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At least 973 records · Page 54Linked to original sources
PubMed2005

Electrical pump-and-probe study of spin singlet-triplet relaxation in a quantum dot.

Spin relaxation from a triplet excited state to a singlet ground state in a semiconductor quantum dot is studied by employing an electrical pump-and-probe method. Spin relaxation occurs via co-tunneling when the tunneling rate is relatively large, confirmed by a characteristic square dependence of the relaxation rate on the tunneling rate. When co-tunneling is suppressed by reducing the tunneling rate, the intrinsic spin relaxation is dominated by spin-orbit interaction. We discuss a selection rule of the spin-orbit interaction based on the observed double-exponential decay of the triplet state.

Journal Article↗
PubMed2006

Resonant inelastic x-ray scattering at the oxygen K resonance of NiO: nonlocal charge transfer and double-singlet excitations.

We report high-resolution polarization-dependent resonant inelastic x-ray scattering (RIXS) at the O K resonance of NiO showing a rich excitation spectrum. We perform multisite Ni6O19 cluster model calculations, revealing that solid state effects are substantial. We identify a nonlocal charge transfer excitation at 4-5 eV and double-singlet creation at 1.75 eV, both exhibiting significant scattering geometry dependence. Apart from an intense band of local charge transfer excitations (above 5 eV) also dd excitations at 1 eV are observed. Finally, we point out that O K RIXS of correlated metal oxides allows a quantitative and consistent determination of the charge transfer energy delta and the Hund coupling energy J(H).

Journal Article↗
PubMed2006

Spin-singlet small bipolarons in Nb-doped BaTiO3.

The magnetic susceptibility and the electrical resistivity of n-type BaTi1-xNbxO3 have been measured over a wide temperature range. It is found that, for 0<x<0.2, dopant electrons form immobile spin-singlet small bipolarons with binding energy around 110 meV. For x=0.2, a maximum in the electrical resistivity around 15 K indicates a crossover from band to hopping transport of the charge carriers, a phenomenon expected but rarely observed in real polaronic systems.

Journal Article↗
PubMed2006

Orbitally driven spin-singlet dimerization in S=1 La4Ru2O10.

Using x-ray absorption spectroscopy at the Ru-L2,3 edge we reveal that the Ru4+ ions remain in the S=1 spin state across the rare 4d-orbital ordering transition and spin-gap formation. We find using local spin density approximation + Hubbard U band structure calculations that the crystal fields in the low-temperature phase are not strong enough to stabilize the S=0 state. Instead, we identify a distinct orbital ordering with a significant anisotropy of the antiferromagnetic exchange couplings. We conclude that La4Ru2O10 appears to be a novel material in which the orbital physics drives the formation of spin-singlet dimers in a quasi-two-dimensional S=1 system.

Journal Article↗
PubMed1982

Evidence of singlet oxygen participation in the chlorophyll-sensitized photooxidation of indoleacetic Acid.

Chlorophyll-sensitized photooxidation of indoleacetic acid (IAA)-with chlorophyll extracted from Pisum sativum L. cv. Alaska W.R.-was determined in the presence of deuterium oxide and known quenchers of singlet oxygen ((1)O(2)) to explore the involvement of (1)O(2) in the reaction. O(2) uptake was measured in light in a buffered aqueous micellar system containing Triton X-100, KCl, chlorophyll, and IAA. The rate of O(2) uptake was zero in darkness. The reaction was stimulated by deuterium oxide and inhibited by sodium azide indicating that (1)O(2) participated in IAA photooxidation. Both mannitol and superoxide dismutase failed to inhibit O(2) uptake suggesting that neither the hydroxyl radical nor the superoxide anion played a significant role in the reaction.

Journal Article↗
PubMed1992

Involvement of Singlet Oxygen in 5-Aminolevulinic Acid-Induced Photodynamic Damage of Cucumber (Cucumis sativus L.) Chloroplasts.

Cucumber (Cucumis sativus L., cv Poinsette) plants were sprayed with 20 millimolar 5-aminolevulinic acid and then incubated in the dark for 14 hours. The intact chloroplasts were isolated from the above plants in the dark and were exposed to weak light (250 micromoles per square meter per second). Within 30 minutes, photosystem II activity was reduced by 50%. The singlet oxygen ((1)O(2)) scavengers, histidine and sodium azide (NaN(3)) significantly protected against the damage caused to photosystem II. The hydroxyl radical scavenger formate failed to protect the thylakoid membranes. The production of (1)O(2) monitored as N,N-dimethyl p-nitrosoaniline bleaching increased as a function of light exposure time of treated chloroplasts and was abolished by the (1)O(2) quencher, NaN(3). Membrane lipid peroxidation monitored as malondialdehyde production was also significantly reduced when chloroplasts were illuminated in the presence of NaN(3) and histidine. Protochlorophyllide was the most abundant pigment accumulated in intact chloroplasts isolated from 5-aminolevulinic acid-treated plants and was probably acting as type II photosensitizer.

