Comparative analysis of ESE modulation from oxidized chlorophyll a and P700 centres in chloroplasts containing 15N nuclei

A comparative analysis of electron spin echo modulation effects from ¹⁵N nuclei in pyrrole rings of Chl a⁺ and P700⁺ has been performed, yielding parameters of hyperfine couplings of the nitrogen nuclei in these paramagnetic centres.     

Comparative modulation analysis of electron spin echo signals from oxidized chlorophyll a in vitro and P 700 centres in chloroplasts

A comparative analysis of electron spin echo modulation effects from ¹⁴N nuclei of pyrrole rings for Chla⁺ and P 700⁺ has been performed, yielding parameters of quadrupole and isotropic hyperfine couplings with nitrogen nuclei in these paramagnetic centres.     

Electron spin resonance analysis of paramagnetic Pd(I) species in Pd(II)-exchanged H-rho zeolite

Zeolite rho was synthesized and Pd(II) exchanged into it. Pd(II) was reduced to paramagnetic Pd(1) by a thermal activation process. The interactions of Pd(I) in zeolite H-rho with oxygen, water, methanol, ammonia, carbon monoxide and ethylene have been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. The ESR spectrum of an activated sample shows the formation of one Pd(I) species. Pd(I) interacts with water vapor or molecular oxygen to form Pd(II)–O₂, indicating decomposition of water. Equilibration with methanol results in a broad isotropic ESR signal which is attributed to the formation of small palladium clusters. ESEM shows that the Pd clusters coordinate one molecule of methanol. Adsorption of ammonia produces a Pd(I) complex containing four molecules of ammonia based upon resolved nitrogen superhyperfine coupling. Adsorption of carbon monoxide results in a Pd(I) complex containing two molecules of carbon monoxide based upon resolved¹³C superhyperfine coupling. ESR and ESEM results indicate that exposure to ethylene leads to two new Pd(I) species each of which coordinates one molecule of ethylene.     

Electron spin echo as a method of ESR tomography

The electron spin echo method combined with pulsed gradients of an external magnetic field has been proposed for studies on the spatial distribution of paramagnetic centres. The relationship has been found between the distribution function of paramagnetic centres and the dependence of the spin echo signal on the parameters of the applied gradients. The resolution of the method depends on the ratio of the inverse phase relaxation time (T₂⁻¹) to the magnetic field-gradient and may be much higher than the resolution of standard ESR tomography. The method has been applied to a model system.     

Saturation transfer spectroscopy: signals sensitive to very slow molecular reorientation

A density matrix-transition rate matrix formalism is employed to compute the eight unique electron paramagnetic resonance signals at the first two harmonics of the applied Zeeman modulation. Calculations are carried out for conditions appropriate for investigating spin-labeled biomolecules with partially saturating microwave fields and Zeeman modulation amplitudes comparable to resonance linewidths. Spectra are computed for rotational correlation times ranging from 10^?7 to 10^?3 s and for modulation frequencies of 50 and 100 kHz. These simulations indicate that of the eight signals the in-phase dispersion signal at the first harmonic of the modulation and the out-of-phase dispersion signal at the first harmonic afford the best sensitivity to molecular motion and the largest signal amplitudes. It is suggested that study of these signals is the method of choice for monitoring slowly tumbling spin labels when signal-to-noise considerations are critical. The conclusions derived from computer simulations are borne out by experimental measurements performed on 10^?3 M solutions of the steroid spin label 17β-hydroxy-4′,4′-dimethylspiro-[5α-androstrance-3,2′-oxazolidin]-3′-oxyl in sec-butylbenzene.     

Spectral editing for 13C NMR signals of deuterated compounds

J-modulation of ¹³C spin echo signals is used to develop a strategy for the analysis of deuterated compounds. On the basis of ¹ J(¹³C²H) and an experimental set-up that allows both ²H and ¹H decoupling, ¹³C subspectra for various groups can be produced. The application of difference methods facilitates the analysis and, finally, leads to the identification of CD, CD₂, CD₃, CHD, CHD₂ and CH₂D groups.     

Electron spin resonance studies on deuterated nitroxyl spin probes used in Overhauser‐enhanced magnetic resonance imaging

