Fluorescence quenching of naphthols by Cu2+ in micelles

Effect of the micelles of anionic, cationic and non-ionic surfactants on the fluorescence quenching of 1- and 2-naphthols has been studied in the presence of copper ion. The excited state lifetime, dynamic and static quenching constants for these systems have been determined. Fluorescence quenching in water and SDS micelle is due to the collision of the fluorophore with the quencher with a small static component. The negatively charged naphtholate ions in the excited state are quenched with significantly higher rates than the neutral naphthol molecules, which are located further inside the mesophase. CTAB micelle is less effective than the SDS micelle for fluorescence quenching. The effect of CTAB on water-assisted excited-state deprotonation has been investigated in the presence of ZnSO₄. For TX-100 micelle there is negligible quenching even at higher concentration of the quencher.     

Superquenching in cyanine pendant poly(L-lysine) dyes: dependence on molecular weight, solvent, and aggregation.

A series of poly(L-lysines) ranging in number of repeat units (N(PRU)) from 6 to 900 has been synthesized and the photophysics of the series and monomer cyanine dye have been studied in solution. In water or aqueous dimethyl sulfoxide, the oligomers and polymers exhibit high sensitivity to fluorescence quenching by oppositely charged electron acceptors; in this study, 9,10-anthraquinone-2,6-disulfonate was used as a quencher for the cationic fluorescent polyelectrolytes. Quenching constants (K(SV)) measured in 50:50 (v/v) dimethyl sulfoxide-water increase monotonically with increase in N(PRU) ranging from 630 M(-1) for monomer to 1.2 x 10(9) M(-1) for dilute solutions of the polymer having N(PRU) approximately 900. The polymers having N(PRU) > 100 exhibit predominantly J-aggregate absorption and fluorescence and enhanced susceptibility to quenching. For the polymers exhibiting strong J-aggregation, the effective exciton domain quenched by a single quencher reaches approximately 100 PRU. The results of this study permit a semiquantitative analysis of superquenching of fluorescent polyelectrolytes in solution and the factors that control it.     

Identifiability of models for intramolecular two-state excited-state processes with added quencher and coupled species-dependent rotational diffusion

The parameters describing the kinetics of excited-state processes can possibly be recovered by analysis of the fluorescence decay surface measured as a function of the experimental variables. The identifiability analysis of a photophysical model assuming errorless time-resolved fluorescence data can verify whether the model parameters can be determined and may lead to the minimal experimental conditions under which this is possible. In this work, we used the method of similarity transformation to investigate the identifiability of three kinetic models utilized to describe the time-resolved fluorescence of reversible intramolecular two-state excited-state processes in isotropic environments: (1) model without added quencher, (2) model with added quencher, (3) model with added quencher coupled with species-dependent rotational diffusion described by Brownian reorientation. Without a priori information, model 1 is not identifiable. For model 2, two sets of quenching rate constants and combinations of excited-state deactivation/exchange rate constants are possible, but they cannot be allocated to a specific excited-state species. For both sets, upper and lower limits on the excited-state deactivation/exchange rate constants can be obtained. For model 3, both spherically and cylindrically symmetric rotors, with no change in the principal axes of rotation in the latter, are considered. The fluorescence delta-response functions I(parallel)(t) and I(perpendicular)(t), for fluorescence polarized parallel and perpendicular, respectively, to the electric vector of linearly polarized excitation, are used to define the sum S(t) identically equal to I(parallel)(t) + 2 I(perpendicular)(t) and the difference D(t) identically equal to I(parallel)(t) - I(perpendicular)(t). The identifiability analysis is performed using the S(t) and D(t) functions. Also for model 3, two sets of kinetic parameters (i.e., quenching rate constants, combinations of deactivation/exchange rate constants, and rotational diffusion coefficients) exist, but these parameters cannot be assigned unequivocally to a specific species. For the three models, an infinite number of alternative spectroscopic parameters associated with excitation and emission are found.     

