Conventional ESR Research Articles

Zero-field Splitting of the Triplet Ground and Excited States of 7H-Benz[de]anthracen-7-ylidene in n-Hexane at 1.7 K: A Hole Burning Study

Spectral holes were burned within the inhomogeneous (0,0) fluorescence excitation line of the T0→T1 transition of 7H-benz[de]anthracen-7-ylidene in an n-hexane matrix at 1.7 K. The complicated spectrum of the obtained holes was interpreted with a model taking into account the zero-field splitting (ZFS) of both the triplet states contributing to the transition, the enforced planar symmetry of the carbene, and the domination of internal conversion in the relaxation of the excited T1 state. Analysis of the hole-burning spectra produced the following ZFS parameters of the ground T0 and excited T1 states: D0 = (0.274 ± 0.001) cm-1 + E1, E0 = (0.0292 ± 0.0002) cm-1 + E1 and D1 = (0.0168 ± 0.0002) cm-1 + E1. The number of possible transitions between different spin sublevels is reduced as a consequence of the symmetry plane in the carbene and one of the ZFS parameters, we chose the smallest, E1, cannot be estimated. The ZFS parameters of the T0 state were also estimated by conventional matrix ESR and were in good agreement with the results of the hole burning experiments.

Self-Assembling Properties of Membrane-Modifying Peptides Studied by PELDOR and CW-ESR Spectroscopies

A new technique is described that is suitable to determine the formation of aggregates from monomeric biomolecules. This technique has been tested in the study of the self-assembling properties of the antibiotic trichogin GA IV which belongs to the class of peptaibols. We have investigated the self-assembling properties of three trichogin analogues by pulsed double resonance in electron spin−echo (PELDOR) spectroscopy combined with conventional continuous wave ESR spectroscopy. In the peptides examined Aib has been substituted by its spin-labeled analogue TOAC at three specific positions of the sequence. More specifically, the magnetic dipole−dipole relaxation of the spin-labeled peptides is measured in glassy polar and apolar solvents at 77 K. Specific assemblies of trichogin molecules are formed in an apolar solvent but addition of a more polar solvent leads to dissociation of the aggregates. The estimates based on experimental data show that each aggregate cluster contains four peptide molecules. Some ...

Hydrogen radioisotopic labelling studies using muonium: properties of thiyl radicals potentially relevant to cellular membrane damage

Thiyl radicals (RS·) are formed labelled with muonium atoms (a radioisotope of hydrogen with a positive muon as the nucleus) and detected using the muon spin rotation (MuSR) method. This approach is shown to be extremely effective in the study of thiyl radicals, in non-aqueous (membrane-like) environments, from which details of the structures of these reactive free radicals and their reaction kinetics were determined. In this regard, the method is superior to conventional ESR spectroscopy, to which thiyl radicals are undetectable in liquid solution, and to pulse-radiolysis experiments, which are limited to aqueous media; the rate constants for the reactions of thiyl radicals with lipids but also with antioxidants such as β-carotene and (SH)glutathione are found to be enhanced in non-aqueous media. Copyright © 2000 John Wiley & Sons, Ltd.

First Interchain Peptide Interaction Detected by ESR in Fully Synthetic, Template-Assisted, Two-Helix Bundles

We have designed and synthesized by solution methods two simple two-helix bundles based on a conformationally constrained cyclo-dipeptide template to the side chains of which two short, 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid spin-monolabeled, 310-helical peptides are covalently tethered. The preferred conformation of the appended chains has been assessed by FTIR absorption. The conclusions are corroborated by an X-ray diffraction analysis of one of the terminally blocked pentapeptide tails. For the first time, a solvent-dependent, inter-helix interaction has been monitored by conventional ESR spectroscopy on fully synthetic peptide systems. Half-field ESR measurements of these side-chain-substituted templates provided an experimental average distance between the two labels that is in good accord with that determined in a molecular modeling study.

