Magnetoelastics of a spin liquid: X-ray diffraction studies of Tb2Ti2O7 in pulsed magnetic fields

We report high resolution single crystal x-ray diffraction measurements of the frustrated pyrochlore magnet Tb2Ti2O7, collected using a novel low temperature pulsed magnet system. This instrument allows characterization of structural degrees of freedom to temperatures as low as 4.4 K, and in applied magnetic fields as large as 30 T. We show that Tb2Ti2O7 manifests intriguing structural effects under the application of magnetic fields, including strongly anisotropic giant magnetostriction, a restoration of perfect pyrochlore symmetry in low magnetic fields, and ultimately a structural phase transition in high magnetic fields. It is suggested that the magnetoelastic coupling thus revealed plays a significant role in the spin liquid physics of Tb2Ti2O7 at low temperatures.

Heat capacity reveals the physics of a frustrated spin tube

We report on theoretical and experimental results concerning the low-temperature specific heat of the frustrated spin-tube material [(CuCl(2)tachH(3)Cl]Cl(2) (tach denotes 1,3,5-triaminocyclohexane). This substance turns out to be an unusually perfect spin-tube system which allows to study the physics of quasi-one-dimensional antiferromagnetic structures in rather general terms. An analysis of the specific-heat data demonstrates that at low enough temperatures the system exhibits a Tomonaga-Luttinger liquid behavior corresponding to an effective spin-3/2 antiferromagnetic Heisenberg chain with short-range exchange interactions. On the other hand, around 2 K the composite spin structure of the chain is revealed through a Schottky-type peak in the specific heat. We argue that the dominating contribution to the peak originates from gapped magnon-type excitations related to the internal degrees of freedom of the rung spins.

Universal magnetic structure of the half-magnetization phase in Cr-based spinels

Using an elastic neutron scattering technique under a pulsed magnetic field up to 30 T, we determined the magnetic structure in the half-magnetization plateau phase in the spinel CdCr2O4. The magnetic structure has a cubic P4{3}32 symmetry, which is the same as that observed in HgCr2O4. This suggests that despite their different zero-field ground states a universal field-induced spin-lattice coupling mechanism is at work in the Cr-based spinels.

Intrinsic magnetoresistance of single-walled carbon nanotubes probed by a noncontact method

The intrinsic magnetotransport effect of the single-walled carbon nanotube (SWNT) has been observed by the cavity perturbation technique, which is a noncontact method for evaluating transport properties. The inverse Q factor of the cavity resonator, which corresponds to the resistance of the sample, shows a linear increase as a function of the magnetic field. The angular and tube diameter dependence of oriented SWNT thin films, and measurements using sorted SWNTs reveal that the observed positive magnetoresistance is due to the Aharonov-Bohm effect of metallic nanotubes.

A portable high-field pulsed-magnet system for single-crystal x-ray scattering studies

We present a portable pulsed-magnet system for x-ray studies of materials in high magnetic fields (up to 30 T). The apparatus consists of a split-pair of minicoils cooled on a closed-cycle cryostat, which is used for x-ray diffraction studies with applied field normal to the scattering plane. A second independent closed-cycle cryostat is used for cooling the sample to near liquid helium temperatures. Pulsed magnetic fields (approximately 1 ms in total duration) are generated by discharging a configurable capacitor bank into the magnet coils. Time-resolved scattering data are collected using a combination of a fast single-photon counting detector, a multichannel scaler, and a high-resolution digital storage oscilloscope. The capabilities of this instrument are used to study a geometrically frustrated system revealing strong magnetostrictive effects in the spin-liquid state.

Neutron diffraction study on the multiple magnetization plateaus in TbB4 under pulsed high magnetic field

We present the first application of pulsed high magnetic fields up to 30 T for neutron diffraction experiments. As the first study, field variations of a couple of magnetic Bragg reflections have successfully been measured in the frustrated antiferromagnet TbB4. The results show that the conventional models fail, and a model, which is a mixture of the XY- and the Ising-type moments, matches for the half-magnetization state. We deduce an interaction that stabilizes an orthogonal moment arrangement as an origin of the unusual magnetization plateaus. Our results demonstrate the powerfulness of the present pulsed magnetic fields neutron diffraction system.

