EPR Parameters Research Articles

Using EPR to follow reversible dihydrogen addition to paramagnetic clusters of high hydride count: [Rh6(PCy3)6H12]+and [Rh6(PCy3)6H14]+

A combined structural/EPR/computational chemistry investigation is reported on the two paramagnetic hydrido-cluster salts [Rh(6)(PCy(3))(6)H(12)][BAr(F)(4)] and [Rh(6)(PCy(3))(6)H(14)][BAr(F)(4)], the latter being formed by reversible addition of H(2) to the former, [BAr(F)(4)](-) = [B{C(6)H(3)(CF(3))(2)}(4)](-). The solid-state structure of [Rh(6)(PCy(3))(6)H(14)][BAr(F)(4)] shows an expanded cluster core compared to previously reported [Rh(6)(PCy(3))(6)H(12)][BAr(F)(4)] indicative of the addition of hydrogen to the cluster surface. This expansion correlates well with the calculated (PH(3) replaces PCy(3)) structures. EPR measurements on [Rh(6)(PCy(3))(6)H(12)][BAr(F)(4)] indicate two isomers at low temperature, which are tentatively assigned as diastereomers that result from locked phosphine rotation and bridging hydride/semi bridging hydride tautomerism. The EPR signal disappears above 60 K which is suggested to occur due to fast Raman-type relaxation-a phenomenon consistent with the calculated small SOMO/SOMO - 1 and SOMO/LUMO gaps. For [Rh(6)(PCy(3))(6)H(14)][BAr(F)(4)] EPR measurements indicate two isomers, the proportion of which change with temperature and deuteration-one axial isomer and one rhombic isomer. DFT calculations on a number of plausible isomers give EPR parameters which fit the experimentally determined rhombic isomer to one in which there is an interstitial hydride in the cluster and thirteen hydride ligands on the surface, while the axial isomer has two dihydrogen-like ligands on the cluster surface. That these isomers lie close in energy comes from both the EPR measurements (as measured from equilibrium constants over a variable temperature range) and DFT calculations. Deuteration of the hydrides should favour the isomer with the lowest zero-point energy and this is the case, with the axial isomer (two D(2) ligands on the surface) being favoured over the rhombic.

Interplay of stereo-electronic, environmental, and dynamical effects in determining the EPR parameters of aromatic spin-probes: INDCO as a test case

An integrated computational strategy for the evaluation of reliable structures and magnetic properties of spin probes and spin labels has been extended to aromatic species. From an electronic point of view, delocalization of the unpaired electron density over aromatic moieties reduces significantly the computed nitrogen isotropic hyperfine coupling constant (A(N)) with respect to values characteristic of aliphatic nitroxides. Solvent effects in not too high polarity media are quite small, but not negligible. At this stage computed A(N) are lower than their experimental counterparts by more than 1 G. Inclusion of vibrational averaging effects by molecular dynamics simulations with a new reliable force field restores full agreement with experiment pointing out the limits of static approaches irrespective of the sophistication of the electronic quantum mechanical method. The generality and computational effectiveness of the proposed integrated approach paves the route toward a reliable analysis of the interplay of stereo-electronic, environmental, and dynamical effects in tuning the properties of large flexible magnetic systems of biological and technological interest.

Copper(ii) complex formation with a linear peptide encompassing the putative cell binding site of angiogenin

Angiogenin is one of the more potent angiogenic factors known, whose activity may be affected by the presence of copper ions. Copper(II) complexes with the peptides encompassing the putative endothelial cell binding domain of angiogenin, Ac-KNGNPHREN-NH(2) and Ac-PHREN-NH(2), have been characterized by potentiometric, UV-vis, CD and EPR spectroscopic methods. The coordination features of all the copper complex species derived by both peptides are practically the same, as predictable because of the presence of a proline residue within their aminoacidic sequence. In particular, Ac-PHREN-NH(2) is really the aminoacidic sequence involved in the binding to copper(II). Thermodynamic and spectroscopic evidence are given that side chain oxygen donor atom of glutamyl residue is involved in the copper binding up to physiological pH. EPR parameters suggest that the carboxylate group is still involved also in the predominant species [Cu(L)H(-2)], the metal coordination environment being probably formed by N(Im), 2N(-), H(2)O in equatorial plane and an oxygen atom from COO(-) in apical position, or vice versa, with the carboxylate oxygen atom in the copper coordination plane and the water molecule confined to one of the apical positions. Moreover, the comparison with the thermodynamic and spectroscopic results in the case of the copper(ii) complex species formed by the single point mutated peptide, Ac-PHRQN-NH(2), provides further evidence of the presence of carboxylate oxygen atom in the copper coordination sphere.