Applications of ESR and CIDEP to mechanistic studies of lignin chemistry

Using time-resolved CIDEP and conventional ESR as well as optical studies much can be elucidated concerning the initial photochemical reactions of lignin model compounds such as α-guaiacoxyacetoveratrone and α-guaiacoxy-β-propioveratrone. The resulting transient radicals are models for intermediates in the processes of photoyellowing of lignin containing mechanical pulps and papers. Model solution experiments which produce evidence for the roles of free radicals in reductive bleaching of mechanical pulps and in the oxidative aging of papers are also described. The more thorough understanding of these complex processes allows the development of strategies to try to “protect” mechanical pulps and papers from light and thermally induced damage and to “repair” any damage which invevitably occurs.

Muon spin relaxation studies of interstitial and molecular motion

The unusual methods of preparation and analysis of spin polarization in μSR spectroscopy, which exploit the unique properties of the positive muon, are introduced in this article. Following a summary overview of applications, particular attention is paid to the problem of spin-lattice relaxation for a muon experiencing a hyperfine interaction with a single unpaired electron. The specific cases considered are the interstitial diffusion of muonium—the 1-electron atom which may be considered as a light isotope of hydrogen—and the molecular dynamics of organic radicals labelled by muonium. Rate equations for the evolution of population in the hyperfine-coupled spin states are solved numerically for various relaxation mechanisms. The formalism is equally valid for conventional ESR studies of paramagnetic states but is pursued specifically to simulate T 1-relaxation in μSR. The simulations are compared with literature data. Also treated is the case of intermittent hyperfine coupling, appropriate to electron capture and loss in semiconductors or soliton motion in polymers; for this, a Monte Carlo approach is used to simulate the muon response. (For low-dimensional motion, the relaxation function is not exponential, so that a unique value of T 1 cannot be defined.) Finally, a proposal is made to implement muon- T 1 ρ measurements in the rotating frame; this is designed for the selective study of electronically diamagnetic muonium states (i.e., those without hyperfine coupling) in the presence of a paramagnetic muonium or radical fraction.

Molecular organization and dynamics of 1-palmitoyl-2-oleoylphosphatidylcholine bilayers containing a transmembrane alpha-helical peptide.

The molecular organization and dynamics have been investigated in membranes consisting of 1-palmitoyl-2-oleoyl-l-alpha-phosphatidylcholine (POPC) and various ratios of a transmembrane alpha-helical peptide, Ac-K2L24K2-amide (L24), in order to gain insights into how the transmembrane portions of membrane proteins are mixed with phospholipids and organized in biological membranes. Particular attention was paid to membranes with high peptide concentrations. The molecular organization and dynamics were studied in the ps-to-micros regime using various spin-labeling techniques. Conventional ESR spectra as well as saturation-recovery curves measured in both the presence and the absence of molecular oxygen showed that phosphatidylcholine spin-labels detect the existence of a single homogeneous environment, indicating that both L24 and POPC are likely to be undergoing fast translational diffusion in L24-POPC membranes of up to 9 mol % peptide. Since 16-18 molecules of phosphatidylcholine are required to surround a transmembrane alpha-helical peptide [Morrow, M. R., Huschilt, J. C., and Davis, J. H. (1985) Biochemistry 24, 5396-5406], L24 must form L24-rich regions at a P/L ratio of 1/10 instantaneously. However, these results suggest that the lipid exchange rates among the bulk, boundary, and L24-rich regions are fast, and that the L24-rich regions must be forming and dispersing rapidly in a time scale shorter than 0.1 micros, the conventional ESR spin-label time scale and the electron spin-lattice relaxation time scale in the presence of molecular oxygen. Although this does not exclude the possibility of the formation of small, stable oligomers of L24, it is unlikely because L24 lacks features that would favor their formation. L24 (9 mol %) increases the hydrophobicity of the central part of the POPC membrane from the level of 1-decanol to that of pure hexane and also increases the hydrophobicity near the membrane surface from the level of 2-propanol to that of 1-decanol. The effect of 9 mol % L24 on the order parameter profile is similar to that of decreasing the temperature by approximately 8 degrees C between 10 and 55 degrees C. It is concluded that L24 is highly miscible in POPC membranes even at high concentrations in the membrane.

