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At least 811 records · Page 45Linked to original sources
PubMed2000

CIDEP effects of intramolecular quenching of singlet and triplet excited states by nitroxide radicals in oligopeptides: a potentially useful new method for investigating peptide secondary structures in solution.

Two hexapeptides, each bearing one photoactive alpha-amino acid (Bin or Bpa) and one nitroxide-containing TOAC residue, have been synthesized and fully characterized. FT-IR absorption measurements indicate that a 3(10)-helical conformation is adopted by these peptides in solution. As two amino acid units separate the photoactive residue from TOAC in the peptide sequences, the two moieties face each other at a distance of about 6 A after one complete turn of the ternary helix. Irradiation by a light pulse from an excimer laser populates the excited states localized on the chromophores. An intramolecular interaction between the singlet (Bin) or triplet (Bin and Bpa) excited states and the doublet state of the TOAC nitroxide makes a spin-selective decay pathway possible, that produces transient spin polarization. In addition, in order to determine whether the intramolecular exchange interaction occurs through-bond or through-space, we have prepared linear and cyclic TOAC-Bin dipeptide units. A CIDEP study revealed that a through-space intramolecular interaction is operative. The observation of spin polarization makes the two helical hexapeptides suitable models to test the possibility of application of this novel technique to conformational studies of peptides in solution.

Cyclic N-Oxides↗
PubMed1993

Effects of oxygen radicals, hydrogen peroxide and water-soluble singlet oxygen carriers on 5- and 12-lipoxygenase.

Inhibition of 5-lipoxygenase from bovine polymorphonuclear leukocytes and 12-lipoxygenase from bovine platelets by active oxygen species has been studied. Oxygen radicals and hydrogen peroxide markedly inhibited 12-lipoxygenase, whereas 5-lipoxygenase activity was only moderately influenced. Singlet oxygen liberated from water-soluble naphthalene endoperoxides was without effect.

12-Hydroxy-5,8,10,14-eicosatetraenoic Acid↗
PubMed2006

Spatial extent of the singlet and triplet excitons in luminescent angular-shaped transition-metal diynes and polyynes comprising non-pi-conjugated group 16 main group elements.

A novel approach based on conjugation interruption has been developed and is presented for a series of luminescent and thermally stable chalcogen-bridged platinum(II) polyyne polymers trans-[{-Pt(PBu3)2C[triple bond]C(C6H4)E(C6H4)C[triple bond]C-}n] (E = O, S, SO, SO2). Particular attention was focused on the photophysical properties of these Group 10 polymetallaynes and comparison was made to their binuclear model complexes trans-[Pt(Ph)(PEt3)2C[triple bond]C(C6H4)E(C6H4)C[triple bond]CPt(Ph)(PEt3)2] and their closest Group 11 gold(I) and Group 12 mercury(II) neighbours, [MC[triple bond]C(C6H4)E(C6H4)C[triple bond]CM] (M = Au(PPh3), HgMe; E = O, S, SO, SO2). The regiochemical structures of these angular-shaped molecules were studied by NMR spectroscopy and single-crystal X-ray structural analyses. Upon photoexcitation, each one has an intense purple-blue fluorescence emission near 400 nm in dilute fluid solutions at room temperature. Harvesting of the organic triplet emissions harnessed through the strong heavy-atom effects of Group 10-12 transition metals was studied in detail. These metal-containing aryleneethynylenes spaced by chalcogen units were found to have large optical gaps and high-energy triplet states. The influence of metal- and chalcogen-based conjugation interrupters on the intersystem crossing rate and on the spatial extent of the lowest singlet and triplet excitons was fully elucidated. We discuss and compare the phosphorescence spectra of these transition-metal diynes and polyynes in terms of the nature of the metal centre, conjugated chain length and Group 16 spacer unit. Our work here indicates that high-energy triplet states in these materials intrinsically give rise to very efficient phosphorescence with fast radiative decays and one could readily observe room-temperature phosphorescence for the platinum polyynes.

Journal Article↗
PubMed2006

Photosensitized oxidation of sulfides: discriminating between the singlet-oxygen mechanism and electron transfer involving superoxide anion or molecular oxygen.

