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PubMedDate not supplied

QSPR analysis of flash points.

A quantitative structure property relationship study of the flash point of a diverse set of 271 compounds provided a general three-parameter QSPR model (R(2) = 0.9020, R(2)(cv) = 0.8985, s = 16.1). Use of the experimental boiling point as a descriptor gives a three-descriptor equation with R(2) = 0.9529. Use of the boiling point predicted by a four-parameter reported relationship gives a three-parameter flash point equation with a R(2) value of 0.9247.

Journal Article↗
PubMed2000

The flash-quench technique in protein-DNA electron transfer: reduction of the guanine radical by ferrocytochrome c.

Electron transfer from a protein to oxidatively damaged DNA, specifically from ferrocytochrome c to the guanine radical, was examined using the flash-quench technique. Ru(phen)2dppz2+ (dppz = dipyridophenazine) was employed as the photosensitive intercalator, and ferricytochrome c (Fe3+ cyt c), as the oxidative quencher. Using transient absorption and time-resolved luminescence spectroscopies, we examined the electron-transfer reactions following photoexcitation of the ruthenium complex in the presence of poly(dA-dT) or poly(dG-dC). The luminescence-quenching titrations of excited Ru(phen)2dppz2+ by Fe3+ cyt c are nearly identical for the two DNA polymers. However, the spectral characteristics of the long-lived transient produced by the quenching depend strongly upon the DNA. For poly(dA-dT), the transient has a spectrum consistent with formation of a [Ru(phen)2dppz3+, Fe2+ cyt c] intermediate, indicating that the system regenerates itself via electron transfer from the protein to the Ru(III) metallointercalator for this polymer. For poly(dG-dC), however, the transient has the characteristics expected for an intermediate of Fe2+ cyt c and the neutral guanine radical. The characteristics of the transient formed with the GC polymer are consistent with rapid oxidation of guanine by the Ru(III) complex, followed by slow electron transfer from Fe2+ cyt c to the guanine radical. These experiments show that electron holes on DNA can be repaired by protein and demonstrate how the flash-quench technique can be used generally in studying electron transfer from proteins to guanine radicals in duplex DNA.

Cytochrome c Group↗
PubMed2000

Ruthenium bisbipyridine complexes of horse heart cytochrome c: characterization and comparative intramolecular electron-transfer rates determined by pulse radiolysis and flash photolysis.

The reaction of [Ru(bpy)2L(H2O)]2+ (bpy = 2,2'-bipyridine, L = imidazole, water) with reduced horse heart cytochrome c results in coordination of [RuII(bpy)2L] at the His 33 and His 26 sites. Coordination at the His 33 site gave a diastereomeric [RuII(bpy)2L]-His-cyt c(II) mixture favoring the lambda-Ru form regardless of the substituent on the bipyridine ligands, while substitution at the more buried His 26 site gave an isomeric distribution that varies according to the substituent on the bipyridine ligands. The diastereomeric aquoproteins (L = H2O) are distinguished by their redox potentials and their conversion to the corresponding fluorescent imidazole proteins. Intramolecular electron transfer between the reduced ruthenium bipyridine and cyt c(III) in [RuII(bpy.)(bpy)L]-His33-cyt c(III) was determined by reductive pulse radiolysis using the aqueous electron as a reducing agent, kret = (2.0 +/- 0.3) x 10(5) s-1, and kret is independent of the sixth ligand L = H2O, imidazole. In addition, the rate constant for intramolecular electron transfer from cyt c(II) to the ruthenium(III) center in [RuIII(bpy)2L]-His33-cyt c(II) was determined by oxidative pulse radiolysis using azide and carbonate radicals. This rate is very sensitive to the nature of the sixth ligand. When L = H2O, the intramolecular electron-transfer rate for the major diastereomer lambda-cis-[RuIII (bpy)2(H2O)]-His33-cyt c(II) is k = 1.1 x 10(4) s-1 and is independent of pH between 5.6 and 8.3. The minor delta-cis-[RuIII(bpy)2(H2O)]-His33-cyt c(II) isomer has pH-dependent electrochemistry and a lower rate of intramolecular electron transfer. Complete conversion from L = H2O to L = imidazole is slow, requiring more than 7 days in 1 M imidazole. A lower limit (k > 2 x 10(6) s-1) for the intramolecular electron-transfer rate constant in [RuIII(bpy)2(L)]-His33-cyt c(II), L = imidazole, could be obtained by pulse radiolysis in the absence of the slower reacting aquo species. This observation is in agreement with the value of 3 x 10(6) s-1 measured by flash photolysis. Earlier pulse radiolysis experiments primarily measured the aquoligated ruthenium protein, while the flash photolysis experiments measured the imidazole-ligated fraction because it is the only species oxidatively quenched in the photoinduced reactions. Intramolecular electron-transfer reactions for a new series of ruthenium bipyridine complexes, [Ru(dabpy)2L]-His33-cyt c proteins (dabpy = 4,4'-diamino-2,2'-bipyridine) (L = imidazole, pyridine, isonicotinamide and pyrazine), proceed with lower driving force, resulting in slower rate constants amenable to measurement by oxidative pulse radiolysis. The electron-transfer rate constants for this series spanned a wide range of the Marcus log k vs delta G plot.

