Sensitive Electron Spin Resonance Research Articles

Oligomeric and Fibrillar α-Synuclein Display Persistent Dynamics and Compressibility under Controlled Confinement.

The roles of α-synuclein in neurotransmitter release in brain neurons and in the Parkinson's disease condition have challenged comprehensive description. To gain insight into molecular mechanistic properties that actuate α-synuclein function and dysfunction, the coupled protein and solvent dynamics of oligomer and fibril forms of human α-synuclein are examined in a low-temperature system that allows control of confinement and localization of a motionally sensitive electron paramagnetic resonance spin probe in the coupled solvent-protein regions. The rotational mobility of the spin probe resolves two distinct α-synuclein-associated solvent components for oligomers and fibrils, as for globular proteins, but with dramatically higher fluidities at each temperature, that are comparable to low-confinement, aqueous-cryosolvent mesophases. In contrast to the temperature-independent volumes of the solvent phases that surround globular and condensate-forming proteins, the higher-fluidity mesophase volume of α-synuclein oligomers and fibrils decreases with decreasing temperature, signaling a compression of this phase. This unique property and thermal hysteresis in the mobilities and component weights, together with previous high-resolution structural characterizations, suggest a model in which the dynamically disordered C-terminal domain of α-synuclein creates a compressible phase that maintains high fluidity under confinement. Robust dynamics and compressibility are fundamental molecular mechanical properties of α-synuclein oligomers and fibrils, which may contribute to dysfunction and inform about function.

4-Amino-TEMPO loaded liposomes as sensitive EPR and OMRI probes for the detection of phospholipase A2 activity

This work aims at developing a diagnostic method based on Electron Paramagnetic Resonance (EPR) measurements of stable nitroxide radicals released from “EPR silent” liposomes. The liposome destabilisation and consequent radical release is enzymatically triggered by the action of phospholipase A2 (PLA2) present in the biological sample of interest. PLA2 are involved in a broad range of processes, and changes in their activity may be considered as a unique valuable biomarker for early diagnoses. The minimum amount of PLA2 measured “in vitro” was 0.09 U/mL. Moreover, the liposomes were successfully used to perform Overhauser-enhanced Magnetic Resonance Imaging (OMRI) in vitro at 0.2 T. The amount of radicals released by PLA2 driven liposome destabilization was sufficient to generate a well detectable contrast enhancement in the corresponding OMRI image.

A sensitive method to investigate the interaction of alpha-crystallin with cortical and nuclear lens lipid membranes derived from a single lens.

The levels of α-crystallin in eye lens cytoplasm decrease with a proportional increase in membrane-bound α-crystallin with age and cataract formation. The influence of differences in lipids composition between cortical membrane (CM) and nuclear membrane (NM) derived from a single lens on α-crystallin binding to these membranes is still unclear. This study aims to develop a sensitive electron paramagnetic resonance (EPR) spin-labeling method to investigate the binding of α-crystallin to the CM and NM derived from total lipids extracted from a single lens of a bovine. No binding of α-crystallin below the surface of CM and small binding of α-crystallin below the surface of NM was observed using cholesterol analogue spin-label (CSL) within these membranes. However, significant binding of α-crystallin on the surface of CM and NM was observed using 4-palmitamido-TEMPO (4PT) spin-label within these membranes. The percentage of membrane surface occupied and the binding affinity for NM are higher than for CM, correlating with less mobility and more order below and on the surface of NM than CM. The mobility decreases with no significant change in order and hydrophobicity below and on the surface of CM and NM with α-crystallin binding. Our results imply that α-crystallin primarily binds on the surface of bovine CM and NM, and such surface binding of α-crystallin to membranes in young and clear lenses may be helpful for membrane stability. However, α-crystallin binding increases hydrophobicity below the surface of the cortical membrane with low cholesterol (CMLC), mimicking the decreased cholesterol content in cataractous lenses, suggesting that α-crystallin binding below the surface of the CMLC forms a hydrophobic barrier for the passage of polar molecules supporting the barrier hypothesis in cataract formation.

