EPR Studies of DOPA–Melanin Complexes with Fe(III)

Paramagnetic centers in 3,4-dihydroxyphenylalanine–melanin and its complexes with Fe(III) were examined by electron paramagnetic resonance (EPR) spectroscopy. Paramagnetic centers of melanin play an important role in detoxification of environment and they reveal high activity in binding of metal ions. Two different signals were observed in EPR spectra: lines of o-semiquinone free radicals and lines of paramagnetic Fe(III). Amplitudes of EPR lines of both free radicals and iron ions decrease with increasing Fe(III) content in melanin–metal ion complexes. Free radical concentrations in the melanin samples, g-factors, amplitudes and line widths of EPR spectra were determined. It was stated that fast spin–lattice relaxation processes exist in both free radical system and paramagnetic iron ions in melanin complexes.     

Application of EPR spectroscopy to examination of gentamicin and kanamycin binding to DOPA-melanin

Electron paramagnetic resonance (EPR) spectroscopy was applied to measure the influence of two aminoglycoside antibiotics: gentamicin and kanamycin on free radical propertis of DOPA-melanin. DOPA-melanin was formed by oxidative polymerization of 3,4-dihydroxyphenylalanine. Different concentrations of gentamicin and kanamycin (from 1·10⁻⁴ to 1·10⁻² M) were used. o-Semiquinone free radicals with ag factor of 2.0043 were found in all studied melanin samples. Their concentrations in the DOPA-melanin-drug complexes were higher than in DOPA-melanin, and increased with the increase of gentamicin and kanamycin concentration. A single EPR line of the analyzed samples (ΔB _pp, 0.48-0.52 mT) indicates that aminoglycoside antibiotics do not create a new type of free radicals in DOPA-melanin. Microwave saturation behavior of the experimental lines indicates the homogeneous broadening of resonance absorption curves for DOPA-melanin and its complexes with aminoglycosides. The EPR lines saturate at low microwave powers. Slow spin-lattice relaxation processes were characteristic for all studied melanin samples.     

Analysis of the Movement of Line-like Samples of Variable Length along theX-Axis of a Double TE104and a Single TE102Rectangular Cavity

Movement of line-like samples with lengths from 5 to 50 mm along thex-axis of the double TE₁₀₄rectangular cavity has been analyzed. The observed dependencies of the EPR signal intensity versus sample position showed: (i) a sharp maximum for sample lengths from 5 to 20 mm; (ii) a plateau, over which the EPR signal intensity remained constant within experimental errors of 0.26–1.07%, for lengths from 30 to 40 mm; and (iii) a “sloping plateau,” which could be approximated by the linear function (correlation,r= 0.98) for sample length 50 mm. Theoretical values of the experimentally observed dependencies of the intensity versus sample position were calculated using the modified sine-squared function and the correlation between observed and theoretically predicted dependencies is very good. The experimental dependence of the EPR signal intensity versus the sample length for samples situated at the same point in the cavity was nonlinear with a maximum for the 40-mm sample. The dependence of the EPR signal intensity upon the movement of a large cylindrical sample (o.d. 4 mm and length 100 mm) along thex-axis of the cavity was similar to that found for the 50-mm sample. However, an additional oscillating signal superimposed on the sloping plateau was observed. The presence of a large sample fixed in the complementary cavity of the double TE₁₀₄cavity caused an additional deformation of the signal intensity for a 30-mm sample which was moving in the first cavity. The primary effect was that the plateau was replaced by a region in which the intensity increased linearly with sample position,r= 0.99. Each of the above phenomena may be a source of significant errors in quantitative EPR spectroscopy. Cylindrical samples to be compared should be of identical length and internal diameter. Accurate and precise positioning of each sample in the microwave cavity is essential.     

Development of a Resonator with Automatic Tuning and Coupling Capability to Minimize Sample Motion Noise for in Vivo EPR Spectroscopy

EPR spectroscopy has been applied to measure free radicals in vivo; however, respiratory, cardiac, and other movements of living animals are a major source of noise and spectral distortion. Sample motions result in changes in resonator frequency, Q, and coupling. These instabilities limit the applications that can be performed and the quality of data that can be obtained. Therefore, it is of great importance to develop resonators with automatic tuning and automatic coupling capability. We report the development of automatic tuning and automatic coupling provisions for a 750-MHz transversely oriented electric field reentrant resonator using two electronically tunable high Q hyperabrupt varactor diodes and feedback loops. In both moving phantoms and living mice, these automatic coupling control and automatic tuning control provisions resulted in an 8- to 10-fold increase in signal-to-noise ratio.     

