Magnetic susceptibility tensor and heme contact shifts determinations in the Rhodobacter capsulatus ferricytochrome c': NMR and magnetic susceptibility studies.

The 1H and 15N resonances of the carbon monoxide complex of ferrocytochrome c' of Rhodobacter capsulatus, a ferrous diamagnetic heme protein, have been extensively assigned by TOCSY-HSQC, NOESY-HSQC, and HSQC-NOESY-HSQC 3D heteronuclear experiments performed on a 7 mM sample labeled with 15N. Based on short-range and medium-range NOEs and H(N) exchange rates, the secondary structure consists of four helices: helix 1 (3-29), helix 2 (33-48), helix 3 (78-101), and helix 4 (103-125). The 15N, 1HN, and 1H(alpha) chemical shifts of the CO complex form are compared to those of the previously assigned oxidized (or ferric) state. From the chemical shift differences between these redox states, the orientation and the anisotropy of the paramagnetic susceptibility tensor have been determined using the crystallographic coordinates of the ferric state. The chi-tensor is axial, and the orientation of the z-axis is approximately perpendicular to the heme plane. The paramagnetic chemical shifts of the protons of the heme ligand have been determined and decomposed into the Fermi shift and dipolar shift contributions. Magnetic susceptibility studies in frozen solutions have been performed. Fits of the susceptibility data using the model of Maltempo (Maltempo, M. M. J. Chem. Phys. 1974, 61, 2540-2547) are consistent with a rather low contribution of the S = 3/2 spin state over the range of temperatures and confirm the value of the axial anisotropy. Values in the range 10.4-12.5 cm(-1) have been inferred for the axial zero-field splitting parameter (D). Analysis of the contact shift and the susceptibility data suggests that cytochrome c' of Rb. capsulatus exhibits a predominant high-spin character of the iron in the oxidized state at room temperature.     

Surface-modified membranes as a matrix for protein purification.

Recently introduced membrane-based chromatographic supports for protein separation are available either with a coupled ligand, e.g., protein A, protein G or ion-exchange groups, or as activated matrices for coupling a desired ligand. The coupling conditions for protein A and immunoglobulin G to an epoxy-activated membrane were determined. The performance of the prepared affinity membranes was investigated using pure rabbit immunoglobulin G and protein A as a model system. For practical application monoclonal antibodies from cell culture supernatant were purified with a prepared protein A membrane and for comparison with a sulphonic acid ion exchange membrane.     

Structural characterisation of a diprotonated ligand lanthanide complex--a key intermediate in lanthanide ion association and complex dissociation pathways

The X-ray structure is reported of a diprotonated Gd(III) tetra-aqua complex of the ligand 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (DOTAM), a thermodynamic intermediate in metal ion association and dissociation pathways.     

Magnetic field induced synthesis and self-assembly of super paramagnetic particles in a protein matrix

Aqueous solution of a globular protein named bovine serum albumin was homogeneously mixed with ferrous and ferric ions and allowed to gel at ambient conditions. Gels were then oxidized using sodium hydroxide, in the presence of magnetic field of magnitude 0.13 T. The effect of magnetic field on the above biomimetic synthesis was a reduction in particle size and a directional assembly of synthesized super paramagnetic particles into a regular pattern in the protein film. The microstructural revelation was complimentary to Mossbauer results and magnetic measurement studies, i.e., an interesting variation in the magnetic behaviour of self-assembled super paramagnetic particles as a function of dc magnetic field induced ordering.     

Magnetic properties of lanthanide organometallic sandwich complexes produced in a molecular beam

The magnetic properties of gas-phase terbium–cyclooctatetraene multi-decker sandwich complexes, Tb_n(C₈H₈)_n+1 were measured using a Stern-Gerlach type magnetic deflection approach. Beams of Tb_n(C₈H₈)_n+1 complexes displayed one-sided deflection toward high field – indicating that fast spin relaxation occurs within the complexes as they pass through the magnetic field. The magnetic moment for Tb_n(C₈H₈)_n+1 (n = 1−5) was evaluated using the Langevin model. Evolution of magnetic moment with the complex size is discussed with electronic structures for oxidation states of Tb^3+/2+ ions, implying the possibility of antiferromagnetic interaction of two adjacent Tb²⁺ ions.     