X-ray magnetic circular dichroism of a valence fluctuating state in eu at high magnetic fields

X-ray magnetic circular dichroism (XMCD) at the Eu L edge in two compounds exhibiting valence fluctuation, namely EuNi_{2}(Si_{0.18}Ge_{0.82})_{2} and EuNi_{2}P_{2}, has been investigated at high magnetic fields of up to 40 T. A distinct XMCD peak corresponding to the trivalent state (Eu(3+)), whose ground state is nonmagnetic (J = 0), was observed in addition to the main XMCD peak corresponding to the magnetic (J = 7/2) divalent state (Eu(2+)). This result indicates that the 5d electrons belonging to both valence states are magnetically polarized. It was also found that the ratio P_{5d}(3+)/P_{5d}(2+) between the polarization of 5d electrons (P_{5d}) in the Eu(3+) state and that of Eu(2+) depends on the material. The possible origin of the XMCD and an explanation of the material dependence of P_{5d}(3+)/P_{5d}(2+) are discussed in terms of hybridization between the conduction electrons and the f electrons.

Undecanuclear mixed-valence 3d-4f bimetallic clusters

Two undecanuclear 3d-4f clusters with the general formula {Mn(III)(4)Mn(IV)Ln(III)(6)}, where Ln = Gd or Tb, were synthesized, with both showing large spin ground states, and the Tb species acting as a single molecule magnet.

Magnetic-field induced phase transitions in a weakly coupled s=1/2 quantum spin dimer system Ba3Cr2O8

By using bulk magnetization, electron spin resonance (ESR), heat capacity, and neutron scattering techniques, we characterize the thermodynamic and quantum phase diagrams of Ba3Cr2O8. Our ESR measurements indicate that the low field paramagnetic ground state is a mixed state of the singlet and the Sz=0 triplet for H perpendicular c. This suggests the presence of an intradimer Dzyaloshinsky-Moriya (DM) interaction with a DM vector perpendicular to the c axis.

Insulator-metal phase transition of Pr(0.6)Ca(0.4)MnO(3) studied by x-ray absorption spectroscopy in pulsed magnetic fields

Evolution of the Mn K-edge x-ray absorption near edge structure (XANES) in Pr(0.6)Ca(0.4)MnO(3) at pulsed magnetic fields has been investigated. A small enhancement of XANES spectra is detected across the magnetic-field-induced transition from the charge- and orbital-ordered (COO) insulator to ferromagnetic metal at 20 K. It is found that the magnetic-field dependence of the enhancement shows clear hysteresis, as seen in the magnetization with metamagnetic transition, suggesting a significant correlation between the change in the XANES and the field-induced collapse of the COO state. The enhancement of the absorption can be explained by an increase of the 4p density of states due to a reduction of hybridization between the 4p state of the central Mn ion with the core hole and the neighboring Mn 3d state. Local structural change around Mn ions is expected to modify the strength of the hybridization.

External magnetic field effects on a distorted kagome antiferromagnet

We report bulk magnetization, and elastic and inelastic neutron scattering measurements under an external magnetic field H on the weakly coupled distorted kagome system, Cu2(OD)3Cl. Our results show that the ordered state below 6.7 K is a canted antiferromagnet and consists of large antiferromagnetic ac components and smaller ferromagnetic b components. By first-principles calculations and linear spin wave analysis, we present a simple spin Hamiltonian with nonuniform nearest neighbor exchange interactions resulting in a system of coupled spin trimers with a single-ion anisotropy that can qualitatively reproduce the spin dynamics of Cu2(OD)3Cl.

A Wheel-Shaped Single-Molecule Magnet of [MnII3MnIII4]: Quantum Tunneling of Magnetization under Static and Pulse Magnetic Fields

The reaction of N-(2-hydroxy-5-nitrobenzyl)iminodiethanol (=H3(5-NO2-hbide)) with Mn(OAc)2* 4 H2O in methanol, followed by recrystallization from 1,2-dichloroethane, yielded a wheel-shaped single-molecule magnet (SMM) of [MnII 3MnIII 4(5-NO2-hbide)6].5 C2H4Cl2 (1). In 1, seven manganese ions are linked by six tri-anionic ligands and form the wheel in which the two manganese ions on the rim and the one in the center are MnII and the other four manganese ions are MnIII ions. Powder magnetic susceptibility measurements showed a gradual increase with chimT values as the temperature was lowered, reaching a maximum value of 53.9 emu mol(-1) K. Analyses of magnetic susceptibility data suggested a spin ground state of S=19/2. The zero-field splitting parameters of D and B 0 4 were estimated to be -0.283(1) K and -1.64(1)x10(-5) K, respectively, by high-field EPR measurements (HF-EPR). The anisotropic parameters agreed with those estimated from magnetization and inelastic neutron scattering experiments. AC magnetic susceptibility measurements showed frequency-dependent in- and out-of-phase signals, characteristic data for an SMM, and an Arrhenius plot of the relaxation time gave a re-orientation energy barrier (DeltaE) of 18.1 K and a pre-exponential factor of 1.63x10(-7) s. Magnetization experiments on aligned single crystals below 0.7 K showed a stepped hysteresis loop, confirming the occurrence of quantum tunneling of the on magnetization (QTM). QTM was, on the other hand, suppressed by rapid sweeps of the magnetic field even at 0.5 K. The sweep-rate dependence of the spin flips can be understood by considering the Landau-Zener-Stückelberg (LZS) model.