EPR characterization of organic free radicals in marine diesel

The intensive unresolved central line associated with organic free radicals in electron paramagnetic resonance (EPR) spectra of petroleum is interpreted as resulting from the superposition of signals from different radical species with very close g values. The mobility of a free radical in crude oil is relatively low due to the high oil viscosity. Aimed at characterizing free radicals in the oil byproduct, the present study is concerned with the investigation of marine diesel (bunker), a low-viscosity oil. Marine diesel was studied by EPR spectroscopy at X-band (9 GHz) in the temperature range from 170 to 400 K. Despite the viscosity at room temperature (2.5·10⁻³ kg/m·s) and the tumbling correlation time for free radicals of about 10⁻⁷ s suggesting a high mobility of free radicals in marine diesel, the EPR spectra at room temperature did not exhibit resolved lines, but only an envelope with a poor signal-to-noise ratio. The same behavior was observed from 170 to 373 K. Above 373 K, the EPR spectrum exhibited resolved lines: a superposition of a septet-quartet, a sextet-quartet and a quintet-quartet group of lines was identified with the following parameters: g = 2.003 ± 0.001, proton hyperfine couplings A = 6.41 ± 0.03 G (septet) and A′ = 1.82 ± 0.02 G (quartet); g = 2.003 ± 0.001, A = 6.21 ± 0.03 G (sextet) and A′ = 1.64 ± 0.02 G (quartet); g = 2.003 ± 0.001, A = 6.16 ± 0.03 G (quintet) and A′ = 1.83 ± 0.02 G (quartet), which were attributed to phenalenyl radicals and their derivatives. Below 373 K, phenalenyl dimerization prevented its detection by EPR.     

A tunable reentrant resonator with transverse orientation of electric field for in vivo EPR spectroscopy

There has been a need for development of microwave resonator designs optimized to provide high sensitivity and high stability for EPR spectroscopy and imaging measurements of in vivo systems. The design and construction of a novel reentrant resonator with transversely oriented electric field (TERR) and rectangular sample opening cross section for EPR spectroscopy and imaging of in vivo biological samples, such as the whole body of mice and rats, is described. This design with its transversely oriented capacitive element enables wide and simple setting of the center frequency by trimming the dimensions of the capacitive plate over the range 100-900 MHz with unloaded Q values of approximately 1100 at 750 MHz, while the mechanical adjustment mechanism allows smooth continuous frequency tuning in the range +/-50 MHz. This orientation of the capacitive element limits the electric field based loss of resonator Q observed with large lossy samples, and it facilitates the use of capacitive coupling. Both microwave performance data and EPR measurements of aqueous samples demonstrate high sensitivity and stability of the design, which make it well suited for in vivo applications.     

Electron paramagnetic resonance of defects with metastable properties in crystalline GaN

An EPR study of GaN revealed the presence of defects exhibiting metastable properties. EPR spectra of two centers (ii1a and ii1b) with axial symmetry along the hexagonal axis of the crystal, which have strongly anisotropic g factors, were observed. The anisotropy of the spectra decreases, and the line shape changes, with increasing temperature. The spectra of the ii1a and ii1b centers disappear at 25 and 50 K, respectively. Subsequent cooling of the samples does not restore the EPR signals, which implies that one observes here phenomena inherent in defects with metastable states. To restore EPR signals, one has to warm the samples to room temperature under very specific conditions. The possible microstructure of the discovered defects is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1818–1823 (October 1998)     

EPR and UV spectrometry investigation of sucrose irradiated with carbon particles

Solid state/EPR (SS/EPR) dosimeters of carbon ions irradiated sucrose are studied with EPR, and their water solutions – with UV spectroscopy. Doses between 20 and 200 Gy are used with linear energy transfer (LET) values for carbon ions of 63, 77, 96 and 230 keV μm^?1. After irradiation all samples show typical for irradiated sucrose EPR and UV spectra. The obtained data are compared with those previously reported for nitrogen particles and gamma rays irradiated sucrose. The identical shape of both the EPR and UV spectra of irradiated with various type radiation samples suggests that generated free radicals are not influenced by the nature of radiation. The lack of difference in the line width of the separate lines or the whole EPR spectrum, obtained for gamma and heavy particles irradiation, suggests negligible spin–spin interaction among the radiation-generated free radicals in the samples. The linear dependence of the EPR response on the absorbed dose radiation is found to be higher when generated by gamma rays, than by the same absorbed dose of heavy particles. In addition, the EPR response for carbon ions is higher than that for nitrogen ions. Water solutions of irradiated sucrose exhibit UV spectrum with absorption maximum at 267 nm, attributed to the recombination products of free radicals. The UV band intensity depends on the absorbed dose radiation. The UV spectra obtained for carbon, nitrogen and gamma rays irradiated sucrose are also compared.     