Methodology and Instrumentation for Electron Paramagnetic Resonance Dosimetry With Tooth Enamel

Abstract When tooth enamel is exposed to ionizing radiation, it generates a dose-dependent concentration of free radical centers (i.e., unpaired electrons). The concentration of these free radical centers is identified and quantified using electron paramagnetic resonance (EPR) spectroscopy in the form of an EPR spectrum. The intensity of the spectrum is proportional to the absorbed dose. Four international intercomparisons have already demonstrated that the EPR tooth enamel dosimetric technique is reliable for retrospective dose assessment in acute and chronic exposure scenarios. Additionally, EPR dosimetry is regarded same as a gold standard for reconstructing the total lifetime dose of individuals using tooth enamel. The accuracy and reproducibility of EPR dose reconstruction depend on the sample preparation, spectrum acquisition, and EPR spectra analysis techniques. So, this paper reviews some of the widely applied and accepted laboratory protocols or methodologies for the EPR dosimetric methods. The minimum detection limit in tooth enamel using this technique was 30 mGy. So, this review aims to share these protocols so that it would be easy to reconstruct the accident doses or chronic exposures with reliable accuracy and precision. Different bands (e.g., L, X, Q, etc.) continuous wave (CW) EPR spectrometers have been used in many historical and accident dose reconstructions; however, due to the availability, moderate price, and not being much influenced by the small amount of moisture in a sample, the X-band has been widely used. A well-developed methodology, a highly sensitive EPR spectrometer, and a well-trained operator are vital for the reliable measurements of absorbed low doses in EPR dosimetry with tooth enamel.

Design Considerations of a Dual Mode X-Band EPR Resonator for Rapid In-Situ Microwave Heating

This paper describes the design considerations for a dual mode X-band continuous wave (CW) Electron Paramagnetic Resonance (EPR) cavity, for simultaneous EPR measurement and microwave heating of the same sample. An elliptical cavity geometry is chosen to split the degeneracy of the TM110 mode, allowing for a well resolved EPR signal with the TM110,a and TM110,b modes resonating at around 10 GHz and 9.5 GHz, respectively, the latter of which is used for EPR measurements. This geometry has the benefit that the TM010 mode used for microwave heating resonates at 6.1 GHz, below the cut off frequency of the X-band waveguide used for the EPR channel, providing effective isolation between the heating and EPR channels. The use of a pair of 9 µm thick copper clad laminates as the flat cavity walls allows for sufficient penetration of the modulation field (Bmod) into the cavity, as well as maintaining a high cavity Q factor (> 5700) for sensitive EPR measurements. Locating the heating port at an angle of 135° to the EPR port provides additional space for easier coupling adjustment and for larger sample access to be accommodated. The associated decrease of EPR signal strength is fully compensated for by using a 7.2 GHz low pass filter on the heating port. EPR spectra using 1.6 mm and 4.0 mm sample tubes are shown at room temperature (298 K) and 318 K for a standard Cu(acac)2 solution, demonstrating the effectiveness of this dual-mode EPR cavity for microwave heating during EPR detection.

Detection of radiation treatment of spices using Electron Spin Resonance Spectroscopy Technique

Use of ionising radiation for commercial sterilisation to increase the hygienic quality of spices is increasing worldwide. Among several detection methods, Electron Spin Resonance (ESR) spectroscopy is one of the reliable techniques for irradiated foods containing some hard and dried parts. Spices are therefore candidates of such a method and radiation treated spices can be detected. The purpose of the present work was to identify the radiation treatment of spices like cinnamon, cardamom, ginger and rosemary using the sensitive technique of ESR spectroscopy. The spice samples were irradiated to 5, 10, 15 and 20 kilogray (kGy) or unirradiated and the samples were dried in vacuum oven and were placed into quartztubes for ESR measurement. On measurements, the unirradiated samples were found to show the ESR-signal at g-value of 2.004 that is typical in case of plant foods containing cellulose. Whereas, the irradiated samples showed a formation of a new paramagnetic structure that appeared in the form of a triplet-signal, attributed to radiation-induced cellulose radical. The clear difference between the nature of ESR signals in case of unirradiated and irradiated samples provided the evidence of the radiation treatment of spices samples. It is therefore concluded that radiation treatment of spices can be identified using sensitive ESR spectroscopy during a storage period of three to ten months.

