Time-resolved fluorescence anisotropy measurements of the adsorption of Rhodamine-B and a labelled polyelectrolyte onto colloidal silica

 The application of time-resolved fluorescence anisotropy measurements (TRAMS) to the investigation of the adsorption of the dye Rhodamine B and a Rhodamine B-labelled cationic polyelectrolyte onto colloidal silica (Ludox) is described. For Rhodamine B the time-resolved fluorescence anisotropy behavior observed can be interpreted using a model consisting of fluorophores with two distinct fluorescence decay lifetimes and two rotational correlation times corresponding to the fluorophore free in solution and bound to the Ludox. Details of the binding obtained from a global analysis of the data are reported. Restricted motion of the fluorescently labelled polyelectrolyte is also observ-ed on adsorption. The considerations for the general application of TRAMS for monitoring adsorption behavior are discussed. Received: 8 July 1998 Accepted: 10 August 1998     

Testing the use of molecular dynamics to simulate fluorophore motions and FRET

Fluorescence resonance energy transfer (FRET) is commonly used to determine the proximity of fluorophores, but usually many assumptions are required to gain a quantitative relationship between the likelihood of energy transfer and fluorophore separation. Molecular Dynamics (MD) simulations provide one way of checking these assumptions, but before using simulations to study complex systems it is important to make sure that they can correctly model the motions of fluorophores and the likely FRET efficiency in a simple system. Here we simulate a well characterised situation of independent fluorophores in solution so that we can compare the predictions with expected values. Our simulations reproduce the experimental fluorescence anisotropy of Alexafluor488 and predict that of AlexaFluor568. At the ensemble level we are able to reproduce the expected isotropic and dynamic motion of the fluorophores as well as the FRET efficiency of the system. At the level of single donor-acceptor pairs, however, very long simulations are required to adequately sample the translational motion of the fluorophores and more surprisingly also the rotational motion. Our studies demonstrate how MD simulations can be used in more complex systems to check if the dynamic orientation averaging regime applies, if the fluorophores have isotropic orientational motion, to calculate the likely values of the orientation factor κ(2) and to determine the FRET efficiency of the system in both dynamic and static orientational averaging regimes. We also show that it is possible in some situations to create system specific relationships between FRET efficiency and fluorophore separation that can be used to interpret experimental data and find any correlations between κ(2) and separation that may influence distance measurements.     

Time-resolved fluorescence spectroscopy for illuminating complex systems

Over the years, the emissive characteristics (spectral, temporal, and polarization) of fluorophores have been widely used to probe a wide variety of systems. Fluorescence lifetime and rotational reorientation time measurements, in particular, offer a means to elucidate key details about complex systems. Further, because fluorescence occurs on the nanosecond (10⁻⁹ s) timescale, competing or perturbing kinetic processes like collisional quenching, solvent relaxation, energy transfer, and rotational reorientation can affect the fluorescence and hence be quantified. Thus, a carefully chosen and “placed” fluorophore can serve as an reporter on a wide range of nanosecond or faster events. This contribution is divided into three sections. The Theory section discusses time-resolved anisotropy and intensity decay kinetics (time and frequency domains), pump–probe spectroscopy, and up-conversion. The second section describes time-correlated single photon counting (TCSPC) and multifrequency phase-modulation fluorescence instruments. The final section is divided into subsections on the use of time-resolved fluorescence: (1) to study solvation dynamics, biochemical systems, polymer photophysics, and organized media; (2) as a tool in the separation sciences, microscopy, and sensing; and (3) coupled with multiphoton excitation strategies.     