The oxidation of diethyl and diphenyl sulfide photosensitized by dicyanoanthracene (DCA), N-methylquinolinium tetrafluoroborate (NMQ(+)), and triphenylpyrylium tetrafluoroborate (TPP(+)) has been explored by steady-state and laser flash photolysis studies in acetonitrile, methanol, and 1,2-dichloroethane. In the Et(2)S/DCA system sulfide-enhanced intersystem crossing leads to generation of (1)O(2), which eventually gives the sulfoxide via a persulfoxide; this mechanism plays no role with Ph(2)S, though enhanced formation of (3)DCA has been demonstrated. In all other cases an electron-transfer (ET) mechanism is involved. Electron-transfer sulfoxidation occurs with efficiency essentially independent of the sulfide structure, is subject to quenching by benzoquinone, and does not lead to Ph(2)SO cooxidation. Formation of the radical cations R(2)S(*+) has been assessed by flash photolysis (medium-dependent yield, dichloroethane>>CH(3)CN>CH(3)OH) and confirmed by quenching with 1,4-dimethoxybenzene. Electron-transfer oxidations occur both when the superoxide anion is generated by the reduced sensitizer (DCA(*-), NMQ(*)) and when this is not the case (TPP(*)). Although it is possible that different mechanisms operate with different ET sensitizers, a plausible unitary mechanism can be proposed. This considers that reaction between R(2)S(*+) and O(2)(*-) mainly involves back electron transfer, whereas sulfoxidation results primarily from the reaction of the sulfide radical cation with molecular oxygen. Calculations indeed show that the initially formed fleeting complex RS(2)(+)...O-O(*) adds to a sulfide molecule and gives strongly stabilized R(2)S-O(*)-(+)O-SR(2) via an accessible transition state. This intermediate gives the sulfoxide, probably via a radical cation chain path. This mechanism explains the larger scope of ET sulfoxidation with respect to the singlet-oxygen process.

Journal Article↗
PubMed2003

QM/MM Car-Parrinello molecular dynamics study of the solvent effects on the ground state and on the first excited singlet state of acetone in water.

We present a hybrid Car-Parrinello quantum mechanical/molecular mechanical (QM/MM) approach that is capable of treating the dynamics of molecular systems in electronically excited states in complex environments. The potential energy surface in the excited state is described either within the restricted open-shell Kohn-Sham (ROKS) formalism or within time-dependent density functional theory (TDDFT). As a test case, we apply this technique to the study of the solvent effects on the ground state and on the first excited singlet state of acetone in water. Our results demonstrate that for this system a purely classical description of the solvent is sufficient, since inclusion of the first solvent shell of 12 water molecules into the quantum system does not show a significant effect on this transition. The excited-state energies calculated with ROKS are red shifted by a constant value compared to the TDDFT results, while the relative variations of the excitation energy for different configurations are in very good agreement. The experimentally observed blue shift of the excitation energy in going from gas phase to condensed phase is well reproduced. Excited-state dynamics carried out with ROKS yield the relaxation of the solute and the rearrangement of the solvent structure on a picosecond timescale. The calculated Stokes shift is in reasonable agreement with experimental data.

Journal Article↗
PubMed2004

Excitonic coupling strength and coherence length in the singlet and triplet excited states of meso-meso directly linked Zn(II)porphyrin arrays.

Excitation-energy transport (EET) phenomena in meso-meso directly linked Zn(II)porphyrin arrays in the singlet and triplet excited states were investigated with a view to electronic coupling strength and coherence length by steady-state and time-resolved spectroscopic measurements. To investigate energy transfer in the triplet states, we modified the Zn(II)porphyrin arrays with bromo substituents at both ends. The coupling strength of the Soret bands of the arrays was estimated to be about 2200 cm-1, and that of the Q bands is about 570 cm-1. The coherence length in the S1 state of the Zn(II)porphyrin arrays was determined to be 4-5 porphyrin units, which is comparable to that of the well-ordered two-dimensional circular structure B850 in the peripheral light-harvesting antenna (LH2) in photosynthetic purple bacteria. This indicates that the Zn(II)porphyrin arrays are well suited for mimicking natural light-harvesting antenna complexes. On the other hand, the rate of energy transfer in the triplet state is estimated to be on the order of 100 microseconds-1, and the very weak coupling between the triplet states (ca. 0.003 cm-1), indicates that the triplet excitation energy is essentially localized on a single porphyrin moiety.

