Collisional Quenching Rate Constants Research Articles

Photophysical Characterization of the Interactions among Tris(2,2′-bipyridyl)ruthenium(II) Complexes Ion-Exchanged within Zirconium Phosphate

We studied the structures, luminescence, and self-quenching properties of tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)(3)(2+)) two-dimensional arrangements within the layers of zirconium phosphate (ZrP). The intercalation of Ru(bpy)(3)(2+) was accomplished using a hydrated form of zirconium phosphate ZrP. Varying the Ru(bpy)(3)(2+)/ZrP intercalation ratio, different Ru(bpy)(3)(2+)-exchanged ZrP loading levels were achieved. The ion exchange of Ru(bpy)(3)(2+) within ZrP produces a red shift in the metal-to-ligand charge-transfer (MLCT) absorption band of the complex from 452 nm in aqueous solution to 460 nm in ZrP. Steady state luminescence spectra of the Ru(bpy)(3)(2+)-exchanged ZrP materials show an increase in the luminescence intensity with an increase in the Ru(bpy)(3)(2+) loading level until about 0.77 M, where subsequent increases in the loading level produce a decrease in the luminescence (self-quenching region). Time-resolved luminescence measurements are consistent with the steady state luminescence measurements. Analysis of the time domain luminescence measurements in loadings higher than 0.77 M leads to the determination of a collisional quenching rate constant of (1.67 +/- 0.05) x 10(6) M(-1) s(-1). Stern-Volmer analysis of the luminescence quantum yield of Ru(bpy)(3)(2+)-exchanged ZrP materials indicates that static quenching is also involved in the Ru(bpy)(3)(2+) self-quenching mechanism. The quantum yield data behavior might be explained by a model that takes into account collisional quenching and the quasi-static Perrin mechanism. The calculation yields a quenching sphere of action of 14.8 A, which is slightly larger than the collisional radii of two Ru(bpy)(3)(2+) ions (12.2 A), as predicted by the model.

Fluorescence study of solubilization of l-α-dilauroylphosphatidylethanolamine in the mixed micelles with monomeric and dimeric cationic surfactants

The mixed micelle formation between l-α-dilauroylphosphatidylethanolamine (DLPE) and a series of monomeric and dimeric cationic surfactants have been studied with the help of steady state fluorescence technique. The micelle formation process has been detected by plotting the emission intensities of solubilized fluorescence probes such as 5(6)-carboxyfluorescein (CF) and pyrene (Py) in the hydrophobic environment of mixed micelles. The mixed micelle process has been found to be facilitated in the presence of monomeric surfactants with higher hydrophobicities. In the case of dimeric surfactants, the steric hindrances generated by both the dimeric head group as well as twin hydrophobic tails have delayed the mixed micellization. The results have been further supported by evaluating the mean aggregation number ( N agg) of the mixed micelles and the collisional quenching rate constant.

Reaction of Cr Atoms with O2at Low Pressures: Observation of New Chemiluminescence Bands from CrO2*

Ground and low-lying electronic states of Cr atoms in the gas phase were generated from photolysis of <TEX>$Cr(CO)_6$</TEX> vapor in He or Ar using an unfocussed weak UV laser pulse and their reactions with <TEX>$O_2$</TEX> and <TEX>$N_2O$</TEX> were studied. When 0.5-1.0 Torr of <TEX>$Cr(CO)_6$</TEX> /<TEX>$O_2$</TEX> /He or Ar mixtures were photolyzed using 295-300 nm laser pulses, broadband chemiluminescence peaked at ~420 and ~500 nm, respectively, was observed in addition to the atomic emissions from <TEX>$z^7P^{\circ}$</TEX>, <TEX>$z^5P^{\circ}$</TEX>, and <TEX>$y^7P^{\circ}$</TEX> states of Cr atoms. When <TEX>$N_2O$</TEX> was used instead of <TEX>$O_2$</TEX>, no chemiluminescence was observed. The chemiluminescence intensities as well as the LIF intensities for those three low-lying electronic states (<TEX>$a^7S_3,\;a^5S_2\;and\;a^5D_J$</TEX>) showed second-order dependence on the photolysis laser power. Also, the chemiluminescence intensities were first-order in <TEX>$O_2$</TEX> pressure, but the presence of excess Ar showed a strong inhibition effect on them. Based on the experimental results, the chemiluminecent species in this work is attributed to <TEX>$CrO_2^*$</TEX> generated from hot ground state Cr atoms with <TEX>$O_2$</TEX>. The apparent radiative lifetimes of the chemiluminescent species and collisional quenching rate constants by <TEX>$O_2$</TEX> and Ar also were investigated.

