Superhyperfine Coupling Research Articles

Structure of the metal-water complex in Ras x GDP studied by high-field EPR spectroscopy and 31P NMR spectroscopy.

The small GTPase Ras plays a key role as a molecular switch in the intercellular signal transduction. On Mg(2+) --> Mn(2+) substituted samples, the first ligand sphere of the metal ion in the inactive, GDP-bound Ras has been studied by continuous wave EPR at 94 GHz (W-band). Via replacement of normal water with (17)O-enriched water, the (17)O--(55)Mn superhyperfine coupling was used to determine the number of water ligands bound to the metal ion. In contrast to EPR data on frozen solutions and X-ray data from single crystals where four direct ligands to the metal ion are found, the wild-type protein has only three water ligands bound in solution at room temperature. The same number of water ligands is found for the mutant Ras(T35S). However, for the alanine mutant in position 35 Ras(T35A) as well as for the oncogenic mutant Ras(G12V), four water ligands can be observed in liquid solution. The EPR studies were supplemented by (31)P NMR studies on the Mg(2+) x GDP complexes of the wild-type protein and the three mutants. Ras(T35A) exists in two conformational states (1 and 2) with an equilibrium constant K(1)(1,2) of approximately 0.49 and rate constants k(1--1) which are much smaller than 40 s(-1) at 298 K. For wild-type Ras and Ras(T35S), the two states can also be observed with equilibrium constants K(1)(1,2) of approximately 0.31 and 0.21, respectively. In Ras(G12V), only one conformational state could be detected.

Theoretical description of hole localization in a quartz Al center: The importance of exact electron exchange

The ``classical'' model of the ${[\mathrm{AlO}}_{4}{]}^{0}$ defect center in irradiated quartz, an Al impurity having replaced a four-coordinated Si atom, is that a hole forms in a nonbonding orbital of an oxygen atom, with consequent asymmetric relaxation along that particular Al-O direction. This model has been proposed years ago, based on the analysis of the electron-paramagnetic-resonance spectra of Al-containing crystalline ${\mathrm{SiO}}_{2}$ and analysis of Hartree-Fock cluster model calculations. Three recent theoretical studies based on first-principle density-functional theory (DFT) and band-structure plane-wave calculations proposed an alternative model where the hole is completely delocalized over four oxygen neighbors to the Al impurity, at 0 K. Using cluster models containing as many as 104 Si and O atoms and various theoretical approaches, we show that the delocalized picture is an artifact of the DFT approach and that a fully localized hole is obtained when an exact treatment of the exchange term is used. The validity of this conclusion is based on the direct comparison of computed and measured quantities such as the ${}^{17}\mathrm{O}$ hyperfine and ${}^{27}\mathrm{Al},$ ${}^{29}\mathrm{Si}$ superhyperfine coupling parameters, the ${}^{27}\mathrm{Al}$ nuclear quadrupole effect, and the derivable local distortion around the defect. This work shows that great care is needed when DFT is used to describe localized holes in insulators.

Copper(II) complexes of inverted porphyrin and its methylated derivatives.

The inverted porphyrins 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH2) and its methylated derivatives 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-carbaporphyrin (2-NCH3CTPPH) and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (2-NCH3-21-CH3CTPPH) stabilize the rare organocopper(II) complexes (CTPP)CuII (1), (2-NCH3CTPP)CuII (2), (CTPPH)CuIIX (3-X), (2-NCH3CTPPH)CuIIX (4-X) (X = Cl-, TFA), and (2-NCH3-21-CH3CTPP)CuIICl (5). The EPR spectra recorded for 1, 2, 4, and 5 revealed typical features diagnostic of the copper(II) electronic structure. The superhyperfine coupling pattern indicates a presence of three nitrogen donors in the first coordination sphere. An addition of HX acid to 1 and 2 to yield the species 3-X and 4-X. The reaction mechanism includes protonation of the inner C(21) carbon accompanied by an axial coordination of anion. Methylation of (2-NCH3CTPP)CuII (2) with methyl iodide resulted in formation of (2-NCH3-21-CH3CTPP)CuIICl (5) which implies an existence of a sigma-carbanion-copper(II) bond in 2. The 2H NMR investigations carried out for the pyrrole deuterated derivatives (CTPP-d7)CuII, (2-NCH3-21-CH3CTTP-d7)CuIICl, and the methyl deuterated (2-NCH3-21-CD3CTPP)CuIICl one confirmed independently the copper(II) electronic structure with the considerable dx2-y2 metal orbital contribution to the SOMO. The redox properties of copper(II) inverted porphyrins were studied by the cyclic and differential pulse voltammetry. The halfwave potentials indicate a metal-centered oxidation of 1 (390 mV) and 2 (343 mV). The dimethylated homologue 5 reveals the reduction process at -224 mV attributed to the CuII/CuI transformation.