Pharmacokinetic Analysis of Free Radicals by in vivo BCM (Blood Circulation Monitoring)-ESR Method

In pharmacokinetic studies, a variety of analytical method including radioisotopic detection and HPLC (high performance liquid chromatography) has been used. In the present investigation, we developed in vivo BCM (Blood Circulation Monitoring)-ESR method, which is a new technique with a conventional X-band ESR spectrometer for observing stable free radicals in the circulating blood of living rats under anaesthesia. Both 5–(PROXYL derivatives) and 6–(TEMPO derivatives) membered nitroxide spin probes with various types of substituent functional group were used. After physicochemical properties of the spin probes such as hyperfine coupling constant (A-value), g-value and partition coefficient as well as chemical stability of the compounds in the fresh blood were obtained, the in vivo BCM-ESR method was performed in normal rats. Several pharmacokinetic parameters such as half-life of the probes, distribution volume, total body clearance and mean residence time were obtained and discussed in terms of their chemical structures. In addition, clearance of a spin probe was related to the urine concentration. The BCM-ESR method was found to be very useful to observe free radicals at the real time. By time-dependent ESR signal decay of spin probes, pharmacokinetic parameters were obtained.

Microelectrode-Detected Electron Spin Resonance (MEDESR) Signals of Free Radicals in Electrolyte Solutions

Abstract The ESR spectra of stable free radical in solution have first been detected through potential change between two electrodes inserted in the solution. The working electrode (φ 50 μm) was biased by a constant current source under microwave irradiation and modulated magnetic field. The AC component of the potential on resonance was preamplified and detected by a lock-in amplifier. The sensitivity of this method, 5 × 106 spins/G, is 103 higher than those of conventional ESR spectrometers.

An E.S.R. study of penetration depth in high- Tc BiSrCaCuO superconductors

X-band microwave penetration depth has been measured in two sol-gel synthesised BiSrCaCuO superconductors (A) 2212 and (B) Pb - doped 2223 in the range 77 K – 300 K by conventional ESR using DPPH decorated pellet samples. The T c ′s were found to be 84 K for A, and 104 K for B which were confirmed by the low field microwave absorption. The widths of observed (Gaussian) ESR lines were related to the penetration depth below T c using Brandt's model for a triangular vortex lattice. From an analysis of the temperature dependence of λ, we found λ o for A as 2400 ± 100 A ̊ and for B as 2582 ± 100 A ̊

Superconductivity of CaxC60 prepared by thermal decomposition of azide: ESR and LFS microwave absorption study

Summary form only given. We have prepared C/sub ax/C/sub 60/ by thermal decomposition of CaN6 and studied the X-evolution of its superconducting properties by low field (LFS) microwave absorption and conventional ESR. Superconducting phase is found at x=5 and also x=6 (with lower fraction) with Tc = 8.4-8.8K. The results of the annealing at different temperature (300-450 /spl deg/C) and of aging effect on the intensity of LFS (and hence the amount of superconducting fraction) and ESR g-value and linewidths are presented. Preliminary results on the C/sub 13/ isotope effect on superconducting T/sub c/ in C/sub a5/C/sub 60/ are also discussed.

Spin-Dependent Recombination Effects in a-SI:H Pin Solar Cell Devices: A New Characterization Technique

ABSTRACTElectrically detected magnetic resonance (EDMR) - also often referred to as spin-dependent photoconductivity - is a particularly attractive technique for the investigation of the electronic properties of semiconductor devices. This method detects the paramagnetic states involved in transport and recombination by recording changes of the sample current induced by a resonant enhancement of the recombination rates. This is in contrast to ESR which monitors the resonant enhancement of microwave absorption. Therefore, EDMR is more sensitive by many orders of magnitude than conventional ESR and can be used to investigate a-Si:H solar cell devices under standard operating conditions.In this report we review the EDMR technique and discuss, in detail, the spin-dependent processes controlling dark and photocurrent of standard a-Si:H pin solar cells. In particular, we show that EDMR is sensitive to changes in the internal field of the i layer. We compare the experimental results to numerical calculations of the potential profiles and of EDMR-related quantities. We arrive at the conclusion that degradation by both high forward current and illumination increases the defect density mainly in the i layer, rather than at the pi interface.