Investigation of Collisional Quenching of CCl2 (ÃB1) in Different Vibrational States

CCl2 free radicals were produced by a pulsed dc discharge of CCl4 (in Ar). Ground eletronic state CCl2 (X) radicals were electronically excited to A 1B1 (0,3,0), (0,4,0), (1,3,0), (0,6,0), (1,4,0), and (2,2,0) vibronic states by a Nd:YAG laser pumped dye laser at 550.56, 541.52, 532.25, 524.31, 523.82, and 523.37 nm, respectively. Experimental quenching data of excited CCl2(A1B1 and a3B1) by CH3OH, C2H5OH, n-C3H7OH, n-C4H9OH, CH2Br2, CH3COCH3, CH3COC2H5, NH(CH3)2, NH(C2H5)2, N(C2H5)3, and n-C6H14 molecules were obtained by observing the time-resolved total laser induced fluorescence(LIF) signal of the excited CCl2 radical, which showed a superposition of two exponential components under the presence of quencher. The quenching rate constants of the A(1B1) and a(3B1) states of CCl2, kA and ka , were derived by analyzing the experimental data according to a proposed three-level-model to deal with the CCl2 (X1A1, A1B1, a3B1) system. It is found that the total collisional quenching rate constants of CCl2 A1B...

Effect of single mutations on the structural dynamics of a DNA repair enzyme, the Escherichia coli formamidopyrimidine-DNA glycosylase--a fluorescence study using tryptophan residues as reporter groups.

The effects on the structure dynamics of the Escherichia coli wild-type formamidopyrimidine-DNA glycosylase (Fpg) protein of the single mutations Lys57-->Gly (FpgK57G), Pro2-->Gly (FpgP2G) and Pro2-->Glu (FpgP2E) were studied by fluorescence techniques, namely: lifetime measurements and acrylamide quenching of the fluorescence of Trp residues. The fluorescence decays of Fpg and its mutant forms were analysed by the maximum-entropy method and lifetime distributions in the range 200 ps to 9 ns were obtained. The lifetime distribution profiles of FpgK57G, FpgP2G and FpgP2E are different from that of wild-type Fpg. Both dynamic and static quenching by acrylamide were observed for all the proteins. At 20 degrees C, the bimolecular collisional quenching rate constant of the FpgP2E fluorescence by acrylamide was only 0.8 M(-1) s(-1) as compared to about 1.4 M(-1) s(-1) for the three other proteins. At 6 degrees C, all the spectroscopic properties of these four proteins are about the same. The analysis of experimental data demonstrates that all three mutations induce a structural reorganization of the Fpg protein. However, only the P2E mutation lead to a reduced accessibility of some Trp residues to acrylamide quenching. It is concluded that the single P2E replacement induces a conformational change leading to a more rigid globular structure as opposed to the wild type and K57G and P2G mutations. The influence of the single mutations on the enzyme activities of the Fpg protein is discussed.

