Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “FLASH”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 901 records · Page 50Linked to original sources
PubMed2003

Flash photolysis and triplet states and free radicals in solution.

A personal account is given of the development of microsecond flash photolysis in George Porter's laboratory at Cambridge in the early 1950s. This made possible, for the first time, the observation of the absorption spectra of the lowest triplet states of many polycyclic aromatic hydrocarbons in fluid solutions. The T-T transitions were found to be very intense with oscillator strengths approaching unity in many cases. Detailed kinetic studies showed that triplet state decay was first order and viscosity dependent and strongly quenched by dissolved oxygen. Several aromatic free radicals such as benzyl were also observed in liquid solution for the first time. The advent of the laser in the early 1960s made it possible to extend flash photolysis studies to the ns and ps and eventually the fs time range. Such studies have revolutionized the study of excited states and free radicals and have made possible advances in the understanding of the dynamics of chemical and biological systems that would have been unimaginable 60 years ago.

Journal Article↗
PubMed2004

Intramolecular electron transfer in diastereomeric naphthalene-amine dyads: a fluorescence and laser flash photolysis study.

Two dyads containing a naphthalene-like chromophore linked to a pyrrolidine-derived moiety, namely (S,S)- and (R,S)-NPX-PYR, have been synthesised by esterification of (S)- or (R)-naproxen (NPX) with (S)-N-methyl-2-pyrrolidinemethanol (PYR) and submitted to photophysical studies (steady-state and time-resolved fluorescence, as well as laser flash photolysis). The emission spectra of the dyads in acetonitrile were characterised by a typical band centred at 350 nm, identical to that of the reference compound (S)-NPX. However the intensities were clearly different, revealing a significant intramolecular quenching in the dyads, as well as a remarkable stereodifferentiation (factor of 1.6). Accordingly, the fluorescence lifetimes of the two dyads were different from each other and markedly shorter than that of (S)-NPX. The quenching mechanism is intramolecular electron transfer, that is thermodynamically favoured. Exciplex formation, that is nearly thermoneutral, does not compete efficiently. The electron transfer rate constants for (S,S)- and (R,S)-(NPX-PYR) were 1.8 x 10(8) and 2.8 x 10(8) s(-1), respectively. By contrast, no significant intramolecular quenching was observed for the excited triplet states (lambda(max)= 440 nm), generated by laser flash photolysis; this is in agreement with the fact that intramolecular electron transfer is thermodynamically disfavoured, due to the lower energy of excited triplets.

Amines↗
PubMed1978

The kinetics and thermodynamics of the reaction of solid-state fully reduced membrane-bound cytochrome oxidase with carbon monoxide as studied by dual-wavelength multichannel spectroscopy and flash photolysis.

1. The results of non-linear optimization studies on the mechanism of reaction of solid-state fully reduced membrane-bound cytochrome oxidase with CO over the 178--203 K range are presented. The analysis is carried out on data obtained by dual-wavelength multichannel spectroscopy at three wavelength pairs (444--463 nm, 590--630 nm and 608--630 nm), which yield three distinct progress curves. The only model that satisfies the triple requirement of a standard deviation within the standard error of the data, a random distribution of residuals and good determination of the optimized parameters is a two-species sequential mechanism: flash photolysis yields unliganded cytochrome oxidase and free CO, which then recombine to form species Ic; Ic is then converted into species IIc, which is identical with the cytochrome oxidase-CO complex existing before flash photolysis. All the thermodynamic parameters describing this model are calculated. 2. On the basis of the data obtained from this paper, together with data from potentiometric studies, magnetic susceptibility measurements and i.r. spectroscopy, the chemical identity of the species is suggested.

Binding Sites↗
PubMed1999

Two-photon and UV-laser flash photolysis of the Ca2+ cage, dimethoxynitrophenyl-EGTA-4.

