Magnetocrystalline anisotropy energy for adatoms and monolayers on non-magnetic substrates: where does it come from?

The substrate contribution to the magnetic anisotropy energy (MAE) of supported nanostructures can be assessed by a site-selective manipulation of the spin-orbit coupling (SOC) and of the effective exchange field Bex. A systematic study of Co adatoms and Co monolayers on the (1 1 1) surfaces of Cu, Ag, Au, Pd and Pt is performed to study common trends in this class of materials. It is found that for adatoms, the influence of the substrate SOC and Bex is relatively small (10-30% of the MAE) while for monolayers, this influence can be substantial. The influence of the substrate SOC is much more important than the influence of the substrate Bex, except for highly polarizable substrates with a strong SOC (such as Pt). The substrate always promotes the tendency to an out-of-plane orientation of the easy magnetic axis for all the investigated systems.

Hyperfine splitting and room-temperature ferromagnetism of Ni at multimegabar pressure

Magnetic and elastic properties of Ni metal have been studied up to 260 GPa by nuclear forward scattering of synchrotron radiation with the 67.4 keV Mössbauer transition of 61Ni. The observed magnetic hyperfine splitting confirms the ferromagnetic state of Ni up to 260 GPa, the highest pressure where magnetism in any material has been observed so far. Ab initio calculations reveal that the pressure evolution of the hyperfine field, which features a maximum in the range of 100 to 225 GPa, is a relativistic effect. The Debye energy obtained from the Lamb-Mössbauer factor increases from 33 meV at ambient pressure to 60 meV at 100 GPa. The change of this energy over volume compression is well described by a Grüneisen parameter of 2.09.

Atomic-scale engineering of magnetic anisotropy of nanostructures through interfaces and interlines

The central goals of nanoscale magnetic materials science are the self-assembly of the smallest structure exhibiting ferromagnetic hysteresis at room temperature, and the assembly of these structures into the highest density patterns. The focus has been on chemically ordered alloys combining magnetic 3d elements with polarizable 5d elements having high spin–orbit coupling and thus yielding the desired large magneto-crystalline anisotropy. The chemical synthesis of nanoparticles of these alloys yields disordered phases requiring annealing to transform them to the high-anisotropy L1₀ structure. Despite considerable efforts, so far only part of the nanoparticles can be transformed without coalescence. Here we present an alternative approach to homogeneous alloys, namely the creation of nanostructures with atomically sharp bimetallic interfaces and interlines. They exhibit unexpectedly high magnetization reversal energy with values and directions of the easy magnetization axes strongly depending on chemistry and texture. We find significant deviations from the expected behaviour for commonly used element combinations. Ab-initio calculations reproduce these results and unravel their origin.

The design and assembly of nanostructures exhibiting ferromagnetic hysteresis at room temperature are recognized goals for high-density data storage. Here, the authors engineer nanostructures with atomically sharp bimetallic interfaces and interlines, which exhibit large magnetic anisotropy and high temperature hysteresis.

Magnetism of FePt surface alloys

The complex correlation of structure and magnetism in highly coercive monoatomic FePt surface alloys is studied using scanning tunneling microscopy, x-ray magnetic circular dichroism, and ab initio theory. Depending on the specific lateral atomic coordination of Fe either hard magnetic properties comparable to that of bulk FePt or complex noncollinear magnetism due to Dzyaloshinski-Moriya interactions are observed. Our calculations confirm the subtle dependence of the magnetic anisotropy and spin alignment on the local coordination and suggest that 3D stacking of Fe and Pt layers in bulk L1_{0} magnets is not essential to achieve high-anisotropy values.