Animals↗
PubMed2001

Product studies and laser flash photolysis on alkyl radicals containing two different beta-leaving groups are consonant with the formation of an olefin cation radical.

1-Bromo-2-methoxy-1-phenylpropan-2-yl (3) and 2-methoxy-1-phenyl-1-diphenylphosphatopropan-2-yl (4) were generated under continual photolysis from the respective PTOC precursors in a mixture of acetonitrile and methanol. The radicals undergo heterolytic fragmentation of the substituent in the beta-position to generate the olefin cation radical (5). Z-2-Methoxy-1-phenylpropene (15) is the major product formed in the presence of 1,4-cyclohexadiene, and is believed to result from hydrogen atom transfer to the oxygen of the olefin cation radical, followed by deprotonation. Laser flash photolysis experiments indicate that reaction between 5 and 1,4-cyclohexadiene occurs with a rate constant of approximately 6 x 10(5) M(-1) s(-1). 2,2-Dimethoxy-1-phenylpropane (18) is observed as a minor product. Laser flash photolysis experiments place an upper limit on methanol trapping of 5 at k <1 x 10(3) M(-1) s(-1) and do not provide any evidence for the formation of reactive intermediates other than 5. The use of two PTOC precursors containing different leaving groups to generate a common olefin cation radical enables one to utilize product analysis to probe for the intermediacy of other reactive intermediates. The ratio of 15:18 is dependent upon hydrogen atom donor concentration, but is independent of the PTOC precursor. These observations are consistent with the proposal that both products result from trapping of 5 that is formed via heterolysis of 3 and 4.

Alkenes↗
PubMed2002

Reactions of N-methyl-N-(4-biphenylyl)nitrenium ion with electron-rich arenes: laser flash photolysis and product studies.

An arylnitrenium ion, N-methyl-N-(4-biphenylyl)nitrenium ion, was generated through photolysis of 1-(N-methyl-N-4-biphenylyl)amino-2,4,6-trimethylpyridinium tetrafluoroborate, and its reactions with various donor-substituted arenes (e.g., 1,3,5-trimethoxybenzene, mesitylene, 1,4-dimethoxybenzene, hexamethylbenzene, etc.) were examined using product analysis and laser flash photolysis. In general, trapping of the short-lived nitrenium ion by the arenes leads to three types of products: (1) the parent amine, N-methyl-N-4-biphenylylamine; (2) an ortho-adduct, where the ring position ortho to the nitrenium ion center is bonded to the arene ring; and (3) an N-adduct, where the nitrenium ion nitrogen is bonded to the trap. Laser flash photolysis studies show that the rates of these trapping reactions vary from 10(4) to 10(9) M(-1) s(-1), depending on the structure of the arene trap. These trapping rate constants do not correlate with the one-electron oxidation potential of the arene, nor with the expected stability of a sigma-complex derived from direct electrophilic aromatic substitution. It is argued that the observed rate constants correspond to initial formation of a pi-complex between the arylnitrenium ion and the arene trap. This complex then forms the observed products.