Journal Article↗
PubMedDate not supplied

Morphology and development of left-handed singlets derived from mirror-image doublets of Stylonychia mytilus.

Mirror-image doublets of Stylonychia mytilus include 2 sets of cortical structures, one with the normal "right-handed" (RH) arrangement, the other with a reversed "left-handed" (LH) arrangement. These sets, however, are incomplete, with certain structures, most notably cirri of the right marginal type, missing near the line of symmetry. When a mirror-image doublet is bisected longitudinally to separate the RH and LH components physically, each fragment undergoes a regeneration process that restores a complete set of cortical structures, including the previously missing cirri of the right marginal type. In the resulting LH cell, all ciliary structures are present in an arrangement that is globally reversed in relation to that found in RH cells; in particular, marginal cirri of the left-marginal type are formed at the cell's right margin, and marginal cirri of the right-marginal type are produced at the cell's left margin. Whereas the regenerated RH fragment always divides and initiates a clone of normal singlets, the LH fragment, though structurally nearly complete, in all cases eventually dies without dividing. The cause of death is starvation due to the formation of an abnormal oral apparatus. In the Discussion, we consider the nature and consequences of a reversal of global positional information.

Animals↗
PubMed1991

Singlet and triplet reactivity in the photoreduction of oxonine(3,7-diaminophenoxazin-5-ium chloride) by iron (II).

The oxazine dye, oxonine (3,7-diaminophenoxazin-5-ium chloride), 1, is photoreduced by Fe (II) sulfate in dilute sulfuric acid. The reaction mechanism is analogous to that for the photo-reduction of thiazine dyes by Fe (II), the most important difference being that reduction of oxonine occurs predominantly from its excited singlet state, S1, rather than from the triplet state, T1. The latter is formed with an intersystem crossing (isc) quantum yield of ca 1.7 x 10(-3). The quenching of S1 by Fe (II) has a rate constant kSQ = 2.2 +/- 0.1 x 10(9) M-1 s-1 and affords the one electron reduced product, semioxonine (R), with a limiting quantum yield, phi SR, of 0.26 +/- 0.02. In contrast, quenching of T1, generated by bromide ion quenching of S1 or by diacetyl sensitization, occurs with KTQ approximately 1.2 x 10(6) M-1 s-1, extrapolated to zero ionic strength, and affords R with a limiting probability, phi TR = 1.1 +/- 0.2. Three possible reasons for the lower quantum yield of the more exothermic S1 reduction are discussed. These are energy transfer from S1 to Fe (II), different rates of escape of R from the encounter complex as a consequence of the different states of protonation of R as initially formed from S1 and T1, and spin allowed back electron transfer in an exciplex formed between S1 and Fe (II). Evidence is also presented for a very low probability (ca 1%) induced isc from the encounter of S1 with paramagnetic Fe (II). Rate parameters for other processes important to the overall reduction mechanism such as disproportionation of R to leucooxonine L and oxonine, k(R)DIS = 1.7 +/- 0.2 x 10(9) M-1 s-1, oxidation of R by Fe (III), k(R)OX = 1.5 +/- 0.1 x 10(5) M-1 s-1, and oxidation of L by Fe (III), kLOX = 1.1 +/- 0.1 x 10(3) M-1 s-1, have also been measured. These results are contrasted with those for the closely related thionine/Fe(II) photoredox reaction, the most well understood system for photogalvanic energy conversion.

Coloring Agents↗
PubMed1993

Detection of the excited singlet state of a deprotonated, reduced flavin.

The excited singlet state of a deprotonated, reduced flavin [1,5-dihydro-N(3)-carboxymethyllumiflavin] in aqueous solution at pH 8 has been detected by laser flash photolysis. The broad absorption band maximized at approximately 490 nm (epsilon = 9.9 x 10(3) M-1 cm-1). The lifetime of the transient was found to be 100 +/- 15 ps. The lifetime was not affected by the presence of pyrimidine dimers, which would be monomerized under these conditions. A longer-lived transient, tentatively identified as the solvated electron, was also detected. The neutral reduced flavin did not give a detectable transient.

DNA Repair↗
PubMed1996

Depopulation of highly excited singlet states of DNA model compounds: quantum yields of 193 and 245 nm photoproducts of pyrimidine monomers and dinucleoside monophosphates.

