E. J. Reijerse

14N-coordination to VO2+ in reduced vanadium bromoperoxidase, an electron spin echo study

Vanadium bromoperoxidase from the brown seaweed Ascophyllum nodosum was studied with electron spin echo envelope modulation (ESEEM) spectroscopy. After comparing the Fourier transformed (FT) ESEEM spectra with those of a number of vanadyl model compounds, it could be concluded that nitrogen is present in the equatorial plane of the vanadyl cation of reduced bromoperoxidase (14N frequencies occurred at 3.1, 4.2, 5.3 and 8.1 MHz). Furthermore, the FT‐ESEEM spectra of reduced bromoperoxidase exhibited an intense 1H modulation (13.8 MHz), which was completely replaced by a deuterium modulation at ∼2 MHz when bromoperoxidase was dissolved in D2O, instead of H2O. These latter data confirm earlier EPR experiments on reduced bromoperoxidase [(1988) Biochemistry 27, 1629–1635], showing that the oxo‐vanadium (IV) ion is coupled to exchangeable protons.

Hyperfine sublevel correlation spectroscopy (HYSCORE ) of disordered solids

In the study of small hyperfine interactions unresolved in the EPR spectrum, in diluted paramagnetic systems, electron spin-echo envelope modulation has proven to be a very useful technique ( 1). For example, in a three-pulse (stimulated-echo ) experiment (2, 3), the nuclear spin frequencies of the nuclei that are coupled to the unpaired electron will appear as a modulation on the electron spin-echo (ESE) intensity which is recorded as a function of the time T between the second and third microwave pulses. The first two microwave pulses, which are separated by time T, will create nuclear spin coherences in the two electron spin ms manifolds, while the last microwave pulse will transform these coherences into a detectable electron spin-echo signal. The stimulated-echo ESEEM experiment has also been demonstrated as a two-dimensional technique, i.e., recording the electron spin-echo amplitude as a function of both times T and T to show the correlation of the nuclear spin coherences of the two electron spin ms manifolds to each other (4). A serious drawback of this experiment is the limited time range available in the 7 direction due to the usually very short T2 relaxation time of the electron spin system. In the last decade many new ESEEM experiments have been introduced (5). In particular, the two-dimensional four-pulse stimulated-echo experiment introduced by P. Hofer et al. as HYSCORE (hyperline sublevel correlation spectroscopy) (6) provides a useful alternative to the classical two-dimensional three-pulse technique. The HYSCORE experiment is derived from the three-pulse experiment by the insertion during the second time interval (see Fig. 1) of a x pulse which exchanges the populations of the ms manifolds. In terms of two-dimensional correlation spectroscopy, this pulse serves as a mixing pulse which creates correlations between nuclear spin transitions associated with the two electron spin ms manifolds. The mixing pulse terminates the evolution interval t, and is followed by the detection interval t2. As in the three-pulse experiment, the nuclear coherences evolving during this time interval are sampled by the last microwave pulse creating an electron spin echo. Compared to the classical

Model calculations of frequency-domain ESEEM spectra of disordered systems

Abstract Model calculations were carried out for electron spin-echo envelope modulation (ESEEM), ENDOR, and pulsed ENDOR spectra of spin systems of one electron coupled with an 1 = 1 2 , I = 1 , or I = 3 2 nucleus. On the basis of these simulations, predictions are made of cases for which resolved powder patterns can be expected. In the I = 1 2 case it is demonstrated that in the ESEEM simulations a degradation of the powder lineshape occurs as compared to the ENDOR simulations. Nevertheless, the shape and the symmetry of the bands may give information about the relative sign of the isotropic and the anisotropic hyperfine interaction and may yield an estimate for the magnitude of the hyperfine interaction. In the I = 1 case, the pure quadrupole frequencies can be observed when the nuclear Zeeman interaction is approximately compensated by the isotropic hyperfine interaction in one Ms manifold. This limitation is much more strict if I = 3 2 . For this nuclear spin the spectra are extremely complicated and virtually no information can be obtained if this “compensation restriction” is not fulfilled.

Comparison of ESEEM, ESE-ENDOR, and CW-ENDOR on 14N in a powder

Abstract Three different techniques were applied for the determination of the 14 N couplings in powdered 0.3% copper-doped nickel(II)bis( NN -diethyl dithiocarbamate): electron spinecho envelope modulation (ESEEM), electron spin-echo ENDOR, and continuous-wave ENDOR. Comparison of the experimental data obtained with these methods shows their complementary nature and suggests that ESEEM and one of the ENDOR techniques should be applied together to obtain maximum information. The various powder spectra were simulated using tensors which are known from single-crystal studies. Intensities and transition probabilities were computed by complete diagonalization of the spin-Hamiltonian matrix. The correspondence with the experimental spectra is quite promising.

