Effect of oxygen and iodine on the optical and magnetic properties of fullerite C<Subscript>60</Subscript>

The effect of oxygen and iodine on the optical and magnetic properties of fullerite C₆₀ is studied by luminescence and EPR spectroscopy within widely varied experimental conditions (temperature of the medium, oxygen or buffer gas pressure, concentration of iodine vapor). It is demonstrated that the efficiency of the singlet oxygen formation when a fullerene sample is irradiated by a neodymium laser at a wavelength of 532 nm and the amplitude of the EPR signal emitted from the unirradiated sample are strongly affected by the concentrations of both oxygen and iodine vapor sorbed by the fullerene sample, as well as by its surface temperature. The spin-spin and spin-lattice relaxation times of paramagnetic centers in fullerite samples studied in the presence of molecular oxygen are determined by the method of microwave radiation absorption saturation.     

High-pressure EPR studies of intermolecular interactions in solids

High-pressure technique as applied to studies of different problems in solid state physics by EPR spectroscopy is presented. An influence of hydrostatic pressure up to 500 MPa on a weak superexchange coupling between Cu(II) ions in monomeric and dimeric copper(II) compounds is described. Both decrease and increase in the coupling is observed under pressure depending on the crystal and molecular structure but in all cases a dominant contribution is due to a shortening of intermolecular distances, whereas molecular structure and molecular vibrations are not affected. In dimeric Cu(II) compounds with resolved fine structure of EPR spectra zero-field splitting is weakly or not at all dependent on pressure whereas a strong dependence on temperature exists. It is discussed as a result of vibronic ligand behaviour which is not volume dependent. For all cases complementary temperature dependences are presented and moreover a behaviour of EPR linewidth under pressure and temperature is shown. An influence of high pressure on spin exchange reactions in solutions and on the phase memory time in electron spin echo experiments is briefly described.     

Translational and Rotational Motions of Albumin Sensed by a Non-Covalent Associated Porphyrin Under Physiological and Acidic Conditions: A Fluorescence Correlation Spectroscopy and Time Resolved Anisotropy Study

The interaction between a free-base, anionic water-soluble porphyrin, TSPP, and the drug carrier protein, bovine serum albumin (BSA) has been studied by time-resolved fluorescence anisotropy (TRFA) and fluorescence correlation spectroscopy (FCS) at two different pH-values. Both rotational correlation times and translational diffusion times of the fluorescent species indicate that TSPP binding to albumin induces very little conformational changes in the protein under physiological conditions. By contrast, at low pH, a bi-exponential decay is obtained where a short rotational correlation time (phi (int) = 1.2 ns) is obtained, which is likely associated to wobbling movement of the porphyrin in the protein binding site. These physical changes are corroborated by circular dichroism (CD) data which show a 37% loss in the protein helicity upon acidification of the medium. In the presence of excess porphyrin formation of porphyrin J-aggregates is induced, which can be detected by time-resolved fluorescence with short characteristic times. This is also reflected in FCS data by an increase in molecular brightness together with a decrease in the number of fluorescent molecules passing through the detection volume of the sample.     

The interaction of <Emphasis Type="Italic">P</Emphasis><Subscript><Emphasis Type="Italic">b</Emphasis></Subscript> centers with oxygen molecules in porous silicon powders

The special features of the interaction of silicon dangling bonds on the surface of porous silicon with oxygen molecules were studied by EPR spectroscopy. The effectiveness of magnetic dipole-dipole interaction between these defects and oxygen molecules decreased as the size of sample granules diminished, because the concentration of defects then increased and, as a consequence, the contribution of their mutual dipole-dipole relaxation grew.     

Photoinduced decarboxylation of 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f]quinoline‐8‐carboxylic acid

Photoinduced reactions of 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f]quinoline‐8‐carboxylic acid (SeQCA) were investigated in alkaline media (aqueous NaOH solutions) by electron paramagnetic resonance (EPR) spectroscopy, following the in situ formation of paramagnetic species. According to UV–Vis and nuclear magnetic resonance investigations, protonation (pH ≈ 11) and deprotonation (pH ≈ 13) of the imino hydrogen of the 4‐pyridone moiety has to be considered, reflected also in the different EPR spectra observed upon irradiation. Photoinduced generation of radicals was found only for carboxylate substituted SeQCA; other studied selenadiazoloquinolone derivatives, together with those substituted at the C(8) position (R = H, COOCH₂CH₃, COOCH₃, COCH₃ or CN), did not generate paramagnetic species during exposure. Consequently, photodecarboxylation was suggested as the decisive step, accompanied by the decomposition of the selenadiazole ring, resulting in the formation of ortho‐hydroxylate anions. EPR parameters elucidated from experimental EPR spectra obtained at pH ≈ 11 and pH ≈ 13 indicate the formation of oxygen‐centered radicals at the decarboxylated 4‐pyridone ring. EPR spin trapping experiments with nitromethane confirmed a very effective photoinduced electron transfer from all the selenadiazoloquinolones investigated. Copyright © 2011 John Wiley & Sons, Ltd.     

