Hierarchical geometric frustration in La3Cu2VO9

The crystallographic structure and magnetic properties of the La3Cu2VO9 were investigated by powder neutron diffraction and magnetization measurements. The compound materializes geometric frustration at two spatial scales, within clusters and between clusters, and at different temperature scales. It is shown by exactly solving the hamiltonian spectrum that collective spins are formed on each clusters at low temperature before inter-clusters coupling operates.Comment: 6 pages, 4 figures. HFM2006 proceeding pape

Examination of the role of methylenetetrahydrofolate reductase in incorporation of methyltetrahydrofolate into cellular metabolism

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154338/1/fsb2002001005.pd

Magnetic properties of the honeycomb oxide Na2_2Co2_2TeO6_6

We have studied the magnetic properties of Na$_2$Co$_2$TeO$_6$, which features a honeycomb lattice of magnetic Co$^{2+}$ ions, through macroscopic characterization and neutron diffraction on a powder sample. We have shown that this material orders in a zig-zag antiferromagnetic structure. In addition to allowing a linear magnetoelectric coupling, this magnetic arrangement displays very peculiar spatial magnetic correlations, larger in the honeycomb planes than between the planes, which do not evolve with the temperature. We have investigated this behavior by Monte Carlo calculations using the $J_1$-$J_2$-$J_3$ model on a honeycomb lattice with a small interplane interaction. Our model reproduces the experimental neutron structure factor, although its absence of temperature evolution must be due to additional ingredients, such as chemical disorder or quantum fluctuations enhanced by the proximity to a phase boundary.Comment: 9 pages, 13 figure

Quasi-One-Dimensional Spin Dynamics in dd-Electron Heavy-Fermion Metal Y1−x_{1-x}Scx_xMn2_2

Slow spin fluctuations ($\nu < 10^{12}$ s$^-1$) observed by the muon spin relaxation technique in Y$_{1-x}$Sc$_x$Mn$_2$ exhibits a power law dependence on temperature ($\nu \propto T^\alpha$), where the power converges asymptotically to unity ($\alpha\rightarrow 1$) as the system moves away from spin-glass instability with increasing Sc content $x$. This linear $T$ dependence, which is common to that observed in LiV$_2$O$_4$, is in line with the prediction of the "intersecting Hubbard chains" model for a metallic pyrochlore lattice, suggesting that the geometrical constraints to t2g bands specific to the pyrochlore structure serve as a basis of the $d$-electron heavy-fermion state.Comment: 5 pages, 4 figures, to appear in J. Phys. Soc. Jp

Formation of collective spins in frustrated clusters

Using magnetization, specific heat and neutron scattering measurements, as well as exact calculations on realistic models, the magnetic properties of the \lacuvo compound are characterized on a wide temperature range. At high temperature, this oxide is well described by strongly correlated atomic $S$=1/2 spins while decreasing the temperature it switches to a set of weakly interacting and randomly distributed entangled pseudo spins $\tilde S=1/2$ and $\tilde S=0$. These pseudo-spins are built over frustrated clusters, similar to the kagom\'e building block, at the vertices of a triangular superlattice, the geometrical frustration intervening then at different scales.Comment: 10 page

Highly Purified Liver Microsomal Cytochrome P450: Properties and Catalytic Mechanism

Recent studies in this laboratory on two forms of cytochrome P450 purified to homogeneity from rabbit liver microsomes are reviewed. The two forms, phenobarbital-inducible P450LM2 and 5,6-benzoflavone-inducible P450LM4, differ in subunit molecular weight, identity of the C-terminal amino acid, optical and EPR spectra, and other properties. As isolated, oxidized P450LM2 is in the low spin state, whereas P450LM4 is largely, but non entirely, in the high spin state. Mechanistic studies have shown the following: (a) P450LM2 may accept two electrons, calculated per heme, from dithionite or NADPH in the presence of catalytic amounts of the reductase, and may donate two electrons to various oxidizing agents, including molecular oxygen. (b) Hydrogen peroxide is formed in the reconstituted system in the presence of NADPH and oxygen, and the amount varies with the substrate added. (c) Hydrogen peroxide and other hydroperoxides apparently donate the oxygen atom inserted into substrate during hydroxylation in the absence of 0 2 and an external donor. (d) Stopped flow spectrophotometry has provided evidence for two distinct oxygenated complexes of the reduced cytochrome. The reductase and cytochrome b5 may play an effector role in increasing the rate of decomposition of the second complex during oxygen insertion into substrate. A scheme is proposed for the mechanism of action of purified P450LM2, based on these and other findings

Highly Purified Liver Microsomal Cytochrome P450: Properties and Catalytic Mechanism

Recent studies in this laboratory on two forms of cytochrome P450 purified to homogeneity from rabbit liver microsomes are reviewed. The two forms, phenobarbital-inducible P450LM2 and 5,6-benzoflavone-inducible P450LM4, differ in subunit molecular weight, identity of the C-terminal amino acid, optical and EPR spectra, and other properties. As isolated, oxidized P450LM2 is in the low spin state, whereas P450LM4 is largely, but non entirely, in the high spin state. Mechanistic studies have shown the following: (a) P450LM2 may accept two electrons, calculated per heme, from dithionite or NADPH in the presence of catalytic amounts of the reductase, and may donate two electrons to various oxidizing agents, including molecular oxygen. (b) Hydrogen peroxide is formed in the reconstituted system in the presence of NADPH and oxygen, and the amount varies with the substrate added. (c) Hydrogen peroxide and other hydroperoxides apparently donate the oxygen atom inserted into substrate during hydroxylation in the absence of 0 2 and an external donor. (d) Stopped flow spectrophotometry has provided evidence for two distinct oxygenated complexes of the reduced cytochrome. The reductase and cytochrome b5 may play an effector role in increasing the rate of decomposition of the second complex during oxygen insertion into substrate. A scheme is proposed for the mechanism of action of purified P450LM2, based on these and other findings

Effects of a Dispersed and Undispersed Crude Oil on Mangroves, Seagrasses and Corals

The primary objective of this study was to evaluate the application of dispersant to spilled oil as a means of reducing adverse environmental effects of oil spills in nearshore, tropical waters. The results of numerous laboratory and field studies have suggested that dispersants may play a useful role in reducing adverse impacts on sensitive and valued environments such as mangroves, seagrasses, and corals. However, the use of dispersants has not been allowed thus far in most situations because of a lack of direct experimental data on the various effects of dispersants and the environmental trade-offs presumed to occur as a result of their application to crude oils. To accomplish this objective, a 21/2- year field experiment was designed in which detailed, synoptic measurements and assessments were made of representative intertidal and nearshore subtidal habitats and organisms (man-groves, seagrass beds, and coral reefs) before, during, and after exposure to untreated crude oil and chemically dispersed oil. The results were in-tended to give guidance in minimizing the ecological impacts of oil spills through evaluation of trade-offs in the relative impacts of chemical dispersion to tropical marine intertidal and subtidal habitats