DFT Calculations of EPR Parameters for Copper(II)-Exchanged Zeolites Using Cluster Models

The coordination environment of Cu(II) in hydrated copper-exchanged zeolites was explored through the use of density functional theory (DFT) calculations of EPR parameters. Extensive experimental EPR data are available in the literature for hydrated copper-exchanged zeolites. The copper complex in hydrated copper-exchanged zeolites was previously proposed to be [Cu(H(2)O)(5)OH](+) based on empirical trends in tetragonal model complex EPR data. In this study, calculated EPR parameters for the previously proposed copper complex, [Cu(H(2)O)(5)OH](+), were compared to model complexes in which Cu(II) was coordinated to small silicate or aluminosilicate clusters as a first approximation of the impact of the zeolitic environment on the copper complex. Interpretation of the results suggests that Cu(II) is coordinated or closely associated with framework oxygen atoms within the zeolite structure. Additionally, it is proposed that the EPR parameters are dependent on the Si/Al ratio of the parent zeolite.

Non-iron [n]Metalloarenophanes

The past 15 years have witnessed the creation of a wealth of fascinating applications for strained [n]ferrocenophanes, in which the aromatic sandwich rings of ferrocene are covalently tethered (n denotes the number of bridging elements). By contrast, the related [n]metalloarenophanes, that is, ansa-complexes not derived from ferrocene, have been neglected for a long time. In this Account, we present the tremendous progress that has been achieved in this field, mostly over the last five years. We focus on systems that have been developed in our laboratories, namely, those based on [M(eta(6)-C(6)H(6))(2)] (where M is V, Cr, or Mo), [Mn(eta(5)-C(5)H(5))(eta(6)-C(6)H(6))], and [M(eta(5)-C(5)H(5))(eta(7)-C(7)H(7))] (where M is V or Cr). We begin by examining the synthetic precursors to [n]metalloarenophanes, the selectively 1,1'-dilithiated sandwich complexes. These species can be isolated and characterized both in the solid state and in solution, and an appreciation of their structural properties is essential for the controlled synthesis of strained [n]metalloarenophanes. About 25 different [n]metalloarenophanes (n = 1, 2) have been obtained from 1,1'-dilithiated sandwich complexes by their stoichiometric reaction with appropriate element dihalides, and most have been fully characterized. X-ray diffraction data confirm the presence of tilted structures, with the extent of the tilt depending on the number of bridging elements, their covalent radii, and the nature of the metal center. The tilt angle, alpha, between the planes of the two carbocyclic ligands represents a good measure of the amount of ring strain present in these species, ranging from 31.23 degrees for a highly strained [1]boravanadoarenophane to 2.60 degrees for the almost unstrained [2]silatrochrocenophane. The strained character of [n]metalloarenophanes is reflected in their rich and unusual reactivity. The thermodynamic driving force for most of the observed transformations is a significant reduction of molecular ring strain, as evidenced by smaller tilt angles in the products. For example, the [1]sila derivatives undergo facile oxidative addition of the strained Si-(arene)C bond to low-valent transition metal complexes. Catalytic reaction with Karstedt's catalyst results in the transition-metal-catalyzed ring-opening polymerization (ROP) of a [1]silatrochrocenophane, yielding a polymeric species (M(W) = 6.4 x 10(3) g mol(-1)). The transition-metal-catalyzed ROP of a paramagnetic [1]silavandoarenophane provides a rare example of a well-characterized macromolecule (M(W) > or = 2.8 x 10(4) g mol(-1)) containing spin-active metal centers in the main chain. The B-B bond of the [2]borametalloarenophanes was found to be particularly susceptible to further functionalizations. Facile oxidative addition was observed, and the resulting [3]diboraplatina derivatives could be successfully employed in the diboration of alkynes to form ansa-bis(boryl)alkenes. This transformation was also accomplished directly from the [2]borametalloarenophanes, that is, a diboration of alkynes under both homogeneous and heterogeneous catalysis conditions. Similarly, the stoichiometric diboration of the N=N double bond of azobenzene by a [3]diboraplatinachromoarenophane is possible. The Si-Si bond in [2]silachromoarenophanes can also be used in further derivatizations, although higher temperatures are required. The bis-silylation of propyne proceeded via palladium mediation under homogeneous conditions to yield ansa-bis(silyl)alkenes. Finally, we discuss the electronic properties of [n]metalloarenophanes developed in our laboratories, revealing correlations between selected NMR, EPR, and UV-visible parameters and molecular distortion.

