X-band Electron Paramagnetic Resonance Spectra Research Articles

Structure of Apolipoprotein A-I N Terminus on Nascent High Density Lipoproteins

Apolipoprotein A-I (apoA-I) is the major protein component of high density lipoproteins (HDL) and a critical element of cholesterol metabolism. To better elucidate the role of the apoA-I structure-function in cholesterol metabolism, the conformation of the apoA-I N terminus (residues 6-98) on nascent HDL was examined by electron paramagnetic resonance (EPR) spectroscopic analysis. A series of 93 apoA-I variants bearing single nitroxide spin label at positions 6-98 was reconstituted onto 9.6-nm HDL particles (rHDL). These particles were subjected to EPR spectral analysis, measuring regional flexibility and side chain solvent accessibility. Secondary structure was elucidated from side-chain mobility and molecular accessibility, wherein two major α-helical domains were localized to residues 6-34 and 50-98. We identified an unstructured segment (residues 35-39) and a β-strand (residues 40-49) between the two helices. Residues 14, 19, 34, 37, 41, and 58 were examined by EPR on 7.8, 8.4, and 9.6 nm rHDL to assess the effect of particle size on the N-terminal structure. Residues 14, 19, and 58 showed no significant rHDL size-dependent spectral or accessibility differences, whereas residues 34, 37, and 41 displayed moderate spectral changes along with substantial rHDL size-dependent differences in molecular accessibility. We have elucidated the secondary structure of the N-terminal domain of apoA-I on 9.6 nm rHDL (residues 6-98) and identified residues in this region that are affected by particle size. We conclude that the inter-helical segment (residues 35-49) plays a role in the adaptation of apoA-I to the particle size of HDL.

Synthesis, crystal structure, and properties of a seven-coordinate manganese(II) complex formed with the tripodal tetradentate ligand tris (N -methylbenzimidazol-2-ylmethyl)amine and salicylate

A seven-coordinate manganese(II) complex with the tripod tetradentate ligand tris(N-methylbenzimidazol-2-ylmethyl)amine (Mentb), [Mn(Mentb)(salicylate)(DMF)](ClO4) ⋅ (DMF), was synthesized and characterized by elemental, electrical conductivity, infrared, and UV-Vis spectral measurements. The crystal structure of the complex has been determined by single-crystal X-ray diffraction. MnII is bonded to a Mentb, a salicylate and dimethylformamide through four nitrogens and three oxygens, resulting in seven-coordination. Cyclic voltammograms of the complex indicate a quasi-reversible Mn3+/Mn2+ couple. The X-band electron paramagnetic resonance spectrum exhibits a six-line manganese hyperfine pattern with g = 2, A = 93, confirming that the material is high-spin Mn(II).

Synthesis, X-ray Structure, Magnetic Resonance, and DFT Analysis of a Soluble Copper(II) Phthalocyanine Lacking C−H Bonds

The synthesis, crystal structure, and electronic properties of perfluoro-isopropyl-substituted perfluorophthalocyanine bearing a copper atom in the central cavity (F(64)PcCu) are reported. While most halogenated phthalocyanines do not exhibit long-term order sufficient to form large single crystals, this is not the case for F(64)PcCu. Its crystal structure was determined by X-ray analysis and linked to the electronic properties determined by electron paramagnetic resonance (EPR). The findings are corroborated by density functional theory (DFT) computations, which agree well with the experiment. X-band continuous-wave EPR spectra of undiluted F(64)PcCu powder, indicate the existence of isolated metal centers. The electron-withdrawing effect of the perfluoroalkyl (R(f)) groups significantly enhances the complexes solubility in organic solvents like alcohols, including via their axial coordination. This coordination is confirmed by X-band (1)H HYSCORE experiments and is also seen in the solid state via the X-ray structure. Detailed X-band CW-EPR, X-band Davies and Mims ENDOR, and W-band electron spin-echo-detected EPR studies of F(64)PcCu in ethanol allow the determination of the principal g values and the hyperfine couplings of the metal, nitrogen, and fluorine nuclei. Comparison of the g and metal hyperfine values of F(64)PcCu and other PcCu complexes in different matrices reveals a dominant effect of the matrix on these EPR parameters, while variations in the ring substituents have only a secondary effect. The relatively strong axial coordination occurs despite the diminished covalency of the C-N bonds and potentially weakening Jahn-Teller effects. Surprisingly, natural abundance (13)C HYSCORE signals could be observed for a frozen ethanol solution of F(64)PcCu. The (13)C nuclei contributing to the HYSCORE spectra could be identified as the pyrrole carbons by means of DFT. Finally, (19)F ENDOR and easily observable paramagnetic NMR were found to relate well to the DFT computations, revealing negligible isotropic hyperfine (Fermi contact) contributions. The single-site isolation in solution and solid state and the relatively strong coordination of axial ligands, both attributed to the introduction of R(f) groups, are features important for materials and catalyst design.

