Effect of germanium on the microstructure and the magnetic properties of Fe–B–Si amorphous alloys

In this work, we present a systematic study on the crystallization kinetics and the magnetic properties of melt-spun Fe₈₀B₁₀Si_10 ? xGe_x (x = 0.0 ? 10.0) amorphous alloys. The activation energy for crystallization, determined by differential scanning calorimetry, displayed a strong dependence on the Ge content, reflecting a deleterious effect on the alloys' thermal stability and their glass forming ability with increasing Ge concentration. On the other hand, the alloys exhibited excellent soft magnetic properties, i.e., high saturation magnetization values (around 1.60 T), alongside Curie temperatures of up to 600 K. Complementary, for increasing Ge substitution, the ferromagnetic resonance spectra showed a microstructural evolution comprising at least two different magnetic phases corresponding to a majority amorphous matrix and to Fe(Si, Ge) nanocrystallites for x ≥ 7.5.     

Quadruplex-based molecular beacons as tunable DNA probes

Molecular beacons (MBs) are fluorescent nucleic acid probes with a hairpin-shaped structure in which the 5' and 3' ends are self-complementary. Due to a change in their emissive properties upon recognition with complementary sequences, MBs allow the diagnosis of single-stranded DNA or RNA with high mismatch discrimination, in vitro and in vivo. Whereas the stems of MB hairpins usually rely on the formation of a Watson-Crick duplex, we demonstrate in this report that the preceding structure can be replaced by a G-quadruplex motif (G4). Intramolecular quadruplexes may still be formed with a central loop composed of 12 to 21 bases, therefore extending the sequence repertoire of quadruplex formation. G4-MB can efficiently be used for oligonucleotide discrimination: in the presence of a complementary sequence, the central loop hybridizes and forms a duplex that causes opening of the quadruplex stem. The corresponding G4-MB unfolding can be detected by a change in its fluorescence emission. We discuss the thermodynamic and kinetic opportunities that are provided by using G4-MB instead of traditional MB. In particular, the intrinsic feature of the quadruplex motif facilitates the design of functional molecular beacons by independently varying the concentration of monovalent or divalent cations in the medium.     

Observation of a photoexcited state of a paramagnetic transition metal complex by time-resolved electron paramagnetic resonance spectroscopy

The first observation of a spin polarized excited state of a paramagnetic metal-complex using time-resolved electron paramagnetic resonance (TREPR) spectroscopy is reported for octaethylporphinatooxovanadium(iv). The TREPR spectra show well resolved orientation dependent hyperfine splitting to the I = 7/2 vanadium nucleus. The reduction of the hyperfine splitting by a factor of 3 compared to the ground state and the observation of a multiplet pattern of spin polarization allow the TREPR spectra to be assigned to the excited quartet state of the complex. The spin polarization patterns evolve with time and it is postulated that this is a result of the equilibration between the lowest excited quartet and doublet states.     

Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: An Investigation of Electron Transfer at Different Temperatures by Multifrequency Time-Resolved and CW EPR

The ambient temperature and low-temperature electron transfer properties of Photosystem I (PS I) from the M688N_PsaA and M668N_PsaB mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 are studied using transient electron paramagnetic resonance (EPR) and continuous-wave (CW) EPR. The two mutations are expected to alter the midpoint potentials of, and the reorganization energies around, the primary electron chlorophyll acceptors A_0A and A_0B, which should lead to a change in the yield and/or rate of electron transfer to the phylloquinone acceptors A_1A and A_1B, respectively. At ambient temperature it is known that both quinone acceptors are active in electron transfer. At low temperature there are at least two fractions that undergo either reversible or irreversible electron transfer. The EPR data of the two PS I variants are used to investigate the relationship between these low-temperature fractions and the ambient temperature electron transfer pathway. The results show that mutation in the PsaA-branch increases the rate of $ {\text{A}}_{{1{\text{A}}}}^{. - } $ to F_X electron transfer at ambient temperature, while the corresponding mutation in the PsaB-branch has no effect on the electron transfer rate observable by transient EPR. An analysis of the complete time/field datasets from both variants suggests that the yield of electron transfer in the branch carrying the mutation is reduced. The mutations have no effect on the low-temperature CW EPR spectra of the iron–sulfur clusters if the samples are frozen under illumination but they both cause a decrease in the yield of reduced F_A and F_B if the samples are frozen in the dark and then illuminated. The PsaA-branch mutation greatly reduces the intensity and changes the polarization pattern of the radical pair $ {\text{P}}_{700}^{ + } {\text{A}}_{1}^{. - } $ . Possible causes of the changes in the polarization pattern are discussed and it is suggested that the mutations introduce structural heterogeneity in the vicinity of the A₀ binding site. No clear correlation between the yield of electron transfer in a particular branch and the yield of stable charge separation is found.     

Algorithmic construction of lumps

We use the singular manifold method to generate lump solutions of a Schrödinger equation in 2+1 dimensions and present three different types of such solutions.     

Acetylcholinesterase-inhibiting alkaloids from Zephyranthes concolor

The bulbs and aerial parts of Zephyranthes concolor (Lindl.) Benth. & Hook. f. (Amaryllidaceae), an endemic species to Mexico, were found to contain the alkaloids chlidanthine, galanthamine, galanthamine N-oxide, lycorine, galwesine, and epinorgalanthamine. Since currently only partial and low resolution (1)H-NMR data for chlidanthine acetate are available, and none for chlidanthine, its 1D and 2D high resolution (1)H- and (13)C-NMR spectra were recorded. Unambiguous assignations were achieved with HMBC, and HSQC experiments, and its structure was corroborated by X-ray diffraction. Minimum energy conformation for structures of chlidanthine, and its positional isomer galanthamine, were calculated by molecular modelling. Galanthamine is a well known acetylcholinesterase inhibitor; therefore, the isolated alkaloids were tested for this activity. Chlidanthine and galanthamine N-oxide inhibited electric eel acetylcholinesterase (2.4 and 2.6 × 10(-5) M, respectively), indicating they are about five times less potent than galanthamine, while galwesine was inactive at 10(-3) M. Inhibitory activity of HIV-1 replication, and cytotoxicity of the isolated alkaloids were evaluated in human MT-4 cells; however, the alkaloids showed poor activity as compared with standard anti-HIV drugs, but most of them were not cytotoxic.     

The electrostatic probe: a tool for the investigation of the Aβ(1-16) peptide deformations using the static modes

We investigate the conformational changes of the Amyloid β(1-16) peptide induced by moving Zn(2+) ions in the solvent, which we call the electrostatic probe. We use our recently developed approach of static modes which allows treating the flexibility of biological molecules at the atomic scale. Starting from an experimental apostructure, we find that several ion impacts allow the transition of the peptide toward its folded conformation, observed experimentally in the presence of Zn(2+) ions. This result shows the ability of our model and its associated software tool to describe properly the conformational changes and opens a new path toward the molecule/molecule docking problem.     

Titanocene(III)-promoted Barbier-type crotylation of carbonyl compounds

A mild, highly regio- and stereoselective method for the crotylation of aldehydes and ketones mediated/catalyzed by titanocene(III) is described. Optimized conditions permit the selective generation of γ-adducts in high yields together with high stereoselectivity, with a predominance of anti stereoisomers.