Electron Spin Echo Signal Research Articles

Isotropic reorientations of fullerene C70 triplet molecules in solid glassy matrices revealed by light-induced electron paramagnetic resonance

Continuous-wave X-band electron paramagnetic resonance (EPR) of fullerene C(70) molecules excited to a triplet state by continuous light illumination was studied in molecular glasses of o-terphenyl and cis/trans-decaline and in the glassy polymers polymethylmethacrylate (PMMA) and polystyrene (PS). Above ∼100 K, a distinct narrowing of EPR lineshape of the triplet was observed, which was very similar for all systems studied. EPR lineshape was simulated reasonably well within a framework of a simple model of random jumps, which implies that the C(70) molecule performs isotropic orientational motion by sudden jumps of arbitrary angles. In simulations, a single correlation time τ(c) was used, varying in the range of 10(-7)-10(-8) s. Near and below 100 K electron spin echo (ESE) signals were also obtained which were found to decay exponentially. Correlation times τ(c) obtained from simulation of the EPR spectra in the slow-motion limit (τ(c) close to 10(-7) s) turned out to be in good agreement with the phase memory times T(M) of the ESE decay, which additionally supports the employed simple model. The observed motional effects provide evidence that the nanostructure of the solid glassy media of different origins is soft enough to allow a large asymmetric C(70) molecule to reorient rapidly. Except for the EPR spectra of the triplet, in the center of the spectra, a small admixture of a narrow line was also observed; its possible nature is briefly discussed.

Small-Amplitude Backbone Motions of the Spin-Labeled Lipopeptide Trichogin GA IV in a Lipid Membrane As Revealed by Electron Spin Echo

Trichogin GA IV is a lipopeptide antibiotic of fungal origin, which is known to be able to modify the membrane permeability. TOAC nitroxide spin-labeled analogues of this membrane active peptide were investigated in hydrated bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) by electron spin echo (ESE) spectroscopy. Because the TOAC nitroxide spin label is rigidly attached to the peptide backbone, it may report on the backbone orientational dynamics. The ESE signal in this system is observed below ∼150 K. Previously, three-pulse stimulated ESE was found to be sensitive to two types of orientational motion of spin-labeled POPC lipid bilayers at these temperatures. The first type is fast stochastic librations, with a correlation time on the nanosecond scale (which also manifests itself in a two-pulse primary ESE experiment). The second type is slow millisecond inertial rotations. In the present work, we find that at low molar peptide to lipid ratio (1:200), where the individual peptide molecules are randomly distributed at the membrane surface, the spin labels show only a fast type of motion. At the high molar peptide to lipid ratio (1:20), a slow motion is also observed. Because at this high concentration trichogin GA IV is known to change its orientation from the in-plane topology to the transmembrane disposition, the observed onset of a slow motion may be safely attributed to the dynamics of peptides, which are elongated along the lipid molecules of the membrane. The possible interrelation between this backbone rotational motion of the peptide antibiotic and the membrane leakage is discussed.

Small-Angle Orientational Motions of Spin-Labeled Lipids in Cholesterol-Containing Bilayers Studied at Low Temperatures by Electron Spin Echo Spectroscopy

Electron spin echo (ESE) study was performed for a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer containing an admixture of lipids (of 1:100 molar ratio) spin-labeled at the polar head or at different positions along the acyl chain and optionally containing additionally cholesterol that substituted POPC lipids in a 50:50 molar ratio. ESE signal was observed below 150-115 K, depending on the label position. Three-pulse stimulated ESE is sensitive to two types of orientational motion of spin labels at these temperatures. The first type of motion (which is also seen in a two-pulse primary ESE experiment) is fast stochastic librations, with a correlation time in the nanosecond scale. The second type of motion is slow millisecond rotations, which in the accessible microsecond time scale are seen as small-angle reorientations. (Restricted rotations also refer to this type of motion.) We found that in presence of cholesterol, fast librations are enhanced at all label positions. This effect may be related to the cholesterol ordering effect. Rotational motions in the presence of cholesterol are remarkably suppressed near the bilayer surface, which may be readily explained by the cholesterol condensing effect. In contrast to that, in the bilayer center rotations in the presence of cholesterol are remarkably enhanced. This effect may be attributed to topological properties of a cholesterol-containing membrane system in which beyond the steroid core, more freedom appears for small-angle motions, as compared with the cholesterol-free case. The found property may be important for physiological temperatures, as well. In particular, it may imply that cholesterol weakens interaction between two leaflets in the bilayer.