INCLUSION AND FLUORESCENE QUENCHING OF 2, 3-DIMETHYLNAPHTHALENE IN β-CYCLODEXTRIN CAVITIES

Abstract— The fluorescence quenching of 2,3-dimethylnaphthalene (DMN) incorporated to β-cyclodextrin (β-CD) cavities by different olefins (fumaronitrile, acrylonitrile, acrylamide and 2-hydroxyethylmethacrylate) has been measured as a function of the β-CD concentration. The quenching efficiency decreases when the β-CD concentration increases, but extrapolation of the data to infinite cavities concentration does not indicate complete protection. These results are interpreted in terms of two quenching processes, one of them taking place between 2,3-dimethylnaphthalene associated to a β-CD cavity and free quencher, and the other between the DMN and the quencher molecule, both associated with a different cavity. The rate constants of both quenching processes and the β-CD quencher association constant are obtained from the dependence of the quenching efficiency with β-CD concentration.     

On spectral relaxation in proteins

During the past several years there has been debate about the origins of nonexponential intensity decays of intrinsic tryptophan (trp) fluorescence of proteins, especially for single tryptophan proteins (STP). In this review we summarize the data from diverse sources suggesting that time-dependent spectral relaxation is a ubiquitous feature of protein fluorescence. For most proteins, the observations from numerous laboratories have shown that for trp residues in proteins (1) the mean decay times increase with increasing observation wavelength; (2) decay associated spectra generally show longer decay times for the longer wavelength components; and (3) collisional quenching of proteins usually results in emission spectral shifts to shorter wavelengths. Additional evidence for spectral relaxation comes from the time-resolved emission spectra that usually shows time-dependent shifts to longer wavelengths. These overall observations are consistent with spectral relaxation in proteins occurring on a subnanosecond timescale. These results suggest that spectral relaxation is a significant if not dominant source of nonexponential decay in STP, and should be considered in any interpretation of nonexponential decay of intrinsic protein fluorescence.     

Interaction between encapsulated excited organic molecules and free nitroxides: communication across a molecular wall

Communication between two molecules, one confined and excited (triplet or singlet) and one free and paramagnetic, has been explored through quenching of fluorescence and/or phosphorescence by nitroxides as paramagnetic radical species. Quenching of excited states by nitroxides has been investigated in solution, and the mechanism is speculated to involve charge transfer and/or exchange processes, both of which require close orbital interaction between excited molecule and quencher. We show in this report that such a quenching, which involves electron-electron spin communication, can occur even when there is a molecular wall between the two. The excited state molecule is confined within an organic capsule made up of two molecules of a deep cavity cavitand, octa acid, that exists in the anionic form in basic aqueous solution. The nitroxide is kept free in aqueous solution. (1)H NMR and EPR experiments were carried out to ascertain the location of the two molecules. The distance between the excited molecule and the paramagnetic quencher was manipulated by the use of cationic, anionic, and neutral nitroxide and also by selectively including the cationic nitroxide within the cavity of cucurbituril. Results presented here highlight the role of the lifetime of the encounter complex in electron-electron spin communication when the direct orbital overlap between the two molecules is prevented by the intermediary wall.     

Fluorescence resonance energy transfer by quencher adsorption into hydrogels containing fluorophores

We synthesized anionic hydrogels containing fluorophores and investigated the adsorption of a cationic quencher having an amino group into hydrogels by fluorescence resonance energy transfer (FRET). FRET from the fluorophore to the quencher in hydrogels was examined by fluorescence intensity and fluorescence decay using a fluorescence spectrophotometer and femtosecond laser spectroscopy. The fluorescence intensity of the fluorophore‐containing hydrogels decreased rapidly with increasing amounts of adsorbed cationic quencher. The fluorescence emission of the fluorophore in the quencher‐adsorbed hydrogels containing fluorophores decayed more rapidly than that of the original hydrogels. The aforementioned result indicates that the fluorescence of the fluorophore‐containing hydrogels is quenched due to FRET from the fluorophore to the quencher as the cationic quenchers can approach the fluorophores in hydrogels by electrostatic interactions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3245–3252, 2006     