Core modified meso-aryl corrole: first examples of CuII, NiII, CoII and RhI complexes.

A variety of metal complexes of 5,10,15-triphenyl-21-monooxa-corrole 4 have been investigated. This monooxa corrole, where one of the pyrrole ring is replaced by a furan moiety, is synthesized by the alpha-alpha coupling reaction of 16-oxa tripyrrane and dipyrromethane. The single crystal X-ray structure of 4 indicates only small deviation of the inner-core heteroatoms from planarity and this macrocycle arrange themselves into a columnar structure. Insertion of metals further flattens the corrole framework. Specifically, oxacorrole 4 binds to Nil(II), Cu(II), and Co(II) with the participation of all heteroatoms in the coordination. However, Rh(I) ion binds to only one imino and one amino nitrogen of the macrocycle. The bond angles at the metal center in the Ni(II) and Rh(I) complexes reveal square planar geometry completed by two CO molecules for Rh(I). The EPR spectra of the paramagnetic that Cu(II) and Col(II) complexes display significant decreases in the metal hyperfine couplings compared with the corresponding porphyrin complexes. The presence of superhyperfine coupling in the Cu(II) complex suggests delocalization of unpaired electron density into the ligand orbitals. Electrochemical studies reveal easier oxidations and harder reductions relative to the corresponding porphyrin derivatives while, the metallated derivatives did not show their characteristic metal reductions due to the high energy of their LUMO.

EPR studies of chromium(V) intermediates generated via reduction of chromium(VI) by DOPA and related catecholamines: potential role for oxidized amino acids in chromium-induced cancers.

The reductions of K2Cr2O7 by catecholamines, DOPA, DOPA-beta,beta-d2, N-acetyl-DOPA, alpha-methyl-DOPA, dopamine, adrenaline, noradrenaline, catechol, 1,2-dihydroxybenzoic acid (DHBA), and 4-tert-butylcatechol (TBC), produce a number of Cr(V) electron paramagnetic resonance (EPR) signals. These species are of interest in relation to the potential role of oxidized proteins and amino acids in Cr-induced cancers. With excess organic ligand, all of the substrates yield Cr species with signals at g(iso) approximately 1.972 (Aiso(53Cr) > 23.9 x 10(-4) cm(-1)). These are similar to signals reported previously but have been reassigned as octahedral Cr(V) species with mixed catechol-derived ligands, [CrV(semiquinone)2(catecholate)]+. Experiments with excess K2Cr2O7 show complex behavior with the catecholamines and TBC. Several weak Cr(V) signals are detected after mixing, and the spectra evolve over time to yield relatively stable substrate-dependent signals at g(iso) approximately 1.980. These signals have been attributed to [Cr(O)L2](L = diolato) species, in which the Cr is coordinated to two cyclized catecholamine ligands and an oxo ligand. Isotopic labeling studies with DOPA (ring or side chain deuteration or enrichment with 15N), and simulation of the signals, show that the superhyperfine couplings originate from the side chain protons, confirming that the catecholamine ligands are cyclized. At pH 3.5, a major short-lived EPR signal is observed for many of the substrates at g(iso) approximately 1.969, but the species responsible for this signal was not identified. Several other minor Cr signals are detected, which are attributed (by comparison with isoelectronic V(IV) species) to Cr(V) complexes coordinated by a single catecholamine ligand (and auxiliary ligands e.g. H2O), or to [Cr(O)L2]- (L = diolato) species with a sixth ligand (e.g. H2O). Addition of catalase or deoxygenation of the solutions did not affect the main EPR signals. When the substrates were in excess (pH > 4.5), primary and secondary (cyclized) semiquinones were also detected. Semiquinone stabilization by Zn(II) complexation yielded stronger EPR signals (g(iso) approximately 2.004).