The electron spin resonance studies were carried out for 2 mm concentration of ¹⁴N‐labeled and ¹⁵N‐labeled 3‐carbamoyl‐2,2,5,5‐tetramethyl‐pyrrolidine‐1‐oxyl, 3‐carboxy‐2,2,5,5‐tetramethyl‐pyrrolidine‐1‐oxyl, 3‐methoxycarbonyl‐2,2,5,5‐tetramethyl‐pyrrolidine‐1‐oxyl and their deuterated nitroxyl radicals using X‐band electron spin resonance spectrometer. The electron spin resonance line shape analysis was carried out. The electron spin resonance parameters such as linewidth, Lorentzian component, signal intensity ratio, rotational correlation time, hyperfine coupling constant and g‐factor were estimated. The deuterated nitroxyl radicals have narrow linewidth and an increase in Lorentzian component, compared with undeuterated nitroxyl radicals. The dynamic nuclear polarization factor was observed for all nitroxyl radicals. Upon ²H labeling, about 70% and 40% increase in dynamic nuclear polarization factor were observed for ¹⁴N‐labeled and ¹⁵N‐labeled nitroxyl radicals, respectively. The signal intensity ratio and g‐value indicate the isotropic nature of the nitroxyl radicals in pure water. Therefore, the deuterated nitroxyl radicals are suitable spin probes for in vivo/in vitro electron spin resonance and Overhauser‐enhanced magnetic resonance imaging modalities. Copyright © 2017 John Wiley & Sons, Ltd.     

Modulation effects in the electron spin echo of trans bis-(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals

Electron spin echo modulation effects have been investigated in trans bis(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals involving ¹⁴N or ¹⁵N nuclei in different positions. The modulation frequencies are shown to be the nitrogen nuclear transition frequencies in radical fragments at m_S = 0 of a triplet electronic spin. Quadrupole splitting parameters for these nitrogen nuclei have been determined.     

On the formation and stability of O− and O−2 radicals in type a zeolites and demonstration of cation interactions

Electron spin resonance studies show tha O^? is formed as the major paramagnetic oxygen species in γ-irradiated Ca₆-A zeolite followed by oxygen adsorption. This is a new method to generate this highly reactive catalytic intermediate. O^?₂ is formed in addition to O^? if oxygen is adsorbed prior to irradiation. In Na₁₂-A zeolite O^? is also seen but it transforms to O^?₂ in several hours. Thus O^? appears to be more stable in divalent exchanged zeolites. By electron spin echo modulation spectrometry interactions fo O^?₂ with Li⁺ have been detected which suggests that oxygen species locations in zeolites can be delineated.     

Electron spin echo envelope modulation of trapped radicals in disordered systems: nitrogen modulation from isotopically substituted chlorophyll a cations

The electron spin echo envelope modulation of the chlorophyll a radical cation has been examined for radicals containing ¹⁴N and ¹⁵N. The modulation is found to be due primarily to the nitrogen nuclei in the heterocycle and the nuclear quadrupole interaction plays a large part in determining the modulation from ¹⁴N. The modulation from ¹⁵N allows limits to be set on the isotropic and anisotropic hyperfine interactions.     

On optically detected ESR spectra from radical cations of liquid hydrocarbon solvents

The optically detected electron spin resonance (OD ESR) method has been employed to study the origin of radical-cation ESR signals in some saturated hydrocarbons with small amounts of 2.5-diphenyloxazol or p-terphenyl under radiolysis. In cyclohexane, the ESR, signal with resolved hyperfine structure was ascribed to c-C₆H₁₀⁺/PPO^? radical-ion pairs produced from primary c-C₆H₁₂⁺/PPO^? ones by monomolecular decay, of cyclohexane radical cations to cyclohexene radical cations. Cis- and trans-decalin under radiolysis accumulate 9,10-octalin which captures solvent holes and form 9,10-octalin radical cations giving a resolved OD ESR spectrum. 9,10-octalin is present in non-irradiated commercial decalin as an impurity. The OD ESR technique has been shown to be very sensitive to some impurities in hydrocarbon solvents.     

Analysis of ESE signal decay modulation for H atoms trapped in 8 M H2SO4 at 77 K

A method is proposed to analyze modulation effects induced in an electron spin echo by weak hyperfine interactions with protons. This method allows one to distinguish the contribution from the protons of the first sphere surrounding a H atom trapped in 8 M H₂SO₄ and that from the matrix protons.     

High-resolution spectroscopy of Pr3+: YAlO3 by stimulated photon-echo envelope modulation

Utilizing a 3-pulse echo sequence, a high-resolution low-field Zeeman study of the hyperfine levels of Pr³⁻ in its excited ¹D₂ state was performed. By extending the observation time for the echo modulation to 100μs. the nuclear spin level structure was determined with a resolution of 15kHz by this purely optical method. Elements of the enhanced nuclear Zeeman and pseudonuclear quadrupole tensor could be deduced.     

Electron spin echo of CH2OH radicals adsorbed in A-type zeolites

Data on the trapping site of radicals in zeolite α-cages have been obtained by the analysis of modulation effects in the electron spin echo of CH₂OH radicals produced by irradiation in A-type zeolite with adsorbed methyl alcohol.     

Pseudo-jahn-teller distortion of pyridine in its lowest triplet state

We have performed an electron spin echo detected electron paramagnetic resonance study of the nitrogen hyperfine interaction in the lowest triplet state of pyridine. It is concluded that the molecule is non-planar in this state owing to pseudo-Jahn-Teller coupling between the ³B₁(nπ) and the close-lying ³A₁(ππ) states.     