Bismuth Qubits in Silicon: The Role of EPR Cancellation Resonances

We investigate electron paramagnetic resonance spectra of bismuth-doped silicon, at intermediate magnetic fields B≃0.1-0.6 T, theoretically and experimentally (with 9.7GHz X-band spectra). We identify a previously unexplored regime of "cancellation resonances," where a component of the hyperfine coupling is resonant with the external field. We show that this regime has experimentally accessible consequences for quantum information applications, such as reduction of decoherence, fast manipulation of the coupled electron-nuclear qubits, and spectral line narrowing.

Sequence-Specific Stereomeric Environment in a DNA Duplex Revealed by a Nucleotide-Independent Nitroxide Probe

In site-directed spin labeling, structural and dynamic information on a parent macromolecule is obtained by monitoring a covalently linked nitroxide radical using electron paramagnetic resonance (EPR) spectroscopy. Our group have developed a method of attaching nitroxide species, such as 1-oxyl-4-bromo-2,2,5,5-tetramethylpyrroline (R5a), to a specific nucleotide position within a target DNA or RNA sequence. The method relies on site-specific introduction of a phosphorothiate during the solid phase chemical synthesis of nucleic acids, and at each given labeling site the nitroxide is attached to one of two phosphorothioate diastereomers (Rp or Sp) in an approximately 50/50 ratio. We have recently reported that variations in DNA structural and dynamic features at the level of an individual nucleotide can be detected using R5a attached to mixed phosphorothioate diastereomers, in which an observed EPR spectrum is presumably a sum of those obtained from either diastereomer (Popova et al., Biochemistry, 2009, 48, 8540-50). In this work, we report X-band EPR spectra of R5a attached to purified Rp and Sp diastereomers at different sites within a B-form DNA duplex. Results are compared to those obtained with mixed nitroxide diastereomers, and advantages and limitations are discussed regarding the necessity of diastereomer separation when probing DNA local environment. Our work is a further step forward in developing a SDSL methodology that may provide a mean for studying structure and dynamics in large DNA molecules.

A Multifrequency High-Field Electron Paramagnetic Resonance Study of CoIIS4Coordination

Advanced electron paramagnetic resonance (EPR) methods have been employed in the study of two high-spin cobalt(II) complexes, Co[(SPPh(2))(2)N](2) (Co(Ph,Ph)L(2)) and Co[(SPPh(2))(SP(i)Pr(2))N](2) (Co(iPr,Ph)L(2)), in which the bidentate disulfidoimidodiphosphinato ligands make up for a pseudotetrahedral sulfur coordination of the transition metal. The CoS(4) core in the two complexes has slightly different structure, owing to the different peripheral groups (phenyl or isopropyl) bound to the phosphorus atoms. To determine the zero-field splitting, notoriously difficult for high-spin cobalt(II), the two complexes required different approaches. For Co(Ph,Ph)L(2), the study of the X-band EPR spectrum of a single crystal as a function of temperature revealed a nearly axial character of the zero-field splitting (ZFS; E/D approximately -0.05). For Co(iPr,Ph)L(2), the combination of the EPR spectra at 9, 95, and 275 GHz revealed a rhombic character of the ZFS (E/D approximately -0.33). The energy difference between the Kramers doublets in Co(Ph,Ph)L(2) and Co(iPr,Ph)L(2) amounts to 24 cm(-1) and 30 cm(-1), respectively. From the X-band EPR spectra of diamagnetically diluted single crystals at fields up to 2.5 T for Co(Ph,Ph)L(2) and 0.5 T for Co(iPr,Ph)L(2), the effective g tensors and cobalt hyperfine tensors have been determined, including the direction of the principal axes in the cobalt sites. The values of the EPR observables are discussed in relation to the structural characteristics of the first (CoS(4)) and second coordination sphere in the complexes.

