ENDOR Spectra Research Articles

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.

Electron paramagnetic resonance, ENDOR and TRIPLE resonance study of some 9,10-anthraquinone radicals in solution

EPR, ENDOR and TRIPLE resonance spectra were recorded for 1,8-dihydroxy-3-methyl-9,10-anthraquinone and 9,10-anthraquinone-2-carboxylic acid anion radicals and the 9,10-anthraquinol-2-carboxylic acid cation radical. EPR spectra were recorded for the 5,8-dideuterio-1,4-dideuterioxy-9,10-anthraquinone anion radical, the 6,7-dideuterio-1,4-dideuterioxy-9,10-anthraquinone anion radical and the 1,4-dihydroxy-9,10-anthraquinol cation radical. The coupling contants of the 1,4-dihydroxy-9,10-anthraquinone anion radical were assigned by deuteriation.

ENDOR Studies of X-Irradiated Single Crystals of Sulphamethoxazole

Abstract Single crystals of sulphamethoxazole were X-irradiated at 273 K. ESR and ENDOR spectra were obtained at 100 K. The free radicals stable at room temperature are formally formed by abstraction of a hydrogen atom from the methyl group of the molecule. The unpaired electron is delocalized in the isoxazole ring. The assignment is supported by comparisons of spin densities obtained experimentally and by semiempirical molecular orbital calculations

Covalently linked porphyrin ubiquinones (O) as model compounds for photosythetic reaction center.

trans- and cis- cyclohexylene linked porphyrin ubiquinones(O) have been synthesized and characterized by NMR and optical spectroscopy. ENDOR spectra of the corresponding porphyrin semiquinone radical anions are discussed.

Electron-nuclear double-resonance investigations on Cr3+ ions in natural MgAl2O4 spinel

The ENDOR spectra of Cr3+ ions in natural MgAl2O4 spinel have been measured at 4.2 K and the hyperfine and quadrupole interactions with the nearest-neighbour 27Al nuclei and with the 53Cr isotope have been investigated. The results show that Cr3+ has the point symmetry 3m, being subjected to the original trigonal expansion at the B sites, but inducing some distortion effects on its surroundings. Further, the isotropic hyperfine constant of the 27Al nuclei has been analysed in an independent bonding model, showing that there is appreciable transfer of unpaired spin to the aluminium ion due to covalency.

Structural information on chromium(V) complexes of 1,2-diols in solution, as determined by isotropic and anisotropic proton ENDOR spectroscopy

Recently, attention has been paid to Cr(V) complexes of polyols or sugars, which are also stable in biological systems. We have tested the usefulness of complementary ENDOR studies, which, as is well-known, can resolve hyperfine splittings not observable by EPR and thus provide information about the interaction between a paramagnetic center and the neighboring magnetic nuclei. The 'H ENDOR spectra recorded at the same temperature (Figure 2) are more revealing because for all complexes they clearly resolve distinct couplings for the different protons. Two coupling constants (AH)i = 0.40 and 1.16 MHz are observed for The finding is consistent with the presence of two classes of nonequivalent protons. The smaller coupling is attributed to the hydrogens of the methyl groups, as expected for a mechanism arising predominantly through atom-to-atom spin polarization. On the other hand, the larger coupling constant derives from the methine protons.

Quantitative studies of davies pulsed ENDOR

The results of quantitative and systematic studies of the dependence of two-pulse spin-echo intensity on the pulse widths and of the Davies ENDOR response on the width of the preparation pulse, t p are reported. A formula that describes the Davies ENDOR response as a function of the selectivity parameter, η η = A n t p is presented. These measurements, which employ a variety of small molecules and metalloenzymes, not only provide information about the Davies ENDOR technique, but also give a quantitative description of the recently described POSHE method for hyperfine selection of heteronuclear pulsed ENDOR spectra through manipulation of pulse widths in the Davies ENDOR sequence. This procedure involves suppression of weakly coupled proton pattern and optimization of strongly coupled heteronuclei signals. It is demonstrated with single-crystal data for Cu(II)-doped Zn(glycinato) 2 and frozen solution data for the blue copper protein Pseudomonas aeruginosa azurin.

An ENDOR study of the temperature dependence of methyl tunnelling

We have studied the motion of the methyl group in the free radical obtained by γ-irradiation of 4-methyl-2,6-di-t-butylphenol (MDBP). The ENDOR spectra have been measured in the temperature range 4.5 to 200 K. The positions of the methyl proton lines are temperature dependent. Their variation is accounted for by considering the transition from the quantum regime of motion of the methyl group at low temperature to the classic regime at higher temperature. This transition is found to occur in a temperature range narrower than that anticipated by Allen's theory.

Model for the porphyrin-DNA binding site: ENDOR investigations of Cu-porphyrins binding to DNA.

