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PubMed2004

Hydroxyl radical generation caused by the reaction of singlet oxygen with a spin trap, DMPO, increases significantly in the presence of biological reductants.

Photosensitizers newly developed for photodynamic therapy of cancer need to be assessed using accurate methods of measuring reactive oxygen species (ROS). Little is known about the characteristics of the reaction of singlet oxygen (1O2) with spin traps, although this knowledge is necessary in electron spin resonance (ESR)/spin trapping. In the present study, we examined the effect of various reductants usually present in biological samples on the reaction of 1O2 with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The ESR signal of the hydroxyl radical (*OH) adduct of DMPO (DMPO-OH) resulting from 1O2-dependent generation of *OH strengthened remarkably in the presence of reduced glutathione (GSH), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), ascorbic acid, NADPH, etc. A similar increase was observed in the photosensitization of uroporphyrin (UP), rose bengal (RB) or methylene blue (MB). Use of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DEPMPO) as a spin trap significantly lessened the production of its *OH adduct (DEPMPO-OH) in the presence of the reductants. The addition of DMPO to the DEPMPO-spin trapping system remarkably increased the signal intensity of DEPMPO-OH. DMPO-mediated generation of *OH was also confirmed utilizing the hydroxylation of salicylic acid (SA). These results suggest that biological reductants enhance the ESR signal of DMPO-OH produced by DMPO-mediated generation of *OH from 1O2, and that spin trap-mediated *OH generation hardly occurs with DEPMPO.

Antioxidants↗
PubMed2001

Cardiac toxicity of singlet oxygen: implication in reperfusion injury.

Oxygen-derived free radicals (O2.-, H2O2, and .OH) that are produced during postischemic reperfusion are currently suspected to be involved in the pathogenesis of tissue injury. Another reactive oxygen species, the electronically excited molecular oxygen (1O2), is of increasing interest in the area of experimental research in cardiology. In this review are discussed the main potential sources of singlet oxygen in the organism, particularly in the myocardium, the various cardiovascular cytotoxic effects induced by this reactive oxygen intermediate, and the growing evidence of its involvement in ischemia/reperfusion injury.

Animals↗
PubMed2000

Singlet oxygen-dependent hydroxyl radical formation during uroporphyrin-mediated photosensitization in the presence of NADPH.

The conversion of singlet oxygen ((1)O2) to hydroxyl radical (*OH) during photosensitization of uroporphyrin (UP) in the presence of NADPH was examined by a spin-trapping technique with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Significant electron spin resonance (ESR) signals of DMPO-OH adduct were observed during irradiation of the UP-NADPH system with visible light. Scavengers of *OH reduced the signal intensity to 3-30% of control, indicating that more than 70% of DMPO-OH results from freely diffusing *OH. The ESR signal was almost completely lost when quenchers of (1)O2 were added, and was enhanced when the amount of deutrated solvent was increased. The appearance of (1)O2, as determined by the oxidation of 2,2,6,6-tetramethyl-4-piperidone (TEMPD), was delayed with an increase in the concentration of NADPH, whereas the production of *OH was upregulated. These observations indicate that conversion of (1)O2 to *OH occurs quickly in the presence of NADPH. Hydrogen peroxide (H2O2) was produced (1)O2-dependently during irradiation of UP in the presence of NADPH. However, neither catalase nor desferrioxamine decreased the DMPO-OH signal, and addition of H2O2 did not increase the signal. SOD increased the signal only slightly. These results suggest that the production of *OH from (1)O2 involves neither superoxide anion radical nor H2O2.

Animals↗
PubMed1989

Singlet oxygen as an ultimately reactive species in Salmonella typhimurium DNA damage induced by methylene blue/visible light.

The specific recognition of various DNA modifications by repair enzymes is exploited for the analysis of DNA damage induced by visible light in the presence of methylene blue in Salmonella typhimurium. The relative frequencies of various endonuclease-sensitive sites and strand breaks are determined in the plasmid pAQ1 of the treated bacteria and are compared with those observed after exposure of isolated DNA to various conditions. This comparison of damage profiles indicates that the cellular DNA damage by illumination in the presence of methylene blue is caused predominantly by the direct action of singlet oxygen. Indirect mechanisms, e.g. involving a generation of superoxide and hydroxyl radicals or the activation of cellular nucleases, do not contribute very much. The damage is dominated by base modifications. These are subject to an efficient repair that is not mediated by uvrABC proteins and therefore most probably involves recognition by specific glycosylases. Revertant frequencies observed under these conditions in the strains TA1535, TA100, TA2638 and TA104 indicate a pronounced mutagenicity of the lesions induced. On the other hand, the DNA damage does not contribute significantly to the cytotoxicity caused by the treatment as an excision repair deficiency (uvrB) has no influence on cell killing.