Single-Step Formation of a Low Work Function Cathode Interlayer and n-type Bulk Doping from Semiconducting Polymer/Polyethylenimine Blend Solution.

The use of polyethylenimine (PEI) as a thin interlayer between cathodes and organic semiconductors in order to reduce interfacial Ohmic losses has become an important approach in organic electronics. It has also been shown that such interlayers can form spontaneously because of vertical phase separation when spin-coating a blended solution of PEI and the semiconductor. Furthermore, bulk doping of semiconducting polymers by PEI has been claimed. However, to our knowledge, a clear delineation of interfacial from bulk effects has not been published. Here, we report a study on thin films formed by spin-coating blended solutions of PEI and poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} [P(NDI2OD-T2)] on indium tin oxide. We observed the vertical phase separation in such films, where PEI accumulates at the bottom and the top, sandwiching the semiconductor layer. The PEI interlayer on ITO reduces the electron injection barrier to the minimum value determined by Fermi level pinning, which, in turn, reduces the contact resistance by 5 orders of magnitude. Although we find no evidence for doping-induced polarons in P(NDI2OD-T2) upon mixing with PEI from optical absorption, more sensitive electron paramagnetic resonance measurements provide evidence for doping and an increased carrier density, at a very low level. This, in conjunction with an increased charge carrier mobility due to trap filling, results in an increase in the mixed polymer conductivity by 4 orders of magnitude relative to pure P(NDI2OD-T2). Consequently, both interfacial and bulk effects occur with notable magnitude in thin films formed from blended semiconductor polymer/PEI solution. Thus, this facile one-step procedure to form PEI interlayers must be applied with attention, as modification of the bulk semiconductor polymer (here doping) may occur simultaneously and might go un-noticed if not examined carefully.

Broadband electron paramagnetic resonance spectroscopy in diverse field conditions using optically detected nitrogen-vacancy centers in diamond

Paramagnetic magnetic resonance, a powerful technique for characterizing and identifying chemical targets, is increasingly used for imaging; however, low spin polarization at room temperature and moderate magnetic fields poses challenges for detecting small numbers of spins. In this work, we use fluorescence from nitrogen-vacancy (NV) centers in diamond to detect the electron paramagnetic resonance (EPR) spectrum of optically inactive target spins under various conditions of field magnitude and orientation. The protocol requires neither direct microwave manipulation of the NV spins nor spectral overlap between NV and target spin resonances, thus enabling broadband detection. This unexpected non-resonant coupling is attributable to a two-phonon process that relaxes NV spins proximate to the fluctuating dipole moment of the target spin, suggesting that the sensitivity is determined by the dipole–dipole coupling strength. This approach holds promise for sensitive EPR detection, particularly in settings where control over the diamond-crystal orientation is difficult. This is notably the case for biological sensing applications, where nanodiamonds are being pursued for their bright, stable fluorescence and biocompatibility.

A Novel Electrochemical-Electron Paramagnetic Resonance Spectroscopic Technique for in-Situ Radical Formation and Detection on Graphite Pencil Electrode.

The unique capability of electron paramagnetic resonance (EPR) to detect and characterize paramagnetic species has been applied to measure free radical intermediates from any oxidative processes. Recently, sensitive benchtop EPR spectrometers have been used to quantify several radical intermediates of drug and pharmaceutical radio sterilization such as ketoconazole and alanine, respectively. During the earlier days of the EPR (also called, electron spin resonance, ESR), simultaneous EC-EPR techniques were developed and accepted as a highly powerful means for identifying paramagnetic intermediates in EC-reactions by using a variety of EC-EPR cells. However, these cells are either expensive or not easy to use, and the electrodes in these cells are made of platinum or precious silver metals. None of these cells could be utilized for the EC-EPR redox processes on the surface of graphite pencil electrodes (GPE) that produced the highest radical intermediate content (see Figure 1). The present work uses a simple, inexpensive flow electrochemical-electron paramagnetic resonance (FEC-EPR) cell for the determination of redox intermediate of different kind of drugs and their derivatives’ solutions. A well-established method for instantaneous electrochemical and EPR measurements with a new EPR-instrument, called EPR-Lab that is equipped with a FEC-EPR cell, flow pump, and OEM-potentiostat, produces data for both electro-analysis and EPR spectroscopy. The new system permits early radicals’ formation detection and analysis. It will also enable the lifetime analysis of radicals’ formation and decay by alternating the applied potential between ‘ON’ and ‘OFF’ states. The method also supports radical detection at a much lower concentration than other electrochemical or chemical oxidation techniques of similar materials. The new system has been used to elucidate the kinetics of the acidified ketoconazole (KTZ) oxidation in vitro in its real forms; tablet, cream, and shampoo extracts as well as in urine model matrix, that could be linked to its delivery in acidic solutions in vivo. Figure 1