We report efficient two-photon and UV-laser flash photolysis of dimethoxynitrophenyl-EGTA-4 (DMNPE-4), a newly-developed photolabile Ca(2+)-specific chelator. This compound exhibits good two-photon absorption at 705 nm, has a low Mg2+ affinity (approximately 7 mM), a Kd for Ca2+ of 19 nM, a quantum yield of 0.20 and changes its Ca2+ affinity by 21,000-fold upon photolysis. Two-photon excitation photolysis (TPP) experiments were performed with a Ti:Sapphire laser in solutions containing DMNPE-4 with either 0 or 10 mM Mg2+ and compared to that of the widely used Ca2+ cage, DM-nitrophen (Kd for Ca2+ 5 nM, Kd for Mg2+ 2.5 microM, quantum yield 0.18, affinity change 600,000-fold). The resulting Ca2+ signals were recorded with the fluorescent Ca2+ indicator fluo-3 and a laser-scanning confocal microscope in the line-scan mode. In vitro, photolysis of DMNPE-4:Ca2+ produced Ca(2+)-release signals that had comparable amplitudes and time courses in the presence and absence of Mg2+. However, photorelease of Ca2+ from DM-nitrophen was obviated by the presence of Mg2+. In patch-clamped isolated cardiac myocytes, equivalent TPP results were obtained in analogous experiments. Single-photon excitation of DMNPE-4 by Nd:YAG laser flashes produced Na-Ca exchange currents of comparable amplitude in the absence and presence of Mg2+. However, only very small currents were observed in DM-nitrophen solution containing 10 mM Mg2+. In conclusion, both DMNPE-4 and DM-nitrophen undergo TPP, however, only DMNPE-4 exhibits efficient release of Ca2+ in the presence of Mg2+.

Animals↗
PubMed1995

[The facial nerve in the petrous bone in thin-layer paratransverse and sagittal magnetic-resonance-tomographic T1-spin-echo and FLASH images].

PURPOSE: It is difficult to effect visualization and delineation of the facial nerve and its neighbouring structures in the temporal bone with conventional MRI examination protocols. We tested temporal bone MRI with 2 mm slices and compared T1-weighted FLASH (TR = 400 ms, TE = 10 ms, 90 degrees flip angle) and spin-echo (TR = 540 ms, TE = 15 ms) sequences. PATIENT AND METHODS: 5 volunteers and 14 patients were examined with the head coil of a 1.0T whole body MRI scanner (Impact, Siemens, Erlangen) with para-transversal images orientated parallel to the inferior outline of the clivus and sagittal images orientated along the brainstem. RESULTS: The facial nerve and its neighbouring structures could be reliably visualized and differentiated along its entire course. The FLASH sequence was superior to the spin-echo sequence. 8 of 11 patients with peripheral facial nerve palsy showed contrast enhancement. In two patients, local swelling of the affected facial nerve was evident. CONCLUSION: The MRI technique tested here seems promising for temporal bone examinations.

Adult↗
PubMed1993

[The initial clinical experiences with a dynamic FLASH-2D sequence in tumors of the head and neck area].

28 patients with clinically confirmed head and neck tumours were examined by MRI. The results of a dynamic FLASH-2D sequence and a Gd-DTPA enhanced T1-weighted sequence were analysed and retrospectively compared with surgery and pathological findings. The extent of tumours was clearly defined by the dynamic technique in 21 cases. Furthermore, it was found to be superior for the purpose of demonstrating contrast behaviour and concerning T-classification. Here the dynamic FLASH-2D sequence proved to be an important supplement for preoperative diagnosis.

Adolescent↗
PubMed1992

Anatomy of a flash. 2. The 'width' of a temporal edge.

In a previous study it has been found by using the Crawford masking paradigm with spatially coextensive stimuli that the end points of even a weak flash have a strong masking effect on the probe pulse, but no threshold elevation was observed when the probe was 100 ms or longer from flash onset and offset. The relationship between the strength of a temporal edge (eg the amplitude of a sudden increase in field luminance) and the duration of masking has not been systematically investigated. In the present experiments the duration of masking produced by a step of luminance as a function of step size was measured. With steps whose amplitude was between ten and five hundred times detection threshold, the duration of masking was about constant, ie 70 and 50 ms for on-responses and off-responses, respectively. This refractory period defined the 'width' of a temporal edge. Unless two suprathreshold unidirectional luminance changes are separated by an interval exceeding the width of a temporal edge, they are perceived as a single change in brightness. A linear systems analysis led to predictions for ramp stimuli that were confirmed by experiment.

Attention↗
PubMed2003

Motion, flash, and flicker: a unified spatiotemporal model of perceived edge sharpening.

Blurred edges appear sharper in motion than when they are stationary. We proposed a model of this motion sharpening that invokes a local, nonlinear contrast transducer function (Hammett et al, 1998 Vision Research 38 2099-2108). Response saturation in the transducer compresses or 'clips' the input spatial waveform, rendering the edges as sharper. To explain the increasing distortion of drifting edges at higher speeds, the degree of nonlinearity must increase with speed or temporal frequency. A dynamic contrast gain control before the transducer can account for both the speed dependence and approximate contrast invariance of motion sharpening (Hammett et al, 2003 Vision Research, in press). We show here that this model also predicts perceived sharpening of briefly flashed and flickering edges, and we show that the model can account fairly well for experimental data from all three modes of presentation (motion, flash, and flicker). At moderate durations and lower temporal frequencies the gain control attenuates the input signal, thus protecting it from later compression by the transducer. The gain control is somewhat sluggish, and so it suffers both a slow onset, and loss of power at high temporal frequencies. Consequently, brief presentations and high temporal frequencies of drift and flicker are less protected from distortion, and show greater perceptual sharpening.