Oxalate decarboxylase requires manganese and dioxygen for activity. Overexpression and characterization of Bacillus subtilis YvrK and YoaN

The Bacillus subtilis oxalate decarboxylase (EC ), YvrK, converts oxalate to formate and CO(2). YvrK and the related hypothetical proteins YoaN and YxaG from B. subtilis have been successfully overexpressed in Escherichia coli. Recombinant YvrK and YoaN were found to be soluble enzymes with oxalate decarboxylase activity only when expressed in the presence of manganese salts. No enzyme activity has yet been detected for YxaG, which was expressed as a soluble protein without the requirement for manganese salts. YvrK and YoaN were found to catalyze minor side reactions: oxalate oxidation to produce H(2)O(2); and oxalate-dependent, H(2)O(2)-independent dye oxidations. The oxalate decarboxylase activity of purified YvrK was O(2)-dependent. YvrK was found to contain between 0.86 and 1.14 atoms of manganese/subunit. EPR spectroscopy showed that the metal ion was predominantly but not exclusively in the Mn(II) oxidation state. The hyperfine coupling constant (A = 9.5 millitesla) of the main g = 2 signal was consistent with oxygen and nitrogen ligands with hexacoordinate geometry. The structure of YvrK was modeled on the basis of homology with oxalate oxidase, canavalin, and phaseolin, and its hexameric oligomerization was predicted by analogy with proglycinin and homogentisate 1,2-dioxygenase. Although YvrK possesses two potential active sites, only one could be fully occupied by manganese. The possibility that the C-terminal domain active site has no manganese bound and is buried in an intersubunit interface within the hexameric enzyme is discussed. A mechanism for oxalate decarboxylation is proposed, in which both Mn(II) and O(2) are cofactors that act together as a two-electron sink during catalysis.

Studies with substrate and cofactor analogues provide evidence for a radical mechanism in the chorismate synthase reaction

Chorismate synthase catalyzes the conversion of 5-enolpyruvylshikimate 3-phosphate (EPSP) to chorismate. The strict requirement for a reduced FMN cofactor and a trans-1,4-elimination are unusual. (6R)-6-Fluoro-EPSP was shown to be converted to chorismate stoichiometrically with enzyme-active sites in the presence of dithionite. This conversion was associated with the oxidation of FMN to give a stable flavin semiquinone. The IC(50) of the fluorinated substrate analogue was 0.5 and 250 microm with the Escherichia coli enzyme, depending on whether it was preincubated with the enzyme or not. The lack of dissociation of the flavin semiquinone and chorismate from the enzyme appears to be the basis of the essentially irreversible inhibition by this analogue. A dithionite-dependent transient formation of flavin semiquinone during turnover of (6S)-6-fluoro-EPSP has been observed. These reactions are best rationalized by radical chemistry that is strongly supportive of a radical mechanism occurring during normal turnover. The lack of activity with 5-deaza-FMN provides additional evidence for the role of flavin in catalysis by the E. coli enzyme.

Bacillus subtilis YvrK is an acid-induced oxalate decarboxylase

Bacillus subtilis has been shown to express a cytosolic oxalate decarboxylase (EC 4.1.1.2). The enzyme was induced in acidic growth media, particularly at pH 5.0, but not by oxalate. The enzyme was purified, and N-terminal sequencing identified the protein to be encoded by yvrK. The role of the first oxalate decarboxylase to be identified in a prokaryote is discussed.

A Secondary beta Deuterium Kinetic Isotope Effect in the Chorismate Synthase Reaction