Proton ENDOR has been observed from frozen solutions (ca. 38K degrees) of copper meso-(4-N-tetra-methylpyridyl)porphyrin (CuTMpyP(4)) complexed with Salmon sperm DNA in water and D2O. Lines from exchangeable protons of the DNA bases have been observed in these ENDOR spectra. Analyses of these ENDOR data show that the separations of these DNA protons from the copper atom are between 3.76 and 3.84 A with angles of 19.5 to 22.5 degrees between the Cu-H vectors and the gz axis. A distant ENDOR response has also been observed from phosphorous nuclei in the DNA backbone. We estimate that the phosphorous atoms producing this ENDOR signal are 7.5-10 A from the copper center of the porphyrin. These ENDOR data combined with results from an earlier NMR investigation have been used to construct a computer simulated model of the binding site in which the porphyrin is partially intercalated and extends into the major groove of DNA. The two GC base pairs at this site are slightly inequivalent. For each, the G imino proton and one of the C amino protons are at appropriate positions to account for the ENDOR signals arising from exchangeable protons. It is unlikely that this inequivalence would persist at room temperature where dynamic processes would give an apparently symmetric interaction. Although the model accounts for all reported experimental data involving tetracationic porphyrin species which have been suggested to be intercalators, it is not a unique solution.

Electron nuclear double resonance spectroscopy of Cr3+ ions in potassium dihydrogen phosphate

The ENDOR spectra of Cr3+ ions in KDP have been measured at 4.2 K and the hyperfine interaction with the nearest neighbour protons has been investigated. The results confirm the EPR model for the defect: the Cr3+ substitutes for K+ with two proton vacancies in the first shell. The relaxation of the lattice ion in the neighbourhood of the impurity is also discussed.

Electron paramagnetic resonance, ENDOR and TRIPLE resonance study of methyl-substituted benzoquinol radical cations

EPR and ENDOR spectra have been recorded for hydroquinone, methylhydroquinone, 2,3-dimethylhydroquinone, 2,5-dimethylhydroquinone, 2,6-dimethylhydroquinone, 2,3,5-trimethylhydroquinone and tetramethylhydroquinone radical cations. The radicals were generated by Ce4+ or Zr4+ oxidation in FSO3H or FSO3H and CH3NO2 solution. The radical cations were stable in strongly acidic media, where they existed in double-protonated form. The splittings of hydroxyl protons were assigned by deuteriation and other coupling constants by the additivity relationship. The cis–trans isomerism of the double-protonated cation radicals showed up clearly in EPR and ENDOR spectra of four of the radicals.

Radical ions of [2.2](2,5)pyrazinophanes: an electron paramagnetic resonance study

In the pseudo-geminal(1a) and pseudo-ortho[2.2](2,5)pyrazinophane (2a) the methylene hydrogens oriented syn towards the aza function can be selectively replaced by deuterium, to give 1b and 2b. The corresponding radical anions were generated in 1,2-dimethoxyethane by reaction of the parent compounds with alkali metal mirror, and their EPR and ENDOR spectra were recorded. General triple resonance, providing the relative signs of the 1H coupling constants, and selective deuteriation of the syn methylene hydrogens led to full assignment in all cases. As anticipated, the magnitudes of the nitrogen and ring proton coupling constants are about one half the magnitudes of those of the corresponding 2,5-dimethylpyrazine radical anion 3˙–. For the two sets of methylene protons, considerably different coupling constants were found; 1a˙–: a(Hsyn)=+2.54, a(Hanti)=+0.24 G; 2a˙–: a(Hsyn)=+1.60, a(Hanti)=+0.30 G. The radical cations of 1a and 2a were prepared by exposure of the parent compounds, as dilute solutions in CFCl3, to 60Co γ-rays at 77 K. The results establish that these are π-radicals rather than σ-radicals despite the fact that the dimethylated monomers (pyrazine radical cations) are clearly σ-species. The results for the pseudo-ortho-derivative cation show that there are two sets of inequivalent CH2 protons.

Angle selection in ESR and ENDOR spectra of rhombic metal complexes: copper glutamate and copper [15N]glutamate spectra

Angle selection in ESR and ENDOR spectra of rhombic metal complexes: copper glutamate and copper [15N]glutamate spectra

Proton ENDOR Studies on Zinc-Doped Copper(II) Acetate Monohydrate Single Crystal

Abstract The proton ENDOR spectra of the zinc-doped copper(II) acetate monohydrate single crystal were measured at 20 K. Hyperfine coupling tensors have been determined for the two protons of the water molecule at the axial position of the paramagnetic copper ion and a proton of the water forming the hydrogen bond with carboxylate oxygen at the equatorial plane of the copper ion. The axial water protons have small positive isotropic hyperfine couplings owing to the pseudo-contact interaction, and the equatorial water proton has a small negative one due to the spin polarization effect of the positive spin on the acetate oxygen through the hydrogen bond. The anisotropic hyperfine tensors of the former protons are interpreted by the dipolar interaction with the unpaired spin on the copper dx2−y2 orbital. The anisotropic interaction of the latter protons indicates that the spin delocalizes onto the oxygen atoms of the acetate bridges. Based on these observed hyperfine interactions, the mechanism of the spin–spin interaction in the dimer complex is discussed.