Journal Article↗
PubMed2004

Singlet excited-state interactions in naphthalene-thymine dyads.

Two thymidine-derived nucleosides 1 and 2 were prepared by attaching a chiral naphthalene to the positions 5' and 3' of the sugar. The resulting dyads, which contain key substructures present in drugs and nucleic acids, exhibit different spatial arrangements (transoid or cisoid) of the fluorophore relative to the thymine unit. Emission measurements on these compounds in the presence of ROH molecules revealed a remarkable intramolecular prescence quenching for dyad 1. The obtained results are consistent with quenching of the singlet excited state of 1 by hydrogen-bond donor solvents. Thus, a physical deactivation process (vibronically induced internal conversion) would be the pathway responsible for the accelerated decay of 1*, favorably competing with fluorescence and intersystem crossing to the triplet. This effect appears to be strongly dependent on the relative spatial arrangement between the naphthalene and thymine units, together with the hydrogen-bonding ability of the employed ROH.

Crystallography, X-Ray↗
PubMed1992

DNA deformation in nucleoprotein complexes between RNA polymerase, cAMP receptor protein and the lac UV5 promoter probed by singlet oxygen.

Singlet oxygen (1O2), generated by exciting an eosin-Tris complex with a high intensity beam of radiation at 532 nm, was used to chemically modify bases in fragments of DNA containing the lac UV5 promoter in the presence of the DNA binding proteins, RNA polymerase and CRP (cAMP receptor protein). Subsequent treatment with piperidine selectively cleaved the DNA at specific modified bases in the sequence. Using this technique we show first that the reactivity of DNA bound by CRP differs in the presence and absence of RNA polymerase. Hence the local conformation of CRP-bound DNA must change during the transition to the open complex. However, no reactivity is observed at the sites of the 40 degrees kinks described in the cocrystal structure (Steitz, 1990). Secondly we show that there is unique CRP-dependent reactivity at a specific site (position -46 on the upper strand) in the open complex. Finally, in the open complex, 1O2 also reacts with sites 90 bp upstream from the transcription start point. This reactivity is qualitatively CRP-independent. We infer that 1O2 reacts at sites where the promoter DNA is significantly distorted, and suggest that the pattern observed reflects the functional orientation of an active transcriptional complex in which the DNA is bent to form an extended loop.

Cyclic AMP Receptor Protein↗
PubMed2002

Ab initio study of the mechanism of singlet-dioxygen addition to hydroxyaromatic compounds: negative evidence for the involvement of peroxa and endoperoxide intermediates.

In this article we report our study of two possible mechanisms of photooxidation of hydroxyaromatic compounds, involving the intermediacy of zwitterionic peroxa intermediates or 1,4-endoperoxides. To study the pathway of the first of them, as yet unexplored by theoretical methods, a simpler system composed of 1,3-butadiene-1-ol and singlet ((1)Delta(g)) dioxygen was considered first, for which calculations were carried out at the CASSCF/MCQDPT2 ab initio level, mostly with the 6-31G* basis set. The cumulative activation barrier to this reaction was found to be 20 kcal/mol and corresponded to a proton transfer (from the hydroxy oxygen atom to the attached oxygen molecule) in the cyclic zwitterionic peroxacyclopenta-3-ene-2-ol intermediate. This intermediate and the proton-transfer transition state were found to have a closed-shell character, which enabled us to estimate the corresponding activation barrier for the phenol-dioxygen system by carrying out optimization at the RHF level and single-point calculations at the MP2, CASSCF, and MCQDPT2 levels of theory. The energy barrier to the reaction was estimated to at least about 40 kcal/mol, rendering this mechanism for the phenol-oxygen system unlikely for nonpolar solvents. Similarly, calculations of the barrier to proton transfer from the 1,4-endoperoxide of phenol to its hydroperoxide were found to exceed 60 kcal/mol, eliminating such a mechanism too, which leaves only the earlier postulated mechanisms involving an initial charge or hydrogen-atom transfer to dioxygen as probable.

