Apparent Lifetime Research Articles

Correlation of agonist structure with acetylcholine receptor kinetics: studies on the frog end-plate and on chick embryo muscle.

The apparent lifetimes of frog end-plate channels activated by several nicotinic agonists have been determined with voltage-jump and fluctuation analysis techniques. The agonists were monoquaternary, N-substituted derivatives of trimethylammonium (TMA). Methyl TMA activated channels which had apparent lifetimes about 3-4 times shorter than acetylcholine (ACh)-activated channels. This result was confirmed with single-channel recordings from embryonic chick skeletal muscle. Channel conductance and voltage dependence of channel lifetime were similar for methyl TMA- and ACh-activated channels. Methyl TMA showed no signs of blocking open end-plate channels. Ethyl TMA, acetylthiocholine, cholinethiol and carbamylcholine all activated channels similar to methyl TMA-activated channels with regard to lifetime. None of these agonists appeared to block end-plate channels in the employed concentrations. 4-ketopentyl TMA, which contains a methylene group in place of the ether oxygen of ACh, sometimes opened end-plate channels with similar apparent lifetimes as those opened by ACh. Single-channel recordings showed that bursts of current from channels activated by 4-ketopentyl TMA have similar durations as do those activated by ACh. Pentyl TMA and benzyl TMA block open end-plate channels even when delivered at doses which elicit very small currents. It is concluded that the ester moiety of ACh serves to stabilize the open conformation of the channel.

On-Line Computer Capability In Fluorescence Spectroscopy

Fluorescence excitation of chlorophyll a and flavin, corrected either by employing the common quantum counter rhodamine B or by comparison with the corresponding absorption spectra, are discussed.absorption are not necessarily identical, as commonly assumed. functions, kernels, is used for smoothing. Cross-correlations with two other, more complex kernels demonstrate their equivalence with the first and fourth derivative. The latter procedures, in turn, have been known for some time to improve wavelength resolution. For the determination of fluorescence quantum efficiencies, are converted from wavelength to wave number on the x-axis, and from energy to quanta absorption on the y-axis. Based on thermodynamic arguments, the fluorescence emission spectrum of chlorophyll a is calculated from its absorption spectrum. Measured and calculated emission are practically identical. Similarly, the integrated absorption spectrum of chlorophyll a was used to calculate the intrinsic fluorescence lifetime and, taking the quantum efficiency of fluorescence into account, the ap-parent lifetime. Continuous fluorescence polarization have been obtained for various commonly used fluorophores. Based on a well-known formula originally deduced by Perrin, angle spectra are calculated from these polarization spectra, which immediately exhibit the angles between the electronic transition dipoles. In addition, for chlorophyll a and flavin, anisotropy spectra have been determined. Taking the dependency of fluorescence polarization on temperature or on the concentration of the fluorophore into account, both its rotational relaxation time and the value of Rc are determined. For all thirteen fluorophores investigated, the corrected and normalized fluorescence excitation and emission are given, including the apparent 0-0 transition energies as deduced from the wavelength of their intersection.

DETERMINATION OF THE GENERATION LIFETIME AND SURFACE GENERATION VELOCITY BY VOLTAGE STEP METHOD

A simple method for determining both the generation lifetime and surface generation velocity is suggested. By this method, using the data of three points from the MOS capacitance transient response to a step voltage, both the generation lifetime and surface generation velocity can be calculated readily. The experimental results show that this method is reasonable, and indicate that neglecting surface generation velocity would result in reducing of apparent generation lifetime.

Evidence for short So 2 lifetimes in the atmosphere: An in-situ measurement of atmospheric so 2 lifetime using cosmic ray produced Sulphur-38

A method for the determination of the in-situ lifetime of atmospheric SO 2 has been developed, using cosmic ray produced radioactive 38S. The 38S production rate by cosmic rays and the 38SO 2 radioactivity of atmospheric samples have been measured. The lifetimes of SO 2 have been obtained by comparison of the 38SO 2 concentrations actually measured and modelled concentrations; the modelling takes into account vertical mixing, dry deposition and radioactive decay, but not in-situ conversion into particulate S (horizontal mixing is neglected in the model). The apparent lifetimes determined by the 38S-method, ranging between 1 2 h and a few h in most cases, are much shorter than lifetimes reported up to now. Lifetimes seem to be strongly influenced by solar radiation. Simple S balance considerations give further evidence for short SO 2 lifetimes in the atmosphere.

