EPR Spectroscopic Studies Research Articles

EPR spectroscopy studies on the structural transition of nitrosyl hemoglobin in the arterial-venous cycle of DEANO-treated rats as it relates to the proposed nitrosyl hemoglobin/nitrosothiol hemoglobin exchange.

In vivo studies show a dynamic cycle in which alpha-nitrosylated hemoglobin is mainly in the relaxed state in arterial blood of rats treated with 2-(N,N-diethylamino)-diazenolate-2-oxide, but converts mainly to the tense state during the arterial-venous transit. A detailed analysis shows that different electron paramagnetic resonance spectra recorded for alpha-nitrosyl hemoglobin in arterial and venous blood at 77 K originate only from a different ratio between 5- and 6-coordinate heme without any change in the concentration of nitrosyl hemoglobin. In venous blood, the five- and six-coordination equilibrium of the alpha-nitrosyl heme is shifted in favor of the 5-coordinate state (58% venous vs. 20% arterial). These results are not consistent with the recently proposed exchange of nitrosyl heme with the beta-93 nitrosothiol group of hemoglobin during the arterial-venous cycle.

Magnetic and optical studies on an S = 6 ground-state cluster [Cr12O9(OH)3(O2CCMe3)15]: determination of, and the relationship between, single-ion and cluster spin Hamiltonian parameters.

The dodecametallic Cr(III) cluster [Cr(12)O(9)(OH)(3)(O(2)CCMe(3))(15)] has a ground spin state of S = 6 characterized by the spin Hamiltonian parameters g(ZZ)() = 1.965, g(XX)() = g(YY)() = 1.960, D(S=)()(6) = +0.088 cm(-)(1), and E(S=)()(6) = 0 (where D and E are the axial and rhombic zero-field splitting parameters, respectively) as determined by multifrequency EPR spectroscopy and magnetization studies. Micro-SQUID magnetization studies reveal steps due to the fine structure of the ground state, with the spacing between the steps in excellent agreement with the D(S=)()(6) value determined by EPR. Analysis of high-resolution optical data (MCD) allows us to determine the single-ion g values and D value (= -1.035 cm(-)(1)) of the constituent Cr(III) ions directly. A vector coupling analysis demonstrates that the cluster ZFS is almost entirely due to the single-ion component. Thus, the relative orientations of the local and cluster magnetic axes can lead to a cluster ZFS of opposite sign to the single-ion value, even when this is the only significant contribution.

Synthesis by inverse emulsion pathway and characterization of conductive polyaniline–poly(ethylene- co-vinyl acetate) blends

Inverted emulsion method was employed for the synthesis of electrically conductive polyaniline–poly(ethylene- co-vinyl acetate) (EVA) blends of different compositions (30, 50, 60, 70 and 80 wt.%) using benzoyl peroxide as a novel oxidant. Polymerization of aniline was carried out in a solution of EVA dissolved in an equimolar mixture of toluene and chlorobenzene as the continuous phase with camphor sulphonic acid as the dopant. The absorption spectra, FT-IR, FT Raman, NMR and EPR spectroscopic studies suggest the presence of chemical interaction (electrostatic) between polyaniline camphor sulphonic acid salt (Pani salt) and EVA. The blends have slightly lower degradation temperatures than the polyaniline salt. The SEM studies revealed both granular and fibrillar morphology while the hot pressed films exhibit fluid like morphology. The blends show relatively higher mechanical strengths and the blend Pani(20)–EVA(80) (wt.%) gave the highest value for elongation at break.

Effects of iron limitation on the respiratory chain and the membrane cytochrome pattern of the Euryarchaeon Halobacterium salinarum.

The effects of iron limitation on the electron transport chain of the extremely halophilic Euryarchaeon Halobacterium salinarum were analyzed. When iron was growth-limiting, the respiratory rates as well as the inhibition pattern of the membranes were significantly different from membranes of iron replete cells. Changes in the availability of iron cause the formation of different respiratory pathways including different entry sites for electrons, different terminal oxidases of the respiratory chain, and drastic changes of the cytochrome composition and of the relative amounts of cytochromes. Under iron-limiting conditions, mainly low-potential cytochromes were measured. EPR spectroscopic studies revealed that the amount of proteins containing iron-sulfur clusters is reduced in membranes under iron-limiting growth conditions. Taken together, our results strongly suggest for the first time an important role of iron supply for the bioenergetics of an Archaeon.