Theoretical studies of the optical and EPR spectra for Fe3+ ion in the MF3:Fe3+ (M=Al, Ga) systems

By analyzing the EPR spectra of Fe 3+ ion in the fluorinde glasses, the local lattice structures around impurity Fe 3+ ion in MF 3 :Fe 3+ (M=Al, Ga) systems have been studied by means of diagonalizing the complete energy matrices of the electron–electron repulsion, the ligand-field and the spin–orbit coupling for a d 5 configuration ion in a trigonal ligand-field. Both the second-order and fourth-order EPR parameters D and ( a − F ) are taken simultaneously in the structural investigation. The results indicate that the local lattice structure around octahedral Fe 3+ center has an expansion distortion for Fe 3+ in MF 3 :Fe 3+ (M=Al, Ga). The expansion distortion may be ascribed to the fact that the radius of Fe 3+ ion is larger than that of Al 3+ ion and Ga 3+ ion, and the Fe 3+ ion will push the fluoride ligands upwards and downwards, respectively. The local lattice structure parameters R= 1.927 A, θ =55.538° for Fe 3+ in AlF 3 :Fe 3+ and R =1.931 A, θ =56.09° for Fe 3+ in GaF 3 :Fe 3+ are determined, respectively, and the EPR spectra of the MF 3 :Fe 3+ (M=Al, Ga) systems are satisfactorily explained.

Global Structure–Activity Analysis in Drug Development Illustrated for Active Cu/Zn Superoxide Dismutase Mimics

AbstractThe analysis of the spectroscopic and electrochemical properties of all reported classes of Cu/Zn superoxide dismutase mimics (SODm) makes it possible to distinguish the geometric specificity of highly active compounds. A linear relationship with EPR parameters characterises the geometry of the first coordination sphere around the metal, which favours catalytic activity. Combined with the appropriate redox potential values, the model of geometric conformation of highly active Cu/Zn SODm serves to streamline further synthetic efforts. This global analysis represents a new approach to treat all classes of compounds intended for a specific biological activity in order to obtain general features that could orient the process of drug discovery.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Validation of broken-symmetry density functional methods for the calculation of electron paramagnetic resonance parameters of dinuclear mixed-valence MnIVMnIIIcomplexes

. The EPR parameters of a series of dinuclear manganese(III,IV) complexes with mono(μ-oxo), bis(μ-oxo), (μ-oxo)(μ-carboxylato), bis(μ-oxo)(μ-carboxylato), and (μ-oxo)bis(μ-carboxylato) bridges were studied by broken-symmetry density functional (DFT) methods. The influence of the exchange-correlation functional on the agreement with experiment has been evaluated systematically for g tensors;55Mn,14N, and1H hyperfine coupling tensors; and Heisenberg exchange couplings.14N and1H hyperfine couplings,55Mn hyperfine anisotropies, g tensors, and exchange couplings are well described by hybrid functionals with moderate exact-exchange admixtures such as B3LYP. The isotropic55Mn hyperfine couplings require larger exact-exchange admixtures. However, the errors of the B3LYP calculations are systematic and may be corrected by a constant scaling factor, providing good predictive power for a wide range of EPR parameters with broken-symmetry DFT and standard functionals. The influence of terminal and bridging ligands on structure, spin-density distributions, and EPR parameters are evaluated systematically. Computed hyperfine and g tensors are not covariant to each other. This may have consequences for spectra simulations. The nature of the broken-symmetry state and the origin of its spin contamination were analyzed by an expansion into restricted determinants, based on paired orbitals.