Characterization of the Lipid Binding Pocket in GM2AP and SapB with EPR Spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy of spin-labeled lipids in complex with the sphingolipid activator proteins, GM2AP and SapB, was utilized to characterize the hydrophobic binding pocket of these lipid transfer proteins. Specifically, the EPR line shapes reveal that the mobility of the labeled lipids within the binding pockets of the transfer proteins are more restricted than when in a lipid bilayer environment and that lipids in GM2AP are slightly more restricted than in SapB. EPR accessibility based relaxation measurements show that the relative ratios of oxygen and water accessibility to sites along the acyl chains in lipids in complex with GM2AP are similar to the profiles obtained for a lipid bilayer albeit with lowered values. The results for SapB are quite different, with the oxygen profile mimicking a lipid bilayer, but there is a higher degree of water accessibility to the acyl chains in the SapB complex, likely because of the location of the lipid at the dimer interface in SapB coupled to dynamics of the dimer.     

Photosynthetic electron transport in the cyanobacteriumSynechocystis sp. PCC 6803: High-Field W-band and X-band EPR study of electron flow through photosystem I

In this work, by using the respective advantages of W- and X-band electron paramagnetic resonance (EPR) spectroscopy techniques to investigate electron transport processes, we have studied the light-induced redox transients of the primary electron donor P₇₀₀ and the secondary acceptor A₁ in photosystem (PS) I complexes of intact cyanobacterial cellsSynechocystis sp. PCC 6803. We found that the kinetic behavior of the cation radical P₇₀₀ ^·+ generated by illumination with continuous light, and the EPR intensity of the radical pair P₇₀₀ ^·+A ₁ ^·? generated upon laser pulse illumination strongly depend on the illumination prehistory (either the sample was frozen in the dark or during illumination). Both these processes were sensitive to the presence of electron transport inhibitors which block electron flow between the two photosystems. In line with our X-band EPR data on the kinetics of light-induced redox transients of P₇₀₀, our high-field W-band EPR study of the radical-pair state P₇₀₀ ^·+A ₁ ^·? shows that photosynthetic electron flow through the PS I reaction center is controlled both on the donor and on the acceptor side of PS I.     

Size-Dependent Effects in EPR and Luminescence Spectra of NH<Subscript>4</Subscript>BPh<Subscript>4</Subscript> Excited States: from Bulk to Nanoparticles

Photoexcited states of NH₄BPh₄ (TPhBA) particles embedded in mesoporous frameworks of different pore sizes were studied by combined electron paramagnetic resonance (EPR), optical and photoluminescence techniques. A distribution of triplet states with short and long electron–hole distances was found. While EPR studies on TPhBA bulk sample suggested that the formation of electron–hole pairs upon the excitation was caused by electron capture on electron traps, the samples in mesoporous frameworks exhibit two ways of the pair formation. The first one is attributed to the capture on phenyl rings and another one is to be thought as the capture of electrons on adsorbed oxygen molecules. These results are also consistent with the thermoluminescence spectra and EPR studies of the photoexcited samples during annealing.     

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.     

Investigation of Novel Quantum Dots/Proteins/Cellulose Bioconjugate Using NSOM and Fluorescence

We investigated the engineered bioconjugate of cadmium selenide core/zinc sulfide shell, (CdSe)ZnS, quantum dots (QDs) with genetically modified proteins using fluorescence spectroscopy, near-field scanning optical microscopy (NSOM) and spectroscopy (NSOS). The protein polymer was allowed to self-assemble to the bacterial microcrystalline cellulose surface through the cellulosic binding domain. Results from the sample containing the QDs/protein/cellulose assemblies suggest that QDs were arrayed along the cellulose surface. The spectroscopic change of spectroscopic properties of the QDs upon bioconjugation, indicating the interaction among the immobilized QDs and between the constructed protein and QDs.     

