Multifrequency EPR studies on the Mn(II) centers of oxalate decarboxylase

Oxalate decarboxylase from Bacillus subtilis is composed of two cupin domains, each of which contains a Mn(II) ion coordinated by four identical conserved residues. The similarity between the two Mn(II) sites has precluded previous attempts to distinguish them spectroscopically and complicated efforts to understand the catalytic mechanism. A multifrequency cw-EPR approach has now enabled us to show that the two Mn ions can be distinguished on the basis of their differing fine structure parameters and to observe that acetate and formate bind to Mn(II) in only one of the two sites. The EPR evidence is consistent with the hypothesis that this Mn-binding site is located in the N-terminal domain, in agreement with predictions based on a recent X-ray structure of the enzyme.     

EPR of new phosphorus-containing centers in synthetic diamonds

The effect is studied of electron and X-ray irradiation on phosphorous centers in synthetic diamonds grown in the P-C medium by the Bars technology. After exposure to X-ray irradiation, a new paramagnetic phosphorus-containing center NP6, in addition to the phosphorous centers NP4 and NP5, is observed in diamonds annealed at a temperature of 2300°C and pressure of 7.5 GPa. The spectrum of NP6 is simulated to give the following parameters: A ₁ = 29.42 G, A ₂ = 23.28 G, A ₃ = 75.85 G, g ₁ = 2.00085, g ₂ = 2.00083, and g ₃ = 2.00083. The NP4-NP6 centers are assumed to be genetically related to the three nitrogenphosphorous centers NP1-NP3 and be formed as a result of the transformation of the tetrahedral environment around the phosphorous atom into an octahedral environment at an annealing temperature of 2300°C. The synthetic diamonds annealed at 2300°C were successively exposed to irradiation with electrons with energies of 3.5 MeV (5×10¹⁷ e/cm²) and annealing at temperatures of 500°C and 700°C. The EPR method is used to find that annealing of the electron-irradiated crystals at 700°C leads to the formation of a new paramagnetic center with spin S = 1 and hyperfine structure (HFS) from one phosphorus atom with the parameters: g = 2.0012, D = 19.7 G, and A(P) = 3.6 G. The center is likely to have an eightvacancy chain structure with a phosphorus atom located at the center.     

EPR spectrum of Cr(CO)4+ in krypton at 20 K

The d⁵ species Cr(CO)₄⁺ has been detected by EPR spectroscopy in samples of Cr(CO)₆ in krypton γ-irradiated at 77 K. The spectra of samples enriched in ¹³C proved the chromiun atom to be tetrahedrally bound to the four CO ligands. The ground state is ⁶A₁ in T_d symmetry.     

Photochemistry of artificial photosynthetic reaction centers in liquid crystals probed by multifrequency EPR (9.5 and 95 GHz)

Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C(60)(1) have been followed by multifrequency time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (X-band) and high field and frequency (W-band). The electron-transfer process has been characterized in the different phases of two uniaxial liquid crystals (E-7 and ZLI-1167). The triad undergoes photoinduced electron transfer, with the generation of a long-lived charge-separated state, and charge recombination to the triplet state, localized in the carotene moiety, mimicking different aspects of the photosynthetic electron-transfer process. Both the photoinduced spin-correlated radical pair and the spin-polarized recombination triplet are observed starting from the crystalline up to the isotropic phase of the liquid crystals. The W-band TREPR radical pair spectrum has allowed unambiguous assignment of the spin-correlated radical pair spectrum to the charge-separated state C(.+)-P-C(60)(.-). The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum and the spin-selective recombination rates have been derived from the time dependence of the spectrum. The weak exchange interaction parameter (J = +0.5 +/- 0.2 G) provides a direct measure of the dominant electronic coupling matrix element V between the C(.+)-P-C(60)(.-) radical pair state and the recombination triplet state (3)C-P-C(60). The kinetic parameters have been analyzed in terms of the effect of the liquid crystal medium on the electron-transfer process. Effects of orientation of the molecular triad in the liquid crystal are evidenced by simulations of the carotenoid triplet state EPR spectra at different orientations of the external magnetic field with respect to the director of the mesophase. The order parameter (S = 0.5 +/- 0.05) has been evaluated.     

Multi-frequency high-field EPR study of iron centers in malarial pigments

The multi-frequency high-field electron paramagnetic resonance (HFEPR) was used to study the magnetic properties of malarial pigment hemozoin and its synthetic analogue, beta-hematin. (FeIII-protoporphyrin-IX)2 dimers containing five-coordinate high-spin FeIII, S = 5/2, are the building blocks of these pigments. The fit of EPR spectra that were acquired in an unprecedented wide range of microwave frequencies of 34 and 94 GHz for hemozoin and 27-500 GHz for beta-hematin yielded a complete set of intrinsic spin Hamiltonian parameters: D = +5.85(1) cm-1, E = 0, g perpendicular = 1.95(1), g parallel = 2.00(1). These results point to the existence of largely axial symmetry of the iron environment in the bulk phase of hemozoin and beta-hematin.     

