GHz Electron Paramagnetic Resonance Spectroscopy Research Articles

Spin coherence in a Mn3 single-molecule magnet

Spin coherence in single crystals of the spin S = 6 single-molecule magnet (SMM) [Mn3O(O2CEt)3(mpko)3]+ (abbreviated Mn3) has been investigated using 230 GHz electron paramagnetic resonance spectroscopy. Coherence in Mn3 was uncovered by significantly suppressing dipolar contribution to the decoherence with complete spin polarization of Mn3 SMMs. The temperature dependence of spin decoherence time (T2) revealed that the dipolar decoherence is the dominant source of decoherence in Mn3 and T2 can be extended up to 267 ns by quenching the dipolar decoherence.

Application of Electron Paramagnetic Resonance (EPR) Oximetry to Monitor Oxygen in Wounds in Diabetic Models

A lack of oxygen is classically described as a major cause of impaired wound healing in diabetic patients. Even if the role of oxygen in the wound healing process is well recognized, measurement of oxygen levels in a wound remains challenging. The purpose of the present study was to assess the value of electron paramagnetic resonance (EPR) oximetry to monitor pO2 in wounds during the healing process in diabetic mouse models. Kinetics of wound closure were carried out in streptozotocin (STZ)-treated and db/db mice. The pO2 was followed repeatedly during the healing process by 1 GHz EPR spectroscopy with lithium phthalocyanine (LiPc) crystals used as oxygen sensor in two different wound models: a full-thickness excisional skin wound and a pedicled skin flap. Wound closure kinetics were dramatically slower in 12-week-old db/db compared to control (db/+) mice, whereas kinetics were not statistically different in STZ-treated compared to control mice. At the center of excisional wounds, measurements were highly influenced by atmospheric oxygen early in the healing process. In pedicled flaps, hypoxia was observed early after wounding. While reoxygenation occurred over time in db/+ mice, hypoxia was prolonged in the diabetic db/db model. This observation was consistent with impaired healing and microangiopathies observed using intravital microscopy. In conclusion, EPR oximetry using LiPc crystals as the oxygen sensor is an appropriate technique to follow wound oxygenation in acute and chronic wounds, in normal and diabetic animals. Nevertheless, the technique is limited for measurements in pedicled skin flaps and cannot be applied to excisional wounds in which diffusion of atmospheric oxygen significantly affects the measurements.

Effects of MnO doping on the electronic properties of zinc oxide: 406 GHz electron paramagnetic resonance spectroscopy and Newman superposition model analysis

MnO-doped ZnO ceramics have been synthesized through the conventional ceramic processing route. Mn2+ ions have been incorporated into the ZnO lattice within the limits of solid solubility. By using X-band-frequency and high-field electron paramagnetic resonance (EPR), we have resolved some of the main electronic transitions for the S = 5/2, I = 5/2 high-spin system and have determined accurately the EPR spin-Hamiltonian parameters. By combining data from crystallographic X-ray diffraction and EPR with the semi-empirical Newman superposition model, we have found the local configurational position of Mn2+ and have confirmed the symmetry of the lattice. The results presented in this contribution indicate that Mn ions substitute at Zn sites in ZnO. The effect of Mn2+ ions on the intrinsic defects becomes remarkable, thus the vacancy related intrinsic defect signals cannot be visible in the EPR spectrum. MnO doping affects the band gap energy of ZnO system which was confirmed via UV-Vis spectroscopy.

Equilibration of tyrosyl radicals (Y356•, Y731•, Y730•) in the radical propagation pathway of the Escherichia coli class Ia ribonucleotide reductase.