Effect of Zn2+ and Cu2+ on free radical properties of melanin fromCladosporium cladosporioides

Effect of metal ions on free radical properties of natural melanin produced by soil fungiCladosporium cladosporioides was studied. The electron paramagnetic resonance (EPR) spectrum of the studied melanin consists mainly of a single line of eumelanin, and only a very weak signal of pheomelanin was observed. o-Semiquinone free radicals form paramagnetic centers in melanin. Diamagnetic Zn²⁺ ions produce an increase in the free radical concentration in melanin. Quenching of melanin EPR lines was obtained for melanin and paramagnetic Cu²⁺ ion complexes. Slow spin-lattice relaxation processes are characteristic for the free radicals in melanin samples and fast spin-lattice relaxation was observed for Cu²⁺ ions. The EPR lines of copper ions saturate at higher microwave powers than the EPR lines of melanin.     

Types of paramagnetic centers in Cu2+ complexes with model neuromelanins

Electron paramagnetic resonance (EPR) spectroscopy was used to examine free radical properties of model neuromelanins obtained by oxidative polymerization of noradrenaline, adrenaline and dopamine. We compared the lineshape of the experimental spectra, type and concentration of free radicals in the analyzed samples. The effect of different concentrations of Cu²⁺ on free radicals in melanins was studied. The total concentration of free radicals (about 10¹⁸ to 10¹⁹ spin/g) in the studied melanins increases as follows: adrenaline-melanin < dopamine-melanin < noradrenaline-melanin. EPR spectra of dopamine-melanin and adrenaline-melanin were a single EPR line (ΔB _pp, 0.50 and 0.55 mT, respectively). o-Semiquinone free radicals with the characteristicg-value of 2.0040 exist in these melanins. EPR spectra of noradrenaline-melanin were a superposition of two lines (ΔB _pp, 0.45 and 0.81 mT). o-Semiquinone free radicals were responsible for the narrower component. Nitrogen free radicals with ag-factor of 2.0030 were probably responsible for the broader component. Paramagnetic copper ions quenched the EPR signals of melanin free radicals in the studied samples. For melanin-Cu²⁺ complexes, broad EPR lines (ΔB _pp, 10–32 mT) of copper ions with ag-value of about 2.1 appeared. The influence of the microwave power on the EPR spectra of these complexes demonstrated the fast spin-lattice relaxation in the copper system in melanins.     

EPR and Magnetic Susceptibility of Na2O–CuO–P2O5 Glasses

Abstract

(50?x/2)Na₂O–xCuO–(50?x/2)P₂O₅ glasses (x=1, 5, 15, or 30 mol%) have been prepared and characterized by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. The shape of the Cu²⁺ EPR spectrum depends on the Cu content, and the corresponding computer simulations suggest that the Cu²⁺ ions occupy two different sites in these glasses: one of them is preponderant at low Cu content and the other is preponderant at high content, in which the Cu²⁺–Cu²⁺ interactions are more important. From EPR parameters, it was found that for the site at low content, the covalency of copper ion bonding with the surrounding ligands is appreciable. The magnetic susceptibility data appear to follow the Curie–Weiss law (χ=C/(T?θ_p)) with negative paramagnetic Curie temperature θ_p indicating antiferromagnetic interactions between Cu²⁺ ions that are more significant in the samples with high Cu content, in agreement with EPR results.     

Special aspects of the temperature dependence of EPR absorption of chemically carbonized polyvinylidene fluoride derivatives

The temperature dependences of electron paramagnetic resonance (EPR) absorption of two samples of chemically carbonized derivatives of polyvinylidene fluoride (PVDF) synthesized under different conditions have been measured in the range of 100–300 K. It has been found that the temperature dependence of the integrated intensity of the EPR signal of both samples is nonmonotonic and does not obey the classical Curie dependence characteristic of free radicals. An analytical expression that is consistent with experimental data and suggests the presence of an activation component of paramagnetism in the test samples has been obtained. The presence of a term independent of temperature in this equation also indicates the paramagnetic contribution of free electrons. The magnitude of the activation energy of the singlet–triplet transitions has been evaluated: δ = 0.067 eV. The HYSCORE spectra of chemically carbonized PVDF derivatives have been obtained for the first time.     

