Quantum dynamics of Mn2+ in dimethylammonium magnesium formate

Dimethylammonium magnesium formate, [(CH₃)₂NH₂][Mg(HCOO)₃] or DMAMgF, is a model used to study high temperature hybrid perovskite-like dielectrics. This compound displays an order-disorder phase transition at about 260 K. Using multifrequency electron spin resonance in continuous wave and pulsed modes, we herein present the quantum dynamics of the Mn²⁺ ion probe in DMAMgF. In the high temperature paraelectric phase, we observe a large distribution of the zero field splitting that is attributed to the high local disorder and further supported by density functional theory computations. In the low temperature ferroelastic phase, a single structure phase is detected and shown to contain two magnetic structures. The complex electron paramagnetic resonance signals were identified by means of the Rabi oscillation method combined with the crystal field kernel density estimation.

Spin dynamics of the S = 5/2 2D triangular antiferromagnet Ba3NbFe3Si2O14

We report pulse-field magnetization, ac susceptibility, and 100 GHz electron spin resonance (ESR) measurements on the S = 5/2 two-dimensional triangular compound Ba3NbFe3Si2O14 with the Néel temperature T(N) = 26 K. The magnetization curve shows an almost linear increase up to 60 T with no indication of a one-third magnetization plateau. An unusually large frequency dependence of the ac susceptibility in the temperature range of T = 20-100 K reveals a spin-glass behavior or superparamagnetism, signaling the presence of frustration-related slow magnetic fluctuations. The temperature dependence of the ESR linewidth exhibits two distinct critical regimes; (i) ΔH(pp)(T) is proportional to (T-T(N))(-p) with the exponent p = 0.2(1)-0.2(3) for temperatures above 27 K, and (ii) ΔH(pp)(T) is proportional to (T-T*)(-p) with T* = 12 K and p = 0.8(1)-0.8(4) for temperatures between 12 and 27 K. This is interpreted as indicating a dimensional crossover of magnetic interactions and the persistence of short-range correlations with a helically ordered state.

Coherent manipulation of electron spins in the {Cu3} spin triangle complex impregnated in nanoporous silicon

We report on coherent manipulation of electron spins in an antiferromagnetically coupled spin triangle {Cu3-X} (X=As, Sb) impregnated in freestanding nanoporous silicon (NS) by using 240 GHz microwave pulses. Rabi oscillations are observed and the spin coherence time is found to be T(2)=1066 ns at 1.5 K. This demonstrates that the {Cu3-X}:NS hybrid material provides a promising scheme for implementing spin-based quantum gates. By measuring the spin relaxation times of samples with different symmetries and environments we give evidence that a spin chirality is the main decoherence source of spin triangle molecules.

Review of chromium (VI) apoptosis, cell-cycle-arrest, and carcinogenesis

Hexavalent chromium combines with glutathione in chloride intracellular channel carrier to form tetravalent and pentavalent chromium in plasma and organelle membranes. It also combines with NADH/NADPH to form pentavalent chromium in mitochondria. Tetravalent- and pentavalent- chromium (directly and indirectly) mediated DNA double strand breaks activate DNA damage signaling sensors: DNA-dependent-protein-kinase signals p53-dependent intrinsic mitochondrial apoptosis, and ataxia-telangiectasia-mutated and ataxia-telangiectasia-Rad3-related signal cell-arrest for DNA repair. Tetravalent chromium may be the most potent species since it causes DNA breaks and somatic recombination, but not apoptosis. Upon further failure of apoptosis and senescence/DNA-repair, damaged cells may become immortal with loss-of-heterozygosity and genetic plasticity.

Fluctuation-induced heat release from temperature-quenched nuclear spins near a quantum critical point

At a quantum critical point (QCP)--a zero-temperature singularity in which a line of continuous phase transition terminates--quantum fluctuations diverge in space and time, leading to exotic phenomena that can be observed at nonzero temperatures. Using a quantum antiferromagnet, we present calorimetric evidence that nuclear spins frozen in a high-temperature nonequilibrium state by temperature quenching are annealed by quantum fluctuations near the QCP. This phenomenon, with readily detectable heat release from the nuclear spins as they are annealed, serves as an excellent marker of a quantum critical region around the QCP and provides a probe of the dynamics of the divergent quantum fluctuations.

Multiphoton coherent manipulation in large-spin qubits

Large-spin Mn2+ ions (S=5/2) diluted in a nonmagnetic MgO matrix of high crystalline symmetry are used to realize a six-level system that can be operated by means of multiphoton coherent Rabi oscillations. This spin system has a very small anisotropy which can be tuned in situ to reversibly transform the system between harmonic and nonharmonic level configurations. Decoherence effects are strongly suppressed as a result of the quasi-isotropic electron interaction with the crystal field and with the 55Mn nuclear spins. These results suggest new ways of manipulating, reading, and resetting spin quantum states which can be applied to encode a qubit across several quantum levels.