Application of ESEEM to study the structure of free radicals

Manifestations of the hyperfine interaction of paramagnetic particles in their electron spin echo signal decay were comprehensively discussed. Mechanochemically activated calcium gluconate was studied using electron spin echo modulation phenomenon and electron-nuclear double resonance techniques and quantum-chemical calculations. Three possible structures are obtained for free radicals in calcium gluconate. To specify the structure of radicals, further investigations are needed.

Dynamical properties and instability of local fluoriteBaF2 structurearound doped Mn2+ ions—EPR and electron spin echo studies

The electron paramagnetic resonance (EPR) and electron spin echo (ESE) were measured at the X-bandfor Mn2+ in a BaF2 crystal in the temperature range 4.2–300 K. In addition to the cubic symmetry centre, twoother lower concentration tetragonal centres were identified. Temperature variations andcomputer simulation of the EPR spectrum confirm that the cubic symmetry of theMnF8 centre isdeformed to two Td tetrahedra of different dimensions at around 45 K. Electron spin relaxation was measured in thetemperature range 4.2–35 K, where the ESE signal was detectable. For higher temperature theMn2+ dynamics produces homogeneously broadened EPR lines. At the lowest temperaturesthe spin–lattice relaxation is governed by ordinary phonon processes with1/T1∼T5. The efficiency of these processes rapidly decreases and at about 11 K a local mode of energy17 cm−1 becomes the relaxation mechanism. Phase relaxation observed as ESE signaldephasing indicates that after the local deformation jumps (tunnelling with frequency4 × 108 s−1)between the two tetrahedral configurations appear, with the energy barrier being the local modeenergy. This motion is directly visible as a resonance-type enhancement of the ESE dephasing rate1/TM around 11 K. Only the cubic centre displays the above dynamics.

Pulsed EPR study of low-dose irradiation effects in L-alanine crystals irradiated with [formula omitted]-rays, Ne and Si ion beams

Low-dose irradiation effects in l-alanine single crystals irradiated with γ -rays, Ne and Si ion beams have been investigated by means of a two-pulse electron spin echo (ESE) technique. An effective phase memory time, T M , was measured from the first stable l-alanine radical, SAR1, and its complex relaxation mechanism is discussed. Both spectral and instantaneous diffusion contributions to the total effective relaxation rate have been extrapolated through the detection of the two-pulse ESE signal as a function of turning angle. The local microscopic concentration of paramagnetic centers C(ions)/ C ( γ -ray) for low-dose heavy-ion irradiation has been deduced from the corresponding spin–spin interaction.

An electron paramagnetic resonance investigation of the electron transfer reactions in the chlorophyll d containing photosystem I of Acaryochloris marina

Electron paramagnetic resonance (EPR) spectroscopy reveals functional and structural similarities between the reaction centres of the chlorophyll d-binding photosystem I (PS I) and chlorophyll a-binding PS I. Continuous wave EPR spectrometry at 12 K identifies iron–sulphur centres as terminal electron acceptors of chlorophyll d-binding PS I. A transient light-induced electron spin echo (ESE) signal indicates the presence of a quinone as the secondary electron acceptor (Q) between P 740 + and the iron–sulphur centres. The distance between P 740 + and Q − was estimated within point-dipole approximation as 25.23 ± 0.05 Å, by the analysis of the electron spin echo envelope modulation.