Pulse radiolysis, electron paramagnetic resonance spectroscopy and theoretical calculations of caffeic acid oligomer radicals

Seven representative compounds isolated from Salvia officinalis, among them caffeic acid, the dimer rosmarinic acid and oligomers of caffeic acid, were investigated with regard to their antioxidant potential both expressed by the radical scavenging activity and the stability and structure of the intermediate radicals. Pulse-radiolytic investigation revealed very high rate constants with both hydroxyl and azide radicals. Evidence from kinetic modelling calculations suggested unusual complex behavior due to the presence of both O 4- and O 3-semiquinones and — in two cases — formation and decay of a hydroxyl radical adduct at the vinyl side chain. EPR spectroscopy studies, which included dihydrocaffeic acid as a model for the saturated side chains of the oligomers, confirmed that the radical structures after oxidation in slightly alkaline solutions are representing dissociated O 4-semiquinones. While according to calculations based on hybrid density-functional theory the other radical structures are valid intermediates, they cannot be observed except by pulse radiolysis due to their fast decay.

Paramagnetic resonance in imidazolate-bridged homobinuclear (copper–copper) and heterobinuclear (copper–zinc) complexes

Bridged homobinuclear (copper–copper) and heterobinuclear (copper–zinc) complexes of diethylenetriamine have been prepared with 2-methyl-imidazole as bridging ligand. EPR spectra of the polycrystalline complexes have been studied at room temperature and also at liquid nitrogen temperature. Low temperature EPR and electronic spectroscopic studies of 50% aqueous DMSO of [(dien)Cu-(MeIm)–Zn(dien)] 3+ solutions show the imidazolate bridged complex to exist mainly over the pH range ∼7.0<pH<10.0. At low pH the 2-MeImH + ion and mononuclear copper and zinc complexes are formed. Above pH >10.0 hydroxide ion splits the imidazolate bridge.

EPR study of substrate binding to the Mn(II) active site of the bacterial antibiotic resistance enzyme FosA: a better way to examine Mn(II).

FosA is a manganese metalloglutathione transferase that confers resistance to the broad-spectrum antibiotic fosfomycin, (1R,2S)-epoxypropylphosphonic acid. The reaction catalyzed by FosA involves the attack by glutathione on fosfomycin to yield the product 1-(S-glutathionyl)-2-hydroxypropylphosphonic acid. The enzyme is a dimer of 16 kDa subunits, each of which harbors one mononuclear Mn(II) site. The coordination environment of the Mn(II) in the FosA x Mn(2+) complex is composed of a glutamate and two histidine ligands and three water molecules. Here we report EPR spectroscopic studies on FosA, in which EPR spectra were obtained at 35 GHz and 2 K using dispersion-detection rapid-passage techniques. This approach provides an absorption envelope line shape, in contrast to the conventional (slow-passage) derivative line shape, and is a more reliable way to collect spectra from Mn(II) centers with large zero-field splitting. We obtain excellent spectra of FosA bound with substrate, substrate analogue phosphate ion, and product, whereas these states cannot be studied by X-band, slow-passage methods. Simulation of the EPR spectra shows that binding of substrate or analogue causes a profound change in the electronic parameters of the Mn(II) ion. The axial zero-field splitting changes from [D] = 0.06 cm(-1) for substrate-free enzyme to 0.23 cm(-1) for fosfomycin-bound enzyme, 0.28 (1) cm(-1) for FosA with phosphate, and 0.27 (1) cm(-1) with product. Such a large zero-field splitting is uncommon for Mn(II). A simple ligand field analysis of this change indicates that binding of the phosphonate/phosphate group of substrate or analogue changes the electronic energy levels of the Mn(II) 3d orbitals by several thousand cm(-1), indicative of a significant change in the Mn(II) coordination sphere. Comparison with related enzymes (glyoxalase I and MnSOD) suggests that the change in the coordination environment on substrate binding may correspond to loss of the glutamate ligand.