Evolution of superconductivity in Ca doped C 60 prepared from azide. Low-field microwave absorption and ESR

Superconducting (SC) behavior of Ca doped C 60 prepared by calcium azide thermal decomposition is studied by low-field signal (LFS) microwave absorption and conventional ESR. SC phase is obtained for the sample prepared with nominal composition of Ca 6C 60 and its volume fraction is significantly enlarged by annealing at 350–400°C for 2 h. It is suggested that the SC compound may have two SC phases with slightly different transition temperatures, T C1 (8.2–8.4 K) and T C2 (8.7–8.8 K). The SC 2 phase with higher T C2 is degraded by annealing at 450–500°C and converts to SC 1. Degradation of SC 2 at high temperature is interpreted to be caused either by (1) thermal reaction of some intercalated Ca with quartz glass and/or (2) dissociation of (CaN) y or N y clusters in the octahedral sites of the Ca 5(CaN) y C 60 or Ca 5N y C 60 lattice.

Annealing effect of the superconducting phase of sodium-nitrogen-C 60 fulleride, prepared from Na-azide

By means of low field signal(LFS) of microwave absorption combined with conventional ESR, the evolution of superconducting phase of sodium doped C 60 prepared from sodium azide is studied upon heat treatment. The superconductivity is found to be improved both regarding the superconducting(SC) volume fraction and the critical temperature(Tc) upon low temperature annealing at 200°C. However, the LFS intensity decreased at a rapid rate upon high temperature(350∼400°C) annealing indicating suppression of superconductivity of Na 3(Na αN β) yC 60 due to thermally activated decomposition of the spacer cluster Na αN β, which has been tentatively assigned to undecomposed NaN 3,(although exact values of α and β are not clear yet). The rate of LFS decrease obeyed Arrhenius relation with the activation energy(Ea)=1.9±0.2eV. The critical temperature Tc is found to decrease smoothly from about 16 K to zero with the rate depending on annealing temperature(T a), suggesting continuous change of SC composition y, and thus a monotonin decrease of the average lattice constant. Clear correlation between improvement or degradation of the SC phase and the peak-to-peak ESR linewidth at room temperature was observed.

Absolute calibration of microwave loss in ESR spectrometers

Studies of field‐dependent microwave losses are useful for the detection and characterization of superconducting materials. The measurements are made in a conventional ESR spectrometer, but until now the data interpretation has been mainly qualitative. A calibration procedure using just a frequency counter, DC voltmeter, and ball bearing is decribed. This allows a quantitative determination of magnetic losses to ∼ 5% accuracy without the need to determine the cavity Q, power level, or field strength.

Kinetic measurement of the erythrocyte sedimentation rate.

A method for estimation of the erythrocyte sedimentation from repeated measurements during the initial part of the sedimentation phase is described. Based on the measurements between 18 and 24 minutes after the beginning of the reaction, the sedimentation at 60 minutes is estimated. The estimate is highly correlated to the measured ESR in the range which the conventional ESR permits (up to about 90 mm). Higher ESR will be estimated from the kinetic data and thus give reproducible and meaningful results of samples which will be reported as "falsely low" by conventional methods. Sedimentations of several hundred mm can thus be estimated suggesting a new use for the ESR, particularly in monitoring patients with severe inflammatory reactions.

Mitogens-DPPC-vesicles interaction: A spin label ESR study

The electron spin resonance spectroscopy with the spin labelling technique has been used to investigate the interaction of sonicated di-palmitoyl-phosphatidylcholine (DPPC) small unilamellar vesicles with concanavalin A (Con A), phytohemagglutinin (PHA) and staphylococcal enterotoxin B (SEB) in the temperature range 0–50°C. Conventional ESR measurements show that the addition of mitogens to the vesicles suspensions up to the number of mitogens/number of vesicles ratio of 60 increases the orientational degree of order of the lipid molecules in the gel phase and the gel→liquid-crystalline phase transition temperature. No effects on the liquid-crystalline phase have been observed, also no matter for the mitogen used. The effectiveness results in the order: ConA>SEB>PHA. Saturation transfer-ESR measurements evidence more restricted rotation of the DPPC molecules in the gel phase in the presence of Con A and SEB than in the presence of PHA if compared to pure DPPC-vesicles. The effective rotational correlation time, τc, of the spin label 5-SASL results about one order of magnitude higher in the presence of 60 mitogen molecules/vesicle. The ESR findings suggest a mitogen-induced increase in the packing density of lipid bilayers which is attributed to a morphological change from sonicated vesicles, with a small radius of curvature, to a more planar mesophase. Con A is more effective than SEB and PHA in inducing this process.