Journal Article↗
PubMed2003

Concerning the reaction between singlet nitrenium ions and water: a computational investigation on competitive reaction paths.

The reaction between singlet nitrenium ions XNH(+) (X = F and Cl) and H(2)O has been investigated by high-level of theory ab initio calculations. The geometries of the involved intermediates, transition structures, and dissociation products have been optimized at the MP2(full)/6-31G(d) level of theory, and accurate total energies have been obtained using the Gaussian-3 (G3) procedure. The reaction commences by the exothermic formation of the F-NH-OH(2) (+) and Cl-NH-OH(2) (+) intermediates, which are in turn able to undergo two distinct low-energy reaction paths, namely, the isomerization to the N-protonated isomers of the hydroxylamines F-NH-OH or Cl-NH-OH, and the eventual extrusion of HF or HCl. The competitive or alternative occurrence of these two processes strictly depends on the nature of the substituent X. In the reaction between FNH(+) and H(2)O, the energy gained in the formation of the complex F-NH-OH(2) (+) from the association between FNH(+) and H(2)O, 52.1 kcal mol(-1), is by far larger than the activation barrier for the loss of HF from F-NH-OH(2) (+), computed as 24.9 kcal mol(-1). In addition, the F-NH-OH(2) (+) intermediate requires 33.0 kcal mol(-1) to overcome the barrier for the isomerization to F-NH(2)-OH(+). Therefore, the reaction between FNH(+) and H(2)O is expected to occur practically exclusively by HF elimination with formation of the HN-OH(+) ionic product. On the other hand, for the reaction between ClNH(+) and H(2)O, it is not possible to get a definitive conclusion on the competitive or alternative occurrence of the two reaction paths. In fact, the transition structure involved in the elimination of HCl from Cl-NH-OH(2) (+) is only 3.4 kcal mol(-1) lower in energy than the transition structure for the isomerization of Cl-NH-OH(2) (+) to Cl-NH(2)-OH(+). In addition, the absolute values of the energy barriers of these two processes, 24.2 and 27.6 kcal mol(-1), respectively, are comparable with the energy gained in the formation of the complex Cl-NH-OH(2) (+) from the association between ClNH(+) and H(2)O, 24.0 kcal mol(-).1 Therefore, the ClNH(+) cation is predicted to react with water significantly slower than FNH(+).

Journal Article↗
PubMed1998

The aggregation in human lens proteins blocks the scavenging of UVA-generated singlet oxygen by ascorbic acid and glutathione.

One hour of UVA irradiation of air-saturated solutions of 2 mg/mL solubilized lens protein aggregates from aged human lens is able to produce on accumulated concentration of more than 2mM 1O2, along with oxidation of 120 nmol/mL of both Trp and His amino acid residues. Increasing concentrations of either sodium azide or ascorbic acid (up to 10 mM) during the irradiation decreased th His destruction by no more than 50-60% with the intact aggregates, but completely prevented the His loss with proteolyzed aggregates. Glutathione (up to 10 mM) was able to protect less than 10% of the aggregate His residues from oxidative damage, whereas His loss was almost completely prevented in the proteolyzed aggregates. Similar data were obtained for teh UVA photolysis of the Trp residues. This finding led us to study the role a protein conformation of these aggregates plays in the diminishing of antioxidant ability to prevent UVA-mediated photolysis of 1O2-sensitive amino acid residues. We found that Trp, His, and Cys are buried in the aggregates and cannot be oxidized by a relatively high concentration of 1O2 generated externally to the protein. Increasing urea denaturation of the aggregates caused exposure of the buried Trp residues as determined by the red shift of the fluorescence maximum and by a marked increase in the acrylamide and iodide fluorescence quenching. The ability of glutathione to prevent Trp oxidation by UVA light correlated directly with the extent of Trp exposure. These data suggest that the aggregation of the lens crystallins during aging produces a barrier, which prevents the access of water-soluble antioxidants to the sites of UVA-dependent singlet oxygen generation. In this case UVA proteolysis of the lens proteins can proceed even in the presence of physiological levels of antioxidants.