Design, structural elucidation, DNA interaction and antimicrobial activities of metal complexes containing tetraazamacrocyclic Schiff bases

Tetraazamacrocyclic Schiff bases, designed and synthesized from 3-hydroxy-4-nitrobenzylideneacetoacetanilide/3-hydroxy-4-nitrobenzylidene-o-acetoacetotoluidide and 1,2-diaminobenzene, are tetradentate ligands and form solid cationic complexes with CuII, NiII, CoII, ZnII, MnII, and VOIV salts in ethanol. Microanalytical data, magnetic moment, electronic, IR, Mass, 13C NMR, IH NMR, and electron spin resonance (ESR) techniques were used to characterize the synthesized compounds. Electronic absorption and IR spectra suggest that all the complexes are square planar, while the oxovanadium complexes are square pyramidal. Elemental analyses and mass spectra suggest that the general formula of the complexes is [ML]X (where L = L1 or L2, X = 2Cl− or ). Electrolytic behavior of the chelates was assessed from conductance data. The X-band ESR spectra of copper and oxovanadium complexes, recorded in dimethyl sulfoxide at 300 and 77 K, are discussed. Binding of the metal complexes with DNA has been investigated by spectroscopic, viscometric, and voltammetric methods. These data suggest that the complexes interact with DNA by intercalative binding. A comparative study of minimum inhibitory concentration values of the ligands and their complexes indicates that the complexes exhibit higher antimicrobial activity than the free ligands. The nuclease activity shows that the copper, manganese, nickel, and vanadium complexes cleave DNA through redox chemistry.

Electron spin resonance of dense Yb-based heavy-fermion compounds: New experimental data

We report the more recent advances in electron spin resonance (ESR) of few undoped Yb-based intermetallic compounds with heavy fermions (HF). The X-band ESR spectra of the Kondo lattices YbBiPt, YbRh 2Pb, and YbT 2Zn 20 (T = Fe, Co) are presented. A comparison with earlier ESR studies in YbRh 2Si 2 and YbIr 2Si 2 shows that the exchange interactions between the Yb 4f electrons and relevant conduction electron (d-, s-, or p-like) bands as well as the hybridization crystalline electric field (CEF) effects should be taken into account in order to develop a reliable model of spin dynamics in the Yb-based HF systems.

Critical Role of Arginine 160 of the EutB Protein Subunit for Active Site Structure and Radical Catalysis in Coenzyme B12-Dependent Ethanolamine Ammonia-lyase

The protein chemical, kinetic, and electron paramagnetic resonance (EPR) and electron spin-echo envelope modulation (ESEEM) spectroscopic properties of ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium with site-directed mutations in a conserved arginine residue (R160) of the active site containing EutB protein subunit have been characterized. R160 was predicted by a comparative model of EutB to play a critical role in protein structure and catalysis [Sun, L., and Warncke, K. (2006) Proteins: Struct., Funct., Bioinf. 64, 308-319]. R160I and R160E mutants fail to assemble into an EAL oligomer that can be isolated by the standard enzyme purification procedure. The R160K and R160A mutants assemble, but R160A EAL is catalytically inactive and reacts with substrates to form magnetically isolated Co(II) and unidentified radical species. R160A EAL activity is resurrected by externally added guanidinium to 2.3% of wild-type EAL. R160K EAL displays catalytic turnover of aminoethanol, with a 180-fold lower value of k(cat)/ K(M) relative to wild-type enzyme. R160K EAL also forms Co(II)-substrate radical pair intermediate states during turnover on aminoethanol and (S)-2-aminopropanol substrates. Simulations of the X-band EPR spectra show that the Co(II)-substrate radical pair separation distances are increased by 2.1 +/- 1.0 A in R160K EAL relative to wild-type EAL, which corresponds to the predicted 1.6 A change in arginine versus lysine side chain length. 14N ESEEM from a hyperfine-coupled protein nitrogen in wild type is absent in R160K EAL, which indicates that a guanidinium 14N of R160 interacts directly with the substrate radical through a hydrogen bond. ESEEM of the 2H-labeled substrate radical states in wild-type and R160K EAL shows that the native separation distances among the substrate C1 and C2, and coenzyme C5' reactant centers, are conserved in the mutant protein. The EPR and ESEEM measurements evince a protein-mediated force on the C5'-methyl center that is directed toward the reacting substrate species during the hydrogen atom transfer and radical rearrangement reactions. The results indicate that the positive charge at the residue 160 side chain terminus is required for proper folding of EutB, assembly of a stable EAL oligomer, and catalysis in the assembled oligomer.