Oxo-vanadium as a spin probe for the investigation of the metal coordination environment of imidazole glycerol phosphate dehydratase

Imidazole glycerol phosphate dehydratase (IGPD) catalyses the dehydration of imidazole glycerol phosphate to imidazole acetol phosphate, an important late step in the biosynthesis of histidine. IGPD, isolated as a low molecular weight and inactive apo-form, assembles with specific divalent metal cations to form a catalytically active high molecular weight metalloenzyme. Oxo-vanadium ions also assemble the protein into, apparently, the same high molecular weight form but, uniquely, yield a protein without catalytic activity. The VO 2+ derivative of IGPD has been investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopy. The spin Hamiltonian parameters indicate the presence of multiple 14N nuclei in the inner coordination sphere of VO 2+ which is corroborated by ENDOR and ESEEM spectra showing resonances attributable to interactions with 14N nuclei. The isotropic superhyperfine coupling component of about 7 MHz determined by ENDOR is consistent with a nitrogen of coordinated histidine imidazole(s). The ESEEM Fourier-transform spectra further support the notion that the VO 2+ substituted enzyme contains inner-sphere nitrogen ligands. The isotropic and anisotropic 14N superhyperfine coupling components are similar to those reported for other equatorially coordinated enzymatic histidine imidazole systems. ESEEM resonances from axial 14N ligands are discussed.

Identification of dimethylbenzimidazole axial coordination and characterization of (14)N superhyperfine and nuclear quadrupole coupling in Cob(II)alamin bound to ethanolamine deaminase in a catalytically-engaged substrate radical-Cobalt(II) biradical state.

Cobalt(II)-(14)N superhyperfine and (14)N nuclear quadrupole couplings in cryotrapped free and ethanolamine deaminase-bound cob(II)alamin have been characterized in the disordered solid state by using X-band electron spin-echo envelope modulation (ESEEM) spectroscopy. Enzyme-bound cob(II)alamin was cryotrapped after formation by substrate-initiated, thermally activated cleavage of the cobalt-carbon bond of adenosylcobalamin. Free dimethylbenzimidazole axial base-on cob(II)alamin was formed by photolysis of the corresponding adenosylcobalamin and cryotrapped in glycerol-aqueous glass. Three-pulse ESEEM experiments were performed by using microwave pulse excitation at the g( perpendicular) value of Co(II) at magnetic field values of 287.0 and 345.0 mT and over a range of tau values from 227 to 1316 ns. Two common sets of (14)N features are distinguished in the ESEEM spectra. One set is assigned to the remote (N1) nitrogen in the dimethylbenzimidazole alpha-axial ligand by using two independent approaches: (a) comparison of ESEEM from cob(II)alamin with ESEEM from cob(II)inamide-ligand model compounds and (b) from the correspondence between the N1 (14)N nuclear quadrupole parameters derived from ESEEM simulations and those computed by using density functional theory. The second set is assigned to the corrin ring (14)N nuclei. The results identify the coenzyme's on-board dimethylbenzimidazole moiety as the alpha-axial ligand to cob(II)alamin in ethanolamine deaminase in the substrate radical-Co(II) biradical catalytic intermediate state. Thus, Co(II) is a pentacoordinate, alpha-axial liganded complex during turnover. We infer that dimethylbenzimidazole is also the alpha-axial ligand to the intact coenzyme in the resting enzyme. A 14% increase in the isotropic hyperfine coupling of the remote dimethylbenzimidazole (14)N nucleus in enzyme-bound versus free base-on cob(II)alamin shows an enhanced delocalization of unpaired spin density from Co(II) onto the axial ligand, which would contribute to the acceleration of the cobalt-carbon bond cleavage rate in situ.