A caged lanthanide complex as a paramagnetic shift agent for protein NMR

A lanthanide complex, named CLaNP (caged lanthanide NMR probe) has been developed for the characterisation of proteins by paramagnetic NMR spectroscopy. The probe consists of a lanthanide chelated by a derivative of DTPA (diethylenetriaminepentaacetic acid) with two thiol reactive functional groups. The CLaNP molecule is attached to a protein by two engineered, surface-exposed, Cys residues in a bidentate manner. This drastically limits the dynamics of the metal relative to the protein and enables measurements of pseudocontact shifts. NMR spectroscopy experiments on a diamagnetic control and the crystal structure of the probe-protein complex demonstrate that the protein structure is not affected by probe attachment. The probe is able to induce pseudocontact shifts to at least 40 A from the metal and causes residual dipolar couplings due to alignment at a high magnetic field. The molecule exists in several isomeric forms with different paramagnetic tensors; this provides a fast way to obtain long-range distance restraints.     

A novel ethacrynic acid sensor based on a lanthanide porphyrin complex in a PVC matrix

Lanthanide porphyrin complexes synthesized by a solid state method were used to prepare a novel ethacrynic acid (EA) sensor. The sensor, based on pentane-2,4-dionato(meso-tetraphenylporphinato)terbium [TbTPP(acac)] with an optimized membrane composition, exhibits a Nernstian response to EA- ion in the concentration range 7.4 x 10(-6)-1.0 x 10(-1) mol l-1 with a pH range from 3.2 to 6.8 and a fast response time of 30 s. The electrode shows improved selectivity towards EA- ion with respect to common co-existing ions compared with the previously reported EA sensor. As electroactive materials, lanthanide porphyrin complexes show better potentiometric response characteristics than copper porphyrin complexes. The effect of solvent mediators and lipophilic ion additives was studied and the experimental conditions were optimized. The electrode was applied to the determination of EA in human urine samples with satisfactory results.     

Magnetic shielding tensor components in a highly strained phosphorus molecule: 1,2,3-triphenylphosphirene

Using the magic-angle spinning (MAS) technique the principal values of the ³¹P NMR magnetic shielding tensor were measured in 1,2,3-triphenylphosphirene: σ₁₁ = −58; σ₂₂ = 23 and σ₃₃ = 636 ppm. The anisotropy is quite large for a three-coordinate phosphorus compound. The high-field component σ₃₃ corresponds to the direction perpendicular to the ring, and is responsible for the high-field resonance in the isotropie phase.     

Ionic liquids as a novel solvent for lanthanide extraction.

Octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) dissolved in an ionic liquids, 1-butyl-3-methyl-imidazolium hexafluorophosphate, greatly enhances extractability and selectivity of lanthanide cations compared to that dissolved in conventional organic solvents; further, the recovery of lanthanides extracted into ionic liquids can be accomplished using several stripping solutions containing complexing agents. The possibility of utilizing ionic liquids as novel separation media in an industrial liquid-liquid extraction process was demonstrated.     

Unusually sharp dependence of water exchange rate versus lanthanide ionic radii for a series of tetraamide complexes

The tetraamide ligand, DOTA-tetra(glycine ethyl ester), forms complexes with the lanthanide(III) cations that exist in solution predominantly as the square antiprism structure with single, slowly exchanging inner-sphere water molecule. Variable-temperature 1H and 17O NMR studies revealed that the bound water lifetimes in these complexes were sharply dependent upon the ionic radius of Ln3+ cation. A novel lanthanide-induced shift technique was used to unmask the bound water 17O resonance of SmL3+ and YL3+ complexes from the bulk water resonance. The bound water lifetime (tauM298) was approximately 800 mus in the EuL3+ complex but became much shorter (several microseconds) for Ln3+ cations with larger and smaller ionic radii. This demonstrates that water exchange is exquisitely fine-tuned in this macrocyclic tetraamide system and that a variety of Ln3+ complexes meet with the exchange requirement, Deltaomega*tauM >/= 1, necessary for an efficient MT agent.     

Unitary group tensor operator algebras for many-electron systems: II. One- and two-body matrix elements

Relying on our earlier results in the unitary group Racah-Wigner algebra, specifically designed to facilitate quantum chemical calculations of molecular electronic structure, the tensor operator formalism required for an efficient evaluation of one- and two-body matrix elements of molecular electronic Hamiltonians within the spin-adapted Gel'fand-Tsetlin basis is developed. Introducing the second quantization-like creation and annihilation vector operators at the unitary group [U(n)] level, appropriate two-box symmetric and antisymmetric irreducible tensor operators as well as adjoint tensors are defined and their matrix elements evaluated in the electronic Gel'fand-Tsetlin basis as single products of segment values. Using these tensor operators, the matrix elements of one- and two-body components of a general electronic Hamiltonian are found. Explicit expressions for all relevant quantities pertaining to at most two-column irreducible representations that are required in molecular electronic structure calculations are given. Relationships with other approaches and possible future extensions of the formalism to partitioned bases or spin-dependent Hamiltonians are discussed.On leave from: Department of Chemistry, Xiamen University, Xiamen, Fujian, PR China.     