A simple method for hyperfine-selective heteronuclear pulsed ENDOR via proton suppression

A major reason for the importance of ENDOR spectroscopy ( 1) in the study of metal complexes is that the technique is inherently broad-banded: all coupled nuclei having spin I > 0 can be detected with comparable sensitivity (2). Unfortunately, at X-band magnetic fields ( -0.3 T at g = 2) the proton ENDOR spectrum often overlaps with and obscures the spectra of important heteronuclei such as 14N, 13C 57Fe or 33S. One solution is to work at higher microwave frequency (2, 3). In adiitioi, three elegant, two-dimensional pulsed-ENDOR techniques (4) have been developed to address this problem by hyperfine selection at X band (57). We now describe a simple one-dimensional pulsed-ENDOR method for hyperfine selection of heteronuclear ENDOR signals through the suppression of the proton pattern. As the technique also leads to enhancement of the intensities of heteronuclear signals, we call it POSHEENDOR (proton suppression, heteronuclear enhancement). Two of the currently available pulsed-ENDOR techniques for hyperiine selection are formally equivalent and can be loosely grouped as ELDOR-ENDOR experiments. Buhlmann et al. (5) used field jumps within a Davies ENDOR sequence (4, 8) to achieve selection, whereas Thomann and Bernard0 (6) used microwave frequency jumps. However, these methods require two-dimensional experiments to obtain a complete hyperfme-selected spectrum and thus need large blocks of time for data acquisition. In addition, they require equipment beyond that necessary for ordinary Davies ENDOR. The third technique, developed by de Beer et al. ( 7)) utilizes a Mims ENDOR sequence (4, 9). In this method, hyperfine selection is achieved by systematic variation of time between the first and second microwave pulses in a stimulated-echo sequence. This procedure is restricted by the time required for a 2D experiment as well as by limitation of the Mims sequence to smaller hypetine couplings. The hyperfine selection technique we describe employs the fact that in a Davies ENDOR sequence,

Chain length-dependent polaron (P −) width in phenylenevinylene oligomers: a solid state ENDOR investigation

We report on the extension of the negative polaron (radical anion) in phenylenevinylene oligomers up to chain length n = 7, where n is the number of phenyl rings. The spin density distribution obtained from solid state pulsed ENDOR spectra is observed to increase with chain length. Local hyperfine interactions as well as the corresponding spin density distribution are reported.

A relativistic effective Hamiltonian for S-state ions

This paper uses the 'effective operator technique' to develop a relativistic formulation to describe the crystalline electric field splitting of an S-state ion. This formulation leads to an effective Hamiltonian which is capable of describing the EPR and ENDOR spectra observed from S-state ions such as Gd3+. It replaces the traditional phenomenological spin Hamiltonian and reveals that all those total angular momentum tensor operators permitted by symmetry considerations are included. It is shown that Kramer's theorem which is valid only for spin states must be modified to apply to only the lowest degenerate level of the ground state of any ion since all tensor operators Tqk(J) where k is odd and k not=1 are allowed because J, and not S, is a good relativistic quantum number.

Photochemically generated carotenoid radicals on Nafion film and silica gel: an EPR and ENDOR study

Carotenoid cation radicals have been observed at 120 K, by EPR and proton ENDOR measurements, to be formed upon 77 K photolysis of thin films of Nafion or silica gel coated with the carotenoids, β-carotene and canthaxanthin. The powder ENDOR spectra consist of resolvable lines due to couplings larger than 2.8 MHz but smaller than 17 MHz assigned to the methyl protons of the carotenoid cation radical and to an α-proton of the planar polyene chain.

Phosphorus-31 ENDOR studies of xanthine oxidase: coupling of phosphorus of the pterin cofactor to molybdenum(V)

31P ENDOR spectra are described for three different molybdenum(V) species in reduced xanthine oxidase samples. The spectra were not affected by removing the FAD from the enzyme, implying that this is located at some distance from molybdenum. Furthermore, in confirmation of the work of J. L. Johnson, R. E. London, and K. V. Rajagopalan [(1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6493-6497], NMR and chemical analysis of the phosphate content of highly purified xanthine oxidase showed there are only three phosphate residues per subunit of the enzyme. It is concluded that the ENDOR features are due to hyperfine coupling of the phosphate group of the pterin cofactor to the molybdenum atom. Evaluation of the dipolar component of the coupling has permitted estimation of the molybdenum-phosphorus distances as 7-12 A. This implies that the cofactor is in an extended conformation in the enzyme molecule. Less detailed 31P ENDOR data on sulfite oxidase are consistent with a similar conformation for the cofactor in this enzyme.

EPR- und ENDOR-Untersuchungen von Benzo[c]phenanthrensemichinonen / EPR and ENDOR Investigations of Benzo[c]phenanthrene Semiquinones

A series of benzo[c]phenanthrene semiquinone diphenylthallium complexes was prepared in pyridine from the corresponding quinones. EPR, ENDOR and TRIPLE spectra provided the proton coupling constants and, in most cases, their assignments and their relative signs. The thallium splittings are discussed in terms of the extent of the twist in the semiquinones and solvation of the complexes.