Fluorescence quenching of photoreaction products in the excited singlet state

The properties of steady-state spontaneous luminescence of a quantum system with a photoproduct with recordable fluorescence under the conditions of dynamic quenching of excited states by extraneous substances were considered. It was shown that the dependence of photoproduct fluorescence intensity and yield on quencher concentration was nontrivial and could not be conveniently used to determine the Stern-Volmer constant. At the same time, the initial form of the luminophore and its photoproduct produced in a kinetically controlled reaction are quenched in such a way that the ratio of their fluorescence intensities increases linearly as the quencher concentration grows. The corresponding equation was used to determine the constant of bimolecular quenching of reaction product excited states. The results were used in an analysis of the experimental fluorescence spectra of flavone (3-hydroxiflavone), whose fluorescence was excited under the conditions of dynamic quenching of the S ₁ state. Our analysis was shown to be applicable to a wide range of compounds with photoreactions accompanied by two-band fluorescence (charge transfer, proton transfer, phosphorescence, complex formation, etc.). It could be used to accurately determine bimolecular contact constants for excited states of photoreaction product molecules. Original Russian Text ? V.I. Tomin, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 3, pp. 580–585.     

Fluorescence quenching of anthracene by indole derivatives in phospholipid bilayers

The quenching of anthracene fluorescence by indole, 1,2-dimethylindole (DMI), tryptophan (Trp) and indole 3-acetic acid (IAA) in palmitoyloleoylphosphatidylcholine (POPC) lipid bilayers was investigated. A very efficient quenching of the anthracene fluorescence in the lipid membrane is observed. Stern-Volmer plots are linear for DMI but present a downward curvature for the other quenchers. This was interpreted as an indication of the presence of an inaccessible fraction of anthracene molecules. By a modified Stern-Volmer analysis the fraction accessible to the quenchers and the quenching constant were determined. The changes in the fluorescence emission spectrum of indole and DMI have been used to calculate the partition constants of these probes into the membranes, and bimolecular quenching rate constants were determined in terms of the local concentration of quencher in the lipid bilayer. The rate constants are lower than those in homogeneous solvents, which may be ascribed to a higher viscosity of the bilayer. No changes in the emission spectra of Trp and IAA are observed in the presence of vesicles, indicating that these probes locate preferentially in the aqueous phase, or in close proximity to the vesicular external interface in a medium resembling pure water. In these cases quenching rate constants were determined in terms of the analytical concentration. In the quenching by DMI a new, red shifted, emission band appears; it is similar to that observed in non-polar solvents and it is ascribable to an exciplex emission. The exciplex band is absent in the quenching by IAA and Trp and only very weakly present when the quencher is indole. From the position of the maximum of the exciplex emission, a relatively high local polarity could be estimated for the region of the bilayer where the quenching reaction takes place.     

Time-resolved kinetic studies on quenching of HCF(A1A'') by alkane and alcohol molecules

HCF(X1A') radicals were produced by laser photolysis of CHFBr2 at 213 nm and were electronically excited from the ground state to A1A'(030) at 492.7 nm with a dye laser pumped by a Nd:YAG laser. With the analysis of the lifetime of the time-resolved total fluorescence signals collected in the reaction cell where the total pressure was fixed to be 14.0 Torr, the quenching data of HCF(A1A') by alkane and alcohol molecules at room temperature were derived from variation of pseudo-first-order rate constant with different quencher pressures. It is found that the quenching rate constants are close to the collision rate constants (10(-10) cm3 molecule(-1) s(-1)), indicating the long-range attractive forces between the collision partners play an important role in the entrance channel of quenching process. Several kinetic models were applied to analyze the mechanism of the quenching process. The complex formation cross sections are calculated with the collision complex model. Correlations of the quenching rate constant for the removal of the HCF(A1A') state with ionization potential of the quenching partners show that the insertion reactive mechanism is probably the dominant reaction channel, which is analogous to the behaviors of other three-atom carbenes in corresponding electronic states.     

pH-dependent optical properties of a poly(phenylene ethynylene) conjugated polyampholyte