Resonance Raman spectroscopic studies in copper reconstituted and hybrid hemoglobins: probe into subunit heterogeneity.

Copper reconstituted hemoglobin (CuHb), copper containing T-state hybrid hemoglobins like alpha2(Ni)beta2(Cu), and alpha2(Cu)beta2(Ni), and intermediate R-state hybrids like alpha2(CO-Fe)beta2(Cu) and alpha2(Cu)beta2(Fe-CO) are studied using resonance Raman (RR) spectroscopy at two different excitation wavelengths. The high frequency RR region in CuHb indicates the presence of both 4- and 5-coordinate forms of Cu(II). In hybrid Hbs, the presence of two distinct metal ion environments within one particular subunit is evident. This is also consistent with previous findings using EPR spectroscopy and sulfydryl reactivity studies on these hybrid Hbs. The low frequency RR region on these copper derivatives of HbA further suggests the existence of two different heme moieties within the subunit.

EPR and NMR spectroscopic studies of [MoL2(MeC identical to CMe)Cp]z (L = P-donor ligand, z = 0 and 1): fluxionality in a metal-alkyne redox pair.

Variable temperature NMR and EPR spectroscopic studies provide rates and activation parameters for alkyne rotation and oscillation, respectively, in the fluxional redox pair [Mo(P(OMe)3)2(MeC identical to CMe)Cp][BF4] (diamagnetic) and [Mo(P(OMe)3)2(MeC identical to CMe)Cp] (paramagnetic).

Crystal growth, EPR and site-selective laser spectroscopy of Gd 3+-activated LiCaALF 6 single crystals

Ce 3+-activated LiCaAlF 6 (LiCAF) and LiSrAlF 6 (LiSAF) single crystals are the most prospective active media for directly pumped UV solid-state tunable lasers. Due to the geterovalent activation nature of these crystals, crystal spectroscopic properties as well as their laser efficiency strongly depend on the actual crystal growth conditions. In order to establish the growth-related peculiarities of activator center formation in LiCAF, concerted crystal growth, EPR and site-selective laser spectroscopic studies were performed. For EPR and optical spectroscopic studies, the Gd 3+-“probe” activation of LiCAF crystal was used here instead of much harder to interpret Ce 3+ activation. The obtained results indicate that up to three types of structurally distinct Gd 3+ centers are formed in the LiCAF crystals with their relative concentrations changing depending on the crystal growth conditions. The results are being applied for growing Ce 3+-activated LiCAF crystals with the desired luminescence and laser performance.

The Generation of Organic Radical Cations: A Guide to Current Practice for EPR Spectroscopic Studies

A wide variety of methods is now available, each with its particular advantages and disadvantages, for generating radical cations for EPR or other spectroscopic studies in fluid solution or in matrix. This field is critically surveyed, together with recent developments in the understanding of the mechanisms of these reactions.

PH-dependent investigations of vanadium(V)-peroxo-malate complexes from aqueous solutions. In search of biologically relevant vanadium(V)-peroxo species.