Measurement of atomic densities using radiation trapping

The apparent lifetimes of the 2P1/2 and 2P3/2 states of Na have been measured as functions of n, the atomic density, and the results compared to theory. It is shown that these measurements made under equilibrium, steady-state conditions may be used to determine n under nonequilibrium, nonsteady-state conditions. This technique of measuring n has advantages over the surface ionization technique, especially for values of n greater than about 1010 cm−3. Extension of the technique to other atoms and molecules is discussed.

Minority carrier lifetime in GaP grown by liquid phase epitaxy for high temperature applications

We have measured the minority carrier lifetime τmin in liquid phase epitaxial GaP material grown for high-temperature device applications, as a function of doping concentration, temperature, and epitaxial layer thickness. In p-type GaP doped with Mg, τmin remains constant with increasing carrier concentration until p≊1018 cm−3, where it decreases rapidly. For nominally undoped n-type material we find that τmin increases by nearly one order of magnitude over the temperature range T = 22–600 °C. The apparent lifetimes in these thin-layer materials increase with layer thickness indicating that surface and interface recombination are important.

Goniometric direction finding for low‐latitude whistlers and their propagation mechanism

In order to study the propagation mechanism of low‐latitude whistlers, their ionospheric exit points have been determined by a triangulation of the goniometer‐polarimeter systems at the two stations of Kagoshima (geomagnetic coordinates 20°24′N, 197°53′E; L=1.22) and Ohgata (22°06′N, 199°51′E; L=1.25), supplemented by the routine observation at Sakushima (24°08′N, 203°19′E; L=1.28). Detailed analyses were made for January 23, 1978, when the whistler activity was most enhanced during one month’s measurement. The difference in temporal variations of the occurrence rate at the three stations and the temporal variation of the single‐valued dispersion enabled us to subdivide the daytime whistlers into three characteristic time intervals and the nighttime ones into two intervals. The characteristics during the whole intervals were found to be satisfactorily accounted for in terms of ducted propagation. The azimuth measurements have indicated that there is present a discontinuity in the characteristics between each two intervals, indicative of the different duct group for different time intervals; in other words, indicative of the identity and persistency of ducts for each period. Then we obtained the ‘apparent lifetime’ of ducts of 1–2 hours. During the last interval for the daytime whistlers, a few ducts were found to be aligned on a certain latitude, which showed a temporal movement, or first increased and then decreased. The simultaneous presence of a few ducts at the same latitude is also seen at night.

A study of desensitization of acetylcholine receptors using nerve-released transmitter in the frog.

1. Desensitization of acetylcholine (ACh) receptors was studied at the frog neuromuscular junction under voltage clamp.2. ACh was applied directly to junctional receptors by stimulating the motor nerve with trains of impulses. End-plate currents (e.p.c.s) were used to estimate the total number of channel openings by the junctional ACh receptors, and miniature end-plate currents (m.e.p.c.s) were used to measure changes in post-synaptic sensitivity. Under the conditions of these experiments the changes in m.e.p.c. amplitudes were shown to be post-synaptic in origin and thus provided a measure of desensitization.3. When the acetylcholinesterase was inhibited with diisopropylfluorophosphate, neostigmine, or collagenase treatment to prolong the duration of the nerve-released ACh in the synaptic cleft, desensitization developed during repetitive stimulation of 1000 impulses at 5-33 impulses/sec and then recovered after the conditioning trains, with a time constant of about 25 sec.4. When the acetylcholinesterase was active so that the duration of ACh in the synaptic cleft resulting from each nerve impulse was brief (< 300 musec), desensitization developed in response to 300-500 pairs of nerve stimuli if the interval between the impulses of each pair was 25 msec or less. When the interval was 30 msec or greater, however, measurable desensitization did not occur, even if the total number of channel openings was many times greater than in the experiments with shorter intervals or inhibited esterase where desensitization readily occurred.5. The desensitization observed to pairs of impulses was enhanced by chlorpromazine and decreased when the post-synaptic membrane was depolarized, properties similar to those described previously for desensitization to bath and ionophoretic application of ACh.6. These results indicate that desensitization to nerve-released transmitter is not a simple consequence of receptor activation, is not due to blockade of the open receptor channels by ACh, and does not result from ACh binding directly to desensitized receptors with a resulting shift in the receptor population towards the desensitized state.7. We suggest that the desensitization observed to nerve-released transmitter is a two-step process with both steps initiated by ACh. In the first step ACh converts some receptors into a desensitizable state which has an apparent lifetime of less than 30 msec; in the second step ACh desensitizes the desensitizable state.