Formation of uracil and orotic acid photodimers, uridine and 5'-UMP photohydrates, TpT photodimers and (6-4)photoproducts, dCpT photohydrates and (6-4)photoproducts and UpU, CpC and CpU photohydrates were studied in neutral deoxygenated aqueous solution at room temperature upon irradiation at either 193 or 254 nm. The photoproducts were identified and quantified and the contribution from photoionization to substrate decomposition, using lambda irr = 193 nm, was separated. The ratio of the quantum yields of respective stable products, eta = phi 193/phi 254, is indicative of the yield of internal conversion from the second to the first excited singlet state, S2-->S1. For the observed photodimers eta decreases from 0.94 for uracil to 0.7 for TpT and further to 0.55 for orotic acid. For the (6-4)photoproducts of TpT and dCpT eta = 0.5-0.8 and for the photohydrates in the cases of UpU, CpC, CpU and dCpT eta ranges from 0.55 to 1.

DNA↗
PubMed1996

Reversible photobleaching of fluorescein conjugates in air-saturated viscous solutions: singlet and triplet state quenching by tryptophan.

Fluorescence recovery after photobleaching (FRAP) measurements on air-saturated aqueous solutions of fluorescein made viscous with glycerol or sucrose revealed a rapid component of fluorescence recovery with exponential time constants of 30-120 microseconds at viscosities of 15-300 cP. The rapid recovery process was not related to fluorophore translational diffusion and was insensitive to fluorophore concentration and the additive used to increase solution viscosity. At constant viscosity, the rate of reversible photobleaching recovery increased 2.5-fold in an O2- vs N2-saturated solution. The relative efficiency of reversible-to-irreversible photobleaching decreased with increasing photobleaching time and/or beam intensity. Reversible photobleaching was also detected for conjugates of fluorescein with dextrans and proteins in viscous media. In screening triplet state quenchers that might influence the reversible recovery, it was found that tryptophan enhanced the rate of reversible photobleaching recovery (two-fold increase at 8 mM) and quenched the fluorescein singlet state (Stern-Volmer constant, 12 M-1). Analysis of fluorescein lifetimes and photobleaching parameters for a series of fluorescein-labeled proteins with different numbers of tryptophans were also carried out. The results provide evidence for an oxygen-dependent, reversible photobleaching mechanism for the fluorescein chromophore involving triplet state relaxation. The identification of reversible fluorescein photobleaching has important implications for FRAP measurements of rapid solute diffusion in biological systems.

Fluoresceins↗
PubMed1997

Reactivity of singlet oxygen toward proteins: the effect of structure in basic pancreatic trypsin inhibitor and in ribonuclease A.

The reactions of singlet oxygen, 1O2, with large peptides have been described previously. It was found that even in these systems, which in their native form are generally not supposed to possess a stable structure in solution, the polypeptide does impede the access of 1O2 to the amino acids that react readily with 1O2. Here we describe the 1O2 reaction with two proteins of well-defined structure. The quenching of 1O2 by bovine pancreatic trypsin inhibitor (BPTI) and by ribonuclease A (RNase A) was compared to that of a solution at the same concentration as those of its constituent amino acids that react readily with 1O2. The proteins were studied in their native form, when partly denatured by splitting their S-S bonds and when fully denatured. It was found that while in the native form the quenching rate constant was seven times lower in BPTI (2.2 vs 15.2 x 10(7) M-1 s-1) and three times lower in RNase A (11.0 vs 32 x 10(7) M-1 s-1) than in a mixture of its constituent amino acid residues, it increased upon denaturation reaching in the fully denatured state the value of the corresponding amino acid mixture. More striking is the effect of the protein structure when comparing the fraction of the encounters between 1O2 and protein, which cause damage to the protein, as reflected in the decrease of its biological activity. This decrease is assumed to be due to the chemical (oxidative) reactions of 1O2 in the protein. In the exceptionally stable BPTI the fraction of such encounters was 0.05 and in RNase A it was 0.2, whereas for the amino acid tryptophan in solution, 0.7 of the collisions with 1O2 led to a chemical reaction.

Animals↗
PubMed2002

Singlet diradicals: from transition states to crystalline compounds.

Singlet diradicals are usually not energy minima. As observed by femtosecond spectroscopy, they readily couple to form final sigma bonds. Substituent effects allow lifetimes to increase into the microsecond range. Taking advantage of the properties of hetero-elements, a diradical has been prepared that is indefinitely stable at room temperature. The availability of diradicals that can be handled under standard laboratory conditions will lead to further insight into their chemical and physical properties, raising the likelihood of practical applications, especially in the field of molecular materials such as electrical conductors and ferromagnets.