DNA Damage↗
PubMed2000

Search for color singlet technicolor particles in p&pmacr; collisions at radicals = 1.8 TeV

We search for color singlet technirho and technipion production in p&pmacr; collisions at sqrt[s] = 1.8 TeV recorded with the Collider Detector at Fermilab. These exotic technimesons are present in a model of walking technicolor. The signatures studied are lepton plus two jets plus E(T) and multijet final states. No excess of events is seen in either final state. We set an upper limit on the technirho production cross section and exclude a region in the technipion mass versus technirho mass plane.

Journal Article↗
PubMed2000

Entropy balance and evidence for local spin singlets in a Kagome-like magnet

We have measured the specific heat C(T) of the S = 3 / 2 Kagome-lattice-containing compound SrCr (9p)Ga (12-9p)O (19). We find little field dependence of the low-temperature C(T), consistent with a low-energy spectrum dominated by many-body singlet excitations. At high temperatures, we recover only approximately 50% of the total R ln4 entropy.

Journal Article↗
PubMed2000

Spin-singlet clusters in the ladder compound NaV2O5

The space group of alpha(')-NaV2O5 turns below T(c) = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton.

Journal Article↗
PubMed2001

Superconducting 2D system with lifted spin degeneracy: mixed singlet-triplet state.

Motivated by recent experimental findings, we have developed a theory of the superconducting state for 2D metals without inversion symmetry modeling the geometry of a surface superconducting layer in a field-effect transistor or near the boundary doped by adsorbed ions. In such systems the twofold spin degeneracy is lifted by spin-orbit interaction, and singlet and triplet pairings are mixed in the wave function of the Cooper pairs. As a result, spin magnetic susceptibility becomes anisotropic and Knight shift retains finite and rather high value at T = 0.

Journal Article↗
PubMed2002

Thermally generated gauge singlet scalars as self-interacting dark matter.

We show that a gauge singlet scalar S, with a coupling to the Higgs doublet of the form lambda(S)S+SH+H and with the S mass entirely generated by the Higgs expectation value, has a thermally generated relic density Omega(S)approximately equal to 0.3 if m(S)approximately equal to (2.9-10.5) (Omega(S)/0.3)(1/5)(h/0.7)(2/5) MeV. Remarkably, this is very similar to the range [m(S) = (6.6-15.4)eta(2/3) MeV] required in order for the self-interaction (eta/4) (S+S)(2) to account for self-interacting dark matter when eta is not much smaller than 1. The corresponding coupling is lambda(S)approximate(2.7 x 10(-10)-3.6 x 10(-9)) (Omega(S)/0.3)(2/5)(h/0.7)(4/5), implying that such scalars are very weakly coupled to the standard model sector.

Journal Article↗
PubMed2002

Collapse of the spin-singlet phase in quantum dots.

We present experimental and theoretical results on a new regime in quantum dots in which the filling factor two-singlet state is replaced by new spin polarized phases. We make use of spin blockade spectroscopy to identify the transition to this new regime as a function of the number of electrons. The key experimental observation is a reversal of the phase in the systematic oscillation of the amplitude of Coulomb blockade peaks as the number of electrons is increased above a critical number. It is found theoretically that correlations are crucial to the existence of the new phases.

Journal Article↗
PubMed2003

Coexistence of ferromagnetism and singlet superconductivity via kinetic exchange.

We propose a novel mechanism for the coexistence of metallic ferromagnetism and singlet superconductivity assuming that the magnetic instability is due to kinetic exchange. Within this scenario, the unpaired electrons which contribute to the magnetization have a positive feedback on the gain of the kinetic energy in the coexisting phase by undressing the effective mass of the carriers involved in the pairing. The evolution of the magnetization and pairing amplitude and the phase diagram are first analyzed for a generic kinetic exchange model and then are determined within a specific case with spin dependent bond-charge occupation.