The established biochemical potential of vanadium has spurred considerable research interest in our lab, with specific focus on pertinent synthetic studies of vanadium(III) with a biologically relevant, organic, dicarboxylic acid, malic acid, in aqueous solutions. Simple reactions between VCl3 and malic acid in water, at different pH values, in the presence of H2O2, led to the crystalline dimeric complexes (Cat)4[VO(O2)(C4H3O5)]2*nH2O (Cat = K+, n = 4, 1; Cat = NH4+, n = 3, 2) and K2[VO(O2)(C4H4O5)]2*2H2O (3). All three complexes were characterized by elemental analysis, FT-IR, and UV/visible spectroscopies. Compound 1 crystallizes in the monoclinic space group P2(1)/c, with a = 8.380(5) A, b = 9.252(5) A, c = 13.714(8) A, beta = 93.60(2) degrees, V = 1061(1) A3, and Z = 4. Compound 2 crystallizes in the triclinic space group P1, with a = 9.158(4) A, b = 9.669(4) A, c = 14.185(6) A, alpha = 104.81(1) degrees, beta = 90.31(1) degrees, gamma = 115.643(13) degrees, V = 1085.0(7) A(3), and Z = 2. Compound 3 crystallizes in the monoclinic space group P2(1)/c, with a = 9.123(8) A, b = 9.439(8) A, c = 10.640(9) A, beta = 104.58(3) degrees, V = 887(1) A3, and Z = 2. The X-ray structures showed that, in 1 and 2, the dimers consist of two (V(V)=O)2O2 rhombic units to which two malate ligands are attached. The ligands are triply deprotonated and, as such, they coordinate to vanadium(V), promoting a pentagonal bipyramidal geometry. In 3, the dimeric (V(V)=O)2O2 rhombic unit persists, with the two doubly deprotonated malate ligands coordinated to the vanadium(V) ions. UV/vis and EPR spectroscopic studies on the intermediate blue solutions of the synthesis reactions of 1-3 support the existence of vanadyl-containing dimeric species. These species further react with H2O2 to yield oxidation of V(IV)2O2 to V(V)2O2 and coordination of the peroxide to vanadium(V). From the collective data on 1-3, it appears that pH acts as a decisive factor in dictating the structural features of the isolated complexes. The details of the introduced structural differentiation in the reported complexes, and their potential relevance to vanadium(V) dicarboxylate systems in biological media are dwelled on.

Incorporation of stable organic radicals into cyclotriphosphazene: preparation and characterization of mono- and diradical adducts.

Stable cyclotriphosphazenes 4 and 5, incorporating one and two carbon radical centers, respectively, have been easily prepared and characterized. EPR spectroscopic studies in fluid solution at room temperature were carried out for both compounds and also for diradical 5 in frozen solvent matrixes. Spectral results are consistent with a triplet or degenerate singlet triplet ground state for 5. Reductive cyclic voltammetry shows a redox couple, being monoelectronic for 4 and bielectronic for 5.

A paramagnetic species with unique EPR characteristics in the active site of heterodisulfide reductase from methanogenic archaea.

Heterodisulfide reductase (Hdr) from methanogenic archaea is an iron-sulfur protein that catalyses the reversible reduction of the heterodisulfide (CoM-S-S-CoB) of the methanogenic thiol coenzymes, coenzyme M (H-S-CoM) and coenzyme B (H-S-CoB). In EPR spectroscopic studies with the enzyme from Methanothermobacter marburgensis, we have identified a unique paramagnetic species that is formed upon reaction of the oxidized enzyme with H-S-CoM in the absence of H-S-CoB. This paramagnetic species can be reduced in a one-electron step with a midpoint-potential of -185 mV but not further oxidized. A broadening of the EPR signal in the 57Fe-enriched enzyme indicates that it is at least partially iron based. The g values (gxyz = 2.013, 1.991 and 1.938) and the midpoint potential argue against a conventional [2Fe-2S]+, [3Fe-4S]+, [4Fe-4S]+ or [4Fe-4S]3+ cluster. This species reacts with H-S-CoB to form an EPR silent form. Hence, we propose that only a half reaction is catalysed in the presence of H-S-CoM and that a reaction intermediate is trapped. This reaction intermediate is thought to be a [4Fe-4S]3+ cluster that is coordinated by one of the cysteines of a nearby active-site disulfide or by the sulfur of H-S-CoM. A paramagnetic species with similar EPR properties was also identified in Hdr from Methanosarcina barkeri.