Tryptophan fluorescence of human hemoglobin. I. Significant change of fluorescence intensity and lifetimes in the T − R transition

The fluorescence spectra and fluorescence lifetimes due to tryptophan residues in HbA, Hb Chesapeake, NES-des-Arg Hb and Hb Kempsey were determined at room temperature. The fluorescence intensity and apparent fluorescence lifetimes decrease when the deoxy or T structure in HbA changes to the oxy or R structure, while no significant difference was observed in Hb Kempsey. The difference of fluorescence behavior was ascribed to the quaternary conformational transition of T- and R-states.

Proof of nanosecond timescale relaxation in apomyoglobin by phase fluorometry

Apomyoglobin was labeled with the fluorescent probe 2-p-toluidinylnaphthalene-6-sulfonic acid (TNS). Apparent phase shift and demodulation lifetimes of bound TNS were measured at various emission wavelengths. The lifetimes increased with increasing wavelength. Similar results were obtained for TNS in the viscous solvent glycerol at 10°C but not for TNS in vitrified or fluid solvent. The wavelength-dependent lifetimes suggest apomyoglobin is relaxing around the TNS molecule during its fluorescent lifetime. Importantly, the apparent phase lifetimes exceeded the apparent modulation lifetimes on the long wavelength side of the emission for TNS in apomyoglobin at 3°C and for TNS in glycerol at 10°C. This result proves the increasing lifetimes are a result of an excited state reaction during the lifetime of the excited state and are not a result of heterogeneity in the fluorescence emission. From the lifetimes on the short wavelength side of the emission the relaxation time of apomyoglobin was estimated to be 18 nsec.

Electron localization in dense nitrogen vapor

The density normalized mobility μn of thermal electrons in saturated nitrogen vapor decreases by two orders of magnitude between n=5.7×1019 and the critical density 6.7×1021 molecule/cm3. This large effect is attributed to temporary anion formation involving resonance rotational states. The process is much more extensive than the quasilocalization of electrons in dense xenon and hydrocarbon vapors. The process is approximately first order in nitrogen at n&amp;lt;2×1021 molecule/cm3, but the order increases progressively with density at n≳2×1021 molecule/cm3. The apparent mean lifetime δτ of the anions at 127 K is 4×10−13 s at n&amp;lt;2×1021 molecule/cm3, but increases at higher densities and reaches 6×10−12 s at n=6.7×1021 molecule/cm3. The value of δτ in the low density gas decreases with increasing temperature.

Demonstration of nanosecond dipolar relaxation in biopolymers by inversion of apparent fluorescence phase shift and demodulation lifetimes.

Demonstration of nanosecond dipolar relaxation in biopolymers by inversion of apparent fluorescence phase shift and demodulation lifetimes.

Solar-wind interactions with the moon: Role of oxygen ions

The solar-wind interacts directly with the lunar surface due to tenuous atmosphere and magnetic field. The interaction results in an almost complete absorption of the solar-wind corpuscles producing no upstream bowshock but a cavity downstream. The solar-wind oxygen ionic species induce and undergo a complex set of reactions with the elements of the lunar minerals and the solar-wind derived trapped gases. The oxygen concentration indegeneous to the lunar surface material is about 60 at.%. Some of these oxygen are displaced from their crystal lattice locations by interactions of the solar-wind corpuscles. A small fraction of these displaced oxygen is in active state. The solar-wind oxygen species flux is about 6×104 cm−2 s−1. Besides inducing and undergoing various reactions these species become trapped as oxygen atoms in the lunar grains. Only a portion of these trapped oxygen atoms is in active state. For the contribution of oxygen atoms and molecules from the lunar surface grains to the atmosphere and their reactions with other species, the diffusion coefficients of oxygen atom and molecule should be known. However their values in the highly radiation-damaged lunar surface material are not known. The coefficients are calculated by using the apparent lifetimes of atomic and molecular oxygen in the lunar material. The atmospheric concentration of oxygen atoms and molecules near the lunar surface are found to be about 20 and 3 cm−3, respectively. These values appear to be very reasonable in comparison with the experimental data. The Apollo 17 lunar orbital UV spectrometer data indicate the atomic oxygen concentration is <8×101 cm−3. The Apollo 17 lunar surface mass spectrometer (sensitivity: 1 count=2×102 molecules cm−3) did not detect any oxygen molecules on the dayside of the Moon, but the sunrise concentration was reported to be 1±×103 cm−3. At the time of the sample collection on the Moon the oxygen content in the trapped gas layer was partly as oxygen atoms and partly as oxygen molecules. At the time of sample analysis on the Earth the concentrations of these two species did not change appreciably.