Bidirectional Electron Transfer in Photosystem I: Determination of Two Distances between P700+ and A1- in Spin-Correlated Radical Pairs

The spin-correlated radical pair [P(700)(+)A(1)(-)] gives rise to a characteristic "out-of-phase" electron spin-echo signal. The electron spin-echo envelope modulation (ESEEM) of these signals has been studied in thylakoids prepared from the wild-type strain of Chlamydomonas reinhardtii and in two site-directed mutants, in which the methionine residue which acts as the axial ligand to the chlorin electron acceptor A(0) has been substituted with a histidine either on the PsaA (PsaA-M684H) or the PsaB (PsaB-M664H) reaction center subunits. The analysis of the time domain ESEEM provides information about the spin-spin interaction in the [P(700)(+)A(1)(-)] radical pair, and the values of the dipolar (D) and the exchange (J) interaction can be extracted. From the distance dependence of the dipolar coupling term, the distance between the unpaired electron spin density clouds of the primary donor P(700)(+) and the phyllosemiquinone A(1)(-) can be determined. The [P(700)(+)A(1)(-)] ESEEM spectrum obtained in wild-type thylakoids can be reconstructed using a linear combination of the spectra measured in the PsaA and PsaB A(0) mutants, demonstrating that electron transfer resulting in charge separation is occurring on both the PsaA and PsaB branches. The [P(700)(+)A(1B)(-)] distance in the point dipole approximation in the PsaA-M684H mutant is 24.27 +/- 0.02 A, and the [P(700)(+)A(1A)(-)] distance in the PsaB-M664H mutant is 25.43 +/- 0.01 A. An intermediate value of 25.01 +/- 0.02 A is obtained in the wild-type membranes which exhibit both spin-polarized pairs.

Electron Spin Echo Study of Molecular Structure and Dynamics: New Approaches Based on Spontaneous Fluctuations of Magnetic Interactions

Modulation phenomena that take place during electron spin echo signal decay have long been used in structural studies of free radicals and their environment. These phenomena are based on coherent dynamic effects, arising from simultaneous excitation (by microwave pulses) of two or more transitions in the EPR spectrum. Recently, a new source of stimulated electron spin echo (ESE) modulation was discovered due to spontaneous changes in the magnetic parameters of radicals during the operation of the pulse sequence. For monoradicals, these changes are caused by intramolecular motions. For radical pairs, additional mechanisms are longitudinal relaxation of spin counterparts and transformations of the paramagnetic partners during chemical reactions. Promising applications of this phenomenon to structural studies of radicals and radical pairs in solids and to investigations of their mobility and chemical transformations are considered.

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes,

A time-resolved Fourier transform electron spin resonance (TR FT ESR) method for determining absolute rate constants of reactive, free radical reactions is described. The phase memory times TM of a family of reacting radicals were measured precisely by electron spin−echo (ESE) detection. In all cases the ESE signal decays show a simple pseudo-first-order behavior. Rate constants of the reactions were extracted from the linear relationship between the reciprocal of the TM values and the alkene concentrations. The validity of the method is demonstrated for the measurement of the rate constant for addition of different phosphinoyl and 2-hydroxy-2-propyl radicals to several acrylates, methyl methacrylate, and vinyl pivalate in organic solvents at room temperature. The results are in excellent agreement with those obtained by other kinetic investigations employing time-resolved steady-state electron spin resonance (TR CW ESR) and optical absorption spectroscopy. The technique is shown to be applicable to systems with rate constants between 104 and 108 M-1 s-1, a range covered by the alkenes examined.

Vector model of electron spin echo envelope modulation due to nuclear hyperfine and zeeman interactions

The transverse electron spin magnetization of a paramagnetic center with effective spinS=1/2 interacting with nonquadrupolar nuclei may be presented as a function of pairs of nuclei magnetization vectors which precess around the effective magnetic field directions. Each vector of the pair starts its precession perpendicular to both effective fields. The free induction decay (FID) signal is proportional to the scalar product of the vectors for nuclear spinI=1/2. The electron spin echo (ESE) signal can be described with two pairs of magnetization vectors. The ESE shape is not equal to two back-to-back FID signals except in the absence of ESE envelope modulation. A recursion relation is obtained which allows calculation of ESE signals for larger nuclear spins in the absence of nuclear quadrupole interaction. This relation can be used to calculate the time course of the ESE signal for arbitrary nuclear spins as a function of the nuclear magnetization vectors. While this formalism allows quantitative calculation of modulation from nuclei, it also provides a qualitative means of visualizing the modulation based on simple magnetization vectors.