Infrared and EPR Spectroscopic Studies of 2-C2H2F and 1-C2H2F Radicals Isolated in Solid Argon

2-fluorovinyl radicals were generated in solid argon by solid-state chemical reactions of mobile F atoms with acetylene and its deuterated analogues. Highly resolved EPR spectra of the stabilized radicals CHF•CH, CDF•CD, CHF•CD, and CDF•CH were obtained for the first time. The observed spectra were assigned to cis-2-fluorovinyl radical based on excellent agreement between the measured (aF = 6.50, aβH = 3.86, aαH = 0.25 mT) hyperfine constants and those calculated using density functional (B3LYP) theory. Analogous experiments carried out using infrared spectroscopy yielded a complete assignment of the vibrational frequencies. An unusual reversible photochemical conversion is observed in which cis-2-fluorovinyl radicals can be partially converted to 1-fluorovinyl radicals by pulsed laser photolysis at 532 nm. Photolysis at 355 nm converts 1-fluorovinyl back to cis-2-fluorovinyl. High-resolution EPR and infrared spectra of 1-fluorovinyl were obtained for the first time. The measured hyperfine constants (aF = 13.71, aH1 = 4.21, aH2 = 1.16 mT) are in good agreement with calculated values.

Novel dinuclear manganese(III) complexes with bi- or tridentate and bridging tetradentate Schiff base ligands: preparation, properties and catalase-like function

Novel mono- and dinuclear manganese(III) complexes: [Mn III(acac)(PA) 2] ( 1), [Mn III(acac)( N-OPh-sal)(EtOH)] ( 2), [Mn III 2(PA) 4(sal- m-xylylene)] ( 3), [Mn III 2( N-OPh-sal) 2(X-sal- m-xylylene)] (X=H ( 4a), 5-MeO ( 4b), 3-MeO ( 4c), 5-Br ( 4d), and 5-NO 2 ( 4e)), and [Mn III 2( N-OPh-sal) 2(X-salpentn)] (X=H ( 5a) and 5-MeO ( 5b)) have been prepared by ligand substitution reactions and characterized, where Hacac, HPA, N-HOPh-Hsal, H 2X-sal- m-xylylene, and H 2X-salpentn denote acetylacetone, picolinic acid, N-hydroxyphenyl-salicylideneamine, N, N′-di-substituted-salicylidene- m-xylylenediamine, and N, N′-di-substituted-salicylidene-1,5-pentanediamine, respectively. Single crystals of complexes 1, 2, 4a, and 5a were used for X-ray crystallographic determination. Complexes 1 and 2 have a mononuclear structure, in which the central manganese(III) ions adopt a distorted octahedral geometry having an elongated axial bond compared to the equatorial bonds due to the Jahn–Teller effect. For complexes 4a and 5a, their detailed structures could not be clarified owing to a disorder of their molecules and quick degradation of the crystals in air. However, the results suggested that the two manganese(III) ions in these complexes are bridged by one sal- m-xylylene or salpentn ligand to form a dinuclear complex, and each complex has an arrangement similar to that of the mononuclear complex 2. The reactivities of these manganese(III) complexes toward H 2O 2 have been found that the dinuclear complexes 4a– e, 5a, and 5b can decompose excess amounts of H 2O 2 (H 2O 2/Mn<100) almost without degradation of the complexes, whereas complex 3 decomposes with an equimolar amount of H 2O 2. Moreover, the disproportionation rates of H 2O 2 have been found to depend on the bridging tetradentate ligands, the substituents on the phenyl rings of the bridging ligands, and the solvents used. On the basis of EPR spectroscopic studies, a series of complexes 4a– e, 5a, and 5b cycle their oxidation levels between Mn 2(III, III) and Mn 2(II, II) with dioxygen evolution during the decomposition of H 2O 2.

Di-tert-butylphosphate Complexes of Mn(II) and Cu(II) as Single-Source Precursors for Metal Phosphate Materials

Abstract Octahedral Mn(II) and Cu(II) di-tert-butylphosphate (L) complexes [ML2(imz)4] have been synthesized and characterized using IR, UV-vis, luminescence, and EPR spectroscopy and single crystal X-ray diffraction studies. Thermal decomposition of both the complexes (probed by TGA, DTA, and DSC) indicate stepwise elimination of iso-butene, imidazole and water to finally yield the respective ceramic materials Mn(PO3)2 or Cu(PO3)2.