Variations of mesospheric structures in different seasons

The SOUSY‐VHF‐Radar has been used to study the diurnal variation of the echo power received from mesospheric structures in different seasons. For the first time phase coding techniques have been applied to such measurements with a height resolution of 300 m during 6 days in summer, 5 days in autumn and 3 days in winter 1978 within the height range between 51 and 118 km. The various structures observed are classified according to characteristic features such as thickness and apparent lifetime. The results indicate the existence of four main types of mesospheric structures — blobs, sheets, thin and thick layers. Their spatial and temporal occurrence, which depends on season, is discussed.

Silk protein production by the immobilized silk gland

The posterior silk glands of the silkworm, (itBombyx mori,) were immobilized in acrylamide gel. This immobilized organ produced silk protein in the presence of amino acids and energy sources. Amino acids began to be incorporated into the immobilized silk glands 2.5 h after incubation, and the incorporation continued for more than 20 h. The apparent lifetime of the posterior silk gland was remarkably elongated by the immobilization.

Fluorescence of the Proton Transferred Excited Species between 4-Amino-2-methyl-5H-[1]benzopyrano[3,4-c]pyridin-5-one and Acetic Acid. Time-Resolved Fluorescence Spectra and Solvent Effects

Abstract The nanosecond time-resolved spectra and apparent lifetime for the fluorescence of the proton transferred excited species (tautomer) between 4-amino-2-methyl-5H-[1]benzopyrano[3,4-c]pyridin-5-one (BPP) and acetic acid have been measured in isooctane (2,2,4-trimethylpentane) at room temperature. It has been established that the rising time of the tautomer fluorescence was ca. 15 ns and the time required for the excited hydrogen-bonded species-tautomer equilibrium was ca. 40 ns. Solvent effects upon the fluorescence characteristics of the tautomer have also been investigated.

Measurements of the vacancy formation enthalpy in aluminum using positron annihilation spectroscopy

The temperature dependence of positron annihilation in aluminum has been investigated over the range 20-435\ifmmode^\circ\else\textdegree\fi{}C by simultaneous measurements of positron lifetime and Doppler broadening of the annihilation spectrum. The analyses of lifetime data are critically examined, especially with respect to the effects of instrumental resolution-function uncertainties. The lifetime and Doppler-broadening data have been analyzed in terms of the two-state trapping model. Application of this model to the measurement of the vacancy-formation enthalpy ${E}_{\mathrm{v}}^{F}$ in aluminum is discussed in light of the observation of a strong temperature dependence of the apparent positron lifetime in the bulk or lattice state above 370\ifmmode^\circ\else\textdegree\fi{}C. The monovacancy formation enthalpy in aluminum, ${E}_{1\mathrm{v}}^{F}=0.66\ifmmode\pm\else\textpm\fi{}0.02$ eV, was determined for temperatures below 372\ifmmode^\circ\else\textdegree\fi{}C. This is compared with values previously reported.

Picosecond time-resolved energy transfer in Porphyridium cruentum. Part I. In the intact alga

The wavelength-resolved fluorescence emission kinetics of the accessory pigments and chlorophyll a in Porphyridium cruentum have been studied by picosecond laser spectroscopy. Direct excitation of the pigment B-phycoerythrin with a 530 nm, 6 ps pulse produced fluorescence emission from all of the pigments as a result of energy transfer between the pigments to the reaction centre of Photosystem II. The emission from B-phycoerythrin at 576 nm follows a nonexponential decay law with a mean fluorescence lifetime of 70 ps, whereas the fluorescence from R-phycocyanin (640 nm), allophycocyanin (660 nm) and chlorophyll a (685 nm) all appeared to follow an exponential decay law with lifetimes of 90 ps, 118 ps and 175 ps respectively. Upon closure of the Photosystem II reaction centres with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination the chlorophyll a decay became non-exponential, having a long component with an apparent lifetime of 840 ps. The fluorescence from the latter three pigments all showed finite risetimes to the maximum emission intensity of 12 ps for R-phycocyanin, 24 ps for allophycocyanin and 50 ps for chlorophyll a. A kinetic analysis of these results indicates that energy transfer between the pigments is at least 99% efficient and is governed by an exp −At 1 2 transfer function. The apparent exponential behaviour of the fluorescence decay functions of the latter three pigments is shown to be a direct result of the energy transfer kinetics, as are the observed risetimes in the fluorescence emissions.