Electron paramagnetic resonance spectroscopy using a single artificial atom

Electron paramagnetic resonance (EPR) spectroscopy is an important technology in physics, chemistry, materials science, and biology. Sensitive detection with a small sample volume is a key objective in these areas, because it is crucial, for example, for the readout of a highly packed spin based quantum memory or the detection of transition metals in biomaterials. Here, we demonstrate a novel EPR spectrometer using a single artificial atom as a sensitive detector of spin magnetization. The artificial atom, a superconducting flux qubit, provides advantages in terms of its strong coupling with magnetic fields. We estimate a sensitivity of ~400 spins·Hz−1/2 with a magnetic sensing volume around 10−14λ3 (50 femtoliters), where λ is the wavelength of the irradiated microwave. Our artificial atom works as a highly sensitive EPR spectrometer with micron-scale area with future opportunity for measuring single spins on the nanometer scale.

Direct Prediction of EPR Spectra from Lipid Bilayers: Understanding Structure and Dynamics in Biological Membranes.

Of the many biophysical techniques now being brought to bear on studies of membranes, electron paramagnetic resonance (EPR) of nitroxide spin probes was the first to provide information about both mobility and ordering in lipid membranes. Here, we report the first prediction of variable temperature EPR spectra of model lipid bilayers in the presence and absence of cholesterol from the results of large scale fully atomistic molecular dynamics (MD) simulations. Three types of structurally different spin probes were employed in order to study different parts of the bilayer. Our results demonstrate very good agreement with experiment and thus confirm the accuracy of the latest lipid force fields. The atomic resolution of the simulations allows the interpretation of the molecular motions and interactions in terms of their impact on the sensitive EPR line shapes. Direct versus indirect effects of cholesterol on the dynamics of spin probes are analysed. Given the complexity of structural organisation in lipid bilayers, the advantage of using a combined MD‐EPR simulation approach is two‐fold. Firstly, prediction of EPR line shapes directly from MD trajectories of actual phospholipid structures allows unambiguous interpretation of EPR spectra of biological membranes in terms of complex motions. Secondly, such an approach provides an ultimate test bed for the up‐to‐date MD simulation models employed in the studies of biological membranes, an area that currently attracts great attention.

Strain-Induced Spin-Resonance Shifts in Silicon Devices

In spin-based quantum information processing devices, the presence of control and detection circuitry can change the local environment of a spin by introducing strain and electric fields, altering its resonant frequencies. These resonance shifts can be large compared to intrinsic spin line-widths and it is therefore important to study, understand and model such effects in order to better predict device performance. Here we investigate a sample of bismuth donor spins implanted in a silicon chip, on top of which a superconducting aluminium micro-resonator has been fabricated. The on-chip resonator provides two functions: first, it produces local strain in the silicon due to the larger thermal contraction of the aluminium, and second, it enables sensitive electron spin resonance spectroscopy of donors close to the surface that experience this strain. Through finite-element strain simulations we are able to reconstruct key features of our experiments, including the electron spin resonance spectra. Our results are consistent with a recently discovered mechanism for producing shifts of the hyperfine interaction for donors in silicon, which is linear with the hydrostatic component of an applied strain.