Investigations on the defect structure and the EPR parameters for AgBr:Ir2+

The defect structure and the EPR parameters (the g factors, the hyperfine structure constants and the superhyperfine parameters) for the tetragonal Ir(2+) center in AgBr are theoretically investigated using the perturbation formulas of these parameters for a 5d(7) ion in tetragonally elongated octahedra. In the calculations, the contributions from the ligand orbital and spin-orbit coupling interactions and the local lattice (elongation) distortion due to the Jahn-Teller effect are taken into account. Related molecular orbital coefficients and the ligand unpaired spin densities are determined quantitatively using a cluster approach. The impurity center is attributed to the substitutional [IrBr(6)](4-) cluster on host Ag(+) site, which suffers the relative elongation of about 0.08 angstrom along C(4) axis due to the Jahn-Teller effect. The calculated EPR parameters based on the above Jahn-Teller elongation show good agreement with the observed values.

Local Structure Determination of Tetragonal Cr2+ Center in CdS Semiconductor

Abstract Recently, many studies for the local structure of 3d5 ions in octahedrally coordinated compounds are made by simulating the EPR parameters on the basis of the complete energy matrix. However, for the 3d4 ions in tetrahedrally coordinated compounds, the studies are relatively fewer. In this work, by diagonalizing the complete energy matrix for a d4 configuration in a tetragonal ligand-field within a strong-field representation, the local structure around Cr2+ in CdS crystal is studied. Our results show that there exists a compression distortion in the local lattice structure. From our calculations, the distortion parameters ΔR = −0.022 A° and Δθ = −1.410° are obtained.

Theoretical studies of EPR g ‐factors and microstructure of the two tetragonal palladium centers in silver chloride

AbstractThe EPR g ‐factors g∥, g⊥ for two tetragonal Pd+ impurity centers (i.e. Pd+ I and II centers) in AgCl crystal are calculated by using the high‐order perturbation formulas of EPR parameters for the 4d9 ion in tetragonal symmetry with the ground state |dx 2–y 2〉. Based on the calculations, the defect models for the two centers are suggested as a substitutional Ag+ ion with (PdCl6)5– or the [(PdCl6)5– – VAg+] cluster. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigations on the EPR Parameters for the Square Planar Cu2+ Centers in K2PdX4 (X = Cl, Br)

The EPR parameters g factors g||, g⊥ and the hyperfine structure constants A|| and A⊥ for the square planar Cu2+ centers in K2-PdX4 (X = Cl, Br) are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion under tetragonally elongated octahedra. In these formulas, not only the contributions from the conventional crystal-field (CF) mechanism, but also those from the charge-transfer (CT) mechanism are taken into account. The related molecular orbital coefficients are uniformly determined from the cluster approach, and the tetragonal field parameters Ds and Dt are obtained from the superposition model and the local structures of the systems. Based on only one adjustable parameter, the present results are in reasonable agreement with the observed values. Importance of the charge-transfer contributions is more significant for ligand Br than that for Cl due to the stronger covalency and much larger spin-orbit coupling coefficient of the former.

Electron paramagnetic resonance of V4+ ions in lanthanum-aluminosilicate glasses

The EPR and optical absorption spectra of V 4+ ions and their relation with the structural features of glasses in the system La 2 O 3 - Al 2 O 3 - SiO 2 are investigated. Three characteristic ranges of the V 4+ EPR parameters are established: I) A ∥ ≈ 167 × 10 -4 cm -1 , g ∥ = 1.937 - 1.938; II) A ∥ ≈ (163 - 164) × 10 -4 cm -1 , g ∥ = 1.939 - 1.940; III) A ∥ ≈ (156 - 159) × 10 -4 cm -1 , g ∥ = 1.945. It is shown that in the region I with low La 2 O 3 content the lanthanum is predominately a modifying ion, and in glass with a high La 2 O 3 concentration lanthanum gradually occupies a site in the glass, bonding silicon-oxygen and aluminum-oxygen tetrahedra, i.e., it becomes a glass former even though its ionic radius is large. Region III glasses with high La 2 O 3 content possess a high softening temperature and anomalously high thermal conductivity (about 12.6 W/(m ∙ K)).