Kinetics of NCl(aΔ) via Photodissociation of ClN3

Kinetic measurements of NCl(a1Δ) metastables produced by the photolysis of chlorine azide (ClN3) at 193 nm are reported. High NCl(a1Δ) density (>1015 molecules/cm3) production facilitates measurements of the NCl(a1Δ) + NCl(a1Δ) self-annihilation rate. A gas phase transient chemical titration scheme is used to calibrate absolute NCl(a1Δ) density. The NCl(a1Δ) bimolecular rate coefficient at room temperature is (7.2 ± 0.9 × 10-12 cm3/(molecule s) and has a magnitude very critical to the development of an NCl(a1Δ) + I(2P3/2) → I*(2P1/2) + NCl(X3Σ-) chemical laser. Despite the large value of the self-annihilation rate, intense I*(2P1/2) emission via transfer from NCl(a1Δ) is strongly observed when mixtures of CH2I2 and ClN3 are photolyzed at 193 nm. Unlike the analogous O2(a1Δ) + I(2P3/2) energy transfer system, the relatively large NCl(a1Δ) bimolecular rate constant suggests that NCl(a1Δ) transport times must be short and will require small chemical mixing regions for efficient lasing. In addition, the 300 K collisional quenching rate constants of NCl(a1Δ) by F2, Cl2, and Br2 are (2.5 ± 1.1) × 10-13, (2.9 ± 0.6) × 10-11, and (1.4 ± 0.2) × 10-10 cm3/(molecule s), respectively.

Measurement of lifetimes and collisional rate constants for 3p levels in neon I, II and IV

Lifetimes and collisional quenching rate constants have been measured for 3p levels in Ne I, Ne II, and Ne IV. The levels were populated by direct excitation from the neon ground state using a pulsed beam of 100 MeV32S ions from the Munich Tandem van de Graaff accelerator. Beam pulses were 2 ns long (FWHM) and had a repetition rate of 78 kHz. Lifetimes were measured by time resolved optical spectroscopy. Collisional rate constants were determined from time spectra for various target gas pressures.

Unfolding Pathway of Apomyoglobin: Simultaneous Characterization of Acidic Conformational States by Frequency Domain Fluorometry

The dynamic properties of the conformational states co-existing during the acid-induced unfolding of tuna apomyoglobin, a single tryptophan-containing protein, have been investigated simultaneously by frequency domain fluorometry. In the transition region, in the absence of salt, the tryptophanyl fluorescence emission arises from a bimodal lifetime distribution. The pH decrease causes a marked broadening of the short-lived distribution component whereas the other component, i.e. the long-lived one, remains unchanged and represented by a very narrow lifetime distribution whose width is similar to that of the native protein. The broadening of the short-lived distribution component observed on lowering the pH indicated that this component arises from fully unfolded molecules. This was further corroborated by acrylamide quenching studies at acidic pH. The collisional quenching rate constant of the short-lived distribution component, i.e. 8·9 × 109 M-1 s-1, was found to be similar to that observed for a fully exposed residue. The long-lived distribution component was characterized by a lower collisional quenching rate constant, i.e.2·3 × 109 M-1 s-1. This value if compared to that determined for the native apoprotein at neutral pH, i.e. 4·0 × 108 M-1 s-1, indicates that the native-like structure surviving the acid-induced transition possesses a large molecular flexibility.

The laser induced fluorescence spectrum and radiative lifetime of PO2

The ultraviolet spectrum of PO2 first observed in absorption by Verma and McCarthy has been studied by laser induced fluorescence for the first time. The spectra are similar in many respects to those observed in the visible system of NO2 and no predissociation is observed. The loss of rotational structure is attributed instead to mixing with one or more near continuous ‘‘background’’ states, with the amount of mixing apparently related to excitation of the bending vibration. The radiative decays are observed to be nonexponential but are accurately described by a double exponential form. This gives collision free radiative lifetimes of about 0.5 and 4.5 μs for the structured and continuous states, respectively, with the effective lifetime of the structured state varying strongly with the amount of mixing. Collisional quenching rate constants are roughly constant at (6±1)×10−10 and (4±1)×10−11 cm3 molecule−1 s−1 for the two states, with the very rapid quenching rate of the structured state probably being for nonradiative transfer to the background continuum. From observations of the wavelength dependence of the fluorescence the ground state ν2 and ν1 frequencies are found to be about 387 and 1117 cm−1, respectively. The emission is very extensive and strongly red shifted and lends further evidence that these states of PO2 are responsible for the chemiluminescence observed in phosphorus/oxygen reactions.

Solvent accessibility of the adenosine 5'-triphosphate catalytic site of sarcoplasmic reticulum CaATPase.

