Echo-detected EPR Spectra Research Articles

EPR and HYSCORE investigation of the electronic structure of the model complex Mn(imidazole) 6: Exploring Mn(II)–imidazole binding using single crystals

The electronic structure of the Mn(II)-imidazole binding was studied by EPR spectroscopy using the model complex Mn(Im) 6 diluted in a single crystal of Zn(Im) 6Cl 2·4(H 2O). The second rank zero-field splitting (ZFS) tensor ( D tensor) of the two sites, a and b, present in the crystal was determined by measuring the orientation patterns of the echo-detected EPR spectra in three different planes at 10 K ( D a = −106, D b = −118, E a = −17, E b = −22 × 10 −4 cm −1. Euler angles with respect to the crystal habitus: α a = 13°, β a = 76°, γ a = 108.5°, α b = 14°, β b = 73.5°, γ b = 103.5°). The contribution of cubic ZFS terms to the spectrum allowed us to determine the orientation of the N–Mn–N directions of the complex as well (Euler angles in the D tensor reference frame α = 100°, β = 23°, γ = 0°, both centers having the same orientation). The hyperfine interactions with 14N were explored by HYSCORE spectroscopy. The correlation patterns and modulation amplitudes in the 2D experiments were studied for different electron spin transitions and orientations of the crystal. Signals of three different pairs of nitrogens were found. The results were analyzed considering that the N–Mn binding directions are principal directions of the hyperfine and nuclear quadrupole tensor of 14N. All three pairs of nitrogens were found to be almost equivalent with an isotropic contribution of A iso ≈ 3.2 MHz and an almost axial anisotropic coupling of 2 T ≈ 1.1 MHz along the N–Mn bonding direction. The nuclear quadrupole principal values are 1.5 MHz along the bonding direction, −0.6 MHz in the direction perpendicular to the imidazole plane, and −0.9 MHz in the direction perpendicular to both.

On the possible manifestation of harmonic-anharmonic dynamical transition in glassy media in electron paramagnetic resonance of nitroxide spin probes

Continuous wave (cw) electron paramagnetic resonance (EPR) and echo-detected (ED) EPR were applied to study molecular motions of nitroxide spin probes in glassy glycerol and o-terphenyl. A linear decrease with increasing temperature of the total splitting in the cw EPR line shape was observed at low temperatures in both solvents. Above some temperature points the temperature dependencies become sharper. Within the model of molecular librations, this behavior is in qualitative and quantitative agreement with the numerical data on neutron scattering and Mossbauer absorption for molecular glasses and biomolecules, where temperature dependence of the mean-squared amplitude of the vibrational motion was obtained. In analogy with these data the departure from linear temperature dependence in cw EPR may be ascribed to the transition from harmonic to anharmonic motion (this transition is called dynamical transition). ED EPR spectra were found to change drastically above 195 K in glycerol and above 245 K in o-terphenyl, indicating the appearance of anisotropic transverse spin relaxation. This appearance may also be attributed to the dynamical transition as an estimation shows the anisotropic relaxation rates for harmonic and anharmonic librational motions and because these temperature points correspond well to those known from neutron scattering for these solvents. The low sensitivity of ED EPR to harmonic motion and its high sensitivity to the anharmonic one suggests that ED EPR may serve as a sensitive tool to detect dynamical transition in glasses and biomolecules.

CW EPR, echo-detected EPR, and field-step ELDOR study of molecular motions of nitroxides ino-terphenyl glass: Dynamical transition, dynamical heterogeneity and β-relaxation

Continuous-wave electron paramagnetic resonance (CW EPR), echo-detected (ED) EPR, and field-step electron-electron double resonance (FS ELDOR) were simultaneously applied to study molecular motions of nitroxide spin probes of two different types in glassyo-terphenyl. A strong linear temperature dependence of the overall splitting of the CW EPR lineshape was found for nitroxide Tempone and only a weak one for a phenyl-ring-containing imidasoline nitroxide. The linear temperature dependence of the splitting is explained within the model of harmonic librations. The assessed libration frequency for Tempone is of the order of 3·1012 rad/s. The observed remarkable difference between the two nitroxides is explained by the different strength of interactions between guest and host molecules and by dynamical heterogeneity of the glass. The nonlinear temperature dependence above 250 K is attributed to the onset of anharmonic motion that is postulated in a number of neutron scattering and Mossbauer spectroscopy studies for molecular glasses and proteins (the so-called dynamical transition). Above 245 K also ED EPR spectra change drastically, which may be explained on the same ground. Magnetization transfer was observed in FS ELDOR for nitroxide Tempone, with a time constant around 10−5 s. It was found to be almost temperature-independent between 160 K and 265 K and was attributed to the Johari-Goldstein β-relaxation process. For the phenyl-ring-containing imidasoline nitroxide this transfer was not observed, which may be explained again by the dynamical heterogeneity of the glass and by small effectivity of the β-relaxation process in this case.