Aging↗
PubMed2001

Spectroscopic studies of the low-lying singlet excited electronic states and photochemical properties of carotenoids.

Investigations of the singlet excited state properties of carotenoids using steady-state fluorescence, transient absorption pump-probe, two-photon excitation, and resonance Raman excitation spectroscopies are described. The application of these experimental techniques to the specific problem of determining the S1 excited energies of carotenoids is discussed in detail, and the recent literature pertaining to the assignment of charge transfer states in carotenoids and states described as having particular pseudoparity elements is reviewed. Hypothetical schemes for how these states may account for some of the dynamic and photochemical behavior of carotenoids are presented.

Carotenoids↗
PubMed2000

Thioredoxin, a singlet oxygen quencher and hydroxyl radical scavenger: redox independent functions.

Thioredoxin is a ubiquitous small protein known to protect cells and tissues against oxidative stress. However, its exact antioxidant nature has not been elucidated. In this report, we present evidence that human thioredoxin is a powerful singlet oxygen quencher and hydroxyl radical scavenger. Human thioredoxin at 3 microM caused 50% inhibition of TEMP-(1)O(2) (TEMPO) adduct formation in a photolysis EPR study. In contrast, Escherichia coli thioredoxin caused 50% inhibition of TEMPO formation at 80 microM. Both E. coli thioredoxin and human thioredoxin inhibited (*)OH dependent DMPO-OH formation as demonstrated by EPR spectrometry. The quenching of (1)O(2) or scavenging of (*)OH was not dependent upon the redox state of thioredoxin. Using a human thioredoxin in which the structural cysteines were mutated to alanine, Trx-C3A, we show that structural cysteines that do not take part in the catalytic functions of the protein are also important for its reactive oxygen scavenging properties. In addition, using a quadruple mutant Trx-C4A, where one of the catalytic cysteines, C35 was mutated to alanine in addition to the mutated structural cysteines, we demonstrated that catalytic cysteines are also required for the scavenging action of thioredoxin. Identification of thioredoxin as a (1)O(2) quencher and (*)OH scavenger may be of significant importance in explaining various redox-related antioxidant functions of thioredoxin.

Alanine↗
PubMed1999

Improved functional recovery of ischemic rat hearts due to singlet oxygen scavengers histidine and carnosine.

There is increasing evidence that reactive oxygen species (ROS) contribute to post-ischemic reperfusion injury, but determination of the specific ROS involved has proven elusive. In the present study electron paramagnetic resonance (EPR) spectroscopy was used in vitro to measure the relative quenching of singlet oxygen (1O2) by histidine and carnosine (beta-alanyl-L-histidine) utilizing the hindered secondary amine 2,2,6,6-tetramethyl-4-piperidone HCl (4-oxo-TEMP). The relative effect of histidine and carnosine on functional recovery of isolated perfused rat hearts was also studied. Functional recovery was measured by left ventricular developed pressure (LVDP), first derivative of left ventricular pressure (dP/dt), heart rate (HR) and coronary flow (CF). EPR measurements and Stern-Volmer plots showed that 400 microM carnosine quenched 1O2 twice as effectively as equimolar histidine in vitro. Moreover, 10 mM histidine improved functional recovery of isolated rat hearts significantly more than 1 mM histidine. Furthermore, 1 mM carnosine improved functional recovery significantly more than equimolar histidine and as effectively as 10 mM histidine. Experiments with 1 mM mannitol, a known hydroxyl radical scavenger, did not show an improvement in functional recovery relative to control hearts, thereby decreasing the likelihood that hydroxyl radicals are the major damaging species. On the other hand, the correlation between improved functional recovery of isolated rat hearts with histidine and carnosine and their relative 1O2 quenching effectiveness in vitro provides indirect evidence for 1O2 as ROS participating in reperfusion injury.

Animals↗
PubMed1998

In vivo lactate editing with simultaneous detection of choline, creatine, NAA, and lipid singlets at 1.5 T using PRESS excitation with applications to the study of brain and head and neck tumors.