Journal Article↗
PubMed2002

Flash photolytic generation of o-quinone alpha-phenylmethide and o-quinone alpha-(p-anisyl)methide in aqueous solution and investigation of their reactions in that medium. Saturation of acid-catalyzed hydration.

o-quinone alpha-phenylmethide was generated as a short-lived transient species in aqueous solution by flash photolysis of o-hydroxy-alpha-phenylbenzyl alcohol, and its rate of decay was measured in HClO4 and NaOH solutions as well as in CH3CO2H, H2PO4-, and HCO3- buffers. These data show that hydration of this quinone methide back to its benzyl alcohol precursor occurs by acid-, base-, and uncatalyzed routes. The acid-catalyzed reaction gives the solvent isotope effect kH+/kD+ = 0.34, whose inverse nature indicates that this reaction occurs via rapid preequilibrium protonation of the quinone methide on its carbonyl oxygen atom followed by rate-determining capture of the ensuing carbocationic intermediate by water, a conclusion supported by the saturation of acid catalysis in concentrated HClO4 solution. o-quinone alpha-(p-anisyl)methide was also generated by flash photolysis of the corresponding benzyl alcohol and of the p-cyanophenol ether of this alcohol as well, and its rate of decay was measured in HClO4 and NaOH solutions and in HCO2H, CH3CO2H, HN3, CF3CH2NH3+, imidazolium ion, H2PO4-, (CH2OH)3CNH3+, (CH3)3CPO3H-, and HCO3- buffers. Acid-, base-, and uncatalyzed hydration reaction routes were again found, and solvent isotope effects as well as saturation of acid catalysis, this time in dilute HClO4, confirmed a preequilibrium mechanism for the acid-catalyzed reaction. Analysis of the buffer data gave buffer-base rate constants that did not conform to the Brønsted relation, consistent with the expected nucleophilic nature of the buffer reactions.

Acids↗
PubMed2002

Reactions of nitrogen oxides with heme models. Characterization of NO and NO2 dissociation from Fe(TPP)(NO2)(NO) by flash photolysis and rapid dilution techniques: Fe(TPP)(NO2) as an unstable intermediate.

Described are studies directed toward elucidating the controversial chemistry relating to the solution phase reactions of nitric oxide with the iron(II) porphyrin complex Fe(TPP)(NO) (1, TPP = meso-tetraphenylporphinato2-). The only reaction observable with clean NO is the formation of the diamagnetic dinitrosyl species Fe(TPP)(NO)2 (2), and this is seen only at low temperatures (K(1) < 3 M(-1) at ambient temperature). However, 1 does readily react reversibly with N2O3 in the presence of excess NO to give the nitro nitrosyl complex Fe(TPP)(NO2)(NO) (3), suggesting that previous claims that 1 promotes NO disproportionation to give 3 may have been compromised by traces of air in the nitric oxide sources. It is also noted that 3 undergoes reversible loss of NO to give the elusive nitro species Fe(TPP)(NO2) (4), which has been implicated as a powerful oxygen atom transfer agent in reactions with various substrates. Furthermore, in the presence of excess NO2, the latter undergoes oxidation to the stable nitrato analogue Fe(TPP)(NO3) (5). Owing to such reactivity of Fe(TPP)(NO2), flash photolysis and stopped-flow kinetics rather than static techniques were necessary for the accurate measurement of dissociation equilibria characteristic of Fe(TPP)(NO2)(NO) in 298 K toluene solution. Flash photolysis of 3 resulted in competitive NO2 and NO dissociation to give Fe(TPP)(NO) and Fe(TPP)(NO2), respectively. The rate constant for the reaction of 1 with N2O3 to generate Fe(TPP)(NO2)(NO) was determined to be 1.8 x 10(6) M(-1) s(-1), and that for the NO reaction with 4 was similarly determined to be 4.2 x 10(5) M(-1) s(-1). Stopped-flow rapid dilution techniques were used to determine the rate constant for NO dissociation from 3 as 2.6 s(-1). The rapid dilution experiments also demonstrated that Fe(TPP)(NO2) readily undergoes further oxidation to give Fe(TPP)(NO3). The mechanistic implications of these observations are discussed, and it is suggested that NO2 liberated spontaneously from Fe(P)(NO2) may play a role in an important oxidative process involving this elusive species.