Journal Article↗
PubMed2003

Singlet-triplet transition tuned by asymmetric gate voltages in a quantum ring.

Wave-function and interaction effects in the addition spectrum of a Coulomb-blockaded many-electron quantum ring are investigated as a function of asymmetrically applied gate voltages and magnetic field. Hartree and exchange contributions to the interaction are quantitatively evaluated at a crossing between states extended around the ring and states which are more localized in one arm of the ring. A gate tunable singlet-triplet transition of the two uppermost levels of this many-electron ring is identified at zero magnetic field.

Journal Article↗
PubMed2004

Low-energy singlets in the Heisenberg antiferromagnet on the kagomé lattice.

The spin-1/2 Heisenberg antiferromagnet on the kagomé lattice, is mapped by contractor renormalization to a spin-pseudospin Hamiltonian on the triangular superlattice. Variationally, we find a ground state with columnar dimer order. Dimer orientation fluctuations are described by an effective O(2) model at energies above an exponentially suppressed clock mass scale. Our results explain the large density of low-energy singlets observed numerically, and the nonmagnetic T2 specific heat observed experimentally.

Journal Article↗
PubMed2004

Geometric frustration, electronic instabilities, and charge singlets in Y2Nb2O7.

Pyrochlore Y2Nb2O7 is studied with density functional calculations. In the ideal pyrochlore structure, no magnetism is found, consistent with experiments, but the band structure is metallic. The phonon dispersions show unstable modes corresponding to charge instabilities. These frustrated instabilities lead to a metal-insulator transition with the formation of "charge singlets". Partial substitution of Ti for Nb results in moment formation due to the occurrence of Ti3+.

Journal Article↗
PubMed2004

Cooper-pair tunneling in junctions of singlet quantum Hall States and superconductors.

We propose tunnel junctions of a Hall bar and a superconducting lead for observing Cooper-pair tunneling into singlet fractional quantum Hall edge states. These tunnel junctions provide a natural means of extracting precise information of the spin polarization and the filling factor of the state. The low energy regime of one of the setups is governed by a novel quantum entangled fixed point.

Journal Article↗
PubMed2005

Dynamical singlets and correlation-assisted Peierls transition in VO2.

A theory of the metal-insulator transition in vanadium dioxide from the high- temperature rutile to the low- temperature monoclinic phase is proposed on the basis of cluster dynamical mean-field theory, in conjunction with the density functional scheme. The interplay of strong electronic Coulomb interactions and structural distortions, in particular, the dimerization of vanadium atoms in the low-temperature phase, plays a crucial role. We find that VO2 is not a conventional Mott insulator, but that the formation of dynamical V-V singlet pairs due to strong Coulomb correlations is necessary to trigger the opening of a Peierls gap.

Journal Article↗
PubMed2005

Magnetic fluctuations of filled skutterudites emerging in the transition region between singlet and triplet states.

In order to clarify magnetic properties of filled skutterudites, we analyze the Anderson model including seven f orbitals hybridized with an au conduction band using a numerical technique. For n=2 corresponding to Pr-based filled skutterudites, where n is the local f-electron number, even if the ground state is a singlet, there remain significant magnetic fluctuations from a triplet state with a small excitation energy. This result can be understood by the fact that f-electron states are clearly distinguished as itinerant and localized ones in the filled skutterudite structure. This picture also explains the complex results for f-electron magnetic susceptibility and entropy for n=1-13.

Journal Article↗
PubMed2005

Yield of singlet excitons in organic light-emitting devices: a double modulation photoluminescence-detected magnetic resonance study.

Double-modulation (DM) photoluminescence (PL) detected magnetic resonance (PLDMR) measurements on poly(2-methoxy-5-(2(')-ethyl)-hexoxy-1,4-phenylene vinylene) are described. In these measurements, the laser excitation power is modulated at 1<f(L)<100 kHz, and the spin-1/2 PLDMR of the PL response at f(L) is monitored by microwave modulation at f(M)<<f(L). The frequency response of the DM-PLDMR is inconsistent with the assumption that the PLDMR is due to delayed PL from nongeminate polaron recombination, which is the basis for previous predictions of the singlet-exciton (SE)-to-triplet exciton (TE) ratio in organic light-emitting devices. Rather, the frequency response is consistent with the assumption that the PLDMR is due to variations in the rate of quenching of SEs by TEs and polarons.

Journal Article↗