Manganese(III,IV) and Manganese(III) Oxide Clusters Trapped by Copper(II) Complexes

Reactions of quinquedentate Schiff base ligands with Mn and Cu ions afforded icosa- and hexadecanuclear mixed-metal clusters in which dinuclear CuII complexes trapped oxo-bridged [MnIII8MnIV4O12] and [MnIII6O6] cores, respectively. Maximum entropy method analysis for synchrotron X-ray diffraction data was used to determine the oxidation states of the Mn ions.

Studies of an Fe9 Tridiminished Icosahedron

The synthesis and structural characterization of a nonanuclear FeIII cage complex is reported. The nine iron centers in [Fe9(mu3-O)4(O3PPh)3(O2CCMe3)13] lie on the vertices of an incomplete icosahedron, with the P atoms of triphenylphosphonate at the other three vertices. The paramagnetic core therefore describes a tridiminished icosahedron. Magnetic studies suggest an S=1/2 ground state for the molecule. Analysis of exchange paths and the susceptibility data point to the interpretation that the cluster can be divided into two nearly decoupled sections: an {Fe6O3} section, with an S=0 ground state, in which three oxo-centered triangles bound a central triangle that is not oxo-centered; and an {Fe3O} triangle with S=1/2. The analysis of the susceptibility data leads to a Heisenberg model based on three significant antiferromagnetic exchange interactions, with values of 173.7 cm-1 in the {Fe3O} triangle, and 30.9 and 19.1 cm-1 within the {Fe6O3} section, while the exchange between them is <1 cm-1. With these assignments, the theoretical low-temperature differential susceptibility is also in very good agreement with measurements up to 50 T. Magnetic measurements in the milli-kelvin range reveal striking hysteresis loops and magnetization reversals associated with a Landau-Zener-Stückelberg (LZS) transition as enhanced by the occurrence of a phonon bottleneck.

Ferromagnetic Exchange Interactions for Cu612+ and Mn612+ Hexagons Sandwiched by Two B-α-[XW9O33]9- (X = AsIII and SbIII) Ligands in D3d-Symmetric Polyoxotungstates

Two novel Cu6 and Mn6 hexagon sandwiched polyoxometalates, [(CuCl)6(AsW9O33)2]12- (1a) and [(MnCl)6(SbW9O33)2]12- (2a), have been synthesized and characterized by X-ray single-crystal analysis and magnetic measurements. These complexes are D3d symmetric and were isolated as [n-BuNH3]+ salts from aqueous solutions: (n-BuNH3)12[(CuCl)6(AsW9O33)2].6H2O (1), rhombohedral, R, a = 20.33(1) A, c = 26.35(2) A, Z = 3 and (n-BuNH3)12[(MnCl)6(SbW9O33)2].6H2O (2). Six Cu (or Mn) atoms, each of which shows 5-fold coordination, make an approximately equatorial hexagon with a first-neighboring Cu...Cu (Mn...Mn) distance of 2.913(2) A (3.248(1) A) and a Cu-O-Cu (Mn-O-Mn) bond angle of 94.5(2) degrees (100.4(2) degrees ). The magnetic behavior investigated by magnetic susceptibility measurements shows the ferromagnetic exchange interactions with J/k = +12.7 K (J/hc = +8.82 cm(-1)) and a S = 3 ground state for 1 and J/k = +0.20 K (J/hc = +0.14 cm(-1)) and a S = 15 ground state for 2, when only J refers to the isotropic magnetic-exchange interactions for first-neighbor atoms of the approximately equilateral Cu6(12+) and Mn6(12+) hexagons. The single-crystal ESR spectroscopy of 1 under the orientation of the magnetic field along a Cu6 hexagon's nearly 6-fold axis equal to the c axis on the variation of temperature supports the S = 3 ground state and allows an estimate of the zero-field (fine-structure) energy separation between Sz = 0 and Sz = +/-1 of D = -0.182 K to be obtained.