Heme↗
PubMed2002

Flash photolysis of (E)-1,2-bis(1-chloro-1-phenylmethyl)cyclopropane. Generation of 1,5-diphenylpentadienyl radical and 1,5-diphenylpentadienylium cation.

The 1,5-diphenylpentadienyl radical (5) is generated from (E)-1,2-bis(1-chloro-1-phenylmethyl)cyclopropane (1) via a two-photon process, either in cyclohexane or in acetonitrile as solvent. Two-laser two-color flash photolysis experiments show that excitation of the benzylic radical generated by homolysis of the first C-Cl bond leads, after ring-opening and proton loss, to the stabilized radical 5. This radical is also generated by photolysis of either (1E,3E)-5-chloro-1,5-diphenyl-1,3-pentadiene (6) or (1E,4E)-1,5-diphenyl-1,4-)pentadiene (7) via one-photon or two-photon processes, respectively. On the other hand, laser flash photolysis of 1 in acetonitrile also produces some 1,5-diphenylpentadienylium cation (10) generated via a one-photon process. Its formation can be explained as due to competitive photoheterolysis leading to a benzylic cation which thermally ring-opens and dehydrohalogenates. Species 10 is more efficiently generated by photolysis of 6 in acetonitrile and undergoes photoisomerization after laser excitation.

Journal Article↗
PubMed2004

Photosensitized reactions of oxime ethers: a steady-state and laser flash photolysis study.

The mechanistic aspects of the photosensitized reactions of a series of oxime ethers were studied by steady-state (product studies) and laser flash photolysis methods. Nanosecond laser flash photolysis studies have shown that chloranil-sensitized reactions of the oxime ethers result in the formation of the corresponding radical cations. The radical cation species react with nucleophiles such as MeOH by clean second-order kinetics with rate constants of (0.7-1.4) x 10(6) M(-1) s(-1). Only a small steric effect is observed in these reactions, which is taken as an indication that the reaction center is not the O-alkyl moiety, but rather somewhere else in the molecule. Product studies in a polar nonnucleophilic solvent (MeCN) revealed that in order for the oxime ether radical cation to react more readily, alpha-protons must be available on the alkyl group. The O-methyl (1), O-ethyl (2), and O-benzyl (3) acetophenone oximes all reacted readily to give acetophenone oxime as the major product (as well as an aldehyde derived from the O-alkyl group), whereas O-tert-butyl acetophenone oxime (4) did not. The product formation can be explained by a mechanism that involves electron transfer followed by proton transfer (alpha to the oxygen) and subsequent beta-cleavage. When using 3 in MeOH, a change in the product formation is observed, the most important difference being the presence of benzyl alcohol rather than benzaldehyde as the major product. On the basis of the data from LFP and steady-state experiments, it is suggested that the competing mechanism under these conditions involves electron transfer, followed by a nucleophilic attack on the nitrogen, a MeOH-assisted [1,3]-proton transfer, and subsequent loss of benzyl alcohol. This mechanism is supported by DFT (B3LYP/6-31G) and AM1 calculations.

Journal Article↗
PubMedDate not supplied

Oxygen uptake occurs faster than sodium pumping in bee retina after a light flash.

When neurones are active there is an entry of Na+, which must subsequently be pumped out, and an increase in their oxygen consumption rate (Qo2). The Na+ pump derives its energy from ATP, splitting it into ADP and Pi, and it has reasonably been proposed that the changes in concentrations of ATP, ADP and Pi lead to a stimulation of the O2 consumption by the mitochondria and hence to a restoration of the stock of ATP. Here we present evidence suggesting that Qo2 must be controlled differently in the retinal photoreceptor cells of the honeybee drone. Stimulation of drone photoreceptors with a flash of light causes an entry of Na+ (ref. 4) and a transient increase in Qo2 that indicates respiration of the right order of magnitude to provide ATP to pump the Na+ out. We report intracellular recordings of changes in intracellular sodium (Nai+) and potassium (Ki+) in response to single light flashes and have compared the time course of extra oxygen consumption (delta Qo2) with these ion changes and other indices of Na+ pumping. We found that the time course of pumping seems to lag behind the time course of delta Qo2. It follows that the mitochondrial respiration must be stimulated by some signal which is generated earlier than the rise in ADP produced by the Na+ pump.

