On the coherent description of diffusion-influenced fluorescence quenching experiments

The fluorescence quenching by electron transfer of a fluorophore, 2,5-bis(dimethylamino)-1,3-benzenedicarbonitrile, to 1,3-dimethyl-2-nitrobenzene, has been studied by means of time-resolved and steady-state experiments at different viscosities and up to large quencher concentrations. Differential Encounter Theory (DET) has been used to rationalize the results, in combination with electron transfer modelled by the Marcus theory. Additionally, the solvent structure and the hydrodynamic effect on the diffusion coefficient have been taken into account. Any simpler model failed to simultaneously fit all the results. The large number of quencher concentrations used is crucial to unambiguously extract the electron transfer parameters.     

A novel method for calculating rate constants of diffusion-influenced reactions

In the present work we suggest a method for calculating rate constants of chemical processes affected by mobility of reactants. The method is based on the encounter theory. Unlike the widely accepted model of collision complexes it provides a general formal solution for practically arbitrary reaction scheme.     

Employing the fluorescence anisotropy and quenching kinetics of tryptophan to hunt for residual structures in denatured proteins

Residual structures in denatured proteins have acquired importance in recent years owing to their role as protein-folding initiation sites. Locating these structures in proteins has proved quite formidable, requiring techniques like NMR. Here in this report, we take advantage of the ubiquitous presence of tryptophan residues in residual structures to hunt for their presence using steady-state fluorescence spectroscopy. The surface accessibility and rotational dynamics of tryptophan in putative residual structures among ten different proteins, namely glucagon, melittin, subtilisin carlsberg, myelin basic protein, ribonuclease T₁, human serum albumin, barstar mutant, bovine serum albumin, lysozyme and Trp-Met-Asp-Phe-NH₂ peptide, was studied using steady state fluorescence quenching and anisotropy, respectively. Five proteins, namely ribonuclease T₁, bovine serum albumin, melittin, barstar and hen egg white lysozyme appear likely to possess tryptophan(s) in hydrophobic clusters based on their reduced bimolecular quenching rates and higher steady-state anisotropy in proportion to their chain length. We also show that the fluorescence emission maximum of tryptophan is insensitive to the presence of residual structures.     

Statistical theory for the kinetics and dynamics of roaming reactions

We present a statistical theory for the effect of roaming pathways on product branching fractions in both unimolecular and bimolecular reactions. The analysis employs a separation into three distinct steps: (i) the formation of weakly interacting fragments in the long-range/van der Waals region of the potential via either partial decomposition (for unimolecular reactants) or partial association (for bimolecular reactants), (ii) the roaming step, which involves the reorientation of the fragments from one region of the long-range potential to another, and (iii) the abstraction, addition, and/or decomposition from the long-range region to yield final products. The branching between the roaming induced channel(s) and other channels is obtained from a steady-state kinetic analysis for the two (or more) intermediates in the long-range region of the potential. This statistical theory for the roaming-induced product branching is illustrated through explicit comparisons with reduced dimension trajectory simulations for the decompositions of H(2)CO, CH(3)CHO, CH(3)OOH, and CH(3)CCH. These calculations employ high-accuracy analytic potentials obtained from fits to wide-ranging CASPT2 ab initio electronic structure calculations. The transition-state fluxes for the statistical theory calculations are obtained from generalizations of the variable reaction coordinate transition state theory approach. In each instance, at low energy the statistical analysis accurately reproduces the branching obtained from the trajectory simulations. At higher energies, e.g., above 1 kcal/mol, increasingly large discrepancies arise, apparently due to a dynamical biasing toward continued decomposition of the incipient molecular fragments (for unimolecular reactions). Overall, the statistical theory based kinetic analysis is found to provide a useful framework for interpreting the factors that determine the significance of roaming pathways in varying chemical environments.     

Oxidation kinetics and fluorescence quenching of coumarin-1 laser dye with peroxodisulfate

Summary The fluorescence of coumarin-1 laser dye (C1; 7-diethylamino-4-methyl-coumarin) proved to be sensitive to the presence of peroxodisulfate (S₂O ₈ ^2– ). The emission intensity decreases with increasing in peroxodisulfate concentration. The decrease is assigned to a quenching process connected with a ground state peroxodisulfate oxidation of the dye. The kinetics of the reaction have been investigated spectrophotometrically at 382 nm. The reaction follows second order kinetics, being first order for each reactant. The reaction rate ispH dependent; higher rates are observed in alkaline media. Addition of sodium dodecyl sulfate (SDS) retards the oxidation process remarkably.