A Binuclear Fe(III)Dy(III) Single Molecule Magnet. Quantum Effects and Models

The binuclear [FeIII(bpca)(mu-bpca)Dy(NO3)4], having Single Molecule Magnet (SMM) properties, belonging to a series of isostructural FeIIILnIII complexes (Ln = Eu, Gd, Tb, Dy, Ho) and closely related FeIILnIII chain structures, was characterized in concise experimental and theoretical respects. The low temperature magnetization data showed hysteresis and tunneling. The anomalous temperature dependence of Mössbauer spectra is related to the onset of magnetic order, consistent with the magnetization relaxation time scale resulting from AC susceptibility measurements. The advanced ab initio calculations (CASSCF and spin-orbit) revealed the interplay of ligand field, spin-orbit, and exchange effects and probed the effective Ising nature of the lowest states, involved in the SMM and tunneling effects.

From an ST = 3 Single-Molecule Magnet to Diamagnetic Ground State Depending on the Molecular Packing of MnIIIsalen-type Dimers Decorated by N,N‘-Dicyano-1,4-naphthoquinonediiminate Radicals

Two manganese(III) tetradentate Schiff-base dimers to which N,N'-dicyano-1,4-naphthoquinonediiminate (DCNNQI) radicals are attached have been selectively synthesized by varying the solvents used in the reactions: [Mn2(5-MeOsaltmen)2(DCNNQI)2].MeOH (1) and [Mn2(5-MeOsaltmen)(2)(DCNNQI)(2)] x 2CH2Cl2.2CH3CN (2) [5-MeOsaltmen2- = N,N'-(1,1,2,2-tetramethylethylene)bis(5-methoxysalicylideneiminate)]. These two complexes share the same molecular core, [(DCNNQI.-)-Mn(III)-(O)2-Mn(III)-(DCNNQI.-)], where -(O)2- is a biphenolate bridge in the out-of-plane dimerized [Mn(2)(5-MeOsaltmen)2]2+ moiety. However, their packing arrangements are completely different. Whereas complex 1 is found to be relatively isolated, strong intermolecular dimerization of the DCNNQI moieties (with the nearest contact being approximately 3.0 A) is observed in 2, forming a one-dimensional chain of [-Mn(III)-(O)2-Mn(III)-(DCNNQI.-)2-](infinity). The magnetic susceptibility of 1 can be modeled with an [S = 1/2, 2, 2, 1/2] four-spin system including strong antiferromagnetic Mn(III)/DCNNQI radical coupling (J(Mn/rad)/kB = -23 K) and ferromagnetic Mn(III)/Mn(III) coupling through the biphenolate bridge (J(Mn/Mn)/kB = +2.0 K). These interactions lead to an ST = 3 ground state that possesses significant uniaxial anisotropy (D(S=3)/kB = -2.1 K). Low-temperature ac and dc magnetic data of 1 reveal its single-molecule magnet behavior with quantum tunneling of the magnetization. By contrast, 2 possesses the diamagnetic ground state induced by dominating Mn(III)-Mn(III) antiferromagnetic interactions mediated by the diamagnetic DCNNQI dimers and/or pi-pi contact along the b axis.

Observation of a half step magnetization in the {Cu3}-type triangular spin ring

We report pulsed field magnetization and ESR experiments on a {Cu3} nanomagnet, where antiferromagnetically coupled Cu(2+) (S = 1/2) ions form a slightly distorted triangle. The remarkable feature is the observation of a half step magnetization, hysteresis loops, and an asymmetric magnetization between a positive and a negative field in a fast sweeping external field. This is attributed to an adiabatic change of magnetization. The energy levels determined by ESR unveil that the different mixing nature of a spin chirality of a total S = 1/2 Kramers doublet by virtue of Dzyaloshinskii-Moriya interactions is decisive for inducing half step magnetization.