Molecular dynamics of nitroxides in glasses as studied by multi-frequency EPR

Pulsed multi-frequency EPR was used to investigate orientational molecular motion of the nitroxide spin probe (Fremy's salt) in glycerol glass near the glass transition temperature. By measuring echo-detected EPR spectra at different pulse separation times at resonance frequencies of 3, 9.5, 95 and 180 GHz, we were able to discriminate between different relaxation mechanisms and characterize the timescale of molecular reorientations (10(-7)-10(-10) s). We found that near the glass transition temperature, the orientation-dependent transverse relaxation is dominated by fast reorientational fluctuations, which may be overlapped with fast modulations of the canonical g-matrix values. The data was interpreted using a new simulation program for the orientation-dependent transverse relaxation rate 1/T2 of nitroxides based on different models for the molecular motion. The validity of the different models was assessed by comparing least-square fits of the simulated relaxation behaviour to the experimental data.

Librational Motion of Spin-Labeled Lipids in High-Cholesterol Containing Membranes from Echo-Detected EPR Spectra

Two-pulse, echo-detected (ED) electron paramagnetic resonance (EPR) spectroscopy was used to study the librational motions of spin-labeled lipids in membranes of dipalmitoylphosphatidylcholine + 50 mol % cholesterol. The temperature dependence, over the range 77–240 K, and the dependence on position of spin-labeling in the sn-2 chain ( n = 5, 7, 10, 12, and 14) of the phospholipid, were characterized in detail. The experimental ED-spectra were corrected for instantaneous spin diffusion arising from static spin-spin interactions, by using spectra recorded at 77 K, where motional contributions are negligible. Simulations according to a model of rapid, small-amplitude librations about an axis whose direction is randomly distributed are able to describe the experimental spectra. Calibrations, in terms of the amplitude-correlation time product, 〈 α 2〉 τ c, were constructed for diagnostic spectral line-height ratios at different echo delay times, and for relaxation spectra obtained from the ratio of ED-spectra recorded at two different echo delays. The librational amplitude, 〈 α 2〉, was determined for a spin label at the 14-C position of the lipid chain from the partially motionally averaged hyperfine splitting in the conventional EPR spectra. The librational correlation time, τ c, which is deduced from combination of the conventional and ED-EPR results, lies in the subnanosecond regime and depends only weakly on temperature. The temperature dependence of the ED-EPR spectra arises mainly from an increase in librational amplitude with increasing temperature, and position down the lipid chain. A gradual transition takes place at higher temperatures, from a situation in which segmental torsional librations are cumulative, i.e., the contributions of the individual segments add up progressively upon going down the chain, to one of concerted motion only weakly dependent on chain position. Such librational motions are important for glass-like states and are generally relevant to high lipid packing densities, e.g., in cholesterol-containing raft domains and condensed complexes.

Echo-Detected Electron Paramagnetic Resonance Spectra of Spin-Labeled Lipids in Membrane Model Systems

The dynamics of spin-labeled lipid chains in the low-temperature phases of dipalmitoyl phosphatidylcholine (DPPC) membranes, with and without equimolar cholesterol, have been investigated by pulsed electron paramagnetic resonance (EPR) spectroscopy. Echo-detected spectra from the two-pulse, primary spin-echo (pulse sequence: π/2-τ-π-τ-echo) are used to detect rapid angular motions, on the time scale of the phase memory time (T 2 M ) that is in the nanosecond regime. Echo-detected spectra from the three-pulse, stimulated spin-echo (pulse sequence: π/2-τ-π/2-T-π/2-τ-echo) are used to detect slow angular motions, on the time scale of the spin-lattice relaxation time (T 1 ) that is in the microsecond regime. Spectra recorded at very low temperature (77 K) are used to correct the two-pulse echo spectra for instantaneous diffusion that arises from dipolar spin-spin interactions between different spin labels. Echo-detected inversion recovery spectra are used to correct the three-pulse echo spectra for intrinsic spin-lattice relaxation and large-scale spectral diffusion induced by nitrogen nuclear spin flips. The dependence of the echo-detected spectral line shapes on the two time delays, r and T, can be simulated adequately by using a simple two-state model to represent the small-amplitude librational motions in the low-temperature membrane phases. The fast librational motion has isotropic character, no singly defined direction of the librational axis, and is segmental in nature, depending on chain position and also on the presence of cholesterol. The slow librational motion is of a more global, cooperative nature, being independent of chain position and cholesterol content.

