Intermediate Plateaux Research Articles

Spin-1/2 Ising–Heisenberg Cairo pentagonal model in the presence of an external magnetic field: effect of Landé g-factors

In the present paper, a study of the magnetic properties of a spin-1/2 Ising-Heisenberg Cairo pentagonal structure is presented. The model has been investigated in Ref. [34] in the absence of external magnetic field. Here, we consider the effects of an external tunable magnetic field. By using the transfer matrix approach, we investigate the magnetic ground-state phase transition, the low-temperature magnetization process, and how the magnetic field influences the various thermodynamic parameters such as entropy, internal energy and specific heat. It is shown that the model exhibits intermediate magnetization plateaux accompanied by a double-peak in the specific heat curve versus temperature. The position of each magnetization jump is in accordance with the merging and/or separation of the two peaks in the specific heat curve. Considering different g-factors for the nodal Ising spins and spin dimers also results in arising different intermediate plateaux and to remarkable alterations of the thermodynamic properties of the model.

Magnetization Process and Magnetocaloric Effect of the Spin-1/2 XXZ Heisenberg Cuboctahedron

Magnetic properties of the spin-1/2 XXZ Heisenberg cuboctahedron are examined using exact numerical diagonalization depending on a relative strength of the exchange anisotropy. While the Ising cuboctahedron exhibits in a low-temperature magnetization curve only one-third magnetization plateau, the XXZ Heisenberg cuboctahedron displays another four intermediate plateaux at zero, one-sixth, one-half and two-thirds of the saturation magnetization. The novel magnetization plateaux generally extend over a wider range of magnetic fields with increasing of a quantum ($xy$) part of the XXZ exchange interaction. It is shown that the XXZ Heisenberg cuboctahedron exhibits in a vicinity of all magnetization jumps anomalous thermodynamic behavior accompanied with an enhanced magnetocaloric effect.

Fractional magnetization plateaus of the spin-12Heisenberg orthogonal-dimer chain: Strong-coupling approach developed from the exactly solved Ising-Heisenberg model

The spin-1/2 Heisenberg orthogonal-dimer chain is considered within the perturbative strong-coupling approach, which is developed from the exactly solved spin-1/2 Ising-Heisenberg orthogonal-dimer chain with the Heisenberg intradimer and the Ising interdimer couplings. Although the spin-1/2 Ising-Heisenberg orthogonal-dimer chain exhibits just intermediate plateaux at zero, one-quarter and one-half of the saturation magnetization, the perturbative treatment up to second order stemming from this exactly solvable model additionally corroborates the fractional one-third plateau as well as the gapless Luttinger spin-liquid phase. It is evidenced that the approximate results obtained from the strong-coupling approach are in an excellent agreement with the state-of-the-art numerical data obtained for the spin-1/2 Heisenberg orthogonal-dimer chain within the exact diagonalization and density-matrix renormalization group method. The nature of individual quantum ground states is comprehensively studied within the developed perturbation theory.

Magnetization Process and Adiabatic Demagnetization of the Antiferromagnetic Spin-1/2 Heisenberg Cubic Cluster

A full energy spectrum of the spin-1/2 Heisenberg cubic cluster is used to investigate a low-temperature magnetization process and adiabatic demagnetization of this zero-dimensional 2x2x2 quantum spin system. It is shown that the antiferromagnetic spin-1/2 Heisenberg cube exhibits at low enough temperatures a stepwise magnetization curve with four intermediate plateaux at zero, one quarter, one half, and three quarters of the saturation magnetization. We have also found the enhanced magnetocaloric effect close to level-crossing fields that determine transitions between the intermediate plateaux.

Studies of homogeneous “biosynthetic” l-threonine dehydratase from Escheria coli K-12 : Some kinetic properties and molecular multiplicity