Chorismate synthase (EC 4.6.1.4) is the shikimate pathway enzyme that catalyzes the conversion of 5-enolpyruvylshikimate 3-phosphate (EPSP) to chorismate. The enzyme reaction is unusual because it involves a trans-1,4 elimination of the C-3 phosphate and the C-6 proR hydrogen and it has an absolute requirement for reduced flavin. Several mechanisms have been proposed to account for the cofactor requirement and stereochemistry of the reaction, including a radical mechanism. This paper describes the synthesis of [4-(2)H]EPSP and the observation of kinetic isotope effects using this substrate with both Neurospora crassa and Escherichia coli chorismate synthases. The magnitude of the effects were (D)(V) = 1.08 +/- 0.01 for the N. crassa enzyme and 1.10 +/- 0.02 on phosphate release under single-turnover conditions for the E. coli enzyme. The effects are best rationalised as substantial secondary beta isotope effects. It is most likely that the C(3)-O bond is cleaved first in a nonconcerted E1 or radical reaction mechanism. Although this study alone cannot rule out a concerted E2-type mechanism, the C(3)-O bond would have to be substantially more broken than the proR C(6)-H bond in a transition state of such a mechanism. Importantly, although the E. coli and N. crassa enzymes have different rate limiting steps, their catalytic mechanisms are most likely to be chemically identical. Copyright 2000 Academic Press.

Specificity of starch synthase isoforms from potato

In higher plants several isoforms of starch synthase contribute to the extension of glucan chains in the synthesis of starch. Different isoforms are responsible for the synthesis of essentially linear amylose chains and branched, amylopectin chains. The activity of granule-bound starch synthase I from potato has been compared with that of starch synthase II from potato following expression of both isoforms in Escherichia coli. Significant differences in their activities are apparent which may be important in determining their specificities in vivo. These differences include affinities for ADPglucose and glucan substrates, activation by amylopectin, response to citrate, thermosensitivity and the processivity of glucan chain extension. To define regions of the isoforms determining these characteristic traits, chimeric proteins have been produced by expression in E. coli. These experiments reveal that the C-terminal region of granule-bound starch synthase I confers most of the specific properties of this isoform, except its processive elongation of glucan chains. This region of granule-bound starch synthase I is distinct from the C-terminal region of other starch synthases. The specific properties it confers may be important in defining the specificity of granule-bound starch synthase I in producing amylose in vivo.

Barley (Hordeum vulgare) oxalate oxidase is a manganese-containing enzyme

Oxalate oxidase (EC 1.2.3.4) catalyses the conversion of oxalate and dioxygen into CO(2) and H(2)O(2). The barley (Hordeum vulgare) seedling root enzyme was purified to homogeneity and shown by metal analysis and EPR spectroscopy to contain Mn(II) at up to 0.80 atom per subunit. The involvement of Mn and neither flavin, Cu nor Fe in the direct conversion of dioxygen to H(2)O(2) makes oxalate oxidase unique. A model of the active site of the holoenzyme based on a homology model of the apoenzyme is proposed.

Evidence for a major structural change in Escherichia coli chorismate synthase induced by flavin and substrate binding

Chorismate synthase (EC 4.6.1.4) catalyses the conversion of 5-enolpyruvylshikimate 3-phosphate (EPSP) into chorismate, and requires reduced FMN as a cofactor. The enzyme can bind first oxidized FMN and then EPSP to form a stable ternary complex which does not undergo turnover. This complex can be considered to be a model of the ternary complex between enzyme, EPSP and reduced FMN immediately before catalysis commences. It is shown that the binding of oxidized FMN and EPSP to chorismate synthase affects the properties and structure of the protein. Changes in small-angle X-ray scattering data, decreased susceptibility to tryptic digestion and altered Fourier-transform (FT)-IR spectra provide the first strong evidence for major structural changes in the protein. The tetrameric enzyme undergoes correlated screw movements leading to a more overall compact shape, with no change in oligomerization state. The changes in the FT-IR spectrum appear to reflect changes in the environment of the secondary-structural elements rather than alterations in their distribution, because the far-UV CD spectrum changes very little. Changes in the mobility of the protein during non-denaturing PAGE indicate that the ternary complex may exhibit less conformational flexibility than the apoprotein. Increased enzyme solubility and decreased tryptophan fluorescence are discussed in the light of the observed structural changes. The secondary structure of the enzyme was investigated using far-UV CD spectroscopy, and the tertiary structure was predicted to be an alpha-beta-barrel using discrete state-space modelling.