Optical Spectroscopy of Mn2+ Ions in CaF2

Excitation and emission spectra of Mn2+ ions in calcium fluoride have been studied using the photoluminescence technique. An analysis of the positions of the Mn2+ crystal-field transitions with aid of the model developed by Curie et al. was performed to determine the relevant crystal-field parameters and the covalency parameters f_s and fp of the Mn2+ ions entering substitutionally for Ca2+ ions in an Oh symmetry cubic environment. The As isotropic and Ap anisotropic superhyperfine coupling constants of Mn2+ in CaF2 are also calculated and compared with those previously measured through the use of EPR technique.

An Electron Spin Resonance Spin-trapping Investigation of the Free Radicals Formed by the Reaction of Mitochondrial Cytochromec Oxidase with H2O2

The reaction of purified bovine mitochondrial cytochrome c oxidase (CcO) and hydrogen peroxide was studied using the ESR spin-trapping technique. A protein-centered radical adduct was trapped by 5, 5-dimethyl-1-pyrroline N-oxide and was assigned to a thiyl radical adduct based on its hyperfine coupling constants of aN = 14.7 G and abetaH = 15.7 G. The ESR spectra obtained using the nitroso spin traps 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and 2-methyl-2-nitrosopropane (MNP) indicated that both DBNBS/.CcO and MNP/.CcO radical adducts are immobilized nitroxides formed by the trapping of protein-derived radicals. Alkylation of the free thiols on the enzyme with N-ethylmaleimide (NEM) prevented 5, 5-dimethyl-1-pyrroline N-oxide adduct formation and changed the spectra of the MNP and DBNBS radical adducts. Nonspecific protease treatment of MNP-d9/.NEM-CcO converted its spectrum from that of an immobilized nitroxide to an isotropic three-line spectrum characteristic of rapid molecular motion. Super-hyperfine couplings were detected in this spectrum and assigned to the MNP/.tyrosyl adduct(s). The inhibition of either CcO or NEM-CcO with potassium cyanide prevented detectable MNP adduct formation, indicating heme involvement in the reaction. The results indicate that one or more cysteine residues are the preferred reductant of the presumed ferryl porphyrin cation radical residue intermediate. When the cysteine residues are blocked with NEM, one or more tyrosine residues become the preferred reductant, forming the tyrosyl radical.

Electron Spin Echo Envelope Modulation Spectroscopy Reveals and Distinguishes Equatorial and Axial Nitrogen Ligands Bound to VO2+

ESEEM (electron spin echo envelope modulation) spectra of a series of vanadyl complexes have for the first time revealed the spectral features that arise from amine nitrogen donors trans to the vanadyl oxo moiety. The complexes [VO(H2O)ada] (1) and [VO(H2O)Hheida] (2) (H2ada, N-(2-acetamido)iminodiacetic acid; H3heida, N-(2-hydroxyethyl)iminodiacetic acid) have recently been synthesized and crystallographically characterized (Hamstra, B. J.; Houseman, A. L. P.; Colpas, G. J.; Kampf, J. W.; LoBrutto, R.; Frasch, W. D.; Pecoraro, V. L. Inorg. Chem. 1997, 36, 4866−4874). In each structure, the ligand is disposed so that the tertiary nitrogen is bound trans to the vanadyl oxo. In solution, the structural integrity of these compounds is maintained as observed by UV/vis and EPR spectroscopies. ESEEM spectra of 1, 2, and [VO(H2O)nta]- (3) (H3nta, nitrilotriacetic acid) reveal that the superhyperfine coupling constants for axial amines are ≅1.3 MHz as opposed to the ≅5.0 MHz superhyperfine couplings typically observed for equatorially coordinated amines. The data indicate that not only can axial nitrogen donors to VO2+ be detected, but they may also be distinguished from nitrogen donors in the equatorial plane of the vanadyl ion. The implications of this study with respect to understanding the coordination environment of VO2+ in the vanadyl-substituted chloroplast F1-ATPase (CF1) and the reduced, inactive form of vanadium bromoperoxidase (VBrPO) are discussed.

Evidence for axial coordination of 5,6-dimethylbenzimidazole to the cobalt atom of adenosylcobalamin bound to diol dehydratase.