EPR Spectroscopic Studies of the Reduction of Chromium(VI) by Methanol in the Presence of Peptides. Formation of Long-Lived Chromium(V) Peptide Complexes

The synthesis and characterization of the first Cr(V) complexes with non-sulfur-containing peptides, which may mimic the chemistry of the intermediates in the formation of Cr-induced peptide-DNA cross-links in vivo, are reported. The reduction of Cr(VI) with methanol in the presence of a number of non-sulfur-containing peptides produced relatively stable Cr(V)-peptide complexes, which were characterized by EPR spectroscopy and electrospray mass spectrometry. The reaction of Cr(VI) with methanol alone (in the absence of peptide ligands) resulted in the formation of two Cr(V)-methanol intermediates, with giso values of 1.9765 and 1.9687. The methanol reduction of Cr(VI) in the presence of the glycine peptides, triglycine, tetraglycine, and pentaglycine resulted in the formation of both Cr(V)-methanol and Cr(V)-peptide intermediates, while only the Cr(V)-peptide complexes were detected in the reactions with the alanine peptides trialanine, tetraalanine, and pentaalanine. Similar EPR signals were observed for all of the Cr(V)-peptide complexes with giso values between approximately 1.986 and approximately 1.979, and AN values of (2.1-2.6) x 10(-4) cm-1.

EPR spectroscopy and hybrid density functional studies of dialkoxyphosphinyl-phenyl iminoxy radicals

The dimethoxyphoshinyl-phenyl and 13 C enriched diisopropoxyphosphinyl-phenyl iminoxy radicals have been prepared by reaction of corresponding oximes with AgO. The magnetic properties of the radicals have been studied by a combined experimental and theoretical approach. The results of B3LYP and B1LYP computations are in excellent agreement with the EPR parameters determined on the basis of simulation of the experimental spectra. A difference observed in experimental hyperfine coupling with the iminoxy 13 C nucleus on going from Z to E isomer of the radical, 16.9 and 24.2 G, respectively, is discussed on the basis of calculated spin density distribution and natural bond orbitals analysis.

Mechanism for the reduction of the mixed-valent MnIIIMnIV[2-OHsalpn]2+ complex by tertiary amines.

The mixed-valent dimanganese(III/IV) complex MnIIIMnIV(2-OHsalpn)2+, 1, is cleanly reduced in acetonitrile by aliphatic tertiary amines to give the dimanganese(III) product MnIII2(2-OHsalpn)2, 2. Thorough characterization of the organic reaction products shows that tributylamine is converted to dibutylformamide and propionaldehyde. Kinetic studies and radical trapping experiments suggest that this occurs via initial single-electron transfer from the amine to 1 coupled with C-H alpha proton transfer from the oxidized amine. EPR spectroscopy and base inhibition studies indicate that coordination of the amine to 1 is a critical step prior to the electron transfer step. Rate data and its dependence on the amine indicate that the ability of the amine to reduce 1 is correlated to its basicity rather than to its reduction potential. Weakly basic amines were unable to reduce 1 irrespective of their reduction potential. This was inferred to indicate that proton transfer from the amine radical cation is also important in the reduction of 1 by tertiary amines. Comparison of the activation energy with reaction thermodynamics indicates that proton transfer and electron transfer must be concerted to explain the rapidity of the reaction. The fate of the amine radical is dependent on the presence of oxygen, and labeling studies show that oxygen in the organic products arises from dioxygen, although incorporation from trace water was also observed. These data indicate that inhibition of the hydrolytic quenching of the amine radical in an aprotic solvent results in a different fate for the amine radical when compared to amine oxidation reactions in aqueous solution. The proposed mechanism gives new insight into the ability of amines with high reduction potential to reduce metal ions of lower potential. In particular, these data are consistent with the ability of small amines and certain amine-containing buffers to inhibit manganese-dependent oxygen evolution in photosynthesis, which arises in some cases as a result of manganese reduction and its concomitant loss from the PS II reaction center.