Two T2-independent J-difference lactate editing schemes for the PRESS magnetic resonance spectroscopy localization sequence are introduced. The techniques, which allow for simultaneous acquisition of the lactate doublet (1.3 ppm) and edited singlets upfield of and including choline (3.2 ppm), exploit the dependence of the in-phase intensity of the methyl doublet upon the time interval separating two inversion (BASING) pulses applied to its coupling partner after initial excitation. Editing method 1, which allows for echo times TE = n/J (n = 1, 2, 3, . . . . ), alters the BASING carrier frequency for each of two cycles so that, for one cycle, the quartet is inverted, whereas, for the other cycle, the quartet is unaffected. Method 2, which also provides water suppression, allows for editing for TE > 1/J by alternating, between cycles, the time interval separating the inversion pulses. Experimental results were obtained at 1.5 T using a Shinnar Le-Roux-designed maximum phase inversion pulse with a filter transition bandwidth of 55 Hz. Spectra were acquired from phantoms and in vivo from the human brain and neck. In a neck muscle study, the lipid suppression factor, achieved partly through the use of a novel phase regularization algorithm, was measured to be over 10(3). Spectra acquired from a primary brain and a metastatic neck tumor demonstrated the presence of lactate and choline signals consistent with abnormal spectral patterns. The advantages and limitations of the methods are analyzed theoretically and experimentally, and significance of the results is discussed.

Aspartic Acid↗
PubMed1997

Fourier Transform Emission Spectroscopy of ScH and ScD: The New Singlet Electronic States A 1 Delta, D 1 Pi, E 1 Delta, and F 1 Sigma-

The emission spectra of ScH and ScD have been investigated in the near infrared and visible using a Fourier transform spectrometer. The molecules were excited in a scandium hollow cathode lamp operated with neon gas and a trace of hydrogen or deuterium. Apart from the X 1 Sigma+ , B 1 Pi, C 1 Sigma+ , and G 1 Pi states reported previously [R. S. Ram and P. F. Bernath, J. Chem. Phys. 105, 2668-2674 (1996)], four additional new singlet electronic states A 1 Delta, D 1 Pi, E 1 Delta, and F 1 Sigma- have been identified below 21 000 cm-1 . The rotational analysis of several vibrational bands of the D 1 Pi-X 1 Sigma+ , D 1 Pi-A 1 Delta, E 1 Delta-B 1 Pi, E 1 Delta-A 1 Delta, and F 1 Sigma- -B 1 Pi transitions has been carried out and spectroscopic constants have been determined. The E 1 Delta state of ScH was not located because of the very weak intensity of the E 1 Delta-B 1 Pi and E 1 Delta-A 1 Delta transitions.

Journal Article↗
PubMed1998

The High-Resolution Vacuum Ultraviolet Emission Spectrum of Molecular Nitrogen from 82.6 to 124.2 nm: Level Energies of 10 Excited Singlet Electronic States

The high-resolution emission spectrum of molecular nitrogen was photographed in the vacuum ultraviolet from 82.6 to 124.2 nm. The use of a low-pressure Penning-type electric discharge source has led to considerably reduced self-absorption at short wavelengths, making it possible to record as many as 283 emission bands. All emission bands have been rotationally analyzed, 215 being observed for the first time, and are reported in a separate publication. Energy values were deduced for the nonpredissociative or only slightly predissociative rovibronic levels in 10 singlet electronic excited states. Copyright 1998 Academic Press.

Journal Article↗
PubMed1998

On the Singlet Spectrum of ZrS.

Three electronic transitions in the singlet manifold of the zirconium sulphide (ZrS) molecule have been rotationally analyzed. They are the B1Pi-X1Sigma+, C1Sigma+-X1Sigma+, and E1Sigma+-X1Sigma+ systems with (0,0) band heads at 924.60, 731.51, and 494.47 nm, respectively. The ground state X1Sigma+ has an equilibrium bond distance of 2.15661(4) Å. Perturbations in the B1Pi and C1Sigma+ states are discussed. Copyright 1998 Academic Press.

Journal Article↗