Escherichia coli ribonucleotide reductase is an α2β2 complex that catalyzes the conversion of nucleotides to deoxynucleotides using a diferric tyrosyl radical (Y(122)(•)) cofactor in β2 to initiate catalysis in α2. Each turnover requires reversible long-range proton-coupled electron transfer (PCET) over 35 Å between the two subunits by a specific pathway (Y(122)(•) ⇆ [W(48)?] ⇆ Y(356) within β to Y(731) ⇆ Y(730) ⇆ C(439) within α). Previously, we reported that a β2 mutant with 3-nitrotyrosyl radical (NO(2)Y(•); 1.2 radicals/β2) in place of Y(122)(•) in the presence of α2, CDP, and ATP catalyzes formation of 0.6 equiv of dCDP and accumulates 0.6 equiv of a new Y(•) proposed to be located on Y(356) in β2. We now report three independent methods that establish that Y(356) is the predominant location (85-90%) of the radical, with the remaining 10-15% delocalized onto Y(731) and Y(730) in α2. Pulsed electron-electron double-resonance spectroscopy on samples prepared by rapid freeze quench (RFQ) methods identified three distances: 30 ± 0.4 Å (88% ± 3%) and 33 ± 0.4 and 38 ± 0.5 Å (12% ± 3%) indicative of NO(2)Y(122)(•)-Y(356)(•), NO(2)Y(122)(•)-NO(2)Y(122)(•), and NO(2)Y(122)(•)-Y(731(730))(•), respectively. Radical distribution in α2 was supported by RFQ electron paramagnetic resonance (EPR) studies using Y(731)(3,5-F(2)Y) or Y(730)(3,5-F(2)Y)-α2, which revealed F(2)Y(•), studies using globally incorporated [β-(2)H(2)]Y-α2, and analysis using parameters obtained from 140 GHz EPR spectroscopy. The amount of Y(•) delocalized in α2 from these two studies varied from 6% to 15%. The studies together give the first insight into the relative redox potentials of the three transient Y(•) radicals in the PCET pathway and their conformations.

Changes in metal specificity due to iron ligand substitutions in reaction centers fromRhodobacter sphaeroides

Bacterial reaction centers have a single nonheme iron that is located between two bound quinones, QA and QB, which are the primary and secondary electron acceptors during photosynthesis, respectively. InRhodobacter sphaeroides, the iron is coordinated by four nitrogen atoms, contributed by histidines at L190, L230, M219, and M266, and two oxygen atoms, contributed by Glu at M234. The roles of these ligands in determining the metal-binding specificity and electron transfer properties of the quinones were investigated by mutagenesis. Each of the four His ligands was changed to Glu, Gln, and Cys, whereas Glu was changed to His, Gln, Cys, and Asp. All mutants supported photosynthetic growth except for those with substitutions of Glu or Cys at L190 or M219. The metal specificity of isolated mutant RCs was determined by measurements using atomic absorption and 35 GHz electron paramagnetic resonance spectroscopy. The M234 mutants had a lesser iron specificity than the wild type with a mole fraction of 0.7 to 0.8 iron but retained a total metal content of 1.0. All His mutants had an even lower iron content with mole fractions of 0.04 to 0.16. The His to Cys at M266 mutant had a significantly greater amount of bound zinc that was further enhanced when the strain was grown in zinc-supplemented media. The charge recombination rates from Q B −. , which ranged from 0.5 to 1 s−1 in the mutants, were comparable to the 1 s−1 value for the wild type. Charge recombination from Q A −. showed complex kinetics, with rates of 15 to 30 s−1 for the L190, L230, and M234 mutants and 200 s−1 for the M266 mutants compared with 8 s−1 for the wild type. The faster rates in the mutants most likely reflected a smaller free energy difference between Q A −. and Φ A − , a nearby bacteriopheophytin, with the smaller energy difference facilitating indirect recombination. All of the mutants transferred electrons to the secondary quinone, with rates (1200 to 4700 s−1) comparable to that of the native (3700 s−1). The data demonstrate that neither the ligands nor the bound metal play a critical role in the electron transfer processes at the acceptor side.

Phospholipid Mesophases at Solid Interfaces: In situ X‐Ray Diffraction and Spin‐Label Studies

AbstractFor Abstract see ChemInform Abstract in Full Text.