Self-Assembly of Pluronic Block Copolymers in Aqueous Dispersions of Single-Wall Carbon Nanotubes as Observed by Spin Probe EPR

The self-assembly of Pluronic block copolymers in dispersions of single-wall carbon nanotubes (SWNT) was investigated by spin probe electron paramagnetic resonance (EPR) spectroscopy. Nitroxide spin labeled block copolymers derived from Pluronic L62 and P123 were introduced in minute amounts into the dispersions. X-band EPR spectra of the SWNT dispersions and of native polymer solutions were measured as a function of temperature. All spectra, below and above the critical micelle temperature (CMT), were characteristic of the fast limit motional regime. The temperature dependence of the 14N isotropic hyperfine coupling, aiso, and the rotational correlation time, tauc, were determined. It was observed that, below the CMT, EPR does not distinguish between chains adsorbed on SWNT and free chains. Above CMT, substantial differences were observed: in the native solution, the Pluronics spin labels experience only one environment, Sm, assigned to spin labels in the corona of the Pluronic micelle, whereas in the SWNT dispersions, in addition to Sm, a second population of nonaggregated, individual chains, Si, is observed. The relative amounts of Sm and Si were found to depend on the relative concentrations of the Pluronic and SWNT. Furthermore, the aggregates formed in the SWNT dispersions do not show the typical increase in chain-end mobility as a function of temperature, observed in the post-CMT regime of the native Pluronic solutions. This suggests a larger dynamical coupling among aggregated chains in the presence of the SWNT as compared to the native micelles. The overall findings are consistent with the formation of a new type of aggregates, composed of a SWNT-polymer hybrid.

Magnetic properties and electron spin resonance of Ecuadorian obsidians. Application to provenance research of archeological samples

Obsidians from major Ecuadorian sources were analyzed by SQUID magnetometry and electron spin resonance (ESR). The association of these two techniques permits to differentiate obsidians from the sources of Cotopaxi volcano and from the Quiscatola and Mullumica–Callejones sources of the Chacana caldera, taking into account various parameters arising from the M vs. H cycles and the X-band ESR spectra. The analysis of 27 archeological samples coming from the prehispanic site of La Maná allowed us to infer that most of them come from the Mullumica–Callejones source.

Spectroscopic evidence for an all-ferrous [4Fe-4S]0 cluster in the superreduced activator of 2-hydroxyglutaryl-CoA dehydratase from Acidaminococcus fermentans.