Form Perception↗
PubMed2002

Flag errors in soccer games: the flash-lag effect brought to real life.

In soccer games, an attacking player is said to be in an offside position if he or she is closer to the opponents' goal line than both the ball and the second-to-last defender. It is an offence for the attacker to be in an offside position and in active play at the moment a fellow team member plays the ball. Assistant referees often make mistakes when judging an offside offence, probably because of optical errors arising from the viewing angle adopted by them (Oudejans, Verheijen, Bakker, Gerrits, Steinbrückner, Beek, 2000 Nature 404 33). Looking more closely at Oudejans et al's data, we show evidence that the flash-lag effect may contribute significantly to these mistakes. Participation of the flash-lag effect in assistant referees' misjudgments would take this perceptual phenomenon from laboratory setups to a real-life situation for the first time.

Fixation, Ocular↗
PubMed2004

Biomolecular cryocrystallography: structural changes during flash-cooling.

To minimize radiation damage, crystal structures of biological macromolecules are usually determined after rapid cooling to cryogenic temperatures, some 150-200 K below the normal physiological range. The biological relevance of such structures relies on the assumption that flash-cooling is sufficiently fast to kinetically trap the macromolecule and associated solvent in a room-temperature equilibrium state. To test this assumption, we use a two-state model to calculate the structural changes expected during rapid cooling of a typical protein crystal. The analysis indicates that many degrees of freedom in a flash-cooled protein crystal are quenched at temperatures near 200 K, where local conformational and association equilibria may be strongly shifted toward low-enthalpy states. Such cryoartifacts should be most important for strongly solvent-coupled processes, such as hydration of nonpolar cavities and surface regions, conformational switching of solvent-exposed side chains, and weak ligand binding. The dynamic quenching that emerges from the model considered here can also rationalize the glass transition associated with the atomic fluctuations in the protein.

Cold Temperature↗
PubMed1998

Evidence for the load-dependent mechanical efficiency of individual myosin heads in skeletal muscle fibers activated by laser flash photolysis of caged calcium in the presence of a limited amount of ATP.

Although a contracting muscle regulates its energy output depending on the load imposed on it ("Fenn effect"), the mechanism underlying the load-dependent energy output remains obscure. To explore the possibility that the mechanical efficiency, with which chemical energy derived from ATP hydrolysis is converted into mechanical work, of individual myosin heads changes in a load-dependent manner, we examined the auxotonic shortening of glycerinated rabbit psoas muscle fibers, containing ATP molecules almost equal in number to the myosin heads, after laser-flash photolysis of caged calcium. Immediately before laser-flash activation, almost all of the myosin heads in the fiber are in the state M.ADP.Pi, and can undergo only one ATP hydrolysis cycle after activation. When the fibers were activated to shorten under various auxotonic loads, the length, force, and power output changes were found to be scaled according to the auxotonic load. Both the power and energy outputs were maximal under a moderate auxotonic load. The amount of M.ADP.Pi utilized at a time after activation was estimated from the amount of isometric force developed after interruption of fiber shortening. This amount was minimal in the isometric condition and increased nearly in proportion to the distance of fiber shortening. These results are taken as evidence that the efficiency of chemomechanical energy conversion in individual myosin heads changes in a load-dependent manner.

Acetates↗
PubMed1997

On the origin of heme absorption band shifts and associated protein structural relaxation in myoglobin following flash photolysis.

The role of the protein structural change monitored by absorption band shifts following flash photolysis of CO from myoglobin is discussed in terms of structure-function relationships. Evidence is presented that the Soret band shift does not depend primarily on the covalent linkage of the heme iron to the protein by using the mutation H93G(L) in which the proximal histidine 93 is replaced by glycine and an exogenous ligand L, which coordinates the heme iron but is not covalently bound to the globin. While CO rebinding kinetics depend strongly on the nature of the exogenous ligand L in H93G(L), the magnitude and time evolution of the Soret band shift in a viscous buffer on the nanosecond time scale are hardly perturbed in all cases studied. Comparison of the Soret band and charge transfer Band III shifts demonstrates that both have a similar time dependence on the nanosecond to microsecond time scale following flash photolysis in viscous solvents. We conclude that the nonexponential kinetics of protein relaxation probed by transient absorption band position shifts involves primarily distal coordinates prior to ligand escape. This result agrees with earlier measurements of Soret band shifts in distal pocket mutants of myoglobin (1). We suggest that the band shifts are primarily a response to changes in the electrostatic field around the heme (a transient Stark shift) associated with changes in protein structure that occur following ligand photodissociation.