Influence of the movement of cylindrical samples with variable internal diameter and variable length along thex-axis of a double TE104 and a single TE102 rectangular cavity on the EPR signal intensity: A sample shape study

The response of the cavity to the movement of cylindrical samples with internal diameters from 0.7 to 4 mm and lengths from 5 to 50 mm along thex-axis of the Bruker double TE₁₀₄ and single TE₁₀₂ rectangular cavity has been analyzed. Independently of sample internal diameter, the experimentally observed dependences of the electron paramagnetic resonance (EPR) signal intensity versus sample position in the cavity showed the following: (i) a sharp maximum for sample lengths from 5 to 20 mm; (ii) a “plateau”, over which the signal intensity remained constant within experimental errors of 0.47–1.16%, for lengths from 30 to 40 mm; and (iii) a “sloping plateau” region, which could be approximated by the linear function (correlationr = 0.96–0.98) for the 50 mm sample. Theoretical predictions of the experimental dependences of the signal intensity versus sample position in the cavity were calculated with the “modified” and “revised” sine-squared function, and the correlation between observed and theoretically computed dependences is very good. Additionally, the experimental dependence of the signal intensity versus the sample internal diameter and length for cylindrical samples situated at the position in the cavity at which the signal intensity was a maximum was likewise numerically approximated by the surface fitting with the Lorentzian cumulative additive function (correlationr = 0.999). The experimental dependence of the signal intensity versus the sample internal diameter for the given sample length is nonlinear. The samples with internal diameters of 0.7 and 1 mm gave the total maximum of signal intensity for the 40 mm sample, however, the samples with internal diameters of 2, 3 and 4 mm gave the total maximal value of signal intensity, which was identical for both the 30 and 40 mm samples. The experimental dependence of the EPR signal intensity versus the sample volume clearly showed that the samples with identical volumes, however, with different shapes, can give significantly different signal intensities (with differences ca. 200–400%). Then, the comparison of cylindrical samples with identical volumes but different shapes may be a serious source of significant errors in quantitative EPR spectroscopy. Cylindrical samples to be compared should be of identical shape. Accurate and precise positioning of each sample in the microwave cavity is essential.     

Egyptian limestone for gamma dosimetry: an electron paramagnetic resonance study

The electron paramagnetic resonance (EPR) properties of limestone from a certain Egyptian site were investigated in order to propose an efficient and low-cost gamma dosimeter. Radiation-induced free radicals were of one type which was produced in the limestone samples at g=2.0066 after exposure to gamma radiation (⁶⁰Co). EPR spectrum was recorded and analyzed. The microwave power saturation curve and the effect of changing modulation amplitude on peak-to- peak signal height were investigated. The response of limestone to different radiation doses (0.5–20 kGy) was studied. Except for the decrease in signal intensities during the first five hours following irradiation, over the period of two months fair stabilities of signal intensities were noticed. From the current results, it is possible to conclude that natural limestone may be a suitable material for radiation dosimetry in the range of irradiation processing.     

Saturation recovery EPR and ELDOR at W-band for spin labels

A reference arm W-band (94 GHz) microwave bridge with two sample-irradiation arms for saturation recovery (SR) EPR and ELDOR experiments is described. Frequencies in each arm are derived from 2 GHz synthesizers that have a common time-base and are translated to 94 GHz in steps of 33 and 59 GHz. Intended applications are to nitroxide radical spin labels and spin probes in the liquid phase. An enabling technology is the use of a W-band loop-gap resonator (LGR) [J.W. Sidabras, R.R. Mett, W. Froncisz, T.G. Camenisch, J.R. Anderson, J.S. Hyde, Multipurpose EPR loop-gap resonator and cylindrical TE₀₁₁ cavity for aqueous samples at 94 GHz, Rev. Sci. Instrum. 78 (2007) 034701]. The high efficiency parameter (8.2 GW^−1/2 with sample) permits the saturating pump pulse level to be just 5 mW or less. Applications of SR EPR and ELDOR to the hydrophilic spin labels 3-carbamoyl-2,2,5,5-tetra-methyl-3-pyrroline-1-yloxyl (CTPO) and 2,2,6,6,-tetramethyl-4-piperidone-1-oxyl (TEMPONE) are described in detail. In the SR ELDOR experiment, nitrogen nuclear relaxation as well as Heisenberg exchange transfer saturation from pumped to observed hyperfine transitions. SR ELDOR was found to be an essential method for measurements of saturation transfer rates for small molecules such as TEMPONE. Free induction decay (FID) signals for small nitroxides at W-band are also reported. Results are compared with multifrequency measurements of T_1e previously reported for these molecules in the range of 2–35 GHz [J.S. Hyde, J.-J. Yin, W.K. Subczynski, T.G. Camenisch, J.J. Ratke, W. Froncisz, Spin label EPR T₁ values using saturation recovery from 2 to 35 GHz. J. Phys. Chem. B 108 (2004) 9524–9529]. The values of T_1e decrease at 94 GHz relative to values at 35 GHz.     