Adenosine Triphosphate↗
PubMed1992

When are simple flashes and floaters ocular emergencies?

One hundred and seventy patients presented to the Bristol Eye Hospital casualty department with flashes and/or floaters during a six month prospective study. A sight-threatening condition was found in 41 patients (24.1%), the most important of which was a retinal break (16.5%). Although there were no specific symptoms which could be correlated to an increased incidence of retinal breaks, those patients who complained of isolated uniocular floaters had an insignificant incidence of breakage, when compared to asymptomatic fellow eyes. Thus while the majority of patients with flashes and floaters do merit an urgent ophthalmological opinion, those who complain of a single, isolated floater can safely be reviewed as routine outpatients.

Adolescent↗
PubMed1997

Cone ERG subnormality to red flash in central retinal vein occlusion: a predictor of ocular neovascularisation?

In patients with unilateral central retinal vein occlusion (CRVO), we retrospectively examined whether cone electroretinogram (ERG) subnormality to red flash (ratio of the b-wave amplitude in the CRVO eye to that in the normal fellow eye < 1) found at the time of diagnosis of the CRVO was a predictor of later ocular neovascularisation. Ganzfeld ERG cone and rod responses had initially been obtained in a consecutive series of 21 patients with unilateral CRVO. Patients were re-evaluated 6-55 months later to determine whether ocular neovascularisation had developed. Of the 21 CRVO eyes, 6 (29%) were subnormal to red in the affected compared with the normal fellow eye. At follow-up, all 6 (100%) patients had developed ocular neovascularisation compared with 1 (7%) of the 15 patients who were supernormal to red (p = 0.00013). Cone ERG subnormality to red flash in CRVO eyes compared with normal fellow eyes may be a predictor of later development of ocular neovascularisation.

Adaptation, Ocular↗
PubMed1997

The prognostic value of flash visual evoked potentials in the assessment of non-ocular visual impairment in infancy.

The results of flash visual evoked potentials (VEPs) in 44 infants blind or severely visually impaired from non-ocular causes are presented, and related to the subsequent visual outcome. Ocular causes of visual impairment were excluded by clinical examination and electroretinography. Using a 2 x 2 contingency table, a significant association between VEP and outcome was demonstrated (chi 2 = 3.51, 1 d.f., p = 0.05). Of 13 infants with normal VEPs, 11 demonstrated substantial visual improvement (negative predictive value = 84.6%). However, of the 31 with abnormal VEPs, only 14 remained severely impaired/blind; the other 17 demonstrating visual improvement (positive predictive value = 45.1%). The sensitivity of the method was high in that 14 of 16 (87.5%) infants who remained impaired/blind had abnormal VEPs, but specificity was low as only 11 of 28 (39.3%) who showed visual improvement had normal VEPs. The accuracy of the technique was therefore low, 25 of 44 (56.8%) being true positive/ negative. With regard to visual outcome when faced with an apparently blind infant, it is important not to be too pessimistic for, as is shown in this study, 28 of 44 demonstrated substantial improvement. There are no absolute indicators of prognosis, but the presence of structural cerebral lesions and a history of either neonatal meningitis or encephalopathy are relatively bad prognostic signs. The flash VEP, despite its limitations, is a useful prognostic tool, particularly in those apparently blind infants whose normal ocular examination/electroretinogram is accompanied by normal VEPs. Those with abnormal VEPs, however, do not necessarily have a poor prognosis, but should be followed-up as maturational changes and/or improvements in function of the sensory pathway will be reflected in the evoked potentials.

Blindness, Cortical↗
PubMed2004

Flash welding of conducting polymer nanofibres.