Optimization Methods for the Design of Sensitive Surface ESR Resonators

Electron spin resonance (ESR) is a powerful spectroscopic technique that has many applications in a wide variety of scientific fields, including chemistry, biology, materials science, and physics. One significant drawback of conventional ESR, however, is its relatively low sensitivity compared to other spectroscopic techniques. Arguably, the most dominant element affecting ESR sensitivity is the microwave resonator used to pick up the ESR signal of the spins. Traditionally, ESR mostly employs a limited set of resonator configurations (e.g., rectangular cavity, dielectric, or loop-gap resonator) that are suboptimal with respect to a wide range of samples. In principle, a smart resonator design can be used to optimize spin sensitivity for a given sample’s properties. In this work, we make use of an efficient Genetic Algorithm (GA) approach to numerically solve, analyze, and optimize a unique class of surface microresonators. The GA is based on a method of moments code, customized directly to render the complexity of a particular resonator’s geometries in our search. The main purposes of the algorithm are to routinely generate more sensitive microresonators, optimized for a predefined sample’s dimensions, and to study the functional relations between the devices’ resonance frequency, quality, and filling factors and their topology, in order to reach a rational optimal design. Preliminary results associated with new, unique, and sensitive surface microresonators are shown and analyzed. Such resonators are cheap and easy to produce on a mass scale with an arbitrary surface geometry.

Broadband ESR Spectroscopy With a Tunable Interferometer

This paper presents a broadband and quantitative electron spin resonance (ESR) spectroscopy technique that is tunable over a wide frequency range. Different from the detection approaches in conventional ESR instrumentation, an interferometer-based method is used to remove radio frequency probing signals at the vector network analyzer (VNA) detector. As a result, high-resolution tuning enables highly sensitive ESR detection. A planar microwave resonator is designed at ~7.6 GHz and incorporated into the interferometer to measure $0.2~\mu \text{g}$ ( $2.8 \times 10^{14}$ spins) 2,2-diphenyl-1-picrylhydrazyl (DPPH) sample at a signal-to-noise ratio (SNR) of ~121 at room temperature. The broadband ESR operation capability is investigated with a broadband meander microstrip line (MML) by measuring $6~\mu \text{g}$ ( $8.4 \times 10^{15}$ ) DPPH between 8 and 13 GHz at a ~194 SNR with 10 kHz VNA intermediate frequency. The obtained sensitivity is significantly higher than that of current broadband ESR methods. In both MR and MML cases, dispersion and absorption ESR signals at room temperature are simultaneously obtained. With MML structures, the quantification of permeability $\mu (\omega )=\mu ' (\omega ) -j\mu ''(\omega )$ is possible. Thus, the ESR techniques are also a highly sensitive ferromagnetic resonance method. More work is needed to further improve ESR sensitivity and quantification accuracy.

New Amino-Acid-Based β-Phosphorylated Nitroxides for Probing Acidic pH in Biological Systems by EPR Spectroscopy.

There is increasing interest in measuring pH in biological samples by using nitroxides with pH-dependent electron paramagnetic resonance (EPR) spectra. Aiming to improve the spectral sensitivity (ΔaX ) of these probes (i.e., the difference between the EPR hyperfine splitting (hfs) in their protonated and unprotonated forms), we characterized a series of novel linear α-carboxy, α'-diethoxyphosphoryl nitroxides constructed on an amino acid core and featuring an (α or α')-C-H bond. In buffer, the three main hfs (aN , aH , and aP ) of their EPR spectra vary reversibly with pH and, from aP or aH titration curves, a two- to fourfold increase in sensitivity was achieved compared to reference imidazoline or imidazolidine nitroxides. The crystallized carboxylate 10 b (pKa ≈3.6), which demonstrated low cytotoxicity and good resistance to bioreduction, was applied to probe stomach acidity in rats. The results pave the way to a novel generation of highly sensitive EPR pH markers.

Synthesis of nanocrystalline calcium aluminate C12A7 under carbon nanoreactor conditions

Recently we have shown that carbon coating supported on the surface of oxide nanoparticles is able to stabilize their size and prevent sintering at high temperatures. The carbon shell can play the role of a nanoreactor where phase or chemical transformations of nanoparticles take place. In the present study the carbon nanoreactor approach was used to study the C12A7:eˉ electride synthesis in the argon atmosphere. For these systems the appearance of free electrons registered by electron paramagnetic resonance (EPR) was observed at moderate temperatures (starting from 1250°C). In the presence of the carbon shell the material maintains relatively high dispersity even at 1450°C, which exceeds the melting temperature for C12A7. The possibility of increasing substitution of oxygen anions with electrons in C12A7@C systems was examined by increasing calcination temperature from 1200 up to 1450°C. Highly sensitive EPR method for qualitative and quantitative characterization of these systems was proposed. It was shown that in absence of the carbon coating conduction electrons appear only at temperatures close to the melting point (above 1360–1380°C). The electride formation inside the carbon shell occurs due to carbothermal reduction of C12A7 nanoparticles encapsulated inside the shell.