Coherent manipulation of electron spins up to ambient temperatures in Cr5+ (S = 1/2) doped K3NbO8

We report coherent spin manipulation on Cr(5+) (S = 1/2, I = 0) doped K(3)NbO(8), which constitutes a dilute two-level model relevant for use as a spin qubit. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At liquid helium temperature the phase coherence relaxation time T2 reaches approximately 10 micros and, with a Rabi frequency of 20 MHz, yields a single-qubit figure of merit Q(M) of about 500. This shows that a diluted ensemble of Cr(5+) (S = 1/2) doped K(3)NbO(8) is a potential candidate for solid-state quantum information processing.

Origin of antiferroelectricity in NH4H2PO4 from first principles

The low-temperature antiferroelectric (AFE) phase of NH4H2PO4 corresponds to H ordering in O-H-O bridges leading to H2PO4 group polarizations perpendicular to the tetragonal c axis and alternating in chains. We determine the microscopic origin of such order by means of first-principles calculations in the framework of the density functional theory. The formation of N-Hcdots, three dots, centeredO bridges with correlated charge transfers and NH4+ group distortions turn out to be essential in stabilizing the AFE configuration against a c-polarized ferroelectric (FE) phase, as well as other FE states polarized perpendicular to the c axis. These FE states lie only a few meV above the AFE phase, which explains the observation of FE-AFE phase coexistence near the AFE transition.

High Resolution 15N NMR of the 225 K Phase Transition of Ammonia Borane (NH3BH3): Mixed Order-Disorder and Displacive Behavior

We report high resolution 15N NMR probing of the solid-solid phase transition of 15N-labeled ammonia borane (NH3BH3) at 225 K. Both the 15N isotropic chemical shift (delta iso) and the spin-lattice relaxation rate (T1-1) exhibited strong anomalies around 225 K. The analysis of T1-1 using the Bloembergen, Purcell, and Pound model showed that the motional correlation time, tau, increased from about 1 to 100 ps and the corresponding Arrhenius activation energy increased from 6 to 14.5 kJ/mol on going through the transition toward lower temperatures. The temperature dependence of delta iso was interpreted by an extension of the Bayer model. The time scale of the underlying motion was found to be in a reasonable agreement with the T1-1 data. These results imply that the NH3 rotor motion plays a pivotal role in the transition mechanism and that the transition is of both order-disorder and displacive type.

The Properties of the [Mn12O12(O2CR)16(H2O)4] Single-Molecule Magnets in Truly Axial Symmetry: [Mn12O12(O2CCH2Br)16(H2O)4]·4CH2Cl2

Detailed studies are reported of a Mn(12) single-molecule magnet (SMM) in truly axial (tetragonal) symmetry. The complex is [Mn(12)O(12)(O(2)CCH(2)Br)(16)(H(2)O)(4)].4CH(2)Cl(2) (2.4CH(2)Cl(2) or Mn(12)-BrAc), obtained by the standard carboxylate substitution method. The complex has an S = 10 ground state, typical of the Mn(12) family, and displays frequency-dependent out-of-phase AC susceptibility signals and hysteresis in single-crystal magnetization vs applied DC field sweeps. Single-crystal high-frequency EPR spectra in frequencies up to 360 GHz exhibit narrow signals that are not overlapping multiplets, in contrast to [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)].2MeCO(2)H.4H(2)O (1 or Mn(12)-Ac), which also crystallizes in an axial (tetragonal) space group but which now is recognized to consist of a mixture of six hydrogen-bonded isomers in the crystal and thus gives multiple, inhomogeneously broadened EPR signals. Similarly, single-crystal (55)Mn NMR spectra on Mn(12)-BrAc display much sharper signals than a single crystal of Mn(12)-Ac, and this allows one Mn(III) signal to show an almost baseline-resolved quintet from quadrupolar splitting ((55)Mn, I = 5/2, 100%), allowing quadrupole coupling parameters (e(2)qQ) to be determined. In addition, it was found that crushing crystals of Mn(12)-BrAc into a microcrystalline powder causes severe broadening and shifts of the NMR resonances, emphasizing the superiority of single-crystal studies. The combined results establish that Mn(12)-BrAc is far superior to Mn(12)-Ac for the study of the intrinsic properties of the Mn(12) family of SMMs in axial symmetry, and for the search for new phenomena such as quantum interference effects caused by higher-order (>2nd-order) transverse terms in the spin Hamiltonian.