Mössbauer spectroscopy for optimising systems for environmental remediation

Mössbauer spectroscopy was used to examine Fe-containing microporous materials zeolite X and AlPO-5 designed for the reduction of Cr(VI) to Cr(III) and Cr(III) uptake. XRD, XRF, EPR and XPS were used to provide supporting information. AlPO-5 was found to only incorporate inactive Fe(III), whilst Fe(II) and Fe(III) were identified in zeolite X across a distribution of sites. Fe coordinates to oxygen atoms belonging to both water molecules and framework positions. EPR confirmed these findings and showed how the site distribution narrows upon dehydration. XPS showed that a distribution of Fe sites exists at the sample surface. Mössbauer spectroscopy was also used to track oxidation and reduction processes in Fe-zeolite X. Over time, gradual Fe oxidation takes place in air-exposed samples. Reduction under hydrogen flow increases the populations of low oxidation state Fe; as the reaction time increases, Fe(II) populations first increase, then decrease as Fe(0) is evolved.     

Multibore sample cell increases EPR sensitivity for aqueous samples

We have performed calculations, verified by experiment, to explain why the sensitivity of biological EPR can be dramatically increased by dividing the aqueous sample into separate compartments. In biological EPR, the major factor affecting sensitivity is the number of spins in the sample. For an aqueous sample at ambient temperature, this is limited by the requirement for a small volume, due to strong non-resonant absorption of microwaves by water. However, recent empirical studies have shown that this volume limitation can be greatly relieved by dividing the aqueous sample into separate volumes, which allows much more aqueous sample to be loaded into a resonant cavity without significant degradation of the cavity quality factor. Calculations, based on the Bruggeman mixing rule, show quantitatively that the composite aqueous sample has a permittivity much less than that of bulk water, depending on the aqueous volume fraction f. Analysis for X-band EPR spectroscopy shows that the optimal volume fraction of an aqueous composite sample, producing maximum sensitivity, is f=0.15, increasing the sensitivity by a factor of 8.7, compared with an aqueous sample in a single tube.     

Thermal induced EPR signals in tooth enamel

Electron paramagnetic resonance (EPR) spectroscopy was used to detect the effects of temperature on powdered human tooth enamel, not irradiated in the laboratory. Samples were heated at temperature between 350 and 450, at 600 and at 1000°C, for different heating times, between 6 min and 39 h. Changes in the EPR spectra were detected, with the formation of new signals. Possible correlation between the changes in EPR spectra and modifications in the enamel and in the mineral phase of bone detected with other techniques is discussed. The implications for dosimetric applications of signals induced by overheating due to mechanical friction during sample preparation are underlined.     

Induced long-range attractive potentials of human serum albumin by ligand binding

Small-angle x-ray scattering and dielectric spectroscopy investigation on the solutions of recombinant human serum albumin and its heme hybrid revealed that heme incorporation induces a specific long-range attractive potential between protein molecules. This is evidenced by the enhanced forward intensity upon heme binding, despite no hindrance to rotatory Brownian motion, unbiased colloid osmotic pressure, and discontiguous nearest-neighbor distance, confirming monodispersity of the proteins. The heme-induced potential may play a trigger role in recognition of the ligand-filled human serum albumins in the circulatory system.     

Comparison of Continuous Wave, Spin Echo, and Rapid Scan EPR of Irradiated Fused Quartz

The E' defect in irradiated fused quartz has spin lattice relaxation times (T(1)) about 100 to 300 μs and spin-spin relaxation times (T(2)) up to about 200 μs, depending on the concentration of defects and other species in the sample. These long relaxation times make it difficult to record an unsaturated continuous wave (CW) electron paramagnetic resonance (EPR) signal that is free of passage effects. Signals measured at X-band (~9.5 GHz) by three EPR methods: conventional slow-scan field modulated EPR, rapid scan EPR, and pulsed EPR, were compared. To acquire spectra with comparable signal-to-noise, both pulsed and rapid scan EPR require less time than conventional CW EPR. Rapid scan spectroscopy does not require the high power amplifiers that are needed for pulsed EPR. The pulsed spectra, and rapid scan spectra obtained by deconvolution of the experimental data, are free of passage effects.     

Probing Protein Secondary Structure Using EPR: Investigating a Dynamic Region of Visual Arrestin

One key application of site-directed spin labeling electron paramagnetic resonance (EPR) spectroscopy is the determination of sequence-specific secondary structure in proteins. Regular secondary structure leads to a periodic variation in both side chain motion and solvent accessibility, two properties easily monitored by EPR techniques. Specifically, saturation recovery (SR) EPR spectroscopy has proven to be useful for making accessibility measurements for multiple protein structure populations by determining individual accessibilities and is, therefore, well suited to study the structure of proteins exhibiting multiple conformations in equilibrium. Here we employ both continuous wave and SR EPR spectroscopy in combination to examine the secondary structure of a short sequence showing conformational heterogeneity in visual rod arrestin. The EPR data presented here clearly distinguish between the unstructured loop and the helical structure formed in the crystallographic tetramer of visual arrestin and show that this region is unstructured in solution.     