Solvent effect on EPR, molecular and electronic properties of semiquinone radical derived from 3,4-dihydroxybenzoic acid as model for humic acid transient radicals: High-field EPR and DFT studies

The g-tensor components and isotropic hyperfine constants of solvent-affected radical anion from 3,4-dihydroxybenzoic acid were computed at UBP86/EPR-II level and correlated with the molecular and electronic structures. The role of solvent was considered in two ways: by using a conductor-like screening model (COSMO) and by adding water molecules, hydrogen bonded to the radical. To reach the best efficiency of the calculations, the theoretical EPR parameters were compared with experimental derived from high-field (406.4 GHz) and X-band spectra. Inclusion of water molecules in the first hydration sphere of the radical appears to be mandatory for correct prediction of its properties.

Electron paramagnetic resonance parameters of Mn4+ ion in h-BaTiO3 crystal from a two-mechanism model

The EPR parameters (g factors g (aEuro-), g (aSyen) and zero-field splitting D) of Mn(4+) ion in h-BaTiO(3) crystal are calculated from the complete high-order perturbation formulas based on a two-mechanism model for the EPR parameters of 3d (3) ions in trigonal symmetry. In the model, not only the widely used crystal-field mechanism, but also the charge-transfer mechanism (which is not considered in crystal-field theory) are included. The calculated results are in reasonable agreement with the experimental values. The relative importance of charge-transfer mechanism to EPR parameters and the defect structure of Mn(4+) centre in h-BaTiO(3) crystal obtained from the calculations are discussed.

Structural dynamics of CaMKII activation

The ubiquitously expressed calcium/calmodulin dependent protein kinase II (CaMKII) functions as a transducer of calcium (Ca2+) signaling by responding to the amplitude, duration, and frequency of Ca2+ transients. During periods of elevated Ca2+, CaMKII is activated by calcium-calmodulin (Ca2+/CaM) binding. A subsequent autophosphorylation at Thr286 allows for Ca2+-independent activity and endows this enzyme with a conformational memory of prior activation. CaMKII activity is regulated by a myriad of factors including CaM binding, autophosphorylation, and catalytic-regulatory domain interactions referred to as autoinhibition. While these variables have been linked to CaMKII function, the underlying structural and dynamic framework of activation and conformational memory is poorly understood. Here we utilize site-directed spin labeling and electron paramagnetic resonance (SDSL-EPR) to explore the conformational changes associated with CaMKII activation and conformational memory. EPR parameters were collected for the regulatory domain where CaM binding and autophosphorylation sites are located. Our results indicate the regulatory domain undergoes significant structural changes between several discrete conformations dependent on autophosphorylation and CaM binding. The CaM binding region is flexible in the apo state but has an induced rigidity in the presence of Ca2+/CaM indicative of a binding event. Investigation of the regulatory domain outside the CaM binding region revealed an increase in protein backbone dynamics with a Thr286Glu autophosphorylation mimic and/or in the presence of Ca2+/CaM. This data provides a structural and dynamic perspective consistent with the current biochemical activation model where CaM binding disrupts autoinhibition by disengaging regulatory and catalytic domains. We predict the enhanced flexibility facilitates Ca2+/CaM binding and may play a role in Ca2+ independent activity. The adjacent regulatory loop showed similar flexibility suggesting this region functions as a hinge between regulatory and catalytic domains allowing for release and reinstatement of autoinhibition.

Copper(II) complexes with peptide fragments encompassing the sequence 122–130 of human doppel protein

Copper(II) complexes of the peptide fragment (Dpl122-130) encompassing the sequence 122-130 of human doppel protein were characterized by potentiometric, UV-Visible, CD and EPR spectroscopic methods. An analogous peptide, in which the aspartate residue was substituted by an asparagine amino acid, was synthesized in order to provide evidence on the possible role of carboxylate group in copper(II) coordination. It was found that the carboxylic group is directly involved in copper(II) coordination at acidic pH, forming the CuLH(2) species with Dpl122-130. This copper(II) complex displayed EPR parameters very similar to those of the analogous complex with the whole doppel protein. At pH higher than 7, the complexes showed magnetic parameters similar to those of the major species of protein formed in the pH range 7-8, with the metal coordination environment consisting of one imidazole and three amide nitrogen atoms. The comparison of Cu-Dpl122-130 binding constant values with those of the prion peptide fragments (PrP106-114), showed that doppel peptide had a higher metal binding affinity at acidic pH whereas the prion peptide fragment binds the metal tightly at physiological pH.