The key enzyme of the fermentation of glutamate by Acidaminococcus fermentans, 2-hydroxyglutarylcoenzyme A dehydratase, catalyzes the reversible syn-elimination of water from (R)-2-hydroxyglutaryl-coenzyme A, resulting in (E)-glutaconylcoenzyme A. The dehydratase system consists of two oxygen-sensitive protein components, the activator (HgdC) and the actual dehydratase (HgdAB). Previous biochemical and spectroscopic studies revealed that the reduced [4Fe–4S]+ cluster containing activator transfers one electron to the dehydratase driven by ATP hydrolysis, which activates the enzyme. With a tenfold excess of titanium(III) citrate at pH 8.0 the activator can be further reduced, yielding about 50% of a superreduced [4Fe–4S]0 cluster in the all-ferrous state. This is inferred from the appearance of a new Mössbauer spectrum with parameters δ = 0.65 mm/s and ΔEQ = 1.51–2.19 mm/s at 140 K, which are typical of Fe(II)S4 sites. Parallel-mode electron paramagnetic resonance (EPR) spectroscopy performed at temperatures between 3 and 20 K showed two sharp signals at g = 16 and 12, indicating an integer-spin system. The X-band EPR spectra and magnetic Mössbauer spectra could be consistently simulated by adopting a total spin St = 4 for the all-ferrous cluster with weak zero-field splitting parameters D = −0.66 cm−1 and E/D = 0.17. The superreduced cluster has apparent spectroscopic similarities with the corresponding [4Fe–4S]0 cluster described for the nitrogenase Fe-protein, but in detail their properties differ. While the all-ferrous Fe-protein is capable of transferring electrons to the MoFe-protein for dinitrogen reduction, a similar physiological role is elusive for the superreduced activator. This finding supports our model that only one-electron transfer steps are involved in dehydratase catalysis. Nevertheless we discuss a common basic mechanism of the two diverse systems, which are so far the only described examples of the all-ferrous [4Fe–4S]0 cluster found in biology.Electronic supplementary materialThe online version of this article (doi:10.1007/s00775-008-0345-z) contains supplementary material, which is available to authorized users.

Oxidation of an o-Iminobenzosemiquinone Radical Ligand by Molecular Bromine: Structural, Spectroscopic, and Reactivity Studies of a Copper(II) o-Iminobenzoquinone Complex

The bis(o-iminobenzosemiquinonato)copper(II) complex 1, containing the radical form [L(*)SQ](1-) arising from the aerial oxidation of the noninnocent ligand 2-anilino-4,6-di- tert-butylphenol, H2L, is readily oxidized by molecular bromine to a bis(o-iminobenzoquinone)copper(II) complex, 2. Thus, a ligand-based oxidative addition is reported for complex 1 containing an electron-rich Cu(II) d(9) metal ion. The crystal structure of the synthesized hexacoordinated complex [Cu(II)(LBQ)2Br2] (2) has been determined by X-ray crystallography at 100 K. Variable-temperature (2-290 K) magnetic susceptibility measurements and an X-band electron paramagnetic resonance spectrum establish the spin state to be St = 1/2 because of localized spin moments mainly in the (d(x(2)-y(2)))(1) orbital of a Cu(II) d(9) ion, indicating clearly the presence of a neutral iminobenzoquinone form, [LBQ](0), of the ligand in 2, as is found also in the X-ray structure. Electrochemical measurements (cyclic voltammograms and coulometry) indicate two successive one-electron reductions of the ligand. The reactivity of complex 2 as an oxidizing agent toward ethanol and triethylamine has been investigated.

EPR STUDY OF NANOSTRUCTURED ZINC FERRITE

Bulk zinc ferrite has been studied in the past. In the bulk form zinc ferrite crystallizes in the normal spinel structure with all the zinc ions occupying the tetrahedral and all the iron occupying the octahedral sites, respectively. But, nanosize zinc ferrite is believed to crystallize in a different way. It is now well established that in a nanosize zinc ferrite there is partial inversion in the cation occupancy, leading to some kind of magnetic ordering in this system much above its Neel temperature of 10 K. Hence, there is ample scope of maneuvering the electrical and magnetic properties of this ferrite. In the present study nanosize zinc ferrite was prepared by a chemical reaction involving the nitrates of Zn and Fe and using the citric acid as the host. The sintering of the precursor was done for 1 h at various temperatures ranging from 300°C to 1000°C. These samples were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR). The average particle sizes in these sintered samples measured by TEM are found to vary from ≈ 10 to 62 nm. The XRD measurements show the formation of single-phase spinel structure in all the samples. The X-band EPR spectrum of the precursor specimen at room temperature consists of two EPR signals: an intense signal at g eff = 2.02 having peak-to-peak line width △H PP = 425 Gauss and other a weak signal at g eff = 4.51 having an asymmetric line shape. As the samples are heat-treated at different temperatures, the EPR signal corresponding to the intense peak gets stabilized at g eff ≈ 2.03, whereas the weak signal gets suppressed progressively. The spin–spin relaxation time in these samples have values ≈ 3 × 10-10 s . The effect of sintering temperature on the properties of the nanostructured specimen of zinc ferrite and the underlying mechanism would be discussed.