EPR of Marine Diesel

Marine diesel was studied by electron paramagnetic resonance (EPR) spectroscopy at X-(9 GHz) and W-bands (94 GHz). The experiments were performed at room temperature (about 300 K). X-band spectra exhibited isotropy, resolved lines and negligible noise, whereas the W-band spectra exhibited a poorer signal-to-noise ratio and anisotropy in g and in hyperfine interactions. Viscosity at room temperature (2.5 · 10⁻³ kg/m · s) and the tumbling correlation time for free radicals (about 10⁻⁷ s) justified the high mobility of free radicals in marine diesel and consequently, the septet-quartet EPR spectrum of organic radicals with parameters g = 2.0028 ± 0.0005, proton hyperfine couplings A = 6.31 ± 0.01 G (septet) and A′ = 1.80 ± 0.01 G (quartet) at X-band. The perinaphthenyl radicals are probably responsible for the septet-quartet EPR spectrum of this oil by-product. Authors' address: Eduardo Di Mauro, Laboratório de Fluorescência e Ressonancia Paramagnética Eletr?nica, Centro de Ciências Exatas, Universidade Estadual de Londrina, C.P. 6014, CEP 86051-970 Londrina, Paraná, Brasil     

EPR of C60 thermal/photochemical reactions with polystyrene and polymethyl methacrylate

Thermal and photochemical free radical reaction products of C₆₀ with polymethyl methacrylate (PMMA) and polystyrene (PS) in orthodichlorobenzene solution were detected by EPR (electron paramagnetic resonance). Thermal radicals (<100°C) of C₆₀/PMMA and C₆₀/PS samples gave single line first-derivative EPR spectra withg=2.0029. Ultraviolet photolysis of a C₆₀/PMMA solid phase sample gave two radical species; whereas, photolysis of a C_60/PS solid phase sample gave only one free radical. EPR signals were also recorded for UV and thermal C₆₀ reaction with free radical initiator, azobis(isobutyronitrile).     

Electron paramagnetic resonance of Ce<Superscript>3+</Superscript> and Nd<Superscript>3+</Superscript> impurity ions in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.13</Subscript>

The electron paramagnetic resonance (EPR) spectra of Ce³⁺ and Nd³⁺ impurity ions in unoriented powders of the YBa₂Cu₃O_6.13 compound are observed and interpreted for the first time. It is demonstrated that, upon long-term storage of the samples at room temperature, the EPR signals of these ions are masked by the spectral line (with the g factor of approximately 2) associated with the intrinsic magnetic centers due to the significant increase in its intensity.     

Electron paramagnetic resonance of NH 3 + radical in potassium sulphate

Room temperature EPR spectra of (NH₄)₂SO₄ doped K₂SO₄ monocrystals irradiated with x-rays show the presence of NH₃ ⁺ radicals. The EPR parameters areg ‖=2.0037 andg ⊥ = 2.0068;¹⁴NA _XX=13.75;A _YY=24.5;A _ZZ=25.5 gauss;¹HA _XX=A _YY=22 andA _ZZ=25 gauss. From the¹⁴N and¹H coupling constants it has been inferred that at room temperature the planar NH₃ ⁺ radical undergoes rotation about theC ₃ axis which corroborates with the equivalence of the protons, but the radical itself is in an asymmetric crystal field environment. The 77K spectra indicate a considerable reduction in the motion of the radical with the free motion almost completely stopped. Part of Ph.D. work of the second author