Animals↗
PubMed1985

The effects of halothane on somatosensory and flash visual evoked potentials during operations.

Intraoperative use of somatosensory evoked potentials (SEP's) to monitor intracranial aneurysm surgery and flash visual evoked potentials (F-VEP's) for parasellar surgery have been routinely employed in our clinic. We found that both EP modalities are sensitive to the changing concentration of our standard hypotensive agent, halothane. The prolongation of the N14-N20 interpeak latency to median nerve stimulation at the wrist, and prolongation of P100 latency with altered configuration of early VEP components to flash light stimulation, appear to be the results of direct pharmacological effects of the agent and not an effect of secondary hypotension. VEP is found easily abolished by halothane at a concentration of 2.0%, while the SEP is more resistant. Halothane is not ideal however when monitoring intraoperative VEP.

Adolescent↗
PubMed1976

Flash photolysis of enzymes.

The photoionization of aromatic residues constitutes a major initial photochemical reaction in the flash photolysis of proteins at gamma greater than 250 nm. The ejected electrons have been observed as eaq- and the disulphide bridge electron adduct, and also must be trapped at unidentified sites. The number of tryptophyl (or tyrosyl) residues photo-ionized at 5 musec delay is approximately equal to the number of exposed residues. The flash photolysis data have been related to inactivation by considering how photolysis of these "photolabile" residues can affect enzymic activity, based on the microstructure and available information about permanent alterations and residue specificities. This analysis indicates that hen lysozyme and papain are inactivated by photolysis of an essential Trp residue, that bovine trypsin is inactivated by photolysis of a Trp residue adjacent to the key catalytic Ser and other pathways initiated by excitation of Tyr and Cys, that the efficient photoionization of Tyr and RNase A is not an important inactivating reaction, and that aromatic residues in subtilisn Carlsberg are photosensitive.

Amino Acids↗
PubMed1992

Flash photolysis of caged compounds in Limulus ventral photoreceptors.

Rapid concentration jumps of Ins(1,4,5)P3 or ATP were made inside Limulus ventral photoreceptors by flash photolysis of the parent caged compounds. In intact ventral photoreceptors, the photolysis flash evokes a maximum amplitude light-activated current; therefore, a procedure was developed for uncoupling phototransduction by blocking two of the initial reactions in the cascade, rhodopsin excitation and G protein activation. Rhodopsin was inactivated by exposure to hydroxylamine and bright light. This procedure abolished the early receptor potential and reduced the quantum efficiency by 325 +/- 90-fold (mean +/- SD). G protein activation was blocked by injection of guanosine-5'-O-(2-thiodiphosphate) (GDP beta S). GDP beta S injection reduced the quantum efficiency by 1,881 +/- 1,153-fold (mean +/- SD). Together hydroxylamine exposure and GDP beta S injection reduced the quantum efficiency by 870,000 +/- 650,000-fold (mean +/- SD). After the combined treatment, photoreceptors produced quantum bumps to light that was approximately 10(6) times brighter than the intensity that produced quantum bumps before treatment. Experiments were performed with caged compounds injected into photoreceptors in which phototransduction was largely uncoupled. Photolysis of one compound, myo-inositol 1,4,5-triphosphate P4(5)-1-(2-nitrophenyl)ethyl ester (caged IP3), increased the voltage clamp current in response to the flashlamp by more than twofold without changing the latency of the response. The effect was not seen with photolysis of either adenosine-5'-triphosphate P3-1-(2-nitrophenyl)ethyl ester (caged ATP) or caged IP3 in cells preloaded with either heparin or (1,2-bis-(o-amino-phenoxy)ethane-N-N-N'-N' tetraacetic acid tetrapotassium salt (BAPTA). The results suggest that photoreleased IP3 releases calcium ions from intracellular stores and the resulting increase in [Ca2+]i enhances the amplification of the phototransduction cascade.

Adenosine Triphosphate↗
PubMed1956

Increased photoreversal of ultraviolet injury by flashing light.