Magnetic properties of the semiconducting lanthanide cuprates Ln2CuO4 and their interpretation: Evidence for a new series of planar copper antiferromagnets

The magnetic susceptibility of the semiconducting lanthanide cuprates Nd₂CuO₄, Pr₂CuO₄, Eu₂CuO₄, and Sm₂CuO₄ has been measured in the range 4–300 K. Below 300 K, the Cu²⁺ ions are ordered antiferromagnetically in the CuO₂ planes of these compounds, and the exchange interactions involving the Ln³⁺ ions are relatively weak. The suceptibility of the Ln³⁺ ions obeys the Curie-Weiss law at elevated temperatures, but deviations from this law occur at lower temperatures. An attempt is made to account for these deviations by fitting theoretical expressions for the susceptibility of isolated Ln³⁺ ions under the influence of a cubic crystal field to the experimental data. Excellent agreement is obtained for Nd³⁺ and Eu³⁺ over the entire temperature range and for Pr³⁺ and Sm³⁺ at elevated temperatures. Deviations at lower temperatures for the latter two ions may be due to structural changes, exchange interactions involving the Ln³⁺ ions, or possibly oxygen nonstoichiometry. The susceptibility parameters derived by fitting the theoretical expressions to the experimental data are also discussed. It is concluded that these compounds form an interesting new series of planar Cu²⁺-ion antiferromagnets.     

Magnetic properties of group VIB hexacarbonyls. Suggestion of a new diamagnetic susceptibility standard

Summary Careful measurements of the magnetic susceptibilities of group VI B metal hexacarbonyls show an ideal diamagnetic behaviour. The properties of W(CO)₆, together with the relatively low price, lead to the suggestion of W(CO)₆ as a new calibration standard for the diamagnetic region.

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Magnetische Eigenschaften von Hexacarbonylen der Gruppe VI B. Vorschlag zu einem neuen diamagnetischen Suszeptibilitätsstandard

Zusammenfassung Sorgfältige Messungen der magnetischen Suszeptibilität der Metallhexacarbonyle der Gruppe VI B zeigen deren ideal diamagnetisches Verhalten. Die physikalischen Eigenschaften des W(CO)₆, zusammen mit dem relativ niedrigen Preis, prädestinieren es als diamagnetischen Eichstandard.

     

4,4'-dithiobisdipicolinic acid: a small and convenient lanthanide binding tag for protein NMR spectroscopy

Pseudocontact shifts (PCS) from paramagnetic lanthanide ions present powerful long-range structure restraints for studies of proteins by nuclear magnetic resonance spectroscopy. To elicit PCSs, the lanthanide must be attached site-specifically to the target protein. In addition, it needs to be attached rigidly to avoid averaging of the PCSs due to mobility with respect to the protein and it must not interfere with the function of the protein. Here, we present a dipicolinic acid reagent that spontaneously forms a disulfide bond with thiol groups of accessible cysteine residues. A minimal number of rotatable bonds between the cysteine side chain and the tag helps to minimise mobility. Combined with the small size of the tag and quantitative tagging yields, these features make it a highly attractive tool for generating structure restraints by paramagnetic lanthanides.     

On the importance of a correct choice of model for lanthanide ion binding in the interpretation of lanthanide-induced N.M.R. shifts: the adamantanone problem

Interpretation of ¹H and ¹³C lanthanide-induced N.M.R. shifts on adamantanone depends critically on the model chosen for lanthanide ion binding.     