Electron spin echo envelope modulation from trapped electrons in a glassy medium

An amplitude modulation of the electron spin echo envelope has been observed for radiation-produced trapped electrons in 10 M NaOD/D₂O at 77°K but not in 10 M N₂OH/H₂O. The modulation has been simulated theoretically by generalizing the single crystal model of Rowan et al. to disordered systems. The modulation has been interpreted as due to dipolar interactions with deuterons in molecules of the second solvation shell around the trapped electrons.     

Spin relaxation and structure of light-induced spin-correlated PCBM−/P3HT+ radical pairs

The electron spin echo (ESE) technique is applied to determine the spin relaxation times of long-lived light-induced radicals and short-term spin-correlated radical pairs (SCRPs) formed by the laser flash of a composite consisting of [6,6]-phenyl-C₆₁-butyric acid methyl ether (PCBM) and poly-(3-hexylthiophene) (P3HT) at 80 K. The ESE signal dependences recorded to measure the longitudinal relaxation times of P3HT⁺/PCBM^? SCRPs and the free P3HT⁺ radical are fitted by the exp(-(t/T ₁)^0.6) dependence with T ₁ values lying in the microsecond time scale. The difference in the transverse spin relaxation times of the P3HT⁺/PCBM^? radical paira appeared after selective and non-selective echo-detected EPR spectrum excitation is explained by the instantaneous diffusion model. Based on the model, the magnetic interaction energy between the electron spins in P3HT⁺/PCBM^? SCRPs is estimated; E/? ～ 10⁶ s^?1.     

Crystal Structure Analysis of the Repair of Iron Centers Protein YtfE and Its Interaction with NO

Molecular mechanisms underlying the repair of nitrosylated [Fe–S] clusters by the microbial protein YtfE remain poorly understood. The X‐ray crystal structure of YtfE, in combination with EPR, magnetic circular dichroism (MCD), UV, and ¹⁷O‐labeling electron spin echo envelope modulation measurements, show that each iron of the oxo‐bridged Fe^II–Fe^III diiron core is coordinatively unsaturated with each iron bound to two bridging carboxylates and two terminal histidines in addition to an oxo‐bridge. Structural analysis reveals that there are two solvent‐accessible tunnels, both of which converge to the diiron center and are critical for capturing substrates. The reactivity of the reduced‐form Fe^II–Fe^II YtfE toward nitric oxide demonstrates that the prerequisite for N₂O production requires the two iron sites to be nitrosylated simultaneously. Specifically, the nitrosylation of the two iron sites prior to their reductive coupling to produce N₂O is cooperative. This result suggests that, in addition to any repair of iron centers (RIC) activity, YtfE acts as an NO‐trapping scavenger to promote the NO to N₂O transformation under low NO flux, which precedes nitrosative stress.     

14N Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy of the Cation Radical P840+, the Primary Electron Donor of the Chlorobium limicola Reaction Center

The electronic structure of the oxidized primary chlorophyll electron donor, P840^+., of the green sulfur bacterium Chlorobium limicola has been investigated using electron spin echo envelope modulation (ESEEM) spectroscopy. This ESEEM investigation of the electron spin density distribution in the radical cation P840^+. in membranes isolated from C. limicola confirms that the electron spin is shared eqully between the two bacteriochlorophyll a molecules. Observation of the small hyperfine couplings to the ring nitrogens by ESEEM gives results that are in agreement with those obtained from ENDOR measurements (S. E. J. Rigby, R. Thapar, M. C. W. Evans and P. Heathcote, FEBS Lett. 350,24–28, 1994) of the large hyperfine couplings to the methyl group protons. These results in combination with the Raman spectroscopy of P840 (U. Feiler, D. Albouy, B. Robert and T. A. Mattioli, Biochemistry 34,11099–11105, 1995) all indicate that the reaction center of green sulfur photosynthetic bacteria is functionally a protein homodimer providing a symmetrical protein environment for the primary electron donor.     

Coordination of vanadyl acetylacetonate with nitrogen-containing donor bases

Conclusions From results obtained in an investigation of modulation effects in the electron spin echo of adducts of VO²⁺acac₂ with nitrogen-containing donor bases, we have been able to determine the constants of HFI of the unpaired electron of the vanadium with the nitrogen nucleus, for a large class of these bases. The values of the HFI constant are in the range 3.6–6 MHz.The sensitivity of modulation effects in ESE to the selection of the base offers a means for determining the relative stability of these adducts.Translated from Zhurnal Strukturnoi Khimii, Vol. 26, No. 3, pp. 53–58, May–June, 1985.Translated from Zhurnal Strukturnoi Khimii, Vol. 26, No. 3, pp. 53–58, May–June, 1985.