Air stability of K 3C 60 superconductors: Low field microwave absorption and ESR study

Using nonresonant microwave absorption in low magnetic field (so called LFS) combined with conventional ESR it is shown that degradation of the superconducting (SC) state of K XC 60 upon exposure to air is a surprisingly slow (e.g. compared to n-type doped conducting polymers) and partially reversible process. The annealing at 100–130°C in high vacuum of air exposed samples restores both LFS and ESR typical for the SC state, suggesting the formation of some unstable (K XC 60O 2) complex at a first stages of oxidation. At longer exposure times superconducting LFS can not be recovered meaning irreversible chemical bonding of 0 to C 60.

Microwave Spectroscopy of Fullerene-Based Molecular Superconductors in Low Magnetic Filed: Mx C60 (MK, Rb, I2, Ga, In), Mx Cn (n 70,76,78,84,90) and Mx(C60)1-y(C70)y(MK, Rb)

Abstract We have used the combined microwave spectroscopy: the nonresonant microwave absorption in low-magnetic field (so-called LFS) together with conventional ESR for the search and study of superconductivity (SC) in the new family of fullerene molecular solids: C60, C70, C76, C78, C84 C90, and their alloys ((C60)1-y(C70)y), doped by different metals (K, Rb, In, and Ga) and iodine I2. These experiments fall into two categories: 1) Search for SC in higher fullerenes and study of the evolution of SC properties upon doping of pure (one component) fullerenes including Cm thin films. 2) Investigation of the Tc change in molecular alloys of Mx (C60)1-y(C70)y type. In the first category of experiments we discuss the evolution of LFS and ESR spectra upon variation of Rb-doping conditions: temperatures of doping: Td (C60), Td(M), and doping time td, and as a result we find the optimal conditions for vapour phase Rb-doping. Here we demonstrate that SC Tc appears and disappears with doping abruptly at a discrete v...

Saturation transfer, continuous wave saturation, and saturation recovery electron spin resonance studies of chain-spin labeled phosphatidylcholines in the low temperature phases of dipalmitoyl phosphatidylcholine bilayers. Effects of rotational dynamics and spin-spin interactions

The saturation transfer electron spin resonance (STESR) spectra of 10 different positional isomers of phosphatidylcholine spin-labeled in the sn-2 chain have been investigated in the low temperature phases of dipalmitoyl phosphatidylcholine (DPPC) bilayers. The results of continuous wave saturation and of saturation recovery measurements on the conventional ESR spectra were used to define the saturation properties necessary for interpreting the STESR results in terms of the chain dynamics. Spin labels with the nitroxide group located in the center of the chain tended to segregate preferentially from the DPPC host lipids in the more ordered phases, causing spin-spin interactions which produced spectral broadening and had a very pronounced effect on the saturation characteristics of the labels. This was accompanied by a large decrease in the STESR spectral intensities and diagnostic line height ratios relative to those of spin labels that exhibited a higher degree of saturation at the same microwave power. The temperature dependence of the STESR spectra of the different spin label isomers revealed a sharp increase in the rate of rotation about the long axis of the lipid chains at approximately 25 degrees C, correlating with the pretransition of gel phase DPPC bilayers, and a progressive increase in the segmental motion towards the terminal methyl end of the chains in all phases. Prolonged incubation at low temperatures led to an increase in the diagnostic STESR line height ratios in all regions of the spectrum, reflecting the decrease in chain mobility accompanying formation of the subgel phase. Continuous recording of the central diagnostic peak height of the STESR spectra while scanning the temperature revealed a discontinuity at approximately 14-17 degrees C, corresponding to the DPPC subtransition which occurred only on the initial upward temperature scan, in addition to the discontinuity at 29-31 degrees C corresponding to the pretransition which displayed hysteresis on the downward temperature scan.