An EPR method for probing surface magnetic fields, dipolar distances, and magnetization fluctuations in single molecule magnets

We outline a spectroscopic method for probing the effective magnetic field B on the surface of crystals of the single molecule magnet (SMM) [(C6H15N3)6Fe8(mu3-O)2(mu2-OH)12]Br7(H2O)Br.H2O, (Fe8Br8). This technique utilizes the line shape changes in the EPR spectra of the organic radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) (g = 2.0036, single peak) adsorbed onto the sample. The temperature dependence of the EPR line shifts scale with the sample's magnetization as measured by a SQUID magnetometer. Analysis of the line shape in terms of dipolar coupling between the DPPH and the SMM molecules on the surface, yields their average dipolar distance. The method's potential for measuring the magnetization fluctuation dynamics is briefly pointed out using the SMM [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-acetate).

Heat capacity and electron spin echo evidence for low frequency vibrational modes and lattice disorder in L-alanine at cryogenic temperatures

With the view of understanding the low frequency (40-50 cm(-1)) motional processes in L-alanine around 4 K, we have carried out heat capacity (CP) and electron spin echo (ESE) measurements on L-alanine and L-alanine-d7. The obtained CP data show the so-called boson peak (seen as a maximum in CP/T3 versus T plots) in the low temperature region (1.8-20 K). The phase memory time, T(M), and spin lattice relaxation time, T1, of the spin probe, the so-called first stable alanine radical (SAR1), *CHCH3COOH, have been measured between 4 and 105 K. The obtained relaxation rate 1/T1 shows an anomalous increase which coincides with the emergence of a boson peak in the low temperature region (4-20 K). Together, the ESE and the CP data confirm the existence of a thermally activated dynamic orientational disorder in the lattices of both compounds below 20 K. The results help explain the discrepancy between the CP data from powders and single crystals of alanine, as well as the proanomalous relaxation mechanisms for SAR1 in these lattices, and they also provide a mechanism for the spin-lattice relaxation process for SAR1 at cryogenic temperatures.

Probing magnetic fields on crystals of the nanomagnet Mn12-acetate by electron paramagnetic resonance

We report on electron paramagnetic resonance (EPR) probing of magnetic fields and magnetic field gradients near the surface of a single crystal of the nanomagnet [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-Ac). As the EPR probe, we utilized a 0.7 mm x 30 microm x 30 microm fibrous needle of the organic conductor N-methylphenazinium-tetracyanoquinodimethane (NMP-TCNQ), which yields an exceptionally sharp peak, with a 0.2 G (approximately 20 microT) width. In the presence of Mn12-Ac, the probe's peak exhibits splitting on temperature lowering, which depends on the orientation of the Zeeman field relative to the axis of easy magnetization of the employed Mn12-Ac crystal. The shifted peaks yield the magnitude of the magnetic field from Mn12-Ac crystal to which the various fibers of the probe are subjected. In conjunction with electron microscopy, the shifts yield the field gradient at the crystal surface and its change with temperature. For Mn12-Ac at 10 K, the surface magnetic field was measured to be in the mT range and its gradient on the order of 50 T/m.

Definitive spectroscopic determination of the transverse interactions responsible for the magnetic quantum tunneling in Mn(12)-acetate

We present detailed angle-dependent single crystal electron paramagnetic resonance (EPR) data for field rotations in the hard plane of the S=10 single molecule magnet Mn(12)-acetate. A clear fourfold variation in the resonance positions may be attributed to an intrinsic fourth-order transverse anisotropy (O(4)/(4)). Meanwhile, a fourfold variation of the EPR line shapes confirms a recently proposed model wherein disorder associated with the acetic acid of crystallization induces a locally varying quadratic (rhombic) transverse anisotropy [O (2)/(2) identical with E(S (2)/(x)-S(2)/(y))]. These findings explain most aspects of the magnetic quantum tunneling observed in Mn(12)-acetate.

1-370 GHz EPR linewidths for K3CrO8: a comprehensive test for the Anderson-Weiss model

Electron paramagnetic resonance (EPR) measurements have been carried out over the frequency range of 1-370 GHz on single crystals of potassium peroxychromate (K3CrO8) with the view of examining the current models of exchange narrowing of EPR signals in solids. K3CrO8 has a simple (tetragonal) lattice structure, can be grown as single crystals pure or diluted with an isostructural diamagnetic host K3NbO8, and its paramagnetism can be described by a very simple (S = 12, I = 0) spin Hamiltonian. The measurements were made at various orientations of single crystals in the Zeeman field, with emphasis on the principal directions of the g-tensor. For essentially all orientations, the linewidth decreases monotonically for measurements at resonance frequencies, omega0, from 1 to about 100 GHz, and then starts to increase at higher omega0. In order to delineate the spin exchange effects from other sources of line broadening, the measurements were repeated with a diluted spin system, K3NbO8 containing approximately/= 0.5 mole % of K3CrO8, representing the broadening effect of all the magnetic field dependent terms, such as the broadening due to the g-strain and sample holder/waveguide magnetization at the high field utilized, up to 14 T. Using these data, the K3CrO8 linewidths were analyzed in terms of the current models of spin exchange narrowing in three-dimensional systems. A reasonably good agreement was found with the Anderson-Weiss model, when modified for various line broadening effects. The accuracy of the analysis procedure was confirmed by the comparison of the presently determined values of the exchange constant, J, and the dipolar field, Hp, with their values obtained by dc magnetic susceptibility measurements and theoretical analysis, respectively; the agreement was within 5% for J (=1.35 K) and about 25% for Hp (160 G). However, some deviations and unusual splittings were noted in measurements at 370 GHz, whose origin remains unclear.