A poly(phenylene ethynylene) conjugated polymer (PPE-NMe(3)(+)-COO(-)) containing tetraalkylammonium groups and carboxylate groups has been synthesized by Sonogashira coupling. Due to the presence of the strong cationic and weak anionic pendant units, the polymer undergoes a pH-induced transition from cationic polyelectrolyte to polyampholyte due to deprotonation of the carboxylic acid units in basic solution. Studies of the pH dependence of the polymers' optical properties reveal changes in absorption oscillator strength and fluorescence quantum efficiency that are triggered by the transition from cationic polyelectrolyte to polyampholyte nature. Stern-Volmer fluorescence quenching of PPE-NMe(3)(+)-COO(-) with a negatively charged quencher 1,4,5,8-naphthalenediimide-N,N-bis(methylsulfonate) (NDS) shows that the polymer fluorescence quenching is amplified at low pH where the polymer is a polycation, whereas the quenching efficiency is considerably less at high pH.     

Polyfluorophore Labels on DNA: Dramatic Sequence Dependence of Quenching

We describe studies carried out in the DNA context to test how a common fluorescence quencher, dabcyl, interacts with oligodeoxynucleoside fluorophores (ODFs)—a system of stacked, electronically interacting fluorophores built on a DNA scaffold. We tested twenty different tetrameric ODF sequences containing varied combinations and orderings of pyrene (Y), benzopyrene (B), perylene (E), dimethylaminostilbene (D), and spacer (S) monomers conjugated to the 3′ end of a DNA oligomer. Hybridization of this probe sequence to a dabcyl‐labeled complementary strand resulted in strong quenching of fluorescence in 85 % of the twenty ODF sequences. The high efficiency of quenching was also established by their large Stern–Volmer constants (K_SV) of between 2.1×10⁴ and 4.3×10⁵ M ^?1, measured with a free dabcyl quencher. Interestingly, quenching of ODFs displayed strong sequence dependence. This was particularly evident in anagrams of ODF sequences; for example, the sequence BYDS had a K_SV that was approximately two orders of magnitude greater than that of BSDY, which has the same dye composition. Other anagrams, for example EDSY and ESYD, also displayed different responses upon quenching by dabcyl. Analysis of spectra showed that apparent excimer and exciplex emission bands were quenched with much greater efficiency compared to monomer emission bands by at least an order of magnitude. This suggests an important role played by delocalized excited states of the π stack of fluorophores in the amplified quenching of fluorescence.     

Kinetic Analysis of the Light-induced Fluorescence Quenching in Light-harvesting Chlorophyll a/b Pigment-Protein Complex of Photosystem II

We carried out a kinetic analysis of the light-induced fluorescence quenching (AF) of the light-harvesting chlorophyll a/b pigment-protein complex of photosystem II (LHCII) that was first observed by Jennings et at (Pho-tosynth. Res. 27, 57–64, 1991). We show that during a 2 min light, 2 min dark cycle, both the light and dark phases exhibit biexponential kinetics; this is tentatively explained by the presence of two types of light-induced quenchers in different domains of aggregated LHCII. Quantitative analysis could be carried out on the faster kinetic component; the slower component that was not completed during the measurement was not amenable for quantitative analysis. Our analysis revealed that the rate of the light-induced decrease of the fluorescence yield depended linearly on the light intensity, which shows that the generation of the quencher originates from a reaction that is first order with respect to the concentration of the excited domains. As shown by the estimated rate constant, pho-togeneration of the quencher is a fast reaction that can compete with other excitation-relaxation pathways. Both the decay and the recovery time constants of AF depended strongly on the temperature. Thermodynamic analysis showed that the fast light-induced decline in the fluorescence was determined by a low fraction of the excited states. Recovery was associated with large decrease in the entropy of activation that indicated the involvement of large structural rearrangements. Macroaggregated LHCII exhibited larger ΔF than small aggregates, which is consistent with the proposed role of aggregated LHCII in thy-lakoid membranes in nonphotochemical quenching.     