Dephasing relaxation of the electron spin echo of the vibronic Cu(H(2)O)(6) complexes in Tutton salt crystals at low temperatures.

Two-pulse electron spin echo (ESE) measurements of the phase relaxation (phase memory time T(M)) were performed in a series of Tutton salt crystals M(I)(2)M(II)(SO(4))(2).6X(2)O (M(I)=NH(4), K, Cs; M(II)=Zn, Mg; X=H, D) weakly doped with Cu(2+) ions (c approximately equal to 10(18) ions/cm(3)) in temperature range 4-60 K where ESE signals were detectable. The ESE decay was strongly modulated with proton (or deuteron) frequencies and described by the decay function V(2tau)=V(0)exp(-btau-mtau(2)) with the mtau(2) term being temperature independent and negligible above 20 K. Various mechanisms leading to the tau- or tau(2)-type ESE decay are reviewed. The m and b coefficients for nuclear spectral diffusion (NSD), electron spectral diffusion (SD), and instantaneous diffusion (ID) were calculated in terms of existing theories and the resulting rigid lattice T(0)(M) times were found to be close one to another within the crystal family with average values: 17.5 micros (NSD protons), 200 micros (NSD deuterons), 8 micros (SD), and 5 micros (ID). The ID dominates but the calculated effective T(M)(0) is longer than the experimental T(M)(0)=2 micros. This is due to a nonuniform distribution of the Cu(2+) ions with a various degree of the disorder in the studied crystals. The acceleration of the dephasing rate 1/T(M) with temperature is due to the mechanisms producing exp(-btau) decay. They are reviewed and two of them were found to be operative in Tutton salt crystals: (a) Excitations to the vibronic levels of energy Delta leading to the temperature dependence 1/T(M)=B exp(-Delta/kT), with the vibronic levels produced by strong Jahn-Teller effect, and (b) spin-lattice relaxation processes being effective above 50 K. Based on the Delta values being on the order of 100 cm(-1), the scheme of vibronic levels in the Tutton salts is presented, and the independence of the Delta on temperature proves that the adiabatic potential surface shape of Jahn-Teller active Cu(H(2)O)(6) complexes is not affected by temperature below 65 K.

Raman spin-lattice relaxation, Debye temperature and disorder effectsstudied with electron spin echo of Cu2+ in Tutton saltcrystals

Spin-lattice relaxation time T1 was determined by the electron spin echo(ESE) method in the temperature range 4-60 K in a series of Tutton saltcrystals MI2MII(SO4)2·6X2O (MI = NH4, K; MII = Zn, Mg; X = H, D) weaklydoped (⩽1018 ions cm-3) with the 63Cu2+isotope. The ESE signal was undetectable at higher temperatures. Therelaxation rate increases over the six decades in the studied temperaturerange with T1 equal to 1 s at 4 K and 0.5 µs at 50 K. Variouspossible relaxation mechanisms are discussed with the conclusion that therelaxation is governed by two-phonon Raman processes without a noticeablecontribution from the reorientations of Cu(H2O)6octahedra between Jahn-Teller distorted configurations. Deuteration of thecrystal has no effect in spin-lattice relaxation. For a few crystals, havingthe largest Cu2+ concentration among the studied crystals, a strongand linear in temperature contribution to the relaxation rate was found below15 K. Possible explanations are discussed with the final conclusion that thiseffect is due to a non-uniform Cu2+ distribution in the hostlattice producing effective relaxation via pairs and triads of the Cu2+ ions. From the T1(T) dependence the Debye temperature ΘDwas determined for the all crystals studied. This varies fromΘD = 166 K for K2Zn(SO4)2·6H2O to ΘD = 238 K for (NH4)2Mg(SO4)2·6H2O. The ΘD values are discussed and usedfor calculation of the sound velocity which was found to be similar in allcrystals and equal to ν = 4150(±150) m s-1.