Magnetic bistability of isolated giant-spin centers in a diamagnetic crystalline matrix

Polynuclear single-molecule magnets (SMMs) were diluted in a diamagnetic crystal lattice to afford arrays of independent and iso-oriented magnetic units. Crystalline solid solutions of an Fe(4) SMM and its Ga(4) analogue were prepared with no metal scrambling for Fe(4) molar fractions x down to 0.01. According to high-frequency EPR and magnetic measurements, the guest SMM species have the same total spin (S=5), anisotropy, and high-temperature spin dynamics found in the pure Fe(4) phase. However, suppression of intermolecular magnetic interactions affects magnetic relaxation at low temperature (40 mK), where quantum tunneling (QT) of the magnetization dominates. When a magnetic field is applied along the easy magnetic axis, both pure and diluted (x=0.01) phases display pronounced steps at evenly spaced field values in their hysteresis loops due to resonant QT. The pure Fe(4) phase exhibits additional steps which are firmly ascribed to two-molecule QT transitions. Studies on the field-dependent relaxation rate showed that the zero-field resonance sharpens by a factor of five and shifts from about 8 mT to exactly zero field on dilution, in agreement with the calculated variation of dipolar interactions. The tunneling efficiency also changes significantly as a function of Fe(4) concentration: the zero-field resonance is significantly enhanced on dilution, while tunneling at ±0.45 T becomes less efficient. These changes were rationalized on the basis of a dipolar shuffling mechanism and transverse dipolar fields, whose effect was analyzed by using a multispin model. Our findings directly prove the impact of intermolecular magnetic couplings on SMM behavior and disclose the magnetic response of truly isolated giant spins in a diamagnetic crystalline environment.     

Trends in stability of the lanthanide series determined for the TBP—Dilute HNO3 extraction systems

The paper concerns the extraction of lanthanides by TBP from dilute solutions of nitric acid. The results show that the relative free energy associated with the investigated process decreases from La to Sm and the increases up to Lu. The occurrence of a minimum in the free energy changes is explained by the change of coordination number in the first hydration sphere of lanthanide aqua ions between Pm and Gd. The results are discussed in terms of the most probable structures of the extracted species. Outer sphere complexa tion seems to be realized by the Ln³⁺ ions transferring into the organic phase. The estimation of the trends in the enthalpy and entropy terms is under preparation. Up to, 1980 Department of Radiochemistry.     

Magnetic effects and viscosity in the nematic phase of a polymer series

Magnetic measurements were made on a series of polyesters in their nematic state and compared to their apparent viscosity at the same temperature. At similar molecular weights, the length of a flexible segment in the main-chain polymer plays an important role in both the magnetic orientation effects and the apparent viscosity behavior. Moreover, on cooling from a perfectly oriented nematic sample, the alignment remains down to room temperature. In contrast with nematic monomers, these polymers behave like solids, even at the lowest velocity of a rotating magnetic field.     

Magnetic circular dichroism and absorption spectra of methylidyne in a krypton matrix

Electronic absorption and magnetic circular dichroism spectra are reported for the A(2)Δ, B(2)Σ(-), and C(2)Σ(+) ← X(2)Π transitions of methylidyne radicals isolated in a Kr matrix at cryogenic temperatures. The results are interpreted in the framework of a model in which the X(2)Π term is split by combination of spin-orbit and crystal-field interactions with the atoms of the host matrix. Analysis of the zeroth moments of the spectra yields an empirical spin-orbit coupling constant A(Π) = 11 ± 2 cm(-1) and orbital reduction factor κ = 0.26 ± 0.05, corresponding to a crystal-field splitting of V(Π) = 43 ± 10 cm(-1) for the X(2)Π term. For the A(2)Δ excited-state term, analysis of the first MCD moments gives a spin-orbit coupling constant of A(Δ) = 4.4 ± 0.9 cm(-1).     

A new potentiometric ibuprofenate ion sensor immobilized in a graphite matrix for determination of ibuprofen in tablets

The characteristics, performance, and application of an electrode, namely, Pt|Hg|Hg₂(IBP)₂|Graphite, where IBP stands for ibuprofenate ion, are described. This electrode responds to IBP with sensitivity of (58.6 ± 0.9) mV decade^− 1 over the range 5.0 × 10^− 5–1.0 × 10^− 1 mol L^− 1 at pH 6.0–9.0 and a detection limit of 3.8 × 10^− 5 mol L^− 1. The electrode is easily constructed at a relatively low cost with fast response time (within 15–30 s) and can be used for a period of 5 months without any considerable divergence in potentials. The proposed sensor displayed good selectivity for ibuprofen in the presence of several substances, especially concerning carboxylate and inorganic anions. It was used for the direct assay of ibuprofen in commercial tablets by means of the standard additions method. The analytical results obtained by using this electrode are in good agreement with those given by the United States Pharmacopeia procedure.