1. Photoreversal of ultraviolet (UV) injury was studied in the ciliate protozoan Tetrahymena pyriformis (geleii) strain W, cultured in the absence of other living organisms. The division pattern of progeny of single animals was followed in hanging drop preparations. 2. A sublethal dose of 450 ergs/mm.(2) of monochromatic UV of wave length 2654 A produces a lag before the first division followed by a period of cessation of fission after the second division. This cessation sometimes lasts as long as 6 weeks, during which time the animals become smaller and rounder and more opaque. Organisms about to resume division increase in size and transparency; after a few divisions the animals regain their normal division rate. 3. The effect of UV ranging in intensity from 5 to 15 ergs/mm.(2)/sec. was found to obey the reciprocity law quite well for the UV effect on the division pattern of T. pyriformis. However, the same dose at lower and at higher intensities was less effective. 4. The effect of a dose of UV delivered at high intensity (19 ergs/mm.(2)/sec.) could be increased by flashing the light, indicating that the system became saturated in the continuous light. 5. A photoreversing dose of monochromatic blue light of wave length 4350 A was found to be more effective when delivered as continuous light at a low intensity, or as intermittent light at a high intensity, rather than as continuous light at the high intensity-indicating that a dark mechanism participates in photoreversal. 6. The time for the dark reaction was determined to be of the order of a few hundredths of a second in experiments in which different lengths of dark period were used while maintaining a constant light period of 0.0025 second. 7. For Colpidium colpoda the efficiency of a given dose of photoreversing light was increased by flashing the light. 8. The present experiments are interpreted in terms of data available in the literature.

Animals↗
PubMed1982

Kinetics of oxygen consumption after a single flash of light in photoreceptors of the drone (Apis mellifera).

The time course of the rate of oxygen consumption (QO2) after a single flash of light has been measured in 300-micrometers slices of drone retina at 22 degrees C. To measure delta QO2(t), the change in QO2 from its level in darkness, the transients of the partial pressure of O2 (PO2) were recorded with O2 microelectrodes simultaneously in two sites in the slice and delta QO2 was calculated by a computer using Fourier transforms. After a 40-ms flash of intense light, delta QO2, reached a peak of 40 microliters O2/g.min and then declined exponentially to the baseline with a time constant tau 1 = 4.96 +/- 0.49 s (SD, n = 10). The rising phase was characterized by a time constant tau 2 = 1.90 +/- 0.35 s (SD, n = 10). The peak amplitude of delta QO2 increased linearly with the log of the light intensity. Replacement of Na+ by choline, known to decrease greatly the light-induced transmembrane current, caused a 63% decrease of delta QO2. With these changes, however, the kinetics of delta QO2 (t) were unchanged. This suggest that the recovery phase is rate-limited by a single reaction with apparent first-order kinetics. Evidence is provided that suggests that this reaction may be the working of the sodium pump. Exposure of the retina to high concentrations of ouabain or strophanthidin (inhibitors of the sodium pump) reduced the peak amplitude of delta QO2 by approximately 80% and increased tau 1. The increase of tau 1 was an exponential function of the time of exposure to the cardioactive steroids. Hence, it seems likely that the greatest part of delta QO2 is used for the working of the pump, whose activity is the mechanism underlying the rate constant of the descending limb of delta QO2 (t).

Animals↗
PubMed1983

Chemical excitation of Limulus photoreceptors. II. Vanadate, GTP-gamma-S, and fluoride prolong excitation evoked by dim flashes of light.

We treated Limulus ventral photoreceptors with the phosphatase inhibitors fluoride, vanadate, and GTP-gamma-S [guanosine-5'0-(3-thiotriphosphate)] under various conditions of illumination and external calcium concentrations. In the dark in low-calcium (1 mM) artificial seawater (ASW), fluoride-induced discrete waves cluster together in time. Under these conditions, the intervals between waves were found to be correlated, and there were excess short intervals beyond the number expected from an exponential interval distribution. To assess the effects of the inhibitors on the light response, we stimulated ventral receptors with a series of dim flashes and averaged the current response under voltage clamp. In ASW, vanadate and GTP-gamma-S prolong the decay of the averaged response to dim test flashes, but prolongation does not always accompany the induction of discrete waves in the dark. Prolongation induced by vanadate in normal-calcium (10 mM) ASW was enhanced in low-calcium (1 mM Ca2+) ASW. Many individual response records suggest that prolongation results from extra discrete waves late in the light response, whereas others reveal long-lasting complex waveforms that cannot easily be resolved into discrete waves. The apparent effect of the inhibitors on the light response is to allow a single photoactivated rhodopsin molecule to produce multiple discrete waves and complex long-lasting events. We suggest that both prolongation of the light response and clustering of waves in the dark result from inhibition of a step in the pathway of visual transduction, in which GTP hydrolysis normally helps to turn off the production of both light-evoked and spontaneous waves.

Animals↗