Electrospinning of Hyaluronic Acid (HA) and HA/Gelatin Blends

Summary: In the present study, electrospinning of hyaluronic acid (HA) and hyaluronic acid/gelatin (HA‐GE) blends in N,N‐dimethylformamide (DMF)/water‐mixed solvents have been investigated. When the volume ratio of DMF to water was in the range of 1.5–0.5, HA solutions could be electrospun into fibrous membranes successfully. The average diameter of HA fibers was about 200 nm. The HA‐GE composite nanofibrous membranes with varied HA/GE weight ratio in the range of 100/20–100/100 have also been successfully fabricated. The average diameter of HA‐GE fibers was in the range of 190–500 nm. The decrease in surface tension could promote fiber formation. Thus, an introduction of DMF that could decrease the surface tension distinctively, without significant change or increase in viscosity of the solution, could bypass the use of blowing‐assisted electrospinning. Our postulated picture is that the lower surface tension could help the ejection of stream with relatively high viscosity and reduce or prevent the droplet formation during the spinning process.

HA/GE (100/80) nanofibrous membrane produced by electrospinning.     

EPR studies of spin-spin interactions between Cu(II) centers in dimeric,hexameric and homo- and heteronuclear tetrameric complexes

Applications of conventional (X-band), High-Field and High-Frequency EPR to study the nature of exchange interactions between Cu(II) centres in different homo- and hetronuclear systems are reviewed. The fine structure EPR spectra have been observed and analyzed in terms of spin-Hamiltonian parameters corresponding to S ≥ 1 for different [Cu₂(β-alanine)₄Cl₂]Cl₂·H₂O dimeric complexes present in one unit cell, Cu(II) dimers possessing different conformations, dimeric species formed between Cu(II) ions and polyfunctional ligand in solution, Cu-Zn-Cu-Zn and Cu-Co-Cu-Co tetrameric heterometallomacrocycles, Cu₄O₄ cubane-like tetramers and for the chain Cu(II) hexamer.     

EPR and ENDOR analysis of Fe3+ impurity centers in fluoroelpasolite lattices

Fe(3+) ions in hexagonal and cubic fluoroelpasolite crystals (A(1)(2)B(I)M(III)F(6)) have been investigated in a combined Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) study. A detailed analysis of the ENDOR spectra for the nearest (19)F and (23)Na shells in X (9.5 GHz) and Q band (34 GHz) allowed the complex EPR spectra to be disentangled and to determine the spin Hamiltonian parameters for the various S = 5/2 Fe(3+) centres. W-band (95 GHz) EPR measurements as a function of temperature were performed to provide unambiguous evidence about the absolute signs of the Zero Field Splitting (ZFS) and SuperHyperFine (SHF) parameters for Fe(3+) in Cs(2)NaAlF(6) as already determined from the ENDOR work. It could be concluded that all principal (19)F hyperfine values were positive, in agreement with earlier assignments in the literature for related systems. A comparative analysis of the (19)F SHF data for Fe(3+) at a perfectly octahedral site in the cubic crystal, and at two slightly trigonally distorted environments in the hexagonal crystals, indicates that the metal-to-ligand distance changes upon doping. The obtained set of parameters concerning one defect in various analogous environments can furthermore be used to test different methods of theoretical calculations for ZFS and SHF values.     

Studies of the optical band positions and EPR g factors for Cu(H2O)6(2+) centers in Tutton salt crystals

The optical band positions and EPR g factors g(i) (i = x, y, z) of Cu(H(2)O)(6)(2+) clusters in pure Tutton salts M(2)Cu(SO(4))(2)·6H(2)O (M = NH(4), Rb) are calculated from the complete diagonalization (of energy matrix) method based on the cluster approach. In the calculation, the superposition model with the structural data is used to obtain the crystal-field parameters. The calculated results are in reasonable agreement with the experimental values, suggesting that the complete diagonalization method and superposition model are effective in the studies of optical and EPR data. The g factors g(i) of Cu(H(2)O)(6)(2+) clusters in Cu(2+)-doped isomorphous diamagnetic Tutton salts M(2)Zn(SO(4))(2)·6H(2)O are also studied from the same method. It is found that the approximately tetragonally compressed Zn(H(2)O)(6)(2+) octahedra in the host crystals change to the approximately tetragonally elongated Cu(H(2)O)(6)(2+) octahedra in the impurity centers. The causes concerning the Jahn-Teller effect are discussed. It appears that in some cases the octahedral environment of an impurity M(I) in crystals differs from that of the replaced host ion, but is close to the one in the isomorphous pure crystals where M(I) is the host ion rather than the impurity ion.     

EPR study of radiolysis of perfluoro(isobutyl-p-methylcyclohexane)

Conclusions Radiation of perfluoro(isobutyl-p-methylcyclohexane) results in the formation of at least three types of perfluorocarbon radicals that have long lives in the liquid. The extremely low probability of their recombination in the liquid at room temperature may be due to steric shielding of the unpaired electron. The lifetime of the LRs that are formed depends on the radiation dose and temperature.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1260–1263, June, 1988.The authors wish to express their appreciation to K. N. Makarov for assistance in this work.     

EPR of glycol-stibine complex of Cr(V)

Conclusions A new complex of Cr(V) with an antimony-containing ligand was obtained and studied by the EPR method. The additional hyperfine structure from the¹²¹Sb and¹²³Sb isotopes was observed for the first time in solutions of complexes of the transition metals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 731–732, March, 1974.