ENDOR-induced EPR Research Articles

Pulsed EPR study of the S′ 3 signal in the Ca 2+-depleted photosystem II

The Ca 2+-depleted photosystem II (PS II) was studied by pulsed EPR spectroscopy. A short (5 s) illumination of the PS II in the S′ 2 state resulted in formation of the doublet S′ 3 signal at g ≈ 2.02 with a splitting of about 15 mT concomitant with the decrease of the modified Mn multiline signal by about 40%. Further illumination up to three minutes led to the formation of another S′ 3 signal with a singlet-like feature at g ≈ 1.98 and to the complete disappearance of the modified Mn multiline signal. Computer simulations of the shape of the doublet S′ 3 signal and the dependence of its electron spin echo amplitude on the microwave field strength suggest that dipole and exchange interactions between two organic radicals are responsible for the doublet S′ 3 signal. The pulsed ENDOR-induced EPR measurements indicate that the ENDOR spectrum of the tyrosine radical Y Z + is associated with the doublet S′ 3 signal only.

ENDOR study of 133Cs hyperfine couplings with SO3 ? radicals in X-irradiated piezoelectric Cs2S2O6 single crystals

The ENDOR technique has been used to study 133Cs hyperfine couplings of SO3– radical ions in X-irradiated Cs2S2O6 single crystals. The orientational dependent parameters for two couplings were obtained from the ENDOR spectra and assigned with the aid of ENDOR-induced EPR. The larger coupling, a(I)= 30.55 MHz, was attributed to the Cs+(2)S(2)O3˙– interaction and the smaller coupling, a(II)= 9.59 MHz, to the crystallographically different product Cs+(2)S(1)O3˙–. The difference in distances was estimated from the experimentally measured hf splittings using a point-dipole approximation, and was found to be in agreement with crystallographic data. A temperature dependence of hyperfine couplings was observed and possible explanations are discussed in terms of the displacements induced by the piezoelectric and pyroelectric properties of the Cs2S2O6 crystal.

Hyperfine-axis orientation in rhombic metal complexes from ENDOR-induced EPR and quadrupolar coupling determination

The ENDOR resonances from ligand nuclei in paramagnetic transition-metal complexes exhibit a shift from the nuclear Larmor frequency because of the dipolar and Fermi contact interactions with the unpaired electron. The dipolar interaction has orientation dependence associated with its magnitude, where the position of the nuclei with respect to the metal atom determines the resonant frequencies and spectral lineshape. In this paper effects of rhombic g and metal hyperfine tensors on the EPR powder pattern and ENDOR spectra are illustrated. The EPR angular selection is modified from that encountered in axial systems and the technique of ENDOR-induced EPR is utilized to elucidate the orientation of magnetic axes with respect to one another. Quadrupole couplings in ENDOR spectra have often been analyzed using an equation from which cross terms have been omitted. The cross terms make important contributions to the spectra in some cases and the effect of these terms has been examined in order to determine when the complete equation for the quadrupole interaction must be utilized to analyze correctly ENDOR spectra.

ENDOR and EPR of defects in relatively stoichiometric β-alumina

ENDOR, ENDOR-induced EPR and EPR measurements have been made of defects, produced by X-irradiation at T<20K, in almost stoichiometric Na beta -alumina single crystals. One of the defects is shown to be an O- centre at the O(5) site; no F+ centres at this site were observed, unlike the case for non-stoichiometric Na beta -alumina. Other defects are also produced and models are proposed for them.

Proton endor-induced EPR with large magnetic field ranges: A new application to polycrystalline samples

A self-tracking ENDOR-induced EPR spectrometer is described, which allows analog correction of the frequency change of the observed ENDOR transition during the magnetic field sweep. Deviations from a linear magnetic field dependence are discussed. New applications in powder samples are given which allow determination of the hfs principal values and axes directions with respect to a non-coaxial g tensor.

ENDOR measurements of defects in sodium β-alumina. I

ENDOR, ENDOR-induced EPR and EPR measurements show that X-irradiation at room temperature of sodium beta -alumina single crystals produces four different paramagnetic defects. One of these is a defect at the O(5) site, probably an F+ centre. Its interaction with the nearest Na and Al nuclei is determined from the ENDOR spectrum. Using the special technique of ENDOR-induced EPR the electric field gradient at these Na nuclei is found to be (0.7+or-0.1)*1040 Vm-2 C-1. A second defect is tentatively identified as an O2- ion in the conduction plane. Models are proposed for the two other defects.

Endor induced electron paramagnetic resonance: Application to the resolution of overlapping spectra

A technique for resolving overlapping EPR spectra is demonstrated in a study of X- and γ-irradiated crystals of 1,1-cyclobutanedicarboxylic acid. Discrimination of overlapping spectra is accomplished by recording the electron paramagnetic resonance desaturation induced by a radiofrequency field which excites nuclear spin transitions of only one of the several paramagnetic species present. This spectrum, referred to as ENDOR induced electron paramagnetic resonance (EI-EPR), is the difference between the ordinary partially saturated EPR spectrum and the EPR spectrum recorded in the presence of a resonant radiofrequency field. An instrumental arrangement for the fast and convenient measurement of EI-EPR at several Zeeman modulation frequencies is described. The technology is employed to resolve the complicated EPR spectrum which arises from three distinct radical species existing in radiation damaged 1,1-cyclobutanedicarboxylic acid. The EI-EPR, electron nuclear double resonance, and electron electron double resonance spectra of the alicyclic and aliphatic radical are recorded as a function of temperature and crystal orientation. EI-EPR is demonstrated to permit resolution of EPR spectra due to different radical species, different molecular conformations of a given species, and radicals in different (magnetically inequivalent) sites in the crystal unit cell. All alicyclic radicals exhibit temperature dependent beta hyperfine interactions although alpha and gamma interactions show little variation with temperature. EI-EPR signal intensities are found to vary with nuclear transition probability, relaxation parameters which characterize the various transitions, and with the applied de magnetic field (i.e., the nuclear Zeeman interaction).

ENDOR of Free Radicals in Solution

Electron nuclear double resonance has been performed on six free radicals in solution. Polar and nonpolar solvents were employed, and both neutral and ionic radicals investigated. The instrumentation is described in detail. Features of the instrument include: pulsed nuclear radio-frequency magnetic fields, rf fields as high as 40 G in the rotating frame, and provision to observe changes in the EPR spectrum caused by nuclear resonance with the ordinary EPR spectrum subtracted (ENDOR-induced EPR). ENDOR mechanisms and adjustment of experimental conditions to achieve optimum signal intensity are discussed. It is established that successful ENDOR experiments can be performed on a large number of free radicals in solution, resulting in improved resolution, simplification of spectra and more accurate hyperfine splittings.