Amplified luminescence quenching of phosphorescent metal-organic frameworks

Amplified luminescence quenching has been demonstrated in metal-organic frameworks (MOFs) composed of Ru(II)-bpy building blocks with long-lived, largely triplet metal-to-ligand charge-transfer excited states. Strong non-covalent interactions between the MOF surface and cationic quencher molecules coupled with rapid energy transfer through the MOF microcrystal facilitates amplified quenching with a 7000-fold enhancement of the Stern-V?lmer quenching constant for methylene blue compared to a model complex.     

Steady-state and time-resolved fluorescence studies on Trichosanthes cucumerina seed lectin

Steady-state and time-resolved fluorescence spectroscopic studies have been carried out on Trichosanthes cucumerina seed lectin (TCSL). The fluorescence emission maximum of TCSL in the native state as well as in the presence of 0.1 M lactose is centered around 331 nm, which shifts to 347 nm upon denaturation with 8 M urea, indicating that all the tryptophan residues of this protein in the native state are in a predominantly hydrophobic environment. The exposure and accessibility of the tryptophan residues of TCSL and the effect of ligand binding on them were probed by quenching studies employing two neutral quenchers (acrylamide and succinimide), an anionic quencher (I(-)) and a cationic quencher (Cs(+)). Quenching was highest with acrylamide and succinimide with the latter, which is bulkier, yielding slightly lower quenching values, whereas the extent of quenching obtained with the ionic quenchers, I(-) and Cs(+) was significantly lower. The presence of 0.1 M lactose led to a slight increase in the quenching with acrylamide and iodide, whereas quenching with succinimide and cesium ion was not significantly affected. When TCSL was denatured with 8 M urea, both acrylamide and succinimide yielded upward-curving Stern-Volmer plots, indicating that the quenching mechanism involves both dynamic and static components. Quenching data obtained with I(-) and Cs(+) on the urea-denatured protein suggest that charged residues could be present in close proximity to some of the Trp residues. The Stern-Volmer plots with Cs(+) yielded biphasic quenching profiles, indicating that the Trp residues in TCSL fall into at least two groups that differ considerably in their accessibility and/or environment. In time-resolved fluorescence experiments, the decay curves could be best fit to biexponential patterns, with lifetimes of 1.78 and 4.75 ns for the native protein and 2.15 and 5.14 ns in the presence of 0.1 M lactose.     

On the localization of water-soluble porphyrins in micellar systems evaluated by static and time-resolved frequency-domain fluorescence techniques

Fluorescence quenching of meso-tetrakis-4-sulfonatophenyl (TPPS(4)) and meso-tetrakis-4-N-methylpyridil (TMPyP) porphyrins is studied in aqueous solution and upon addition of micelles of sodium dodecylsulfate (SDS), cetyltrimethylammonium chloride (CTAC), N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and t-octylphenoxypolyethoxyethanol (Triton X-100). Potassium iodide (KI) was used as quencher. Steady-state Stern-Volmer plots were best fitted by a quadratic equation, including dynamic (K(D)) and static (K(S)) quenching. K(S) was significantly smaller than K(D). Frequency-domain fluorescence lifetimes allowed estimating bimolecular quenching constants, k(q). At 25 degrees C, in aqueous solution, TMPyP shows k(q) values a factor of 2-3 higher than the diffusional limit. TPPS(4) shows collisional quenching with pH dependent k(q) values. For TMPyP quenching results are consistent with reported binding constants: a significant reduction of quenching takes place for SDS, a moderate reduction is observed for HPS and almost no change is seen for Triton X-100. Similar data were obtained at 50 degrees C. For CTAC-TPPS(4) system an enhancement of quenching was observed as compared to pure buffer. This is probably associated to accumulation of iodide at the cationic micellar interface. The attraction between CTAC headgroups and I(-), and repulsion between SDS and I(-), enhances and reduces the fluorescence quenching, respectively, of porphyrins located at the micellar interface. The small quenching of TPPS(4) in Triton X-100 is consistent with strong binding as reported in the literature.     