Electron spin relaxation of vibronic Cu(H2O)6 complexes inK2Zn(SO4)2·6H2O single crystals

The low temperature (6 K) EPR spectrum of Cu2+ inK2Zn(SO4)2·6H2O is characteristic for the ground statea|x2-y2⟩-b|z2⟩ with essentially the |x2-y2⟩ stateand 4.5% admixture of the |z2⟩ state due to the zero-point motions.Temperature variations of the g and A parameters are characteristic fortwo-well vibronic dynamics and the vibronic averaging of the g-factor iswell described with the simple Silver-Getz model above 60 K with energydifference δ12 = 68 cm-1 between the wells. However, thismodel does not reproduce the A(T) dependence, which seems to be influencedby temperature induced changes in the unpaired electron delocalization ontoligands. Electron spin relaxation was measured at low temperatures up to 55 Kwhere the electron spin echo signal was detectable. At low temperatures, inthe static Jahn-Teller limit, the Cu(H2O)6 complexes are stronglylocalized in the deepest potential well and the slow vibronic dynamics isoverdominated by two-phonon Raman processes in electron spin-latticerelaxation. The relaxation rate is described by 1/T1 = aT9I8(ΘD/T)with transport integral I8 and the Debye temperature ΘD = 170 K.Electron spin echo decay is strongly modulated by dipolar coupling with 1Hand 39K nuclei and the decay function is V(2τ) = V0exp (-aτ-mτ2). The quadratic term dominates in the staticJahn-Teller limit (below 18 K) and describes the decay produced by thenuclear spectral diffusion. For higher temperatures the exp (-aτ) termdominates and describes an effect of the intrawell excitations with phaserelaxation rate 1/TM = a + bexp (-Δ/kT) with Δ = 67 cm-1being the energy of the first excited vibronic level. The excitations producea clear broadening of the Fourier transform electron spin echo (FT-ESE)spectra (at 18 K), where peaks from potassium and hydrogen nuclei have beenidentified.

Frequency Dependence of EPR Signal-to-Noise

Direct measurements of electron spin-echo signal and noise in well-characterized X-band and S-band spectrometers agree with predictions of frequency dependence based on first principles. For the particular spectrometers compared, the echo at 9.52 GHz was 9.5 times larger than the echo at 2.68 GHz, after scaling for differences in spectrometer gain. The calculated ratio was 7.6. This result contrasts with prior predictions that the frequency dependence would be much greater.

Soliton dynamics in TTF — CA

Electron Spin-Echo (ESE) measurements on mixed-stack charge transfer compound tetrathiafulvalene (TTF)-chloranil (CA) are reported. Below Neutral-Ionic transition temperature, the ESE signals related to the spin solitons were observed. Comparing the phase memory time with the inverse of the EPR line-width, it was found that a crossover from fast motion of the spin solitons to slow motion took place around 70K. In addition, the spin-lattice relaxation measurements suggested the thermal fluctuation of the dimerized lattice above 60K.

Structural Modifications of DMPC Vesicles upon Interaction with Poly(amidoamine) Dendrimers Studied by CW-Electron Paramagnetic Resonance and Electron Spin−Echo Techniques

This report describes a computer-aided CW- and pulsed-electron paramagnetic resonance (EPR) investigation on the structural modification of dimyristoylphosphatidylcoline (DMPC) vesicles, which occur upon interaction with starburst dendrimers (SBDs). Probes used for this study included doxyl-functionalized stearic acids, with the doxyl group attached at different positions of the stearic chain (5DXSA, 12DXSA, and 16DXSA). Mainly mobility and polarity parameters were evaluated from the analysis of the CW-EPR spectra, whereas the analysis of the decay and modulation of the electron spin-echo (ESE) signal provided information on the structural environment of the paramagnetic center. Due to the interaction with the SBD surface, the vesicle structure became more rigid and ordered. The enhanced rigidity of the structure also caused the tilting of the chains of about 50° with respect to the surface line. The permeability of water at the chain beginning level increased, thus increasing the rotational mobility of the probe. The perturbing effects lessened toward the end of the chains. A fraction of 16DXSA (15%) was in the bent conformation, with the chain inserted into the lipid layer and the two polar heads at the external surface. The interaction with protonated dendrimers caused the swelling of the vesicle structure. This study indicated that the bilayer structure is modified but only partially perturbed by the addition of the dendrimers, and the integrity of the vesicle, as a model cell membrane, is preserved after the interaction with the dendrimers. This is encouraging for the use of the SBDs as drug and gene carriers.