Characterisation of the molybdenum-responsive ModE regulatory protein and its binding to the promoter region of the modABCD (molybdenum transport) operon of Escherichia coli

Molybdenum-dependent repression of transcription of the Escherichia coli modABCD operon, which encodes the high-affinity molybdate transporter, is mediated by the ModE protein. This regulatory protein was purified as an N-terminal His6-tagged derivative and characterised both with and without the N-terminal oligohistidine extension. Equilibrium centrifugation showed that ModE is at least a 57-kDa homodimer. Circular dichroism spectroscopy indicated that when molybdate or tungstate bind to ModE there is little change in its alpha-helical content, but a major change in the environment of tryptophan and tyrosine residues occurs. Addition of molybdate or tungstate to the protein results in almost 50% quenching of the fluorescence attributed to tryptophan. Titration of fluorescence quenching showed that two molecules of molybdenum bind to each dimer of ModE with a Kd of 0.8 microM. DNA mobility-shift assays showed that ModE requires molybdenum, or tungstate, to bind with high affinity (approximate Kd of 30 nM ModE) to the modABCD promoter region. In accord with ModE's role as a molybdenum-dependent transcriptional repressor, DNase I footprinting experiments showed that the ModE-molybdenum complex binds to a single 31-bp region around the transcription start of the modABCD promoter. This region contains a 6-base palindromic sequence CGTTAT-N12-ATAACG.

Studies with flavin analogs provide evidence that a protonated reduced FMN is the substrate-induced transient intermediate in the reaction of Escherichia coli chorismate synthase

Chorismate synthase catalyzes the 1,4-elimination of phosphate and the C-(6-pro-R) hydrogen from 5-enolpyruvylshikimate 3-phosphate (EPSP) to generate chorismate. Although this reaction does not involve an overall change in redox state, the enzyme requires reduced FMN. To investigate the role of the flavin in catalysis we have employed chemically modified flavins: 1- and 5-deaza-, 2- and 4-thio-, 6-hydroxy-, 8-nor-6-methyl-, 8-methyl-sulfonyl-, 8-chloro-, 8-fluoro-, 8-nor-methyl-, 8-S-methyl-, 8-methoxy, 8-mercapto- and 8-amino-FMN. Photoreduction of 4-thio-FMN in the presence of chorismate synthase at pH 7.5 produced a reduced flavin species with an absorbance maximum at lambda = 410 nm indicative of monoanionic, reduced 4-thio-FMN. Binding of 8-mercapto- and 6-hydroxy-FMN to chorismate synthase in the presence of EPSP or (6R)-6-fluoro-EPSP resulted in an increase of the flavin analogs' pKa values by 4 and 1 pH units, respectively. On the basis of these findings it is concluded that chorismate synthase preferentially binds neutral flavin species, including the protonated reduced form, rather than anionic flavin species in the presence of EPSP or the 6-fluoro-substrate analog. Further support for this conclusion was obtained using 5-deaza- and 4-thio-FMN. Addition of EPSP to enzyme-bound, reduced 5-deaza-FMN produced spectral changes consistent with protonation of the flavin. Photoreduction of 4-thio-FMN in the presence of enzyme and the (6R)-6-fluoro-EPSP generated a reduced flavin species with absorbance properties of a neutral, reduced 4-thio-flavin. These results and their implications for the nature and kinetic properties of an observed flavin intermediate are discussed in the context of a possible role of reduced flavin as an electron donor to bound EPSP.