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Kinetik der Oxidation und Fluoreszenzlöschung des Laserpigments Coumarin-1 mit Peroxodisulfat

Zusammenfassung Die Fluoreszenz des Laserpigments Coumarin-1 (C1; 7-Diethylamino-4-methyl-coumarin) reagiert auf die Anwesenheit von Peroxodisulfat (S₂O ₈ ^2– ). Die Emissionsintensität nimmt mit steigender Peroxodisulfatkonzentration ab. Die Abnahme wird einem Löschvorgang zugeschrieben, der mit einer Oxidation des Grundzustands durch Peroxodisulfat verbunden ist. Die Kinetik der Reaktion wurde bei 382 nm spektrophotometrisch untersucht. Sie verläuft nach zweiter Ordnung (nach erster Ordnung bezüglich jedes Reaktanden). Die Reaktionsgeschwindigkeit istpH-abhängig; in alkalischen Medien werden höhere Geschwindigkeitskonstanten gefunden. Zusatz von Natriumdodecylsulfat (SDS) hemmt die Oxidation beträchtlich.

     

Rate constant for diffusion-influenced ligand binding to receptors of arbitrary shape on a cell surface

The theory of ligand binding to receptors on a cell surface suggested by Berg and Purcell and generalized by Zwanzig and Szabo uses the assumption that receptors are circular absorbing disks on an otherwise reflecting sphere. One of the key ingredients of this theory is a solution for the rate constant for ligand binding to a single circular receptor on a reflecting plane. We give an exact solution for the rate constant for binding to a single elliptic receptor and an approximate solution for binding to a single receptor of more general shape. The latter was tested by Brownian dynamics simulations. We found that the approximate formula predicted the rate constant with better than 10% accuracy for all studied receptor shapes. Using our solutions one can find the rate constant for ligand binding to a cell covered by N noncircular receptors by means of the Zwanzig-Szabo formula.     

The theory and practice of thermoanalytical kinetics of solid-state reactions

on kinetics have become a feature of ICTAC Congresses and are generally well-attended and result in lots of useful discussion. The Workshop at ICTAC 13 continued the tradition with about 50 attendees. The workshop was chaired by Haruhiko Tanaka and Michael Brown. Six introductory presentations were made on the subject, followed by the discussion summarized below. The six introductory speakers were: (1) Ranjit K. Verma; (2) Mike Reading; (3) Crisan Popescu; (4) Erwin Kaiserberger; (5) Mike Brown; (6) Haruhiko Tanaka.     

荧光猝灭法测定壳聚糖含量

在pH 6.3的NaH2PO4-Na2HPO4缓冲溶液中,壳聚糖对荧光素的荧光强度具有明显的猝灭作用,且在一定浓度范围内,其猝灭程度与加入的壳聚糖浓度成线性关系,据此建立了一种新的测定壳聚糖含量的荧光猝灭分光光度法。该方法的回归方程为ΔF=64.02+42.28ρ(mg/L),R2=0.9942,线性范围为0.50~10.0 mg/L,检出限为0.27 mg/L。样品测定的RSD为4.5%(n=6),平均回收率为99.3%。采用该方法可测定复杂样品中的壳聚糖含量。     

Properties of excited singlet states of N-arylurethanes : Photo-fries reactions,fluorescence, quenching and sensitization

The deactivation of the first excited S(ππ^*) states of N-arylurethanes (produced upon irradiation with UV light) by emission (fluorescence), chemical reaction (photo-Fries rearrangement and fragmentation), energy transfer to quenchers, and radiationless transitions to ground and triplet states is investigated. Arylurethanes exhibit fluorescence (λ_f ≈ 295–350 nm, φ_f ≈ 10^?2, τ_f ≈ 1–6 ns) and phosphorescencs (λ_p ≈ 370–410 nm). The variations of the quantum yields of the fluorescence and of the photo-Fries rearrangement of N-arylurethanes by substituents and solvents are essentially due to variations of the rate constants for the radiationless processes. Fluorescence and photo-Fries reactions can be quenched by diffusion-controlled energy transfer to aliphatic ketones. Quenching is accompanied by sensitization of the ketone fluorescence. The urethane fluorescence and photo reactions may be sensitized by aromatic hydrocarbons. The results of all the quenching and sensitization experiments demonstrate that the photo-Fries reactions of N-arylurethanes proceed via the first excited singlet states of the urethanes.     

Amplified fluorescence quenching and biosensor application of a poly (para-phenylene) cationic polyelectrolyte

The quenching behavior of a water-soluble cationic poly (para-phenylene) bearing quaternized ammonium side groups (P-NEt ₃ ⁺ ) was studied. P-NEt ₃ ⁺ is efficiently quenched by sodium anthraquinone-2,6-disulfonate (AQS) and sodium 1,4,5,8-naphthalenediimide-N,N’-bis (methylsulfonate) (NDS) in aqueous solution via a photo-induced electron-transfer mechanism. Absorption spectra of the NDS/P-NEt ₃ ⁺ ion-pair complex indicated formation of a stable charge-transfer complex in the ground state. A large spectral shift and band broadening occurred during AQS/P-NEt ₃ ⁺ complex formation, which is believed to arise due to P-NEt ₃ ⁺ conformational changes induced by hydrophobic interactions. Finally, a protein sensor that relies on the quenching behavior of P-NEt ₃ ⁺ was designed based on the quencher-tether-ligand (QTL) approach. AQS tethered to biotin (AQS-E-Biotin) was used along with P-NEt ₃ ⁺ to sense avidin.     