Interaction of VO2+ ion with human serum transferrin and albumin

The complexation of VO(2+) ion with the high molecular mass components of the blood serum, human serum transferrin (hTf) and albumin (HSA), has been re-examined using EPR spectroscopy. In the case of transferrin, the results confirm those previously obtained, showing that VO(2+) ion occupies three different binding sites, A, B(1) and B(2), distinguishable in the X-band anisotropic spectrum recorded in D(2)O. With albumin the results show that a dinuclear complex (VO)(2)(d)HSA is formed in equimolar aqueous solutions or with an excess of protein; in the presence of an excess of VO(2+), the multinuclear complex (VO)(x)(m)HSA is the prevalent species, where x=5-6 indicates the equivalents of metal ion coordinated by HSA. The structure of the dinuclear species is discussed and the donor atoms involved in the metal coordination are proposed on the basis of the measured EPR parameters. Two different binding modes of albumin can be distinguished varying the pH, with only one species being present at the physiological value. The results show that the previously named "strong" site is not the N-terminal copper binding site, and some hypothesis on the metal coordination is discussed, with the (51)V A(z) values for the proposed donor sets obtained by DFT (density functional theory) calculations. Finally, preliminary results obtained in the ternary system VO(2+)/hTf/HSA are shown in order to determine the different binding strength of the two proteins. Due to the low VO(2+) concentration used, the recording of the EPR spectra through the repeated acquisition of the weak signals is essential to obtain a good signal to noise ratio in these systems.

Electronic properties of pentacoordinated heme complexes in cytochrome P450 enzymes: search for an Fe(i) oxidation state

The cytochrome P450s are versatile enzymes that catalyze a range of monoxygenation reactions. Their catalytic cycle includes molecular oxygen binding and two reduction and protonation steps to create the active species, Compound I. In an anaerobic environment, however, only two reduction steps starting from the resting state can take place. Currently, very little information is known on this doubly-reduced species; therefore, we have performed a detailed density functional theory (DFT) and combined quantum mechanics/molecular mechanics (QM/MM) study on this complex. In principle, the doubly-reduced pentacoordinated heme can exist in two possible electronic configurations, namely an Fe(I) with closed-shell heme or an Fe(II) coupled to a heme anion radical [Fe(II) Por(-*)]. Our calculations show that there are several close-lying spin states with a [Fe(II) Por(-*)] configuration and these states are much lower in energy than the alternative [Fe(I) Por(0)] situation. We have calculated spectroscopic parameters of the lowest lying sextet spin state, including IR spectra and EPR parameters.

Copper(ii) complexes of prion protein PEG11-tetraoctarepeat fragment: spectroscopic and voltammetric studies

Spectroscopic (UV-Vis and EPR) and voltammetric studies have been carried out on the copper(II) complexes with the Ac-PEG11-(PHGGGWGQ)4-NH2 (L) polypeptide. In the ratios Cu : L 3 : 1 and 4 : 1, the two [Cu3(L)H(-6)] and [Cu4(L)H(-8)] complex species have been characterized at neutral pH values. All the copper atoms occupy similar coordination sites formed by imidazole, peptidic nitrogen atoms and carbonyl oxygen atoms in a square base pyramidal geometry. Voltammetric measurements on these systems point out the cooperativity in the electron transfer processes among the copper(II) sites during their reduction. NO interaction with these polynuclear copper species is characterized by the reduction of the copper sites through the formation of two different intermediate complex species. When an excess of the Ac-PEG11-(PHGGGWGQ)4-NH2 ligand is considered, frozen solution EPR parameters and UV-Vis spectroscopic data identify the [Cu(N(im))4]2+ chromophore, which does not interact with NO.