It was demonstrated by electron paramagnetic resonance (EPR) spectroscopy that organic radical intermediates disappeared and cob(II)alamin accumulated upon suicide inactivation of diol dehydratase by 2-methyl-1,2-propanediol. The resulting EPR spectra showed that the eight hyperfine lines due to the divalent cobalt atom of cob(II)alamin further split into triplets by the superhyperfine coupling to the 14N nucleus. Essentially the same superhyperfine splitting of the octet into triplets was observed with [14N]- and [15N]apoenzyme. When the adenosyl form of [14N2]- and [15N2]imidazolyl analogues of the coenzyme [Toraya, T., and Ishida, A. (1991) J. Biol. Chem. 266, 5430-5437] was used with unlabeled apoenzyme, the octet showed superhyperfine splitting into triplets and doublets, respectively. Therefore, it was concluded that cobalamin is bound to this enzyme with 5,6-dimethylbenzimidazole coordinating to the cobalt atom. This conclusion is consistent with the fact that the consensus sequence forming part of a cobalamin-binding motif, conserved in methionine synthase and some of the other cobalamin enzymes, was not found in the deduced amino acid sequences of the subunits of diol dehydratase. Adenosylcobinamide methyl phosphate, a coenzyme analogue lacking the nucleotide moiety, underwent cleavage of the cobalt-carbon bond upon binding to the enzyme in the presence of substrate, forming a cob(II)inamide derivative without nitrogenous base coordination, as judged by EPR and optical spectroscopy. Therefore, this analogue may be a useful probe for determining whether the replacement of the 5, 6-dimethylbenzimidazole ligand by a histidine residue takes place upon binding of cobalamin to proteins.

Mapping the unpaired electron density in [Pt(bipy)L2]- (bipy=2,2′-bipyridine; L=Cl-, CN-, 13CN-): A combined EPR–ENDOR–theoretical study

The X-band EPR and electron–nuclear double resonance (ENDOR) spectra of the formally , 17-electron radical anions [Pt(bipy)L2]− (bipy = 2,2′-bipyridine; L = Cl−, CN−, 13CN−) are reported. The 195Pt (I = 1/2, 34%) anisotropic hyperfine matrices are analysed in terms of both 5d and 6p contributions to the singly occupied molecular orbitals (SOMOs). For L = Cl− there is 5.0% 5dyz and 7.6% 6pz character, for L = CN−there is <1% 5dyz and 13.1% 6pz. The bulk of the unpaired electron density is delocalised about the bipy π* system and ENDOR spectra reveal the superhyperfine couplings to the 1H, 14N and 13C (L = 13CN−) nuclei. The unpaired electron densities in the 14N 2pz orbitals contributing to the π* systems are calculated to be 12% and 4% for L = Cl− and CN−, respectively. The cyanide π-orbitals make little contribution to the SOMO for [Pt(bipy)(CN)2]−. There is a good agreement with density functional theory (DFT) MO calculations on the radical anions. DFT calculations also show the atomic orbital compositions of the lowest unoccupied (LU) MO of [Pt(bipy)L2] and the SOMO of [Pt(bipy)L2]− to be virtually identical.

EPR Investigation of the Electronic Structure of Mononuclear Copper(I) Nitric Oxide Adduct Formed upon Low-Pressure Adsorption of NO onto Cu/ZSM-5 Zeolite

The adsorption of NO onto copper-exchanged ZSM-5 zeolite has been studied by EPR spectroscopy. A distinct Cu+−NO species possessing an end-on η1 bent structure has been identified. Its EPR spectrum with a well-defined superhyperfine coupling with 63,65Cu nuclei and hyperfine coupling with 14N was interpreted in terms of completely anisotropic g and hyperfine tensors with noncoincident axes (monoclinic symmetry). The spin Hamiltonian parameters of this adduct have been analyzed in detail, leading to a semiquantitative molecular orbital correlation picture of the complex. The model developed shows that the unpaired electron resides mainly on the angularly coordinated NO and the copper superhyperfine structure arises from delocalization of the unpaired electron density onto Cu a‘ orbitals (3dz2, 3dxz, and 4s). The total spin density on copper is found to be equal to 0.2 and is shared among 3dz2 (0.079), 3dxz (0.021), and 4s (0.1) orbitals. The remaining part of the unpaired electron density is localized on n...

