron theoretical calculations show that the existence of the quantum critical point at temperatures approaching zero affects not only physical properties of ferropericlase at low temperatures but also its properties at P-T of the earth’s lower mantle.
Sterols and triterpenes are structurally diverse bioactive molecules generated through cyclization of linear 2,3-oxidosqualene. Based on carbocationic intermediates generated during initial substrate ...preorganization step, oxidosqualene cyclases (OSCs) are roughly segregated into protosteryl cation group that mainly catalyzes tetracyclic products and dammarenyl cation group which mostly generates pentacyclic products. However, in contrast to well-studied cascade of ring-forming reactions, little is known about the mechanism underlying the initial substrate folding process. Previously, we have identified a cucurbitadienol synthase (Bi) and its null allele bi (C393Y) from cucumber. By integration of homology modeling, residue coevolution and site-directed mutagenesis, we discover that four covarying amino acids including C393 constitute a dynamic domain that may be involved in substrate folding process for Bi. We also reveal a group of co-conserved residues that closely associated with the segregation of plant OSCs. These residues may act collaboratively in choice of specific substrate folding intermediate for OSCs. Thus, our findings open a door to engineer plant OSCs from four-ringed skeleton catalysts into five-ringed producer.
The atomic and electronic structure of vanadium phosphide one- to four-atomic-layer thin films and their composites with zinc oxide substrate are modelled by means of quantum chemistry. Favorable ...vanadium phosphide to ZnO orientation is defined and found to remain the same for all the structures under consideration. The electronic structure of the composites is analyzed in detail. The features of the charge and spin density distribution are discussed.
Evolutionary pressure on residue interactions, intramolecular or intermolecular, that are important for protein structure or function can lead to covariance between the two positions. Recent ...methodological advances allow much more accurate contact predictions to be derived from this evolutionary covariance signal. The practical application of contact predictions has largely been confined to structural bioinformatics, yet, as this work seeks to demonstrate, the data can be of enormous value to the structural biologist working in X-ray crystallo-graphy, cryo-EM or NMR. Integrative structural bioinformatics packages such as
can already exploit contact predictions in a variety of ways. The contribution of contact predictions begins at construct design, where structural domains may need to be expressed separately and contact predictions can help to predict domain limits. Structure solution by molecular replacement (MR) benefits from contact predictions in diverse ways: in difficult cases, more accurate search models can be constructed using
modelling when predictions are available, while intermolecular contact predictions can allow the construction of larger, oligomeric search models. Furthermore, MR using supersecondary motifs or large-scale screens against the PDB can exploit information, such as the parallel or antiparallel nature of any β-strand pairing in the target, that can be inferred from contact predictions. Contact information will be particularly valuable in the determination of lower resolution structures by helping to assign sequence register. In large complexes, contact information may allow the identity of a protein responsible for a certain region of density to be determined and then assist in the orientation of an available model within that density. In NMR, predicted contacts can provide long-range information to extend the upper size limit of the technique in a manner analogous but complementary to experimental methods. Finally, predicted contacts can distinguish between biologically relevant interfaces and mere lattice contacts in a final crystal structure, and have potential in the identification of functionally important regions and in foreseeing the consequences of mutations.
Exchange bias in graphitic C/Co composites Hsu, Hua-Shu; Chang, Yu-Ying; Chin, Yi-Ying ...
Carbon (New York),
April 2017, 2017-04-00, 20170401, Letnik:
114
Journal Article
Recenzirano
Odprti dostop
The exchange bias (EB) effect, which is the shift of the hysteresis loop of a ferromagnet in direct contact with an antiferromagnet, is highly advantageous for the development of spintronics ...applications. Carbon (C) has been considered as a potential material in next generation electronics production as well as spintronics devices beyond silicon. Here we show experimental evidence for an EB in C/Co composites. The significant EB needs thermal annealing to occur. X-ray absorption spectra and transmission electron microscopy data of these samples reveal that Co carbides in as grown samples decompose and form graphitic C/Co interfaces after annealing. Using x-ray magnetic circular dichroism we have detected the C spins that are responsible for the EB. These results inspire a theoretical model to investigate the magnetic interactions in graphitic C/Co interfaces and interpret the observed results.