Reactivity of MnII with Superoxide. Evidence for a [MnIIIOO]+ Unit by Low-Temperature Spectroscopies

Manganese superoxide dismutase cycles between the MnIII and MnII states to produce oxygen and hydrogen peroxide from superoxide. The formation of an adduct has been suggested, but its nature remains questionable because both [MnIIOO-] and [MnIIIOO2-] redox states have been proposed. Study of the reactivity of superoxide with manganese complexes is of current interest. The reaction of [(L)MnII]2+ [L = N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine] with potassium superoxide has been investigated at low temperature in an anhydrous solvent using various techniques. Upon the addition of ca. 2 equiv of potassium superoxide, the [(L)MnII]2+ colorless solution turned blue and the UV-vis spectrum displayed a band at 590 nm (165 M(-1) cm(-1)) and a shoulder at 430 nm (100 M(-1) cm(-1)). Electrospray ionization mass spectrometry showed a peak (m/z = 434.1) assigned to [(L)MnO2]+. The X-band electron paramagnetic resonance spectrum parallel mode displayed a six-line signal separated by 6.6 mT and centered at 86 mT (g = 8.1). These results support the formation of an [MnIIIOO]+ adduct.

Reduction Potential Tuning of the Blue Copper Center in Pseudomonas aeruginosa Azurin by the Axial Methionine as Probed by Unnatural Amino Acids

The conserved axial ligand methionine 121 from Pseudomonas aeruginosa azurin (Az) has been replaced by isostructural unnatural amino acid analogues, oxomethionine (OxM), difluoromethionine (DFM), trifluoromethionine (TFM), selenomethionine (SeM), and norleucine (Nle) using expressed protein ligation. The replacements resulted in < 6 nm shifts in the S(Cys)-Cu charge transfer (CT) band in the electronic absorption spectra and < 8 gauss changes in the copper hyperfine coupling constants (AII) in the X-band electron paramagnetic resonance spectra, suggesting that isostructural replacement of Met resulted in minimal structural perturbation of the copper center. The slight blue shifts of the CT band follow the trend of stronger electronegativity of the ligands. This trend is supported by 19F NMR studies of the fluorinated methionine analogues. However, the order of AII differs, suggesting additional factors influencing AII. In contrast to the small changes in the UV-vis and EPR spectra, a large variation of > 227 mV in reduction potential was observed for the series of variants reported here. Additionally, a linear correlation was established between the reduction potentials and hydrophobicity of the variants. Extension of this analysis to other type 1 copper-containing proteins reveals a linear correlation between change in hydrophobicity and change in reduction potential, independent of the protein scaffold, experimental conditions, measurement techniques, and steric modifications. This analysis has also revealed for the first time high and low potential states for type 1 centers, and the difference may be attributable to destabilization of the protein fold by disruption of hydrophobic or hydrogen bonding interactions that stabilize the type 1 center.

Consistent simulation of X- and Q-band EPR spectra of an unsymmetric dinuclear [formula omitted] complex

Simulation of X- and Q-band electron paramagnetic resonance (EPR) spectra of an unsymmetric dinuclear [ Mn 2 II , III L ( μ - OAc ) 2 ] ClO 4 complex ( 1), (L is the dianion of 2-{[ N, N-bis(2-pyridylmethyl)amino]methyl}-6-{[ N-(3,5-di- tert-butyl-2-hydroxybenzyl)- N-(2-pyridylmethyl)amino]methyl}-4-methylphenol) was performed using one consistent set of simulation parameters. Rhombic g-tensors and hyperfine tensors were necessary to obtain satisfactory simulation of the EPR spectra. The anisotropy of the effective hyperfine tensors of each individual 55Mn ion was further analyzed in terms of intrinsic hyperfine tensors. Detailed analysis shows that the hyperfine anisotropy of the Mn III ion is a result of the Jahn-Teller effect and thus an inherent character. In contrast, the anomalous hyperfine anisotropy of the Mn II ion is attributed as being transferred from the Mn III ion through the spin exchange interaction. The anisotropy parameter for the Mn II is deduced as D II = −1.26 ± 0.2 cm −1. This is the first reported D II value for a Mn II ion in a weakly exchange coupled mixed-valence Mn 2 II , III complex with a bis-μ-acetato-bridge. The d xy 1 d yz 1 d zx 1 d x 2 - y 2 1 electronic configuration of the Mn III ion in 1 is revealed by the negative sign of its intrinsic hyperfine tensor anisotropy, Δ a III = a z − a x, y = −46 cm −1. Lower spectral resolution of the Q-band EPR spectrum as compared to the X-band EPR spectrum is associated to large line width broadening of the x- and y-components in contrast to the z-component. The origins of the unequal distribution of line width between the z- and x-, y-components are discussed.