“Biosynthetic” l-threonine dehydratase (EC 4.2.1.16) was purified to a homogeneous state with 29% yield of total activity from Escherichia coli K-12. The homogeneity of the enzyme was shown by polyacrylamide gel disc electrophoresis in the presence of dodecyl sulphate. The enzyme consisted of equal subunits having a molecular weight of about 57 000. The polyacrylamide gel disc electrophoresis has shown that the native enzyme consisted of a set of oligomeric forms. The multiplicity of molecular organization of the enzyme was reflected in complicated kinetic behaviour: at pH > 9 on the plots of initial reaction rate ( v) versus initial substrate concentration ([ S] 0) there were four inflexion points (two intermediate plateaux), the position and deepness of which depended on enzyme concentration. At pH 8.3 on the v versus [ S] 0 plots appeared two inflexion points (one intermediate plateau), the position of which practically did not depend on enzyme concentration in the reaction mixture, but strongly depended on the enzyme concentration in the stock solution. Repeated polyacrylamide gel disc electrophoresis of several oligomeric forms, isolated by the first electrophoresis, has shown that the oligomeric forms underwent a slow polymerization. It was suggested that “biosynthetic” l-threonine dehydratase from E. coli K-12 is a set of multiple oligomeric forms, having different kinetic parameters. Probably, each form of the enzyme has a “simple” kinetics characterized by hyperbolic or sigmoidal shape of v versus [ S] 0 plots. The rate of equilibrium installation between the oligomeric forms was small in comparison with the enzyme reaction velocity, that lead to the complex kinetic curves, appearing as a result of summing up of the kinetics inherent to the individual forms.

PH-dependent intermediate plateaux in the kinetics of the reaction catalyzed by “biosynth” l-threonine dehydratase of Escherichia coli K-12

1. 1.|A method for the determination of l-threonine dehydratase ( l-threonine hydro-lyase (deaminating), EC 4.2.1.16) activity, based on measurements of the amount of 2-oxobutyrate formed in the threonine dehydratase reaction by means of an auxiliary lactate dehydrogenase system (EC 1.1.1.27), was described. 2. 2.|The kinetics of the reaction catalyzed by the “biosynthetic” l-threonine dehydratase in a crude extract from Escherichia coli K-12 cells were studied. Four inflexion points ( i.e., two intermediate plateaux) on the plot of initial reaction rate (ν) versus initial substrate concentration ([ S] 0) were distinctly revealed at pH > 9. 3. 3.|The allosteric inhibitor l-isoleucine, at certain pH values, displaced the sigmoid shape of the plot of v versus [ S] 0 to higher substrate concentrations; the magnitude of the maximal reaction rate ( V) did not change. At high concentrations of enzyme protein, l-isoleucine, at low concentrations, removed the intermediate plateau on the plot of v versus [ S] 0 and hence “activated” the enzyme in the region of high substrate concentrations, whilst the second intermediate plateau was demonstrated in these curves as in the usual sigmoidal plots. 4. 4.| dl-Norvaline “normalized” the kinetics of the reaction catalyzed by the enzyme and at the same time decreased the value of V. 5. 5.|Treatment of the enzyme with low concentrations of HgCl 2 (7.5·10 −5 M) led to the complete desensitization of the enzyme in relation to the inhibitory effect of l-isoleucine at pH 9.6 and “normalized” the effects of dl-norvaline on the reaction kinetics. The plot of v versus [ S] 0 for the desensitized enzyme was characterized by the absence of the inflexion points (intermediate plateaux) which were characteristic for the native enzyme at this pH value, although the plot did not become hyperbolic. Treatment of the enzyme at pH 8.2 with lower concentrations of HgCl 2 (1.0·10 −5 M) led to selective desensitization, namely, the sensitivity to the “normalizing” effect of dl-norvaline was completely lost but the sensitivity to the inhibitory effect of l-isoleucine was partially restored. It was suggested that the enzyme molecule possesses two spatially divided binding sites for these allosteric effectors. 6. 6.|The shape of the plot of v versus [ S] 0 depends on the concentration of enzyme protein, namely, at low enzyme concentrations two intermediate plateaux were revealed on the plot of v versus [ S] 0. At the same time, an increase in enzyme concentration led to the annihilation of the intermediate plateau demonstrated at low substrate concentrations and to the displacement of the second intermediate plateau found at high substrate concentrations towards lower substrate concentrations. 7. 7.|Low concentrations of l-isoleucine removed this second intermediate plateau only at high enzyme concentrations, and hence “activated” the enzyme at about the substrate concentrations at which the second intermediate plateau was revealed. 8. 8.|The data obtained were discussed, taking into consideration the possible effects of allosteric ligands on the dissociation of the enzyme oligomer and on the conformation states of subunits in the oligomer itself.