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We investigated the effects of excited many-electron states in the optical control of the magnetic state in undoped Mott-Hubbard insulator. To derive the spin Hamiltonian in material under optical ...pumping one has used a many-electron approach based on the X-operator representation. Extending the projection operators approach on arbitrary energy spectra of the Mott-Hubbard insulator, we obtained the Hamiltonian of superexchange interaction in analytical form. The Hamiltonian includes the spin-exciton variables which are usually missing in discussion on the magnetic response to optical pumping. The superexchange is also not additive over contributions from the ground and optical excited states, and nonzero contributions to the Dzyaloshinskii-Moriya interaction are induced in insulators with different spins at the ground and excited cell states. As a test, a microscopic background for the optical induced superexchange was analyzed in La2CuO4 (further La214) and FeBO3 with spins 1/2 and 5/2, respectively.
We considered the effect of optical pumping at the f−f transitions frequency on d−f superexchange in the rare-earth antiferromagnetic ferroborate Nd0.5 Gd0.5 Fe3 (BO3)4 between Nd3+ ions in the ...excited states of the 4G5/2, 2G7/2, and Fe3+ in the ground state 6A1. In the ferroborate, d−f superexchange is directly related to the strong magnetoelectric coupling observed in the ground state of the material. We show that under optical pumping at the frequency of f−f transitions 4I9/2 ↔ (4G5/2 + 2G7/2) in the Nd3+ ion, the nature of the d−f interaction changes to FM. The phenomena can be explained by the occupation of the excited Nd3+ "spin pure" states with spin 1/2, or their mixing to the optically excited states with spin 3/2 due to the spin-orbit interaction. Near optically excited ions Nd3+, magnetic frustrations change from FM ordering under the d−f AFM interaction to AFM ordering under the FM interaction in the basal hexagonal plane.
Gd endohedral complexes of C82 fullerenols were synthesized and mass spectrometry analysis of their composition was carried out. It was established that the synthesis yields a series of fullerenols ...Gd@C82Ox(OH)y (x = 0, 3; y = 8, 16, 24, 36, 44). The atomic and electronic structure and properties of the synthesized fullerenols were investigated using the density functional theory calculations. It was shown that the presence of endohedral gadolinium increases the reactivity of fullerenols. It is proposed that the high-spin endohedral fullerenols are promising candidates for application in magnetic resonance imaging.
Coelenteramide-containing fluorescent proteins are products of bioluminescent reactions of marine coelenterates. They are called ‘discharged photoproteins’. Their light-induced fluorescence spectra ...are variable, depending considerably on external conditions. Current work studies a dependence of light-induced fluorescence spectra of discharged photoproteins obelin, aequorin, and clytin on excitation energy. It was demonstrated that photoexcitation to the upper electron-excited states (260–300nm) of the discharged photoproteins initiates a fluorescence peak in the near UV region, in addition to the blue-green emission. To characterize the UV fluorescence, the light-induced fluorescence spectra of coelenteramide (CLM), fluorophore of the discharged photoproteins, were studied in methanol solution. Similar to photoproteins, the CLM spectra depended on photoexcitation energy; the additional peak (330nm) in the near UV region was observed in CLM fluorescence at higher excitation energy (260–300nm). Quantum chemical calculations by time depending method with B3LYP/cc-pVDZ showed that the conjugated pyrazine-phenolic fragment and benzene moiety of CLM molecule are responsible for the additional UV fluorescence peak. Quantum yields of CLM fluorescence in methanol were 0.028±0.005 at 270–340nm photoexcitation. A conclusion was made that the UV emission of CLM might contribute to the UV fluorescence of the discharged photoproteins. The study develops knowledge on internal energy transfer in biological structures – complexes of proteins with low-weight aromatic molecules.
Ultraviolet fluorescence of coelenteramide, fluorophore of coelenteramide-containing proteins. Display omitted
•UV band appears in discharged photoprotein fluorescence at 260–300nm excitation.•UV band (330nm) appears in coelenteramide fluorescence at 260–300nm excitation.•Benzene and pyrazine-phenolic moieties contribute to UV coelenteramide fluorescence.
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