The CaATPase of rabbit skeletal sarcoplasmic reticulum was labeled at or near the ATP catalytic site with fluoresceinyl isothiocyanate (FITC), and the accessibility of the attached probe to the bulk solvent was determined by I- quenching of its fluorescence. The quenching of free FITC was also measured. In both cases, the quenching was of the Stern-Volmer type and collisional quenching rate constants were obtained over the pH range 5-8 in the presence of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and with added Ca2+, vanadate, or phosphate. The fluorescence intensity and susceptibility to quenching by I- of free FITC were insensitive to the added ligands. In all cases, the intensity decreased with pH, as predicted from the known properties of FITC mono- and dianions. The collisional quenching rate constants increased at lower pH, as expected for I- quenching of a molecule with decreasing negative charge due to protonation. When FITC was attached to the CaATPase, the FITC fluorescence intensity and I- collisional quenching rate constants were sensitive to ligand binding as well as pH. The changes in fluorescence intensity with acidity, when compared to the results for free FITC, indicated the pKa of the FITC was reduced 0.6 unit when it was attached to the CaATPase. Excited-state lifetime measurements indicated that ligand effects at constant pH were not due to protonation-induced changes in FITC quantum yield but to conformational changes of the CaATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Vibrational quenching of small molecular ions in neutral collisions

Selected ion flow drift tube techniques have recently been developed for the measurement of molecular ion vibrational relaxation rate constants. The first systematic studies of ion vibrational relaxation show that the collisional quenching rate constants are generally large and generally decrease with relative energy in the thermal range. This is a clear indication that the quenching is dominated by the long-range attractive forces. A model is proposed in which transient complex formation is followed by vibrational predissociation in the complex. If one utilizes ion-neutral vibrational relaxation and three-body association of the same ion-neutral reactants, a vibrational predissociation rate constant in the range 10/sup 9/-10/sup 10/ s/sup -1/ is found applicable to a number of cases. The magnitudes of vibrational relaxation rate constants in some cases yield new insights into ion-molecule interaction potentials. 62 references, 4 figures, 2 tables.

Time-resolved emission studies of the kinetic behaviour of Ca(4 3 P J ) and Sr(5 3 P J ) following pulsed dye-laser excitation. Electronic energy exchange between Ca(4 3 P J ) and Sr(5 3 P j ), energy pooling and decay measurements on Ca(4 3 P J ), Sr(5 3 P J ) and Sr(6 3 S 1)

The kinetics of Ca(4 3PJ), Sr(5 3PJ) and Sr(6 3S1) have been studied by time-resolved emission following pulsed dye-laser excitation. Ground-state calcium atoms, Ca[4s2(1S0)], in the presence of Sr[5s2(1S0)], were optically excited to the first electronically excited state, Ca[4s4p(3P1)], by means of a dye-laser operating at λ= 657.3 nm [Ca(4 3P1)â†� Ca(4 1S0)]. Following rapid Boltzmann equilibration within the Ca(43PJ) manifold, fast, near-resonant energy transfer took place with Sr(5 1S0) to yield Sr(5 3PJ), together with energy pooling to yield Sr(6 3S1). The following transitions were monitored in the time-resolved mode using pre-trigger photomultiplier gating and boxcar integration with repetitive pulsing on a slow-flow system, kinetically equivalent to a static system: Ca(4 3P1)→ Ca(4 1S0)+hν(λ= 657.3 nm), Sr(5 3P1)→ Sr(5 1S0)+hν(λ= 689.3 nm) and Sr(6 3S1)→ Sr(5 3P0)+hν(λ= 679.1 nm). Detailed measurements of the time dependence of the emission profiles at λ= 657.3 and 689.3 nm demonstrated the rapid establishment of the equilibrium Ca(4 3PJ)+ Sr(5 1S0)⇌ Ca(4 1S0)+ Sr(5 3PJ) and its sustenance throughout kinetic decays. A further comparison of the first-order decay coefficients for the decays of Ca(4 3PJ) and Sr(5 3PJ) with those for Sr(6 3S1) demonstrated energy pooling between Ca(4 3PJ)+ Sr(5 3PJ) and Sr(5 3PJ)+ Sr(5 3PJ). Finally, the measured first-order decay coefficients, k′, for Ca(4 3PJ) and Sr(5 3PJ) in chemical equilibrium with one another are seperated quantitatively into their component parts of spontaneous emission, diffusion, the equilibrium constant connecting the 3PJ states and collisional quenching rate constants. Rate constants for quenching of Ca(4 3PJ) and Sr(5 3PJ) by Kr, Xe, H2 and D2 derived from earlier measurements on the individual atoms are found to be quantitatively consistent with the decay profiles under equilibrium conditions.