The transient kinetics of Escherichia coli chorismate synthase: substrate consumption, product formation, phosphate dissociation, and characterization of a flavin intermediate

Chorismate synthase is the seventh enzyme of the shikimate pathway and catalyzes the conversion of 5-enolpyruvylshikimate 3-phosphate (EPSP) to chorismate. The reaction involves the 1,4-elimination of phosphate and the C-(6proR) hydrogen of the substrate with unusual anti stereochemistry and requires a reduced flavin cofactor. This paper describes the kinetics of the formation and decay of a flavin intermediate, EPSP consumption, chorismate and phosphate formation, and phosphate dissociation during single and multiple turnover experiments, determined using rapid reaction techniques. The kinetics of phosphate dissociation using the substrate analogues (6R)-[6-2H]EPSP and (6S)-6-fluoro-EPSP have also been determined. The observations are consistent with a nonconcerted chorismate synthase reaction. The flavin intermediate is not simply associated with the conversion of substrate to product because it forms before the substrate is consumed. The transient spectral changes must be associated primarily with events such as protonation of the reduced flavin, a charge transfer complex between reduced flavin and an aromatic amino acid, or a conformational change in the protein. This does not rule out the direct role of flavin in catalysis.

Binding of the oxidized, reduced, and radical flavin species to chorismate synthase. An investigation by spectrophotometry, fluorimetry, and electron paramagnetic resonance and electron nuclear double resonance spectroscopy

Chorismate synthase (EC 4.6.1.4) binds oxidized riboflavin-5'-phosphate mononucleotide (FMN) with a KD of 30 microM at 25 degrees C, but in the presence of 5-enolpyruvylshikimate-3-phosphate (EPSP), the KD decreases to ca. 20 nM. Similar effects occur with the substrate analogue (6R)-6-fluoro-EPSP (KD = 36 nM) and chorismate (KD = 540 nM). Fluorescence of oxidized FMN is slightly quenched in the presence of chorismate synthase. Addition of EPSP or the (6R)-6-fluoro analogue causes a shift of the fluorescence from 520 to 495 nm. Chorismate causes no shift in, but a quenching of, the fluorescence emission maximum. In the presence of EPSP, (6R)-6-fluoro-EPSP, or chorismate, the neutral flavinsemiquinone is generated. The electron paramagnetic resonance (EPR) line width of the flavin radical is indicative of a neutral flavinsemiquinone. Frozen solution electron nuclear double resonance (ENDOR) of the radical with (6R)-6-fluoro-EPSP shows a number of proton ENDOR line pairs. The largest splitting is assigned to a hyperfine coupling to the methyl group beta-protons at position 8 of the isoalloxazine ring. The hyperfine-coupling (hfc) components have values of A perpendicular = 8.07 MHz and A parallel = 9.60 MHz, giving Aiso of 8.58 MHz, consistent with a neutral semiquinone form. The isotropic hfc coupling of the 8-methyl protons with (6R)-6-fluoro-EPSP decreases by about 0.5 MHz when chorismate is bound, indicating that the spin density distribution within the isoalloxazine ring system depends critically on the nature of the ligand. The redox potential of FMN in the presence of chorismate synthase was 95 mV more positive than that of free FMN (at pH 7.0), equivalent to a 1660-fold tighter binding of reduced FMN. The pH dependence of the redox potential of chorismate synthase-bound FMN exhibits a slope of -30 mV per pH unit between pH 6 and 9, indicating that the two-electron reduction of the flavin is associated with the uptake of one proton; this, and the UV-visible spectrum, is consistent with the reduced flavin being bound to chorismate synthase in its monoanionic form.

Escherichia coli chorismate synthase catalyzes the conversion of (6S)-6-fluoro-5-enolpyruvylshikimate-3-phosphate to 6-fluorochorismate. Implications for the enzyme mechanism and the antimicrobial action of (6S)-6-fluoroshikimate