A pulsed EPR study of clustering of Yb 3+ ions incorporated in GeO 2 glass

The structural aspects of clustering of Yb 3+ ions and the paramagnetic behavior of these clusters have been investigated in GeO 2 glasses doped with 140–1100 ppm by weight of Yb 2O 3 using time-domain electron paramagnetic resonance (EPR) spectroscopic techniques. The echo-detected EPR (EDEPR) spectra of Yb 3+ ions and their unusual dependencies on microwave power and magnetic field have been found to be indicative of the formation of clusters of these rare earth ions in GeO 2 glass that behave as non-Kramers type spin systems. The magnetic field and concentration dependence of phase relaxation rates of Yb 3+ in these glasses further substantiate such a scenario and indicate the formation of clusters of Yb 3+ ions. A comparison of the EDEPR spectra with calculated cw-EPR line shapes yields a semi-quantitative measure of the typical cluster size of ⩾3 Yb atoms and intra-cluster Yb–Yb distances of 3.5–4.0 Å in these glasses at doping levels of ⩾350 ppm of Yb 2O 3.

Direct EPR detection of transient and continuous wave signals at 2.5 GHz

Direct detection of free induction decays and electron spin echoes, and the recording of echo-detected EPR spectra and electron spin echo envelope modulation patterns at a microwave frequency of 2.5 GHz is demonstrated. This corresponds to the measurement of the transverse magnetization in the laboratory frame, rather than in the rotating frame as usually done by down-converting the signal (homodyne detection). An oscilloscope with a 6-GHz analog bandwidth, a sampling rate of 20 GigaSamples per second, and a trigger frequency of 5 GHz for the edge trigger and 750 MHz for the advanced trigger, is used in these experiments. For signal averaging a 3-GHz microwave clock divider has been developed to synchronize the oscilloscope with the frequency of the EPR signal. Moreover, direct detection of continuous wave EPR signals at 2.5 GHz is described.

Spatial distribution of Nd3+ dopant ions in vitreous silica: A pulsed electron paramagnetic resonance spectroscopic study

The structural aspects of clustering of Nd3+ ions in Nd2O3-doped SiO2 glasses and the effect of Al3+ codoping on these clusters have been investigated with pulsed electron paramagnetic resonance (EPR) techniques over a temperature range of 1.5–4 K. The Nd2O3 concentrations in these glasses range between 700 and 2400 ppm by weight. The Nd3+ echo-detected EPR (EDEPR) spectra of the Al-free glasses show indications of weak exchange coupling between Nd3+ ions due to clustering. The EDEPR spectra also suggest that the local coordination environment of Nd3+ ions is affected by codoping with Al. The electronic spin-lattice relaxation rates are found to be not sensitive to the spatial distribution of Nd3+ ions over the entire temperature range of measurements. On the other hand, the concentration dependence of phase relaxation rates show clear evidence of clustering of Nd3+ ions in Al-free glasses, even at the lowest Nd2O3 doping levels. These Nd3+ clusters are found to break up and homogenize with an increase in the average Nd–Nd distance on codoping with Al. Analyses of the hyperfine sublevel correlation spectrum of a Nd and Al codoped glass indicate that the homogenization of Nd3+ clusters is possibly a consequence of the formation of Nd–O–Al linkages.

Libration motion of guest spin probe molecules in organic glasses: CW EPR and electron spin echo study

Echo-detected (ED) EPR spectra of nitroxide spin probes dissolved in glassy materials provide evidence that guest molecules in these media undergo fast librational motion. Theory of spin relaxation of a librating molecule is presented. The mean squared amplitude, 〈 α 2〉, of this motion which can be derived from continuous wave (CW) EPR spectral splitting is found to depend linearly on temperature in the low temperature region. This may be ascribed to thermal harmonic vibrations. The slope of the linear dependence varies from glass to glass and seems to correlate with the strength of the intermolecular bonds and with a degree of the fragility of the glass. Above the glass transition temperature 〈 α 2〉 increases sharply. Different applications are discussed: study of molecular properties of glass, intracellular glass formation in plant tissues, structural investigations.