Augmentation of pulmonary reactions to quartz inhalation by trace amounts of iron-containing particles

Fracturing quartz produces silica-based radicals on the fracture planes and generates hydroxyl radicals (.OH) in aqueous media. .OH production has been shown to be directly associated with quartz-induced cell damage and phagocyte activation in vitro. This .OH production in vitro is inhibited by desferrioxamine mesylate, an Fe chelator, indicating involvement of a Fenton-like reaction. Our objective was to determine if Fe contamination increased the ability of inhaled quartz to cause inflammation and lung injury. Male Fischer 344 rats were exposed 5 hr/day for 10 days to filtered air, 20 mg/m3 freshly milled quartz (57 ppm Fe), or 20 mg/m3 freshly milled quartz contaminated with Fe (430 ppm Fe). High Fe contamination of quartz produced approximately 57% more reactive species in water than quartz with low Fe contamination. Compared to inhalation of quartz with low Fe contamination, high Fe contamination of quartz resulted in increases in the following responses: leukocyte recruitment (537%), lavageable red blood cells (157%), macrophage production of oxygen radicals measured by electron spin resonance or chemiluminescence (32 or 90%, respectively), nitric oxide production by macrophages (71%), and lipid peroxidation of lung tissue (38%). These results suggest that inhalation of freshly fractured quartz contaminated with trace levels of Fe may be more pathogenic than inhalation of quartz alone.

Synthesis of Cr(IV)-GSH, its identification and its free hydroxyl radical generation: a model compound for Cr(VI) carcinogenicity

Current models of Cr(VI) carcinogenesis suggest an important role for Cr(IV) as an intermediate, toxic, carcinogenic species, but direct chemical evidence has been lacking. This is because Cr(IV) is a highly reactive oxidation state of Cr and few Cr(IV)-based compounds are known that can be used as a model compound containing a biological ligand. This study reports the isolation of such a stable Cr(IV) complex. The Cr(IV)-GSH complex has been synthesized through the reaction of Cr(VI) with GSH. Its electron paramagnetic resonance (EPR) spectrum exhibits g = 1.9629 and a peak-to-peak line width of 480 G in aqueous medium as well as in the powder form. Magnetic susceptibility measurements showed that the compound has a magnetic moment of 2.53 Bohr magneton per Cr, establishing that the Cr ion has two unpaired electrons, hence its identity as Cr(IV). The Cr(IV)-GSH complex is able to generate hydroxyl (.OH) radical in the presence of molecular oxygen in aqueous medium. Catalase inhibited the .OH radical generation while H2O2 enhanced it, indicating that the .OH radical was generated via a Fenton-like reaction, H2O2 being generated as an intermediate in the reduction of molecular oxygen. Metal ion chelators, deferoxamine and 1,10-phenanthroline, attenuated the generation of Cr(IV)-mediated .OH radical. In the case of deferoxamine, a deferoxamine-derived free radical was generated as shown by EPR measurements. The results imply that Cr(IV) may play an important role in the mechanism of Cr(VI)-induced carcinogenesis and Cr(IV)-GSH can be used as a model compound to study the role of Cr(IV) in this mechanism.

Squaric acid as an internal standard for temperature measurements in 13C MAS NMR

Squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) is suggested as an internal standard for temperature measurements in 13C MAS NMR studies in the temperature range from 373 K to 520 K. Compared to previously utilized standards, squaric acid has several important advantages, such as fast kinetics of the phase transition, higher temperature range for 13C studies, and an almost constant sample volume through the transition temperature range. Thus, we found squaric acid to be suitable for the probe calibration with the reference temperature at 373.2 K. Moreover, upon partial deuteration the reference temperature can be gradually increased to 520 K. Additionally, the described procedure makes it possible to quantitatively estimate the temperature gradient across the sample. We also discuss the effect of spinning related stress on the temperature measurement technique and how this effect could be minimized.