Correlation between activity and dynamics of the protein matrix of phosphorylase b.

Quenching of the tryptophan fluorescence of phosphorylase b was studied by using iodide and acrylamide. Steady-state measurements indicated that all indole side chains were accessible to the nonionic quencher, although only 3 out of the total of 12 residues could be quenched by I-. From Stern--Volmer plots and the fluorescence lifetime data, it was concluded that the quenching was mainly of dynamic character. The value of the collisional quenching rate constant was found to be an order of magnitude less than that obtained in the case of fully exposed tryptophans. The relatively high activation energy, 30.9 kJ/mol, of the diffusion-controlled process and the value of the activation entropy suggest that the diffusion takes place in a fluctuating, structured medium. In spite of the application of sensitive fluorescent techniques, no gross conformational changes were found in the presence of acrylamide. However, the catalytic rate of the glycogen synthesis was decreased with the residual activity of the enzyme, proportional to the concentration of the probe. Binding of activator (AMP) and substrates (glucose 1-phosphate and glycogen) was found to be unaffected by acrylamide in concentrations applied (0--0.8 M). In a similar manner, activation enthalpy did not change in the presence of the quencher either. The complete reversibility of both activity inhibition and fluorescence quenching ruled out the irreversible denaturation of the enzyme or the covalent modification of any of the functional groups. We concluded that a model, suggesting the cross-correlation of activity and fluctuation, was consistent with the experimental findings.

Energy transfer in the 1Austate of glyoxal. III. Deuterium effects

Collision-free lifetimes and collisional quenching rate constants are reported for a number of single vibrational levels of glyoxal-d2 (1Au) and glyoxal-hd (1A″) and are compared with previous results for glyoxal-h2. There was some success in correlating the dependences of collision-free decay rates on specific vibrational level and on isotopic substitution with the predictions of intramolecular, nonradiative decay theory. There are strong isotope effects on the collision-free lifetimes, but the rate constants for collision depopulation of a vibronic level, which increase sharply with energy in the excited state, are strikingly isotope independent. The increase is believed to be primarily a result of increased intersystem crossing rate and is compared with a mixed state model proposed by Freed.

Collisional relaxation of electronically excited copper: 3d9 4s2 (2D5/2)

A flash photolysis experiment whereby optically metastable copper atoms (3d9 4s2 2D5/2) were produced in a high temperature (600 °K) reaction cell by the photodecomposition of copper halides (CuCl and CuI) is described. The subsequent decay of these low-lying electronically excited copper atoms was monitored by time resolved atomic resonance absorption techniques to provide collisional quenching rate constants for relaxation of the 2D5/2 state by He, Ar, Xe, N2, CO, CO2, SF6, and O2.

Kinetic investigation of electronically excited tin atoms, Sn(5 1S 0), by time-resolved attenuation of atomic resonance radiation

A kinetic study of electronically excited tin atoms in the (5 p 2 1S 0) state, 2.128 eV above the 5 3P 0 ground state, is described. These optically metastable species were generated by the pulsed irradiation of SnCl 4 at relatively low wavelength (λ > 165 nm) and monitored photochemically in absorption by time-resolved attenuation of the atomic resonance transition at λ = 291.45 nm [6 d( 3D 0 1) ← 5 p 2( 1S 0)]. The resulting method, involving the use of a rapid response precision logarithmic signal converter which permitted ln ( I 0/ I tr) to be measured directly, was found to be sensitive and yielded reliable kinetic data for the 1S state. Collisional quenching rate constants are given for the gases He, Xe, H 2, O 2 and SnCl 4, constituting the first reported kinetic data for this atomic state. The data are compared with those for the low lying 1D state of the atom as well as with those of the analogous states of other atoms in Group IV, namely, carbon and lead. The data are discussed in terms of correlations based on both the weak and strong (J, Ω coupling) spin orbit coupling approximations.