荧光猝灭法测定次黄嘌呤及机理研究

研究了次黄嘌呤猝灭伊文思蓝的荧光猝灭机理并根据次黄嘌呤对伊文思蓝荧光的猝灭作用建立了测定次黄嘌呤的新方法.线性回归方程为:ρ=6.292×103F-1-26.49,相关系数为0.9990,线性范围为0.20～20.0 μg/mL,检出限为0.14 μg/mL,相对标准偏差(RSD)为2.5%.求算了伊文思蓝与次黄嘌呤的形成常数K=2.63×102L/moL,实验了pH、放置时间、干扰离子等对测定的影响.     

荧光猝灭法测定痕量亚硝酸根

基于在酸性介质中,NO-2与藏红T直接反应,使藏红T的荧光猝灭,加入强碱弱酸盐对体系有增敏作用,据此提出了荧光法测定痕量NO-2的新方法。方法的线性范围为2.8～294ng/mL,方法的检出限为2.8ng/mL。可直接用于水样及蔬菜中痕量NO-2的测定。     

荧光猝灭法测定痕量NO2-

在HCl介质中,NO2-与5-氨基-1,2,3,4-四氢-1,4酞嗪二酮(ATPD)发生亚硝化反应,使ATPD的荧光猝灭,且其荧光猝灭程度与NO2-量呈线性关系,从而建立了测定痕量NO2-的新方法。该法线性范围为0.067~1.172μg/mL,方法检出限为0.70μg/L。本法已用于雨水中痕量NO2-的测定。     

Kinetics of partly diffusion controlled reactions. I. Transient and apparent transient effect in fluorescence quenching

Analysis of decay curves of electronically excited molecules A* versus time in presence of quencher B leads to the determination of kinetic data for the reaction (i.e. for diffusion limited reactions, the experimental collisional distance σ′ greater than the true collisional distance σ) and the sum D of diffusion coefficients of both reactants). Experimental fluorescence steady state measurements (Stern-Volmer representation) are inconsistent with calculated curves using classical Smoluchowski's model with σ′and D. The difference between this has been interpreted by complex formation in the ground state between A and B. But, this assumption is unnecessary if we take into account a “static” quenching arising from B molecules located at distances between σ and σ′ from A*. A theoretical model based upon this principle is described; good agreement between the model and experimental results was obtained.     

Steady-state fluorescence quenching applications for studying protein structure and dynamics

Fluorescence quenching methods are useful to obtain information about the conformational and/or dynamic changes of proteins in complex macromolecular systems. In this review steady-state methods are described and the data interpretation is thoroughly discussed. As a special case of fluorescence quenching mechanism, fluorescence resonance energy transfer (FRET) phenomenon is also presented. Application of a FRET based method to characterize the temperature dependence of the flexibility of protein matrix is clearly demonstrated.     

Theoretical explanation of nonexponential OH decay in reactions with benzene and toluene under pseudo-first-order conditions

OH radical reactions with benzene and toluene have been studied in the 200-600 K temperature range via the CBS-QB3 quantum chemistry method and conventional transition-state theory. Our study takes into account all possible hydrogen abstraction and OH-addition channels, including ipso addition. Reaction rates have been obtained under pseudo-first-order conditions, with aromatic concentrations in large excess compared to OH concentrations, which is the case in the reported experiments as well as in the atmosphere. The reported results are in excellent agreement with the experimental data and reproduce the discontinuity in the Arrhenius plots in the 300 K < T < 400 K temperature range. They support the suggestion that the observed nonexponential OH decay is caused by the existence of competing addition and abstraction channels and by the decomposition of thermalized OH-aromatic adducts back to reactants. We also find that the low-temperature onset of the nonexponential decay depends on the concentration of the aromatic compounds and that the lower the concentration, the lower the temperature onset. Under atmospheric conditions, nonexponential decay was found to occur in the 275-325 K range, which corresponds to temperatures of importance in tropospheric chemistry. Branching ratios for the different reaction channels are reported. We find that for T > or = 400 K the reaction occurs exclusively by H abstraction. At 298 K, ipso addition contributes 13.0% to the overall OH + toluene reaction, while the major products correspond to ortho addition, which represents 43% of all possible channels.