Role of chromium(IV) in the chromium(VI)-related free radical formation, dG hydroxylation, and DNA damage

A reactive Cr(IV) ester was synthesized from a reaction of CrO3 with 2,4-dimethyl-2,4-pentanediol as verified by electron spin resonance (ESR) and magnetic susceptibility measurements. ESR spin trapping studies demonstrate that the Cr(IV) ester is capable of generating hydroxyl free radical (.OH) through a Fenton-like mechanism (Cr(IV)+H2O2-->Cr(V)+.OH+OH-) with a concomitant generation of Cr(V) species (g = 1.9787). Cr(IV) caused DNA strand breaks as measured by electrophoretic assays. H2O2 enhanced the DNA strand breaks via .OH formation by a Cr(IV)-mediated Fenton-like reaction. In the Cr(IV)/H2O2 system, formate did not block Cr(V) formation, but prevented DNA damage, indicating that .OH radicals, and not Cr(V), caused the DNA damage. Reaction of Cr(VI) with ascorbate was also used as a source of Cr(IV). Incubation of Cr(VI), ascorbate, and DNA caused DNA strand breaks. A free radical trap, 5,5-dimethyl-1-pyrroline (DMPO), only slightly inhibited the DNA damage. Addition of Mn(II), which inhibited Cr(IV), caused significant protection. H2O2 enhanced the DNA damage via Cr(IV)-mediated .OH radical generation and Mn(II) inhibited the damage, again showing that Cr(IV) and its related .OH generation caused DNA strand breaks. HPLC measurements showed that .OH radicals generated by a Cr(IV)-mediated Fenton-like reaction generated 8-hydroxy-2'-deoxyguanosine from 2'-deoxyguanosine. The results demonstrate that Cr(IV) and its generated .OH radicals are capable of damaging DNA. Moreover, in comparison with Cr(V), Cr(IV) is a more potent DNA damaging agent.

Chromium (IV)-mediated fenton-like reaction causes DNA damage: implication to genotoxicity of chromate

This paper describes our deoxyribonucleic acid (DNA) damage and Fenton activity studies on Cr(IV) that were undertaken to examine a role of Cr(IV) in the biochemical mechanism of genotoxicity of Cr(VI) compounds. A Cr(IV)-containing compound, a Cr(IV)-ester, was synthesized and used as the first model compound for Cr(IV) studies. Electron spin resonance measurements demonstrated that in the presence of H2O2, this Cr(IV) compound acts as a potent Fenton-like reagent, i.e., it generates OH radical and a Cr(V) species (g = 1.9787) via: Cr(IV) + H2O2-->Cr(V) + OH + OH-. Measurements of OH yield as a function of H2O2 concentration showed that Cr(IV) is more potent than Cr(V) in generating OH. DNA damage assays demonstrate that Cr(IV) and the Cr(IV)/H2O2-generated OH radical can cause DNA strand breaks. The DNA damage caused by OH radical can be effectively blocked by OH radical scavengers. These results thus constitute the first direct evidence for DNA damage by Cr(IV) and Cr(IV)-mediated Fenton-like reaction and support the conjecture that Cr(IV) plays a significant role in the biochemical mechanism of the genotoxic reactions of Cr(VI) compounds and that this effect can be modulated by suitable antiradical agents.

Hydroxyl radical generation by coal mine dust: possible implication to coal workers' pneumoconiosis (CWP)

Occupational exposure to coal mine dust causes coal workers' pneumoconiosis (CWP) and other pulmonary diseases by mechanisms that remain unclear. Because the hydroxyl radicals (.OH) may play an important role in the pathogenesis of CWP, we studied the potential role of bituminous coal mine dust samples for catalyzing the generation of .OH from hydrogen peroxide (H2O2). These coal mine dusts evaluated represented two geographic areas with diversity in CWP prevalence. Electron spin resonance (ESR), with the aid of spin trapping techniques, was used to measure the .OH radical generation. Bituminous coal mine dusts representing the Pittsburgh seam in the eastern United States and Blind Canyon seam in the mid-western United States were used together with a standard coal dust obtained from the National Institute of Standards and Technology, Gaithersburg, MD. All the coal mine dust samples generated varying levels of .OH radicals from H2O2 in the presence of a .OH spin trap 5,5-dimethyl-l-pyrroline-N-oxide (DMPO). .OH radical generation by the coal from H2O2 was effectively inhibited by deferoxamine and catalase, but only partially inhibited by superoxide dismutase. Metal chelators DETAPAC and EDTA enhanced the radical generation. These results indicated that the Fenton reaction is predominantly involved in the generation of .OH radicals from H2O2. The .OH-generating potential of all the coal dusts showed a positive correlation with the surface iron content of coal mine dusts. In addition, the potential to induce lipid peroxidation by the coal samples exhibited a good correlation with the available surface iron. Based on the results presented here, we propose that higher concentrations of surface iron in coal mine dust may be involved in the generation of increased levels of .OH radicals and may play an important role in the development of CWP in different coal mining areas.