Theoretical evidence for metastable cyclic ozone

OZONE is a vital component of the atmosphere. Knowledge of its properties is important to the understanding of its involvement in chemical and photo-induced reactions which can affect the stability of the ozone concentration in the upper atmosphere. The reactivity of ozone and its unusually low thermochemical stability with respect to partial dissociation are difficult to reconcile with an apparently multiply bonded symmetric bent structure. This has led to speculation1,2 concerning alternative forms of ozone such as the equilateral triangular cyclic form (isoelectronic with cyclopropane) from which facile partial dissociation could be envisioned. To have an important role such a form would presumably need to be thermally accessible from the ground state and bound with respect to partial dissociation. The spectroscopic structure determination of the symmetric bent ground state3–10 has been finalised10 (re (O–O) 127.1 pm, angle 116.8°), and there was recently experimental confirmation11,12 of excited states of this species13–16,22 which are bound with respect to dissociation, including the predicted 3B2 state13–17,22. Investigations of bound excited states which are stable to partial dissociation have been stimulated by the sequential observation during O2 radiolysis experiments12,18,19 of two short-lived ‘ozone precursors’ O3α and O3β following recombination of ground state O2 and O. O3β may be vibrationally excited ground state ozone20, whereas the O3α precursor which shows absorption12 at ∼ 315 nm in the ultraviolet (and possibly21 the emission observed at 1500 cm−1 in the infrared) has not yet been given an identity beyond an apparent lifetime of 5 µs. The species showing the single infrared emission21 is interesting as D3h cyclic ozone is expected to show one (degenerate) infrared active fundamental, fewer than any alternative structure. Theoretical evidence for the stability of cyclic ozone has been conflicting. In 1973 Wright1 calculated it to be 26 kJ mol−1 more stable than the ground state; in 1972 and 1975 Hay et al.13,22 calculated it to be less stable by ∼ 150kJ mol−1 (that is, above partial dissociation energy ΔH° = 101 kJ mol−1) and in 1974 Shih et al.14 have concluded it is 63 kJ mol−1 above the ground state. Shih et al.14 have recognised that considerable uncertainty still exists in their computation on total and relative energies, and their calculated ground state structural parameters differ significantly from experiment. We have reinvestigated the relative stability of cyclic ozone and symmetric bent ozone and have obtained a total energy for the ground state of−224.7710 hartree which is 471 kJ mol−1 lower than the previous best model wavefunction of Hay et al.13 who predicted cyclic ozone to be above dissociation. In other words, the relative shortcoming in total energy in all previous computations on each conformer has been around five times the key energy difference between ground state ozone and the partial dissociation products, ΔH° = 101 kJ mol−1. The computed optimum geometric parameters (bond length re = 126.4pm and bond angle αe = 117.3°) are sufficiently close to the experimental values to lend some confidence to our model wavefunctions in regard to structural predictions. The corresponding wavefunction for cyclic ozone has an energy expectation of−224.7637 hartree at an optimum bond length of re = 143.5 pm (bond angle 60°). On this evidence the difference in the energy of the local minima on the ozone ground state conformation hypersurface is less than one fifth of the partial dissociation energy. We, therefore, confidently predict the existence of a bound species of ozone having an equilateral triangle conformation. We believe this species may be the O3α precursor, which in the experiments referred to18,19 is presumably vibrationally excited.

Kinetic studies of reactions involving ground state Bi(64S) atoms by time-resolved resonance fluorescence

We describe a kinetic study of reactions involving Bi(64S) which is generated photochemically from BiMe3 by pulsed irradiation. The ground state atoms were monitored by time-resolved resonance fluorescence at λ= 306.77 nm [Bi(74P½→ 64S)] in the “single-shot” mode following optical excitation to the 74P½ state, which was effected by means of a high intensity microwave-powered atomic flow spectroscopic source. The fluorescence intensity was calibrated experimentally against the density of Bi(64S) atoms and absolute rate constants k3, defined according to the equation –d[Bi]/dt=k3[Bi(64S)]. [C2H2, C2H4][He], were then determined for the removal of the atom in the presence of the gases C2H2 and C2H4. The resulting values for k3, namely, k3C2H2= 3.3 ± 0.3 × 10–33 and k3C2H4= 8.7 ± 0.4 × 10–33 cm6 molecule–2 s–1(300 K) are compared with the analogous data (k′3) derived previously using time-resolved absorption of atomic resonance radiation, and discussed in terms of the modified Beer–Lambert law, Itr=I0 exp [–ε(cl)γ]. Finally, we calculate radiation trapping effects in our system assuming the atomic line profiles to be the summation of Voigt profiles over the nuclear hyperfine interaction components. Solutions of the diffusion equation for radiation trapping using the boundary conditions for infinite slab geometry, cylindrical geometry or spherical geometry indicate that, whilst apparent radiative lifetimes for the transition are considerably enhanced, sensible linearity between fluorescence intensity and particle density may be expected over wide concentration ranges. The implications of this result to both time-resolved resonance fluorescence measurements and steady fluorescence measurements of the Stern–Volmer type are discussed.