Evaluation of the distance between the hydrogen atom and hydrogen molecule in the tunneling reaction HD + D → H + D 2 in an argon matrix at 20 K

A hydrogen atom-hydrogen molecule pair entirely isolated in an Ar matrix has been investigated by high-resolution ESR spectroscopy from 4.2 to 23 K. A doublet ESR spectrum of the hydrogen atom was separated by 51.18 mT and further split isotropically into nine lines with superhyperfine coupling constants of ca. 0.06 mT and peak intensities of 1:1:2:1:2:1:2:1:1 in a solid Ar matrix containing HD (3.3 mol%) at 20 K. The superhyperfine structure can be attributed to HHD and HD 2 pairs. The HD 2 pair is a product of the tunneling reaction HD + D → HD 2. The mean distance between the hydrogen atom and the hydrogen molecule in the tunneling reaction in the Ar matrix at 20 K is experimentally determined to be 1.73 Å.

CuIIlocation and adsorbate interaction in CuII-exchangedsynthetic Na-omega gallosilicate EPR and electron spin echo modulation studies

The location of CuII and its interaction with adsorbates in CuII-exchanged synthetic Na-omega gallosilicate have been studied by EPR and electron spin echo modulation (ESEM) spectroscopies. These results are compared with those of CuII-exchanged Na-omega aluminosilicate and also those of L and offretite gallosilicates of similar channel type and size, and the differences are discussed. In general, similar results to those for CuNa-omega gallosilicate are obtained for CuNa-omega aluminosilicate. It is concluded that, in fresh hydrated omega material, CuII is in a main channel coordinating to three water molecules and to framework oxygens in the main channel wall. A minor CuII diaquo species not seen in the gallosilicate is observed in the aluminosilicate. Upon evacuation at increasing temperature, CuII moves from the main channel to a gmelinite cage. Dehydration at 410°C produces one CuII species located in a six-ring window of a gmelinite cage, based on a lack of broadening of its EPR lines by oxygen. In L and offretite gallosilicates, there is evidence for back migration of CuII from a hexagonal prism into a main channel to coordinate with adsorbates. However, in omega the back migration from a gmelinite cage to a main channel seems to be blocked, as shown by very slow changes in the EPR spectra and differing coordination numbers for methanol, ethanol and propanol to CuII when alcohols are adsorbed. CuII does not form a complex with propanol or larger adsorbates in omega gallosilicate. It is suggested that, in omega, small adsorbates must diffuse into a gmelinite cage where CuII is located, to form CuII–adsorbate complexes. The slow changes in the EPR spectra correspond to the time for adsorbate diffusion into a gmelinite cage. CuII interacts with one molecule each of ethylene and acetonitrile, based on EPR and ESEM analyses. CuII forms a square-planar complex containing four molecules of ammonia, based on resolved nitrogen superhyperfine coupling.

Electron-nuclear double-resonance analysis of diatomic sulfur and selenium defects in NaBr and NaI.

In this work, a single-crystal electron-nuclear double-resonance (ENDOR) study of NaBr:${\mathrm{S}}_{2}^{\mathrm{\ensuremath{-}}}$, NaI:${\mathrm{S}}_{2}^{\mathrm{\ensuremath{-}}}$, and NaBr:${\mathrm{Se}}_{2}^{\mathrm{\ensuremath{-}}}$ is presented. These defects have in common that the paramagnetic p lobes are parallel to the [001] axis. For all cases, the angular variation of two sets of $^{23}\mathrm{Na}$ and one set of halide ENDOR transitions is investigated. The corresponding superhyperfine (SHF) and nuclear-quadrupole coupling tensors are determined. The two sodium interactions can be explained in terms of the nearest-neighboring ${\mathrm{Na}}^{+}$ ions. The halide interaction is caused by eight next-nearest-neighboring halide ions. The ENDOR results can only be explained in terms of a monovacancy model in which ${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ (X=S, Se) ion is replacing a single halide ion on a lattice site. The nearest-neighboring ${\mathrm{Na}}^{+}$ ions are found to be displaced outwards. The linewidth of the EPR signals can be simulated using the ENDOR SHF data. \textcopyright{} 1996 The American Physical Society.