An ESEEM investigation of single crystals and powders of copper-dopedl-histidine hydrochloride monohydrate

In this paper we present an investigation of the remote nitrogen of imidazole in copper-dopedl-histidine hydrochloride monohydrate, which is a model system for a wide range of biologically important copper-imidazole complexes. Since these systems are mostly not available in a crystalline form it is interesting to investigate to what extent information that can be obtained from these disordered systems using ESEEM techniques is reliable. From single crystal ESEEM and HYSCORE experiments we have determined the hyperfine and quadrupole coupling tensors of the remote nitrogen in this model system. Additionally, we determined information on these tensors from powdered material, independent of the single crystal results, using orientation selective multifrequency and two-dimensional experiments. The availability of the coupling tensors from the single crystal investigation allowed an assessment of the applicability of the employed powder techniques to this important class of systems. The two-dimensional stimulated echo experiment was found to be a very useful spectroscopic tool for the study of these systems when combined with simulations.

An efficient general algorithm for the simulation of magnetic resonance spectra of orientationally disordered solids

Abstract We present an efficient and general algorithm for the simulation of EPR powder spectra. The method is based on the accumulation of a large number of single-crystal spectra. Only 2–10% of these spectra are computed by exact diagonalization of the spin-Hamiltonian matrix, whereas the other spectra are obtained by separate bicubic spline interpolation procedures for the spectral positions and intensities of all relevant transitions. The energy-level tracking scheme required for the correct interpolation of the resonance position uses the inproduct matrix of the eigenvectors of the spin-Hamiltonian matrices calculated for two subsequent orientations of the magnetic field. The procedure can be easily included in any general spin-Hamiltonian program. The merits of the method are demonstrated with the simulation of the spectrum of Re(IV) in TiO ( S = 3 2 , I = 5 2 , two isotopes Dzz = 40,000 × 10−4cm−1) at X band. The computing time as compared to that of an algorithm without interpolation was reduced by a factor of 10 without deterioration of the resulting simulated powder spectrum.

An electron spin-echo envelope modulation study of 14N nuclear hyperfine and quadrupole coupling in copper(II)/nickel(II) bisN,N-di-n-butyl-dithiocarbamate)

Abstract The electron spin-echo envelope modulation technique (ESEEM) is used to determine the 14N hyperfine and quadrupole tensors in copper-doped nickel(II) bis(N,N-di-n-butyl-dithiocarbamate). The line positions and peak intensities of the Fourier-transformed ESEEM spectra are interpreted using simulated spectra, obtained by diagonalizing the full spin-Hamiltonian matrix. The 14N quadrupole tensor can be accounted for theoretically by extended Huckel molecular orbital calculations. For the interpretation of the 14N hyperfine tensor, however, an unrestricted molecular orbital calculation is necessary. Therefore, one should be cautions in drawing conclusions about spin-density distributions from small hyperfine couplings of nuclei in higher coordination spheres.

Magnetic resonance studies on porous alumina doped with iron and chromium

A combined magic-angle spinning (MAS) NMR and electron paramagnetic resonance (EPR) study has been performed on Fe- and Cr-doped alumina as a function of the heat-treatment temperature. Between 300 and 600 °C large quantities of tetra- and penta-coordinated Al were produced in porous xerogels. The pentacoordinated Al species have been found to be related to the catalytic activity. It has been found that the Fe ions accelerate the transition from γ- to α-Al2O3 and were mainly found within the alumina granules. On the other hand, Cr ions in high oxidation states (Cr5+, Cr6+) hampered the transition from γ- to α-Al2O3 and it has been proposed that they are preferentially situated on the surfaces of the granules.

Pulsed EPR for studying silver clusters

Abstract The cationic silver clusters of different nuclearity have been produced by radiolysis of zeolite A and SAPO molecular sieves containing Ag + as exchangeable cations. The pulsed EPR spectroscopy has been applied for studying the local environment of silver cluster in order to understand the mechanism of cluster formation and stabilization. The electron spin echo modulation (ESEM) results on Ag 6 n + cluster in dehydration zeolite A indicate that the hexameric silver is stabilized only in sodalite cages which are surrounded by α-cages containing no water molecules. Trimeric silver clusters formed in hydrated A zeolites strongly interact with water, thus the paramagnetic center can be considered as a cluster-water adduct. In SAPO-molecular sieves, silver clusters are formed only in the presence of adsorbed alcohol molecules. From ESEM it is determined that Ag 4 n + in SAPO-42 is stabilized in α-cages, where it is directly coordinated by two methanol molecules. Dimeric silver, Ag 2 + in SAPO-5 and SAPO-11 is located in 6-ring channels and interacts with three CH 3 OH molecules, each in different 10-ring or 12-ring channels. The differences of Ag 2 + stability in SAPO-5 and SAPO-11 are also discussed.

Estimation of higher-order magnetic spin interactions of Fe(III) and Gd(III) ions doped in α-alumina powder with multifrequency EPR

Fe(III) and Gd(III) ions in α-alumina (A12O3) exhibit spin states ofS = 5/2 andS = 7/2 respectively. The magnitude of the zero-field interaction (ZFI) (D = 0.10−0.15 cm−1) gives rise to an inter Kramers doublet splitting in the same order of magnitude as the X-band electron paramagnetic resonance (EPR) quantum (0.3 cm−1). It is demonstrated that through a careful step-by-step analysis and spectral simulation of EPR spectra taken at D-band (130 GHz), Q-band (35 GHz), and X-band (9 GHz) at room temperature, the (relative) sign and magnitude of the ZFI parameters, b20, b40, and b43, can be reliably estimated.