Silica radical-induced DNA damage and lipid peroxidation

In recent years, more attention has been given to the mechanism of disease induction caused by the surface properties of minerals. In this respect, specific research needs to be focused on the biologic interactions of oxygen radicals generated by mineral particles resulting in cell injury and DNA damage leading to fibrogenesis and carcinogenesis. In this investigation, we used electron spin resonance (ESR) and spin trapping to study oxygen radical generation from aqueous suspensions of freshly fractured crystalline silica. Hydroxyl radical (.OH), superoxide radical (O2.-) and singlet oxygen (1O2) were all detected. Superoxide dismutase (SOD) partially inhibited .OH yield, whereas catalase abolished .OH generation. H2O2 enhanced .OH generation while deferoxamine inhibited it, indicating that .OH is generated via a Haber-Weiss type reaction. These spin trapping measurements provide the first evidence that aqueous suspensions of silica particles generate O2.- and 1O2. Oxygen consumption measurements indicate that freshly fractured silica uses molecular oxygen to generate O2.- and 1O2. Electrophoretic assays of in vitro DNA strand breakages showed that freshly fractured silica induced DNA strand breakage, which was inhibited by catalase and enhanced by H2O2. In an argon atmosphere, DNA damage was suppressed, showing that molecular oxygen is required for the silica-induced DNA damage. Incubation of freshly fractured silica with linoleic acid generated linoleic acid-derived free radicals and caused dose-dependent lipid peroxidation as measured by ESR spin trapping and malondialdehyde formation. SOD, catalase, and sodium benzoate inhibited lipid peroxidation by 49, 52, and 75%, respectively, again showing the role of oxygen radicals in silica-induced lipid peroxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

Generation of hydroxyl radical by chromate in biologically relevant systems: role of Cr(V) complexes versus tetraperoxochromate(V)

While Cr(V) species and .OH radicals have been suggested to play significant roles in the mechanism of chromate-related carcinogenesis, controversy still exists regarding the identity of the Cr(V) species and their role in the generation of .OH radicals. Some recent studies have suggested that the primary Cr(V) species involved is the tetraperoxochromate(V) (CrO8(3-)) ion, which produces .OH radical either on decomposition or by reaction with H2O2. The present study utilized ESR and spin trapping techniques to probe this mechanism. The results obtained show that (i) CrO8(3-) is not formed in any significant quantity in the reaction of chromate with biologically relevant reductants such as glutathione, glutathione reductase, NAD(P)H, ascorbate, vitamin B2, etc. (ii) Decomposition of CrO8(3-), or its reaction with H2O2 does not generate any significant amount of .OH radicals. (iii) The major Cr(V) species formed are complexes of Cr(V) with reductant moieties as ligands. (iv) These Cr(V) complexes generate .OH radicals from H2O2 via Fenton-like reaction. The present study thus disagrees with the recently proposed "tetraperoxochromate(V) theory of carcinogenesis from chromate." Instead, it suggests an alternative mechanism, which might be labeled as "the Cr(V)-complexation-Fenton reaction model of carcinogenesis from chromate.

Chromate-mediated free radical generation from cysteine, penicillamine, hydrogen peroxide, and lipid hydroperoxides

The Cr(VI)-mediated free radical generation from cysteine, penicillamine, hydrogen peroxide, and model lipid hydroperoxides was investigated utilizing the electron spin resonance (ESR) spin trapping technique. Incubation of Cr(VI) with cysteine (Cys) generated cysteinyl radical. Radical yield depended on the relative concentrations of Cr(VI) and Cys. The radical generation became detectable at a cysteine:Cr(VI) ratio of about 5, reached its highest level at a ratio of 30, and declined thereafter. Cr(VI) or Cys alone did not generate a detectable amount of free radicals. Similar results were obtained with penicillamine. Incubation of Cr(VI), Cys or penicillamine and H2O2 led to hydroxyl (.OH) radical generation, which was verified by quantitative competition experiments utilizing ethanol. The mechanism for .OH radical generation is considered to be a Cr(VI)-mediated Fenton-like reaction. When model lipid hydroperoxides such as t-butyl hydroperoxide and cumene hydroperoxide were used in place of H2O2, hydroperoxide-derived free radicals were produced. Since thiols, such as Cys, exist in cellular systems at relatively high concentrations, Cr(VI)-mediated free radical generation in the presence of thiols may participate in the mechanisms of Cr(VI)-induced toxicity and carcinogenesis.

Vanadate-mediated hydroxyl radical generation from superoxide radical in the presence of NADH: Haber-Weiss vs Fenton mechanism

The mechanism of hydroxyl (.OH) radical generation from O2- and H2O2 by vanadate [V(V)] and the role of NADH in this reaction have been investigated using electron spin resonance (ESR) and spin trapping techniques. The results show that the reaction of V(V) with O2- (generated via xanthine/xanthine oxidase) does not generate any ESR detectable V(IV) ion or .OH radical and the addition of H2O2 has little effect on the radical yield. In the presence of NADH, however, the xanthine/xanthine oxidase/V(V) system generates .OH as well as V(IV), the formation of both of which could be suppressed by superoxide dismutase. Catalase inhibits the .OH formation but enhances V(IV) generation. Reaction of V(V) with NADH alone in the presence of phosphate buffer also causes .OH radical generation albeit at a much reduced rate, and superoxide dismutase reduces the .OH yield. These observations indicate, in contrast to earlier reports, that O2- does not reduce V(V) to V(IV) in the absence of NADH. It is concluded that vanadate generates the .OH radical via not a Haber-Weiss but a Fenton-like reaction [V(IV) + H2O2-->V(V) + .OH+OH-], the V(IV) and H2O2 being generated by V(V)-stimulated, O(2-)-dependent NADH oxidation.