Oximate-Bridged Trinuclear Dy−Cu−Dy Complex Behaving as a Single-Molecule Magnet and Its Mechanistic Investigation

Lanthanide ions are supposed to be promising candidates for the elements of single-molecule magnets (SMMs) because of the large magnetic momentum and anisotropy. We have established the [Dy2Cu] complex as a new SMM. A plausible mechanism for quantum tunneling of magnetization is proposed for the first time among the 4f-3d heterometallic SMMs. The magnetic coupling parameter between Dy and Cu ions was well-defined as -0.155 K.

Ferromagnetic ordering in a new nickel polyborate NiB12O14(OH)10

A new nickel polyborate, NiB12O14(OH)10 was synthesized using boric acid as a flux. This material has two-dimensional borate layers with a quasi-square lattice of Ni2+. The Ni2+ ions locate in the plane of the two-dimensional layer, bridged through BnO(n+1) chains in the plane and connected with the three-membered ring borate groups out of the plane. The dc and ac magnetic susceptibility, magnetization and specific heat measurements show that this material undergoes a weak ferromagnetic phase transition at Tc = 5.8 K. At T < Tc, a metamagnetic phase transition is observed at about 5 T, associated by a spin-flop, suggesting the ferromagnetic ordering is induced by an antiferromagnetic interaction. A broad maximum in the ac susceptibility at TM = 23-24 K indicates an intermediate short-range ordering.

Transfection of antisense core 2 beta1,6-N-acetylglucosaminyltransferase-1 cDNA suppresses selectin ligand expression and tissue infiltration of B-cell precursor leukemia cells

B-cell precursor (BCP) leukemia cells infiltrate into peripheral organs and the disease often relapses. Inhibition of tissue infiltration may improve the treatment outcome of BCP-leukemia patients. Selectin ligand has been suggested to play an important role in the infiltration of leukemia cells. However, the regulation mechanisms and involvement in tissue infiltration of selectin ligand expression in BCP-leukemia cells are not fully understood. In this study, we report that BCP-leukemia cells express selectin ligand on O-sialoglycoproteins. Core 2 beta1,6-N-acetylglucosaminyltransferase-1 (C2GnT-1) is mainly expressed in BCP-leukemia cells. Transfection of the antisense C2GnT-1 cDNA resulted in a significant reduction of either selectin ligand expression or selectin-dependent cell adhesion in BCP-leukemia cell line KM3 cells. Migration ability into mouse peripheral organs was reduced significantly in the antisense transfectant. These findings suggest that C2GnT-1 regulates selectin ligand expression. Downregulation of the selectin ligand expression level inhibits tissue infiltration of BCP-leukemia cells. C2GnT-1 may be a candidate of therapeutic target for the inhibition of infiltration of leukemia cells.

Secretion of bacterial xenobiotic-degrading enzymes from transgenic plants by an apoplastic expressional system: an applicability for phytoremediation

In search of an effective method for phytoremediation of wastewater contaminated with organic compounds, we investigated the application of an apoplastic expressional system that secretes useful bacterial enzymes from transgenic plants into hydroponic media through the addition of a targeting signal. We constructed transgenic Arabidopsis expressing the aromatic-cleaving extradiol dioxygenase (DbfB), which degrades 2,3-dihydroxybiphenyl (2,3-DHB), and transgenic tobacco expressing haloalkane dehalogenase (DhaA), which catalyzes hydrolytic dechlorination of 1-chlorobutane (1-CB). Although crude leaf extracts of transgenic plants expressing cytoplasm-targeted degradative enzymes showed higher activity than did those from transgenic plants expressing apoplast-targeted enzymes, the hydroponic media of the latter showed 23.2 times (DbfB) and 76.4 times (DhaA) higher activity than plants containing the cytoplasm-targeted enzymes. Addition of crystalline 2,3-DHB to 100 mL of the hydroponic medium of transgenic or wild-type seedlings revealed that only medium from the transgenic Arabidopsis expressing apoplast-targeted DbfB showed rapid ring cleavage of 2,3-DHB. Transgenic tobacco expressing apoplast-targeted DhaA also resulted in the accumulation of the dehalogenation product 1-butanol in the hydroponic medium and showed a higher tolerance to 1-CB than wild-type or transgenic plants expressing cytoplasm-targeted DhaA. These results demonstrate the usefulness of the apoplastic expression of bacterial recombinant proteins in phytoremediation.