The absorption of light by a material generates heat through non-radiative energy dissipation and exothermic photochemical reactions. In nanostructured materials, the heat generated through photothermal processes will be confined within the individual nanostructures when heat transfer to neighbouring nanostructures and the environment is slow. This leads to unprecedented photothermal effects that cannot be observed in bulk materials, especially when a strong, pulsed light source is used. Here we demonstrate an enhanced photothermal phenomenon with conducting polymer nanofibres in which a camera flash causes instantaneous welding. Under flash irradiation, polyaniline nanofibres 'melt' to form a smooth and continuous film from an originally random network of nanofibres. This photothermal effect can be used to form asymmetric nanofibre films, to melt-blend polymer-polymer nanocomposites rapidly and to photo-pattern polymer nanofibre films.

Microscopy, Electron↗
PubMed2005

A FlAsH-based FRET approach to determine G protein-coupled receptor activation in living cells.

Fluorescence resonance energy transfer (FRET) from cyan to yellow fluorescent proteins (CFP/YFP) is a well-established method to monitor protein-protein interactions or conformational changes of individual proteins. But protein functions can be perturbed by fusion of large tags such as CFP and YFP. Here we use G protein-coupled receptor (GPCR) activation in living cells as a model system to compare YFP with the small, membrane-permeant fluorescein derivative with two arsen-(III) substituents (fluorescein arsenical hairpin binder; FlAsH) targeted to a short tetracysteine sequence. Insertion of CFP and YFP into human adenosine A(2A) receptors allowed us to use FRET to monitor receptor activation but eliminated coupling to adenylyl cyclase. The CFP/FlAsH-tetracysteine system gave fivefold greater agonist-induced FRET signals, similar kinetics (time constant of 66-88 ms) and perfectly normal downstream signaling. Similar results were obtained for the mouse alpha(2A)-adrenergic receptor. Thus, FRET from CFP to FlAsH reports GPCR activation in living cells without disturbing receptor function and shows that the small size of the tetracysteine-biarsenical tag can be decisively advantageous.

Animals↗
PubMed2005

Continuous flash suppression reduces negative afterimages.

Illusions that produce perceptual suppression despite constant retinal input are used to manipulate visual consciousness. Here we report on a powerful variant of existing techniques, continuous flash suppression. Distinct images flashed successively at approximately 10 Hz into one eye reliably suppress an image presented to the other eye. The duration of perceptual suppression is at least ten times greater than that produced by binocular rivalry. Using this tool we show that the strength of the negative afterimage of an adaptor was reduced by half when it was perceptually suppressed by input from the other eye. The more completely the adaptor was suppressed, the more strongly the afterimage intensity was reduced. Paradoxically, trial-to-trial visibility of the adaptor did not correlate with the degree of reduction. Our results imply that formation of afterimages involves neuronal structures that access input from both eyes but that do not correspond directly to the neuronal correlates of perceptual awareness.

Figural Aftereffect↗
PubMed2002

Product studies and laser flash photolysis of direct and 2,4,6-triphenylpyrylium-zeolite Y photocatalyzed degradation of fenvalerate.

Direct irradiation of fenvalerate (FV) in aqueous solution (4 x 10(-3) M) gives rise to 2,3-diarylisohexanitrile through the homolytic CO-CH(i-Pr) bond cleavage, followed by a rapid decarboxylation of the primary carbonyloxy radical, and radical recombination. The unusual exclusive formation of the cross combination product suggests that the persistent free radical effect is operative in this system and controls product formation. Spectroscopic evidence of the intermediacy of benzylic radicals decaying in microsecond time scale has been obtained by laser flash photolysis. These studies also reveal a remarkable attenuation of radical reactivity toward oxygen resulting from cyano substitution at the radical center. TPY photocatalyzed degradation of FV also leads to decarboxylation products accompanied by oxidative mineralization (38%). The TPY-photocatalyzed degradation can occur through two operating mechanisms involving the generation of OH(*) radicals and/or FV(*+) radical cations. Evidence for the latter intermediate has been obtained by laser flash photolysis in acetonitrile solution (detection of TP(*) radical) and by the quenching by FV of the emission from the TPY solid.

Acetonitriles↗