ESR Spin-trapping of a Protein-derived Tyrosyl Radical from the Reaction of Cytochrome cwith Hydrogen Peroxide

The reaction of horse heart cytochrome c with hydrogen peroxide was investigated using the ESR spin-trapping technique and the nitroso spin traps 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and 2-methyl-2-nitrosopropane (MNP). The ESR spectra obtained using both spin traps were typical of an immobilized nitroxide and indicated that the adduct was a macromolecule. The intensity of the ESR spectrum corresponding to the DBNBS/*cytochrome c radical adduct was greatly enhanced by performing the reaction under anaerobic conditions, which suggested that the spin trap was competing with O2 for reaction with the radical site(s). Nonspecific proteolysis of either the DBNBS or the MNP adducts revealed isotropic three-line spectra. In addition, a high resolution ESR spectrum for the protease-treated MNP cytochrome c-derived protein radical adduct was obtained. The superhyperfine couplings detected in this spectra were identical to those detected from an authentic MNP/tyrosyl adduct. Carbon-13 labeling of the aromatic ring positions of tyrosine yielded additional hyperfine coupling, demonstrating that the radical site was definitely located on the ring of tyrosine. Mass spectrometry detected as many as four DBNBS/.cytochrome c-derived adducts from the reaction of cytochrome with H2O2. Thus, it would appear four radical sites are formed during the reaction, at least one of which is tyrosine.

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.

Asymmetric functionalisation of aza macrocycles. Syntheses, crystal structures and electrochemistry of [Ni(Bz[9]aneN3)2][PF6]2and [Pd(Bz[9]aneN3)2][PF6]2·2MeCN (Bz[9]aneN3= 1-benzyl-1,4,7-triazacyclononane)

1-Benzyl-1,4,7-triazacyclononane (Bz[9]aneN3) has been prepared and its co-ordination chemistry with NiII and PdII investigated. The crystal structure of the complex [Ni(Bz[9]aneN3)2][PF6]2 revealed six independent Ni–N distances, affording a distorted-octahedral co-ordination geometry. Four lie in the range 2.089(4)–2.123(4)Å, while the distances to the benzyl amine N-donors are significantly longer at 2.262(4) and 2.251(4)Å. The benzyl amine N-donors occupy adjacent sites at the metal centre affording a syn geometry. Cyclic voltammetry of [Ni(Bz[9]aneN3)2][PF6]2 in MeCN at a platinum electrode showed a reversible NiII–NiIII oxidation at E½=+0.76 V vs. ferrocene–ferrocenium at 235 K. This process was irreversible at higher temperatures. The ESR spectrum of [Ni(Bz[9]aneN3)2]3+ exhibits an axial signal, with g‖= 2.03 and g⊥= 2.15, consistent with a tetragonally elongated octahedral co-ordination geometry at NiIII. The crystal structure of [Pd(Bz[9]aneN3)2][PF6]2·2MeCN revealed the Pd atom on an inversion centre with the ligands bound via two N-donors from each, Pd–N 2.0588(19) and 2.0605(19)Å. The benzyl amine N-donors do not interact with the metal centre, Pd ⋯ N 3.402(4)Å. Cyclic voltammetry of [Pd(Bz[9]aneN3)2][PF6]2 in MeCN showed a reversible PdII–PdIII couple at E½=+0.36 V at 298 K. The ESR spectrum of [Pd(Bz[9]aneN3)2]3+ exhibits an axial signal, with g‖= 2.007 and g⊥= 2.117. Superhyperfine coupling (A‖= 26.9 G) is observed to two N-donors in the axial component consistent with a tetragonally elongated octahedral co-ordination geometry at the PdIII.

ENDOR investigation of S2–, SSe–and Se2–defects in NaCl

A single-crystal ENDOR study of S2–, Se2– and SSe– defects in NaCl is presented. ENDOR signals corresponding with interactions with the nearest neighbouring Na+ ions are found. For each interaction, the superhyperfine (SHF) and nuclear quadrupole coupling tensors are determined. These ENDOR results can be explained only by assuming a monovacancy model, i.e. the S2–, Se2– or SSe– ion replaces a single Cl– ion on a lattice site. Analysis of the ENDOR data also reveals that the nearest neighbouring Na+ ions are displaced with respect to their lattice position. The size of the defect ions suggests that they will be incorporated into the NaCl lattice under great strain.