Exposure of rats to hyperoxia: alteration of lavagate parameters and macrophage function

Exposure of rats to hyperoxia (100% oxygen for 64 h) resulted in striking alterations in the properties of samples obtained by bronchoalveolar lavage. The yield of neutrophils, lymphocytes, and red blood cells was increased, while the number of harvested alveolar macrophages decreased. The acellular lavage fluid level of protein was elevated, indicating lung damage. However, acellular phospholipid levels were unchanged. The ability of alveolar macrophages to produce reactive forms of oxygen in response to zymosan was significantly decreased by oxygen exposure. This impaired function was not fully explained by a decrease in viability of these phagocytes. In contrast, stimulant-induced chemiluminescence was elevated after hyperoxia. This rise was not due to a change in cellular antioxidant levels or to a discernible increase in arachidonic acid metabolites. However, it was associated with increased cellular lipid peroxidation.

Generation of free radicals in reactions of Ni(II)-thiol complexes with molecular oxygen and model lipid hydroperoxides

The generation of free radicals from reactions of nickel(II)-thiol complexes with molecular oxygen and model lipid hydroperoxides was investigated by electron spin resonance (ESR) utilizing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap. Incubation of nickel(II) [Ni(II)] with cysteine in an aerobic environment generated hydroxyl (.OH) radical, which then reacted with cysteine to generate a carbon-centered alkyl (.R) radical. Radical generation was inhibited under a nitrogen atmosphere. Model lipid hydroperoxides, cumene hydroperoxide, and t-butyl hydroperoxide enhanced the yield of these radicals and also generated an alkoxyl (.OR) radical. Radical yield decreased by approximately half under a nitrogen atmosphere. Although histidine did not cause radical formation in the reaction between Ni(II) and cumene hydroperoxide under aerobic conditions, the addition of histidine to a mixture containing Ni(II), cysteine, and cumene hydroperoxide under the same experimental conditions increased the yield of .R radical but lowered the yield of .OR and .OH radical adducts. It thus appears that histidine caused the .OH attack to be more site-specific. Similar results were obtained utilizing t-butyl hydroperoxide. Penicillamine or N-acetylcysteine yielded similar results except that under aerobic conditions, reaction between Ni(II) and N-acetylcysteine without hydroperoxide did not generate a significant concentration of free radicals. Under the same experimental conditions, cystine did not generate any detectable free radicals, suggesting an important role of the -SH group in Ni(II)-mediated free radical generation. The results indicate that free radical generation from the reaction of Ni(II)-thiol complexes and molecular oxygen, and/or lipid hydroperoxides, may play an important role in the mechanism(s) of Ni(II) toxicity and carcinogenesis.

Generation of free radicals from model lipid hydroperoxides and H2O2 by Co(II) in the presence of cysteinyl and histidyl chelators

Electron spin resonance spin trapping was utilized to investigate the generation of free radicals from cumene hydroperoxide (cumene-OOH), tert-butyl hydroperoxide (tert-butyl-OOH), and H2O2 at pH 7.2 by Co(II) in the presence of cysteinyl and histidyl chelating agents. The spin trap used was 5,5-dimethyl-1-pyrroline N-oxide. Incubation of Co(II) with cumene-OOH or tert-butyl-OOH did not generate any detectable amounts of free radicals. However, in the presence of glutathione, cysteine, penicillamine, or N-acetylcysteine, Co(II) generated cumene-OOH-derived carbon-centered radicals, cumene alkoxyl radicals, and hydroxyl (.OH) radicals. Oxidized glutathione and cystine used instead of reduced glutathione or cysteine did not generate any free radical, indicating an important role of the -SH group in radical generation. While the addition of diethylenetriaminepentaacetic acid (DTPA) prevented radical generation, deferoxamine had only a slightly inhibitory effect. Similar results to those obtained using cumene-OOH were obtained utilizing tert-butyl-OOH in place of cumene-OOH. The yields of free radicals were in the order of glutathione > cysteine > penicillamine > N-acetylcysteine. Incubation of Co(II) with cumene-OOH ort-butyl-OOH in the presence of the histidyl oligopeptide Gly-Gly-His also generated lipid hydroperoxide-derived free radicals, with the yield being comparable to that obtained using thiols. In contrast, histidine, anserine, homocarnosine, or carnosine did not cause any free radical generation from Co(II) and lipid hydroperoxides. Incubation of Co(II) with H2)2 produced only a small amount of .OH radicals.(ABSTRACT TRUNCATED AT 250 WORDS)