Chorismate synthase catalyzes the conversion of 5-enolpyruvylshikimate-3-phosphate to chorismate. It is the seventh enzyme of the shikimate pathway, which is responsible for the biosynthesis of aromatic metabolites from glucose. The chorismate synthase reaction involves a 1,4-elimination with unusual anti-stereochemistry and requires a reduced flavin cofactor. The substrate analogue (6S)-6-fluoro-5-enolpyruvylshikimate-3-phosphate is a competitive inhibitor of Neurospora crassa chorismate synthase (Balasubramanian, S., Davies, G. M., Coggins, J. R., and Abell, C. (1991) J. Am. Chem. Soc. 113, 8945-8946). We have shown that this analogue is converted to 6-fluorochorismate by Escherichia coli chorismate synthase at a rate 2 orders of magnitude slower than the normal substrate. The decreased rate of reaction is consistent with the destabilization of an allylic cationic intermediate. The formation of chorismate and 6-fluorochorismate involves a common protein-bound flavin intermediate although the fluoro substituent does influence the spectral characteristics of this intermediate. The fluoro substituent also decreased the rate of decay of the flavin intermediate by 280 times. These results are consistent with the antimicrobial activity of (6S)-6-fluoroshikimate not being mediated by the inhibition of chorismate synthase but by the inhibition of 4-aminobenzoic acid synthesis as previously proposed (Davies, G. M., Barrett-Bee, K. J., Jude, D. A., Lehan, M., Nichols, W. W., Pinder, P. E., Thain, J. L., Watkins, W. J., and Wilson, R. G. (1994) Antimicrobial Agents and Chemotherapy 38, 403-406).

Escherichia coli chorismate synthase: a deuterium kinetic-isotope effect under single-turnover and steady-state conditions shows that a flavin intermediate forms before the C-(6proR)-H bond is cleaved

We report the observation of a deuterium kinetic isotope effect for the conversion of 5-enolpyruvylshikimate-3-phosphate into chorismate (6proR2HV = 1.13 +/- 0.03) using recombinant chorismate synthase from Escherichia coli. Similar isotope effects were observed for the decay of a spectroscopically characterized flavin intermediate (6proR2Hk = 1.17 +/- 0.04) during single-turnover experiments. The main rate-limiting steps and C-(6proR)-H bond breaking are therefore distinct and both must occur after the formation of the flavin intermediate and either before or concomitant with its decay.

The Greater Vancouver Mental Health Service Society: 20 years' experience in urban community mental health

Caring for people in the community with persistent and disabling mental illnesses presents a major challenge to government, planners and mental health professionals. The success with which mentally disabled people are integrated into community life says much about the society in which we live. This article describes the experience of the Greater Vancouver Mental Health Service Society in offering community-based mental health services to persons with schizophrenia and other major mental disorders over the past 20 years. The key to its success lies in a decentralized, relatively non hierarchical organizational structure which allows committed and skilled multidisciplinary teams to work with patients and their families in their community. The resulting services are fully integrated within the fabric of the community and are responsive to local needs. Partnerships among professionals, patients, families and community agencies result in work that is creative, productive and effective.

[Catabolic properties of 5',5"-linked dinucleosidephosphates in rat liver nuclei]

We report on the enzymatic degradation of 14C-labelled 5',5"-linked dinucleoside triphosphates Gp3A, 2-O-mGp3A, 2'dGp3A, 2'3"ddGp3A and m7Gp3A in rat liver nuclei. The 2'-desoxy- and 2',3'-didesoxy compounds are poorer substrates than the other cap-structured dinucleotides investigated.

[P1, P3-(5'-guanosyl-5''-[14C]adenosyl)triphosphate: preparation of the cap parent compound and its catabolic properties towards rat liver subcellular fractions (author's transl)]

We report on the chemical preparation of 14C-labeled P1, P3-(5'-guanosyl-5''-adenosyl)triphosphate (1) which is the parent compound of 5'-terminal cap structures of most eukaryotic mRNAs. Incubation experiments of cap (1)- and non cap-structured nucleotides as Ap2A (2) as well as their ribose ring opened derivatives (3, 4) with rat liver subcellular fractions show that cap-degrading nucleases being able to unblock 5' termini are present in nuclei but not in mitochondria.