Griffiths like phase in nanocrystalline manganite La0.50Ca0.50MnO3 studied by magnetic susceptibility and electron spin resonance

The Griffiths like phase embedded into paramagnetic matrix of mixed valence orthorhombic nanocrystalline manganite La0.50Ca0.50MnO3 is experimentally studied by means of sensitive magnetic susceptibility and electron spin resonance. A comparative study of the nano- and polycrystalline manganites indicates the temperature interval of the Griffiths like phase located above the Curie temperature of nanocrystalline one. The upper temperature limit for an existence of Griffiths like phase, revealed by the sensitive electron spin resonance, is 10 K higher than that of the magnetic susceptibility. This proves that the high frequency field is able to excite the relatively smaller magnetic clusters.

Injectable LiNc-BuO loaded microspheres as in vivo EPR oxygen sensors after co-implantation with tumor cells

Electron paramagnetic resonance (EPR) oximetry is a technique which allows accurate and repeatable oxygen measurements. We encapsulated a highly oxygen sensitive particulate EPR spin probe into microparticles to improve its dispersibility and, hence, facilitate the administration. These biocompatible, non-toxic microspheres contained 5–10 % (w/w) spin probe and had an oxygen sensitivity of 0.60±0.01µT/mmHg. To evaluate the performance of the microparticles as oxygen sensors, they were co-implanted with syngeneic tumor cells in 2 different rat strains. Thus, tissue injury was avoided and the microparticles were distributed all over the tumor tissue. Dynamic changes of the intratumoral oxygen partial pressure during inhalation of 8 %, 21 %, or 100 % oxygen were monitored in vivo by EPR spectroscopy and quantified. Values were verified in vivo by invasive fluorometric measurements using Oxylite probes and ex vivo by pimonidazole adduct accumulation. There were no hints that the tumor physiology or tissue oxygenation had been altered by the microparticles. Hence, these microprobes offer great potential as oxygen sensors in preclinical research, not only for EPR spectroscopy but also for EPR imaging. For instance, the assessment of tissue oxygenation during therapeutic interventions might help understanding pathophysiological processes and lead to an individualized treatment planning or the use of formulations with hypoxia triggered release of active agents.

Response to Tetrahydrobiopterin and Endothelial Nitric Oxide Synthase Uncoupling

Although we certainly recognize the excellent work done by Tsikas et al1 using purified endothelial NO synthase (eNOS) and incubation with tetrahydrobiopterin (H4B) at much higher concentrations (see Reference 4 and Figure in Reference 1) than what is found in vivo by many independent groups including us,2–4 we respectfully disagree based on the following grounds. First, accurate assessment of the uncoupling state of eNOS and determination of endogenous H4B content to the level of picomoles per milligram of protein in vivo can be successfully achieved. A large body of published work from many independent groups including ours has confirmed this consistently.2–4 In our case, we used the highly sensitive electron spin resonance method to measure superoxide production and high-performance liquid chromatography …

Noise in Frequency-Sensitive ESR Detectors

AbstractIn this paper we present a method to compute analytical expressions for the phase and frequency noise in integrated CMOS LC tank oscillators. Our approach combines advanced MOS transistor modeling using the EKV model with the solution of stochastic differential equations modeling the behavior of the noisy oscillator. Using these results, we can calculate the noise floor in a new class of electron spin resonance (ESR) detectors presented in Yalcin and Boero (2008) and thereby provide estimates for the limit of detection for this type of frequency sensitive ESR detectors. The theoretical results are verified against simulations performed in a commercial simulation tool as well as measured data obtained from a CMOS prototype of the frequency sensitive ESR detector.