Investigating magnetically aligned phospholipid bilayers with EPR spectroscopy at 94 GHz

In this paper, we report our initial results on studying magnetically aligned phospholipid bilayers (bicelles) at high magnetic fields (approximately 3.4 T) with electron paramagnetic resonance (EPR) spectroscopy at 95 GHz (W-band). In order to characterize this system for W-band EPR studies, we have utilized the nitroxide spin probe 3beta-doxyl-5alpha-cholestane to demonstrate the effects of macroscopic bilayer alignment. At W-band due to the increase in magnetic field strength (when compared to X-band studies at 9.5 GHz) (S. M. Garber et al., J. Am. Chem. Soc. 121, 3240-3241 (1999)), we were able to examine magnetically aligned phospholipid bilayers at two orientations with the bilayer normal oriented either perpendicular or parallel (upon addition of YbCl3) with respect to the direction of the static magnetic field. Additionally, at a magnetic field of 3.4 T (g=2 resonance at W-band), we were able to study the parallel alignment with a lower concentration of Yb3+, thereby eliminating the possible unwanted effects associated with lanthanide-protein interactions and paramagnetic shifts and/or line broadening induced by the lanthanide ions. The development of this new spin label alignment technique will open up a whole new area of investigation for phospholipid bilayer systems and membrane protein EPR studies at high magnetic fields.     

Effect of nitrogen dioxide on the EPR property of lithium octa-n-butoxy 2,3-naphthalocyanine (LiNc-BuO) microcrystals

Lithium octa-n-butoxy-naphthalocyanine (LiNc-BuO) is a stable free radical that can be detected by electron paramagnetic resonance (EPR) spectroscopy. Previously we have reported that microcrystals of LiNc-BuO exhibit a single sharp EPR peak, whose width varies linearly with the partial pressure of paramagnetic molecules such as oxygen and nitric oxide. In this report, we present the effect of nitrogen dioxide (NO2), which is also a paramagnetic molecule, on the EPR properties of LiNc-BuO. The gas-sensing property of LiNc-BuO is attributed to the open molecular framework of the crystal structure which is arranged with wide channels capable of accommodating large molecules such as NO2. The EPR linewidth of LiNc-BuO was highly sensitive to the partial pressure of NO2 in the gas mixture. The line-broadening was quick and reversible in the short-term for low concentration of NO2. However, the EPR signal intensity decreased with time of exposure, apparently due to a reaction of NO2 with LiNc-BuO crystals to give diamagnetic products. The results suggested that LiNc-BuO may be a useful probe for the determination of trace amounts of NO2 using EPR spectroscopy.     

Combustion synthesized MgAl2O4:Cr phosphors—An EPR and optical study

Magnesium aluminate (MgAl₂O₄) doped with trivalent chromium (Cr³⁺) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer-Emmet-Teller (BET) adsorption isotherms and diffuse-reflectance UV-vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110 K. The EPR spectrum exhibit resonance signals at g=5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr³⁺ ions. The luminescence of Cr³⁺-activated MgAl₂O₄ exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper ²E_g→⁴A_2g ground state of Cr³⁺ ions. By correlating EPR and optical data the crystal field splitting parameter (D_q), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr³⁺ ions with the ligands and the Cr³⁺ ions are in distorted octrahedral environment.     

Study on the effect of PEG in ionic transport for CMC-NH<Subscript>4</Subscript>Br-based solid polymer electrolyte

The present study investigates the ion transport properties and structural analysis of plasticized solid polymer electrolytes (SPEs) based on carboxymethyl cellulose (CMC)-NH₄Br-PEG. The SPE system was successfully prepared via solution casting and has been characterized by using electrical impedance spectroscopy (EIS), Fourier transform infrared (FTIR) spectroscopy, and x-ray diffraction (XRD) technique. The highest conductivity of the SPE system at ambient temperature (303 K) was found to be 1.12?×?10^?4 S/cm for un-plasticized sample and 2.48?×?10^?3 S cm^?1 when the sample is plasticized with 8 wt% PEG. Based on FTIR analysis, it shows that interaction had occurred at O–H, C=O, and C–O moiety from CMC when PEG content was added. The ionic conductivity tabulation of SPE system was found to be influenced by transport properties and amorphous characteristics as revealed by IR deconvolution method and XRD analysis.