Cyano-Bridged MnIII3MIII (MIII = Fe, Cr) Complexes: Synthesis, Structure, and Magnetic Properties

Two cyano-bridged tetranuclear complexes composed of Mn(III) salen (salen = N,N'-ethylene bis(salicylideneiminate)) and hexacyanometalate(III) (M = Fe, Cr) in a stoichiometry of 3:1 have been selectively synthesized using {NH2(n-C12H25)2}3[M(III)(CN)6] (M(III) = Fe, Cr) starting materials: [{Mn(salen)(EtOH)}3{M(CN)6}] (M = Fe, 1; Cr, 2). Compounds 1 and 2 are isostructural with a T-shaped structure, in which [M(CN)6]3- assumes a meridional-tridentate building block to bind three [Mn(salen)(EtOH)]+ units. The strong frequency dependence and observation of hysteresis on the field dependence of the magnetization indicate that 1 is a single-molecule magnet.

A Single-Chain Magnet Formed by a Twisted Arrangement of Ions with Easy-Plane Magnetic Anisotropy

A unique "single-chain quantum magnet" was formed by an alternating arrangement of high-spin FeII/low-spin FeIII ions. Although the predominant spin-carrier components (high-spin FeII) possess easy-plane magnetic anisotropy (D > 0), twisted arrangement of easy-planes along the chain axis defines a novel class of one-dimensional Ising system.

A Heterometal Single-Molecule Magnet of [MnIII2NiII2Cl2(salpa)2]

The tetranuclear complex [MnIII2NiII2Cl2(salpa)2] (salpa = N-(2-hydroxybenzyl)-3-amino-1-propanol) has a spin ground state of S = 6 and was confirmed to be an SMM based on a steplike feature of the magnetization hysteresis loop at 0.55 K.

Spin-frustrated (VO)36+-triangle-sandwiching octadecatungstates as a new class of molecular magnets

Spin-frustrated polyoxometalates, K(11)H[(VO)(3)(SbW(9)O(33))(2)].27H(2)O (1) and K(12)[(VO)(3)(BiW(9)O(33))(2)].29H(2)O (2), containing approximately equilateral and isosceles (VO)(3)(6+)-triangles (V(IV)...V(IV) separation of 5.4-5.5 A) sandwiched by two diamagnetic alpha-B nonatungstate ligands ([SbW(9)O(33)](9)(-) and [BiW(9)O(33)](9)(-)) with approximate D(3)(h) symmetry, are found to show magnetization jumps with distinct hysteresis for the S = (1)/(2) <--> S = (3)/(2) level crossing under fast sweeping pulsed magnetic fields (approximately 10(3) T/s) at T < or = 0.5 K. This unusual phenomenon is attributed to the theoretical prediction of half step magnetization, which is expected for an antiferromagnetic spin triangle with antisymmetrical Dzyaloshinky-Moriya interaction. The degeneracy of the S = (1)/(2) states for 1 is removed by slightly lower symmetry effects of triangular structure for 2. The calorimetry of 1 and 2 shows the heat capacity anomaly at 2 < or = T < or = 20 K which is associated with a thermal excitation from the S = (1)/(2) ground states to the S = (3)/(2) state at zero field. Zero-field splitting energies (5-7 K) between S = (1)/(2) and S = (3)/(2) states for 1 and 2, readily estimated by the level-crossing field for the magnetization, allow us to measure the hyperfine-structural 22 lines due to three equivalent I = (7)/(2) (51)V nuclei, the fine-structural triplet line of the S = (3)/(2) excited state, and the g anisotropy on the high-frequency ESR spectra. The spin-frustrated (VO)(3)(6+)-triangle for 1 and 2 is a good model of the magnetization between pure quantum states S = (1)/(2) and (3)/(2) and provides a new class of single-molecule magnets.