Spin-label HF-EPR of lipid ordering in cholesterol-containing membranes

Conventional oxazolidine spin-labelled lipids have the axial14N-hyperfine tensorz-axis directed along the long axis of the lipid chain. Investigation of lateral ordering of the lipids in membranes requires measurement of thex-y Zeeman anisotropy of the nonaxialg-tensor at high fields. Both the lateral and transverse ordering of the lipid chains in membranes of dimyristoyl phosphatidylcholine containing 40 mol% cholesterol in the liquid-ordered phase have been studied with 94 GHz electron paramagnetic resonance spectroscopy. This has been done by using probe amounts of phosphatidylcholine systematically spin-labelled at positionsn along the length of thesn- 2 chain [n-PCSL, 1-acyl-2-(n-(4,4-dimethyloxazolidine-N-oxyl) stearoyl)-sn-glycero-3-phosphocholine]. Nonaxial (gxx−gyy) anisotropy of the spin-labelled lipid chains is detected over a wide range of temperature throughout the liquid-ordered phase. The transverse profile of lateral ordering with position,n, of chain labelling follows the profile of the rigid steroid nucleus of cholesterol. It becomes progressively averaged towards the terminal methyl group of thesn- 2 chain, in the region of the flexible hydrocarbon chain of cholesterol. The nonaxial lipid ordering may be related to lipid domain formation in membranes containing cholesterol and saturated-chain lipids.

In vivo EPR dosimetry of accidental exposures to radiation: experimental results indicating the feasibility of practical use in human subjects

Low frequency electron paramagnetic resonance (EPR) provides the potential advantage of making accurate and sensitive measurements of absorbed radiation dose in teeth in situ, i.e. without removing the teeth from the potential victim. The potential limiting factors for making such measurements are: (1) whether low frequency EPR is sufficiently sensitive to detect radiation-induced signal in human teeth; (2) whether sufficient sensitivity can be maintained under in vivo conditions. In this manuscript, we summarize results indicating that this approach is feasible. Using 1.2 GHz EPR spectroscopy, we found that the lower limit for these measurements in isolated human teeth is 0.2 Gy or lower. Measurements of radiation-induced EPR signals in the teeth of living rats were achieved with sufficient sensitivity to indicate that, when taking into consideration the larger mass of human teeth, similar measurements in human teeth in situ would provide sensitivity in the dose range for potential accidental exposures. We estimate that the current lower limit for detecting radiation doses in human teeth in situ (in vivo) is 0.5–1.0 Gy; this would be sufficient for determining if a person has been exposed to potentially life threatening doses of ionizing radiation. The limiting factor for sensitivity appears to be background signals rather than signal/noise, and there are feasible means to overcome this problem and further increase sensitivity. The additional instrumental developments required to make an effective in vivo EPR dosimetric spectrometer for the measurements in teeth in human subjects in situ, seem quite achievable.

Noninvasive in Vivo Monitoring of Drug Release and Polymer Erosion from Biodegradable Polymers by EPR Spectroscopy and NMR Imaging

Biodegradable polymers have attracted much attention as implantable drug delivery systems. Uncertainty in extrapolating in vitro results to in vivo systems due to the difficulties of appropriate characterization in vivo, however, is a significant issue in the development of these systems. To circumvent this limitation, noninvasive magnetic resonance techniques, electron paramagnetic resonance (EPR) and magnetic resonance imaging (MRI), were applied to characterize drug release and polymer degradation in vitro and in vivo. MRI makes it possible to monitor water content, tablet shape, and response of the biological system such as edema and encapsulation. The results of the MRI experiments give the first direct proof in vivo of postulated mechanisms of polymer erosion. Using nitroxide radicals as model drug releasing compounds, information on the mechanism of drug release and microviscosity inside the implant can be obtained by means of 1.2GHz EPR spectroscopy. To be able to attribute nitroxide mobility to a particular layer of the implant, sandwich-like tablets were manufactured, taking advantage of the distinct spectral features of nitroxides containing different isotopes of nitrogen (15N vs 14N). The use of both noninvasive methods to monitor processes in vivo leads to new insights in understanding the mechanisms of drug release and polymer degradation.