Vibronic Behavior and Electron Spin Relaxation of Jahn−Teller Complex Cu(H2O)62+ in (NH4)2Mg(SO4)2·6H2O Single Crystal

Temperature dependences of the CW-EPR spectrum as well as the electron spin−lattice relaxation time T1 and phase memory time TM determined by electron spin echo were measured for Cu2+ ions in (NH4)2Mg(SO4)2·6H2O single crystal. The dependences are dominated by vibronic behavior of the Cu(H2O)62+ complex and connected to the dynamic Jahn−Teller effect producing reorientations between two lowest energy wells of the adiabatic potential surface. Below 70 K the static Jahn−Teller effect is observed, and the system is strongly localized, by the local strains, in the deepest potential well leaving higher wells not populated. Above this temperature the second well becomes progressively populated, and a rapid averaging of the gz and gy factors as well as corresponding hyperfine splittings appears. The Boltzmann population of these two wells is achieved at 160 K. Simultaneously with g factors averaging a continuous broadening of the hyperfine lines is observed with line shape transformed from Gaussian at 70 K to Lorentzian at 160 K. The averaging and broadening processes are thermally activated with energy barrier δ12 = 108 ± 3 cm-1 = 156 K = 1.26 kJ/mol being the energy difference between the two deepest potential wells. Electron spin relaxation was measured below 50 K where electron spin-echo signal was detectable. Spin−lattice relaxation is driven by the direct and Raman processes with relaxation rate 1/T1 = aT + bT5 as expected for dynamic Jahn−Teller systems. Spin−spin phase relaxation described by the phase memory time TM depends on temperature as 1/TM = a + bT + c exp(−Δ/kT) with Δ = 102 ± 2 cm-1. At low temperatures the higher energy well is not populated; thus, Δ can be assigned as the energy of the first excited vibronic level in the deepest well. The Δ and δ12 are temperature-independent, indicating that adiabatic potential surface is not affected by temperature. We suggest that the deviations of experimental data from theoretically predicted vibronic g-factors averaging observed for Cu2+ in many Tutton salt type crystals are not due to temperature variations of the local strains or barrier height but are due to the fact that Boltzmann population of the potential wells cannot exist at low temperatures. This effect is especially pronounced for Cu2+ ions in (NH4)2Mg(SO4)2·6H2O since the energy Δ of the first vibronic level is lower than the energy difference δ12 between adjacent wells. In such case the phonon-assisted tunneling jumps between the energy wells induced by two-phonon Raman processes via virtual state of energy δ12 become to be effective when kT ≥ δ12/2, and the Boltzmann population of the second well is achieved via direct phonon process when thermal phonons of energy kT ≥ δ12 are available.

Spin-echo in the phosphorescent triplet state of crystalline 2-indanone

The relative ordering of the three sublevels of the phosphorescent triplet state of 2-indanone was determined by correlating optically detected magnetic resonance (ODMR) spectra taken with the sample in a Zeeman field with calculated spectra. Spin coherence was observed in the phosphorescent triplet state of 2-indanone in neat and doped single crystals by conventional and real time detection. The Hahn electron spin-echo signal was observed in real time by a pulse sequence which terminates with a continuous-wave low power field (LPF). The Fourier transform of the echo decay as a function of τ yielded a peak at 9.0 MHz which was attributed by isotopic substitution studies to nuclear coupling of the triplet electrons via the α-protons in the molecule. Dephasing of the Hahn and stimulated spin echo in 2-indanone due to energy trapping was measured with chemically mixed crystals of 2-indanone prepared with varying concentrations of quinoxaline. Energy transfer to quinoxaline which served as a deep trap contributed to the dephasing of the echo.

Design and Fabrication of Copper-Film Loop–Gap Resonators

Abstract The design, production, and performance of relatively simple, economical, and easy-to-build loop–gap resonators made of copper-clad Teflon (CuFlon) are described. Resonators were built at microwave frequencies between 1 and 9.4 GHz. A C-band continuous wave spectrum, an L-band electron spin-echo signal, and an L-band saturation-recovery signal for irradiated fused silica standard samples are presented.