A Dinuclear MnIII-CuII Single-Molecule Magnet

The reaction of 1/3 equivalent of CuCl2.2H2O with MnCl2.4H2O and 5-bromo-2-salicylideneamino-1-propanol (H(2)5-Br-sap) in methanol gave dark brown crystals of [MnIIICuIICl(5-Br-sap)2(MeOH)] (1). Complex 1 has an alkoxo-bridged dinuclear core of MnIII and CuII ions, which have elongated octahedral and square-planar coordination geometries, respectively. In dc magnetic susceptibility measurements, chi(m)T values increased as the temperature was lowered, followed by a sudden decrease below 20 K. This behavior is indicative of the occurrence of intramolecular ferromagnetic interactions, and fitting gave an S=5/2 spin ground state with an exchange coupling constant J(MnCu) of +78 cm(-1). Magnetization data collected as a function of temperature and applied magnetic field were analyzed by using a spin Hamiltonian with isotropic Zeeman and axial zero-field splitting (ZFS) terms, and a negative D(5/2) value (-1.86 cm(-1)) was obtained. A high-field EPR (HFEPR) spectrum (342.0 GHz) at 4.2 K was composed of four peaks, and two additional peaks at higher magnetic field appeared as the temperature was increased. The temperature dependences in the HFEPR spectra are indicative of a negative D(5/2) value, and fitting of the data gave D(5/2)=-1.81 cm(-1). In the ac magnetic susceptibility measurements, frequency dependent in-phase (chi(m)') and out-of-phase (chi(m)'') signals with peak maxima at 0.7-1.5 K were observed and small peaks below 0.7 K appeared. The ac susceptibility data supports that 1 is a single-molecule magnet (SMM). Arrhenius plots for the chi(m)'' peaks from 0.7-1.5 K gave the re-orientation energy barrier (DeltaE) of 10.5 K with a pre-exponential factor of 8.2x10(-8) s.

Competing spin phases in geometrically frustrated magnetic molecules

We identify a class of zero-dimensional classical and quantum Heisenberg spin systems exhibiting anomalous behavior in an external magnetic field B similar to that found for the geometrically frustrated kagome lattice of classical spins. Our calculations for the isotropic Heisenberg model show the emergence of a pronounced minimum in the differential susceptibility dM/dB at B(sat)/3 as the temperature T is raised from 0 K for structures based on corner-sharing triangles, specifically the octahedron, cuboctahedron, and icosidodecahedron. As the first experimental evidence we note that the giant Keplerate magnetic molecule {Mo(72)Fe(30)} (Fe(3+) ions on the 30 vertices of an icosidodecahedron) exhibits this behavior. For low T when B approximately B(sat)/3 two competing families of spin configurations exist of which one behaves magnetically "stiff" leading to a reduction of dM/dB.

Divergence of mobile genetic elements involved in the distribution of xenobiotic-catabolic capacity

Bacteria adapt rapidly to environmental stimuli, such as exposure to xenobiotics. Mobile genetic elements (MGEs) play a major role in such bacterial adaptation, via the dispersal of catabolic capacity; and, in fact, genes that encode the degradation enzymes for xenobiotics are often located on MGEs. The list of reported catabolic MGEs keeps growing as researchers continue to isolate and characterize xenobiotic degraders and the corresponding catabolic genes. Major catabolic MGEs include (conjugative) plasmids, transposons, and conjugative transposons. Catabolic transposons can be divided into class I elements (composite transposons) and class II elements (Tn 3 family transposons). This review includes a comprehensive list of naturally occurring discrete catabolic MGEs, together with a brief description for each. While MGEs are often rather large, genome-wide or large-scale sequence analyses have provided useful information on the whole genetic structure of MGEs, with clues to their function (transfer, maintenance, catabolism, etc.) and behavior in a complex natural environment. This review also gives an insight into MGE functions, based on the complete sequencing of several catabolic plasmids and two Pseudomonas genomes.

Characterization and identification of genes essential for dimethyl sulfide utilization in Pseudomonas putida strain DS1

Microbial dimethyl sulfide (DMS) conversion is thought to be involved in the global sulfur cycle. We isolated Pseudomonas putida strain DS1 from soil as a bacterium utilizing DMS as a sole sulfur source, and tried to elucidate the DMS conversion mechanism of strain DS1 at biochemical and genetic level. Strain DS1 oxidized DMS to dimethyl sulfone (DMSO(2)) via dimethyl sulfoxide, whereas the oxidation was repressed in the presence of sulfate, suggesting that a sulfate starvation response is involved in DMS utilization by strain DS1. Two of the five DMS-utilization-defective mutants isolated by transposon 5 (Tn 5) mutagenesis had a Tn 5 insertion in the ssuEADCBF operon, which has been reported to encode a two-component monooxygenase system (SsuED), an ABC-type transporter (SsuABC), and a small protein (SsuF), and also to play a key role in utilization of sulfonates and sulfate esters in another bacterium, P. putida strain S-313. Disruption of ssuD and SsuD enzymatic activity demonstrated that methanesulfonate is a metabolic intermediate of DMS and desulfonated by SsuD. Disruption of ssuC or ssuF also led to a DMS-utilization-defective phenotype. Another two mutants had a defect in a gene homologous to pa2354 from P. aeruginosa PAO1, which encodes a putative transcriptional regulator, while the remaining mutant had a defect in cysM encoding O-acetylserine (thiol)-lyase B.