Generation of free radicals from hydrogen peroxide and lipid hydroperoxides in the presence of Cr(III)

Free radical generation from H2O2 and lipid hydroperoxides in the presence of Cr(III) was investigated by electron spin resonance (ESR) spin trapping methodology. Incubation of Cr(III) with H2O2 at physiological pH generated hydroxyl (.OH) radical, the yield of which reached saturation level in about 6 min. Deferoxamine reduced the .OH radical yield by only about 20%, diethylenetriamine pentaacetic acid (DTPA) reduced it by about 70%, while cysteine, glutathione, and NADH exhibited no significant effect. The yield of .OH radical formation also depended on the pH being 15 times higher at pH 10 than that at pH 7.2. At pH 3.0, .OH radical generation became nondetectable, and addition of H2O2 to Cr(III) solution did not affect the intensity of the Cr(III) ESR signal while at pH 10, addition of H2O2 reduced the Cr(III) intensity by about 40%, showing that reaction of Cr(III) with H2O2 occurred only at higher pH. Incubation of Cr(III) with the model lipid hydroperoxides, cumene hydroperoxide and t-butyl hydroperoxide, generated lipid hydroperoxide-derived free radicals. Addition of deferoxamine or DTPA had a minor inhibitory effect on that generation. These results show that Cr(III) is capable of producing free radicals from H2O2 and lipid hydroperoxides, which may have significant implications regarding the mechanism of chromium-induced carcinogenesis.

One-electron reduction of vanadium(V) by flavoenzymes/NADPH

The biochemical mechanism underlying vanadate-stimulated NAD(P)H oxidation is controversial. Some reports favor an exclusive role for a superoxide (O2(-)-mediated radical chain reaction, while others cite data that suggest a contribution from O2(-)-independent enzymatic pathways. We recently reported that a vanadium(IV) species accumulates over a period of about 30 min in phosphate-buffer mixtures of vanadate, NAD(P)H, and a flavoenzyme such as glutathione reductase, lipoyl dehydrogenase, or ferredoxin-NADP+ oxidoreductase. The concentration of this vanadium(IV) species was found to depend critically on the simultaneous presence of the enzyme and NAD(P)H, but not on superoxide dismutase, or a nitrogen atmosphere. It was thus concluded that the flavoenzyme/NAD(P)H system acts as a vanadate reductase. However, a subsequent report put forth an alternative hypothesis in which the accumulation of this vanadium(IV) species is ascribed to direct reduction of vanadate by NAD(P)H itself, starting when buffer-dissolved molecular O2 and H2O2 have been depleted. We have reexamined our earlier data, and carried out new measurements to evaluate the effect of dissolved oxygen and related factors on the kinetics of vanadium(IV) generation in vanadate/NAD(P)H/flavoenzyme mixtures. The new data support our earlier suggestion that the above-mentioned flavoenzymes can indeed act as NAD(P)H-dependent vanadate reductases.

Mechanisms of hydroxyl free radical-induced cellular injury and calcium overloading in alveolar macrophages

Excessive production of reactive oxygen radicals by alveolar macrophages is proposed to play an important role in oxidative lung injury. A major product oxygen radical formation is the highly reactive hydroxyl radical (.OH) generated via a biologic Fenton reaction. In addition to its known ability to induce lipid peroxidation, recent studies have suggested that the .OH may exert its cytotoxic effect through the alteration of [Ca2+]i homeostasis. To test this potential mechanism as well as to investigate the relationship between .OH and Ca2+ overloading in cytotoxic injury, isolated rat alveolar macrophages were exposed to externally generated radical system, H2O2 (0.01 to 1 mM) and Fe2+ (1 mM) and their [Ca2+]i levels and cell injury were monitored using quantitative fluorescence microscopy with the aid of the specific Ca2+ indicator, Fura-2, and membrane integrity indicator, propidium iodide. Electron spin resonance measurements using the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) confirmed the production of the .OH radical by this system. Upon the addition of the radicals, the macrophages displayed a rapid initial rise in [Ca2+]i which was followed by a slower but more pronounced [Ca2+]i elevation that reached a level 3 to 5 times higher than the basal level. This process preceded cell death as evident by nuclear propidium iodide fluorescence. Depletion of extracellular Ca2+ inhibited both the [Ca2+]i response and cell injury. Preincubation of the cells with the Ca2+ channel blocker verapamil or .OH radical scavenger mannitol similarly inhibited the [Ca2+]i rise and loss of viability. Firefly luciferase assay of cellular ATP content demonstrated that the alterations in [Ca2+]i following .OH treatment preceded the depletion of ATP.(ABSTRACT TRUNCATED AT 250 WORDS)