Journal Article↗
PubMed1987

Antiarthritic gold compounds effectively quench electronically excited singlet oxygen.

Although certain gold [Au(I)] compounds have been used effectively in the treatment of rheumatoid arthritis for some years, the molecular basis for such therapeutic action has been unclear. One possible mechanism of the action of Au(I) compounds is that they protect unsaturated membrane lipids and proteins against oxidative degradation caused by activated phagocytes that are not properly regulated. In this study it has been shown that superoxide ion (O-2.), a product of activated phagocytes, can be oxidized to electronically excited singlet oxygen (O1(2)delta g), an agent that is capable of peroxidation of unsaturated fatty acid derivatives. It has also been shown that antiarthritic Au(I) compounds are effective deactivators of O1(2)delta g with quenching constants on the order of 10(7) M-1 sec-1.

Arthritis, Rheumatoid↗
PubMed1980

Cross-linking of lens crystallins in a photodynamic system: a process mediated by singlet oxygen.

In a dye-sensitized photooxidation system, lens crystallin polypeptides become cross-linked, and a blue fluorescence that is associated with the proteins is produced. These changes are similar to those seen in vivo in the aging human lens. Evidence implicating singlet oxygen as the causative agent of the effects in vitro is presented, and the possibility that this species may play a role in aging and cataractogenesis in vivo is discussed.

Aging↗
PubMed1995

Direct detection and quantification of singlet oxygen during ischemia and reperfusion in rat hearts.

To detect singlet oxygen (1O2) in postischemic reperfused hearts, 5,8-endoperoxide, an oxidation product of beta-carotene, was used as a marker for 1O2 generation and was quantified using high-performance liquid chromatography (HPLC). Isolated rat hearts were subjected to ischemia for 5, 10, 20, 30, and 60 min followed by 10 min of reperfusion with buffer containing 25 microM beta-carotene. The coronary effluent was collected, extracted, and injected into the HPLC unit. The production of 5,8-endoperoxide was maximum during the first 2 min of reperfusion. Maximal accumulated amount of 1O2 was observed in hearts subjected to 60-min ischemia (36.2 +/- 1.7 nmol.10 min-1.g-1) as compared with 10-min ischemia (6.2 +/- 1.0 nmol.10 min-1.g-1). There was a good correlation between the amount of 1O2 production and cardiac function. Treatment with 25 mM histidine significantly decreased 5,8-endoperoxide from 7.02 +/- 0.47 to 0.98 +/- 0.11 nmol.min-1.g-1 (P < 0.01) and improved cardiac function in the group with 60-min ischemia. This study demonstrates that 1) the present method is useful and reliable for the measurement of 1O2 in the heart, 2) 1O2 production during reperfusion is dependent on the duration of initial ischemia, and 3) 1O2 is one of the major factors in postischemic reperfusion injury.

Animals↗
PubMed2001

Ultraviolet irradiation of titanium dioxide in aqueous dispersion generates singlet oxygen.

We previously reported that irradiation of titanium dioxide (TiO2) in ethanol generates both singlet oxygen (1O2) and superoxide anion (O2*-) as measured by EPR spectroscopy. The present study describes the production of reactive oxygen species upon irradiation of TiO2 in aqueous suspension as determined by EPR spectroscopy using 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP) and 5,5-dimethyl-pyrroline-N-oxide (DMPO). Photoproduction of 1O2 by suspended TiO2, detected as 2,2,6,6-tetramethyl-4-piperidone-N-oxyl (4-oxo-TEMPO), was measured in water and deuterium oxide (D2O) in the presence or absence of sodium azide (NaN3) and under air or argon atmospheres. Production of a DMPO-OH adduct was examined in 4-oxo-TMP containing medium in the presence or absence of dimethyl sulfoxide (DMSO). The signal for the DMPO spin adduct of superoxide anion was not observed in aqueous conditions. Kinetic analysis revealed that 1O2 was produced at the surface of irradiated TiO2 in aqueous suspension as was observed in ethanol. Kinetic analysis revealed that the formation of DMPO-OH adduct reflects oxidation of DMPO by 1O2 rather than the trapping of the hydroxyl radical produced by the reaction of photo-exited TiO2 and water. The production of large amounts of 1O2 by TiO2 in aqueous suspension as compared to those in ethanol and possible formation of hydroxyl radical in aqueous suspension but not in alcohol, suggest that irradiation of TiO2 in aqueous environments is biologically more important than that in non-aqueous media.

Antioxidants↗