Phthalate catabolic gene cluster is linked to the angular dioxygenase gene in Terrabacter sp. strain DBF63

Phthalate is a metabolic intermediate of the pathway of fluorene (FN) degradation via angular dioxygenation. A gene cluster responsible for the conversion of phthalate to protocatechuate was cloned from the dibenzofuran (DF)- and FN-degrading bacterium Terrabacter sp. strain DBF63 and sequenced. The genes encoding seven catabolic enzymes, oxygenase large subunit of phthalate 3,4-dioxygenase (phtA1), oxygenase small subunit of phthalate 3,4-dioxygenase (phtA2), cis-3,4-dihydroxy-3,4-dihydrophthalate dehydrogenase (phtB), [3Fe-4S] or [4Fe-4S] type of ferredoxin (phtA3), ferredoxin reductase (phtA4), 3,4-dihydroxyphthalate decarboxylase (phtC) and putative regulatory protein (phtR), were found in the upstream region of the angular dioxygenase gene (dbfA1A2), encoded in this order. Escherichia coli carrying phtA1A2BA3A4 genes converted phthalate to 3,4-dihydroxyphthalate, and the 3,4-dihydroxyphthalate decarboxylase activity by E. coli cells carrying phtC was finally detected with the introduction of a Shine-Dalgarno sequence in the upstream region of its initiation codon. Homology analysis on the upstream region of the pht gene cluster revealed that there was an insertion sequence (IS) (ISTesp2; ORF14 and its flanking region), part of which was almost 100% identical to the orf1 and its flanking region adjacent to the extradiol dioxygenase gene ( bphC1) involved in the DF degradation of Terrabacter sp. strain DPO360 [Schmid et al. (1997) J Bacteriol 179:53-62]. This suggests that ISTesp2 plays a role in the metabolism of aromatic compounds in Terrabacter sp. strains DBF63 and DPO360.

Recent developments in molecular techniques for identification and monitoring of xenobiotic-degrading bacteria and their catabolic genes in bioremediation

The pollution of soil and water with xenobiotics is widespread in the environment and is creating major health problems. The utilization of microorganisms to clean up xenobiotics from a polluted environment represents a potential solution to such environmental problems. Recent developments in molecular-biology-based techniques have led to rapid and accurate strategies for monitoring, discovery and identification of novel bacteria and their catabolic genes involved in the degradation of xenobiotics. Application of these techniques to bioremediation has also improved our understanding of the composition, phylogeny, and physiology of metabolically active members of the microbial community in the environment. This review provides an overview of recent developments in molecular-biology-based techniques and their application in bioremediation of xenobiotics.

Molecular detection and diversity of polycyclic aromatic hydrocarbon-degrading bacteria isolated from geographically diverse sites

Nineteen polycyclic aromatic hydrocarbon (PAH)-degrading bacteria were isolated from environmental samples in Kuwait, Indonesia, Thailand, and Japan by enrichment with either naphthalene or phenanthrene as a sole carbon source. Sequence analyses of the 16-S rRNA gene indicated that at least seven genera (Ralstonia, Sphingomonas, Burkholderia, Pseudomonas, Comamonas, Flavobacterium, and Bacillus) were present in this collection. Determination of the ability of the isolates to use PAH and its presumed catabolic intermediates suggests that the isolates showed multiple phenotypes in terms of utilization and degradation pathways. The large subunit of the terminal oxygenase gene (phnAc) from Burkholderia sp. strain RP007 hybridized to 32% (6/19) of the isolates, whilst gene probing using the large subunit of terminal oxygenase gene (pahAc) from Pseudomonas putida strain OUS82 revealed no pahAc-like genes amongst the isolates. Using three degenerated primer sets (pPAH-F/NR700, AJ025/26, and RieskeF/R), targeting a conserved region with the genes encoding the large subunit of terminal oxygenase successfully amplified material from 6 additional PAH-degrading isolates. Sequence analyses showed that the large subunit of terminal oxygenase in 4 isolates was highly homologous to the large subunit of naphthalene dioxygenase gene from Ralstonia sp. strain U2. However, we could not obtain any information on the oxygenase system involved in the naphthalene and/or phenathrene degradation by 7 other strains. These results suggest that PAH-degrading bacteria are diverse, and that there are still many unidentified PAH-degrading bacteria.