Molecular dynamics and spatial distribution of TOAC spin-labelled peptaibols studied in glassy liquid by echo-detected EPR spectroscopy

2,2,6,6-tetramethylpiperidine-l-oxyl-4-carboxylic acid (TOAC) spin-labelled analogues of the Aib-rich peptide (peptaibol) Trichogin GA IV are investigated in a glassy methanol/glycerol (70/30 v/v%) system, using conventional CW EPR and electron spin echo spectroscopy. Echo-detected (ED) EPR spectra indicate that the labels undergo restricted orientational motion (libration). Comparison with the small molecular spin probe Tempone shows that the dynamics of peptide molecules is determined by their structure and not by the surrounding medium. At the terminal positions (position 1 and 8) TOAC residues were found to be more flexible than at the central 4th position. Instantaneous diffusion mechanism in electron spin echo, which also contributes to the ED EPR spectra, was employed for deriving the local concentration of peptaibols. This approach may serve as a tool for investigation of peptide aggregation. In the studied model glassy solution the peptaibols were found to be randomly distributed.

Librational motion of guest spin probe molecules in glassy media

Echo-detected (ED) EPR spectra of nitroxide spin probes dissolved in glassy materials evidence that guest molecules in these media undergo fast librational motion. The mean square amplitude 〈 α 2〉 of this motion can be simply derived from continous wave (CW) EPR spectra by measuring the field separation between the two outer spectral peaks. By comparing the ED and CW EPR data the correlation time τ c of librational motion could be evaluated. Above the glass transition temperature 〈 α 2〉 was found to increase sharply.

Studies of Framework Iron in Zeolites by Pulsed ENDOR at 95 GHz

High frequency (95 GHz, W-band) pulsed ENDOR measurements were carried out on the 57Fe-containing zeolites: Fe-sodalite (FeSOD), Fe-L (FeLTL), Fe-mazzite (FeMAZ), and Fe-ZSM5 (FeMFI), where 57Fe(III) was introduced during synthesis. The echo-detected EPR spectra of all zeolites investigated, recorded at 1.8 K, show mainly the |−5/2〉 to |−3/2〉 EPR transition. Accordingly, the ENDOR spectra exhibit only two 57Fe ENDOR transitions at 67.8−68.8 and 39.0−39.6 MHz, corresponding to MS = −5/2 and −3/2, respectively. From these frequencies isotropic hyperfine couplings of −29.0, −29.3, −29.5, and −29.6 MHz were derived for 57FeSOD, 57FeL, 57FeMAZ, and 57FeMFI, respectively. On the basis of an earlier assignment of the g = 2 signal in FeSOD to Fe(III) in tetrahedral framework sites it is concluded that hyperfine couplings in the range −29.0 to −29.6 MHz are characteristic of 57Fe(III) in zeolite frameworks. In contrast to the X-band 57Fe ENDOR signals, the W-band signals are free from second- and third-order cont...

FT Pulsed and CW EPR Studies of Stable Molecular Spin Crystals

Abstract The potential application of time domain EPR spectroscopy to an organic molecular spin system was examined, particularly focusing upon quantum spin transient nutation phenomena. A nitronyl nitroxide-based biradical with an excited triplet state (S=1), p-phenylenebis(α-nitronylnitroxide), was adopted as a simple model system for studying the nutation spectroscopy which enables us to discriminate between S≥1 and S=½ states. In the low temperature region below 10 K, an S=1 spin of the themally excited biradical molecule and an S=½ spin of an impurity molecule were found to coexist in the crystalline solid state of p-phenylenebis(α-nitronylnitroxide). Field-swept echo-detected EPR spectra reproduced the cw spectra of the biradical powder. Echo-detected nutation spectra exhibited two distinct maxima of the echo intensity at the nutation frequencies expected for S=1 and S=½ spins, indicating that the biradical solid was identified to be a mixture of the S=1 and S=½ species as expected from the static s...

Molecular Motion of the Cholestane Spin Label in a Multibilayer in the Gel Phase Studied Using Echo-Detected EPR

Echo-detected EPR spectra of the cholestane spin label in a macroscopically oriented lipid multibilayer have been obtained at temperatures between 50 and 210 K. The drastic temperature dependence which the spectra reveal was attributed to relaxation induced by the orientational oscillations of the spin-label molecule. The scheme of computer simulations of echo-detected spectra for the model of uniaxial orientational oscillations (librations) is described in detail. The simulated spectra show a remarkable dependence on the orientation of the motional axis in the molecular frame. The simulation and experimental results agree when the projection of this axis in the plane of the nitroxide fragment makes an angle of 70° ± 10° with the direction of the NO bond. This was attributed to librational motion of the label about the long axis of the cholestane moiety.

The reaction center triplet state of the photosynthetic bacterium Rhodobacter sphaeroides R26: Electron spin echo spectroscopy of a single crystal

Electron spin echo detected EPR spectra have been obtained of the triplet state of a single crystal of the reaction center of Rhodobacter sphaeroides R26. The crystal is found to be monoclinic and to contain four magnetically inequivalent sites. The magnetic relaxation behaviour of this triplet has been investigated and the lifetimes of the triplet sublevels have been determined.