MULTIFREQUENCY EPR STUDY OF RADIATION-INDUCED DEFECTS IN CHLORAPATITE

Gamma-irradiated chlorapatite, synthesized from a CaCl2 fl ux, has been investigated by powder and single-crystal electron paramagnetic resonance (EPR) spectroscopy at X- and W-band frequencies, including in situ high-T X-band EPR. The powder EPR spectra, particularly high-T X-band spectra and high-resolution W-band spectra, reveal a new hole-like center, H(III), in addition to two previously reported hole-like centers, H(I) and H(II). Center H(III) is characterized by an electron spin ½ and hyperfi ne interaction with one 35 Cl nucleus, suggesting a structural model consisting of a hole trapped by a substitutional oxygen ion adjacent to a Cl – ion vacancy in the anion column. This discovery of center H(III) also lends support to the structural model already proposed by other authors for center H(II). Single-crystal X-band EPR spectra also disclose a new electronic center, E(I). The structure model for center E(I) includes an electron trapped at an isolated Cl – ion vacancy in the anion column, corresponding to center F E(II) in fl uorapatite and similar to the well-known F center in alkali halides.

Magnetic properties of gadolinium-doped β-tricalcium phosphate

The frequency of radiant energy used in the majority of electron spin resonance (ESR) spectrometers is approximately 9 GHz (X-band), in the medium-frequency microwave region. Recently, high frequency microwave (Q-band) has been used to measure ESR spectra because high frequency ESR enhances the sensitivity. In the present work, ESR spectra (X-band, Q-band) of gadolinium-doped β-tricalcium phosphate (β-Ca 3(PO 4) 2:Gd, β-TCP:Gd) were investigated. X-band and Q-band ESR spectra were recorded. Q-band ESR spectra was observed clearer than X-band ESR spectra. The ESR spectra were assigned to the typical fine structure of the Gd 3+ ion. The crystal-field parameters obtained at room temperature were determined to be b 2 0 = − 267.0 , b 4 0 = − 1.5 , and b 6 0 = − 6.4 c m − 1 .

Electron paramagnetic resonance study ofZnAl2S4 spinel

Single crystals of ZnAl2S4 spineldoped by paramagnetic Cr3+ and Mn2+ ions have been studied by the electron paramagnetic resonance (EPR) technique. Thecrystal field symmetry around the impurity ions has been determined from theangular behaviour of X-band EPR spectra. The anisotropic EPR signal of theCr3+ ions shows splitting into 31 narrow lines due to the super-hyperfine interaction betweenunpaired electron spins of the chromium centres and nuclear spins of six neighbouringAl27 (I = 5/2) ions. It has beenestablished that the Cr3+ ions are located at the octahedral sites in the spinel structure, and the super-hyperfineinteraction results from a weak covalent bounding with the Al atoms. The EPR signals of theCr3+ paramagnetic centres show no fine-structure splitting due to a perfectlycubic symmetry of the local crystal field in the octahedral sites of theZnAl2S4 spinel structure. A weak EPR signal consisting of six componentshas been ascribed to the transitions between hyperfine levels of theMn2+ (I = 5/2,S = 5/2) ions located at tetrahedral sites, while the fine-structure splitting of eachcomponent could be resolved only for special orientations of the sample in theexternal magnetic field. The parameters of the EPR signal of both chromiumand manganese centres indicate that there is an essential covalence in theZnAl2S4 spinel crystal. Verynarrow linewidths (∼2 Oe) of the Cr3+ EPR signal components point to very high homogeneity and quality of theZnAl2S4 crystals.