The mechanism of pyrene fluorescence quenching by selective and nonselective quenchers during sol–gel transition

The process of pyrene fluorescence quenching by potassium iodide, acrylamide, and gaseous oxygen in silane sol during sol–gel transition was investigated. Pyrene fluorescence emission spectra were recorded vs concentrations of the added quencher on consecutive days of the gelling process. When using 0, 20, and 100% oxygen in the gas mixture as a quencher, time-resolved measurements were also made. On this basis, Stern–Volmer constants were determined on consecutive days of the gelling process, availability of fluorophore molecules to the quencher, and also rate constants of pyrene quenching by acrylamide, iodide ions, and oxygen were specified. Moreover, diffusion coefficients were determined for acrylamide and potassium iodide in the silane gel formed. The mechanism of fluorophore quenching in sol of growing viscosity and in gel was discussed in view of using this carrier in the construction of sensors and optical biosensors.     

Quenching of tryptophan fluorescence in various proteins by a series of small nickel complexes

A series of twelve anionic, cationic, and neutral nickel(II) complexes have been synthesized and characterized. The interaction of these complexes with bovine serum albumin (BSA), human serum albumin (HSA), lysozyme (Lyso), and tryptophan (Trp) has been studied using steady-state fluorescence spectroscopy. Dynamic and static quenching constants have been calculated, and the role played in quenching by the ligand and complex charge investigated. The nickel complexes showed selectivity towards the different proteins based on the environment surrounding the Trp residue(s). Only small neutral complexes with hydrophobic ligands effectively quenched protein fluorescence via static quenching, with association constants ranging from 10(2) M(-1) (free Trp) to 10(10) M(-1) (lysozyme), indicating a spontaneous and thermodynamically favorable interaction. The number of binding sites, on average, was determined to be one in BSA, HSA and free Trp, and two in lysozyme.     

Quenching of the excited singlet state of acridine and 10-methylacridinium cation by thio-organic compounds in aqueous solution

In this work, the lowest excited singlet states of acridine (Acr), acridinium (AcrH⁺) and 10-methylacridinium (AcrMe⁺) are quenched by sulfur-containing amino acids and carboxylic acids in aqueous solution. Both steady-state and time-resolved fluorescence techniques were used to monitor the quenching of fluorescence. Stern–Volmer plots of the fluorescence intensity showed a static component (K_S) to the quenching. The experimental K_S values were compared to theoretical K_S values for outer-sphere complexes based on Debye–Hückel theory and the Fuoss equation. The general agreement between experimental and theoretical K_S values indicate that the static quenching can be attributed to non-fluorescing ion pairs associated as simple outer-sphere complexes. The computed values of the interionic distances of the ion pairs are consistent with the ion pairs of the Z_AZ_Q=−1 and −2 cases being solvent-separated ion pairs while those of the Z_AZ_Q=−3 case are contact ion pairs. The effect of the reactants’ charges on the quenching rate constants (dynamic component) was observed for the reactions of AcrMe⁺ with the anionic forms of the quenchers (having charges Z_Q=−1, −2 and −3). The rate constants (extrapolated to ionic strength, μ=0) for the quenching processes were determined to be 0.3–5.3×10¹⁰ M⁻¹ s⁻¹ depending on the ionic charge (Z_Q) of the quencher used. These trends in the quenching rate constants are rationalized with a quenching scheme for electron transfer. Analogous quenching rate constants for alanine and glycine were found to be at least an order of magnitude lower. Photoinduced electron transfer from the sulfur atom of the quencher molecule to the acridine excited singlet state is suggested to be the most likely mechanism of the process under discussion.     

Studies on the role of peroxy and hydroperoxy groups in polymer photodegradation

Interactions of carbonyl singlet and triplet excited states with peroxides and hydroperoxides were examined in order to ascertain the reasons why the latter two compounds act as nearly diffusion controlled quenchers of ketone-containing polymer photodegradation. From singlet state fluorescence quenching and laser flash photolysis measurements of triplet decays, direct interaction with the carbonyl excited states was ruled out. These processes occurred with rate constants at least one to two orders of magnitude below the diffusion controlled limit. Trapping of the 1,4-biradical precursor to chain scission, thereby preventing its disproportionation was also tentatively eliminated. Results obtained using benzoyl peroxide suggest that quencher photodissociation into alkoxy radicals might represent the key step in the quenching of ketone-containing polymer photodegradation.