LIGHT QUENCHING OF FLUORESCENCE: A NEW METHOD TO CONTROL THE EXCITED STATE LIFETIME AND ORIENTATION OF FLUOROPHORES

Abstract Experimental studies have recently demonstrated that fluorescence emission can be quenched by laser light pulses from modem high-repetition rate lasers, a phenomenon we call “light quenching.” In this overview article, we describe the possible effects of light quenching on the steady-state and time-resolved intensity and anisotropy of fluorophores. One can imagine two classes of experiments. Light quenching can occur within the single excitation pulse, or light quenching can be accomplished with a second time-delayed quenching pulse. The extent of light quenching depends on the amplitude of the emission spectrum at the quenching wavelength. Different effects are expected for light quenching by a single laser beam (within a single laser pulse) or for a time-delayed quenching pulse. Depending upon the polarization of the light quenching beam, light quenching can decrease or increase the anisotropy. Remarkably, the light quenching can break the usual z-axis symmetry of the excited state population, and the measured anisotropy (or polarization) depends upon whether the observation axis is parallel or perpendicular to the propagation direction of the light quenching beam. The polarization can increase to unity under selected conditions. Quenching with time-delayed light pulses can result in step changes in the intensity or anisotropy, which is predicted to result in oscillations in the frequency-domain intensity and anisotropy decays. These predicted effects of light quenching, including oscillations in the frequency-domain data, were demonstrated to occur using selected fluorophores. The increasing availability and use of pulsed laser sources requires consideration of the possible effects of light quenching and offers the opportunity for a new class of two-pulse or multiple-pulse time-resolved experiments where the sample is prepared by the excitation pulse and subsequent quenching pulses to modify the excited state population, followed by time- or frequency-domain measurement of the optically prepared excited fluorophores.     

Aggregation of Poly(γ‐benzyl L‐glutamate)s Followed by Time‐Resolved Emission Anisotropy

Syntheses of poly(γ‐benzyl L ‐glutamate)s (PBLGs) labeled with various fluorophores (tryptophan, dansyl, and anthracene) having different molecular weights are reported. Association of PBLG chains was studied by time‐resolved emission anisotropy in the solvents supporting the aggregation process (1,4‐dioxane and tetrahydrofuran) and in N,N‐dimethylformamide, where the aggregates were not formed. The influence of molecular weight and polymer concentration on PBLG association was studied as well. The limiting emission anisotropy (r_∞) and rotational correlation times (ϕ) were determined. The chain relaxation dynamics were compared with the fluorescence lifetimes of the fluorophores and spectroscopically suitable labels were selected. Tryptophan was found to be an inconvenient fluorophore for the association study of PBLGs because of its short excited‐state lifetime. Dansyl and anthracene fluorophores, however, proved to be suitable labels for the chain dynamics study of PBLGs in solution. The mobilities of PBLG chains in 1,4‐dioxane were slower than those in tetrahydrofuran and N,N‐dimethylformamide because of PBLG association in this solvent.     

Rotational dynamics of UVITEX-OB in alkanes, alcohols and binary mixtures

Steady-state and time resolved fluorescence anisotropy measurements were carried out to study the rotational diffusion dynamics of UVITEX-OB (U-OB) in series of alcohols, alkanes and binary mixtures of toluene and butanol at room temperature. The experimentally measured rotational reorientation times were compared with those estimated by the hydrodynamic and molecular models developed for microscopic friction. The experimental results are in good agreement with theoretical slip hydrodynamics and a deviation towards subslip behavior is noted. Also a faster rotation of the probe in binary mixture of toluene and butanol is noted as compared to that in alcohols and alkanes.     

Anisotropy resolved multidimensional emission spectroscopy (ARMES): A new tool for protein analysis

Structural analysis of proteins using the emission of intrinsic fluorophores is complicated by spectral overlap. Anisotropy resolved multidimensional emission spectroscopy (ARMES) overcame the overlap problem by the use of anisotropy, with chemometric analysis, to better resolve emission from different fluorophores. Total synchronous fluorescence scan (TSFS) provided information about all the fluorophores that contributed to emission while anisotropy provided information about the environment of each fluorophore. Here the utility of ARMES was demonstrated via study of the chemical and thermal denaturation of human serum albumin (HSA).     

The decay of the fluorescence anisotropy of tryptophan residues in fungal lipase fromHumicola lanuginosa

Pulse nanosecond fluorescence anisotropy decay has been used to study the mobility of tryptophan residues within fungal lipase fromHumicola lanuginosa. The decay of emission anisotropy of protein in native, inhibited and mutated form has been investigated in buffered water and 50% v/v glycerol solutions. The rotational motions of the lipase were analyzed in terms of two different kinetic models. It was found that the fluorescence emission anisotropy decay can best be desribed with two rotational correlation times: 0.63 and 5.45 ns in water and 0.98 and 10.70 ns and in 50% v/v glycerol solution. Using the same experimental conditions the decay of inhibited and mutatedH. lanuginosa lipase showed a similar biexponential character. These results are interpreted in terms of local or segmental motion arising from a mass of about 1083 daltons which corresponds to the ‘lid’-helix fragment of the enzyme.     

Recent progress in nanomaterial-enhanced fluorescence polarization/anisotropy sensors

In this review, we summarize the number of scientific publications in the field of FP/FA sensor in recent five years, and introduce the recent progress of FP/FA sensor based on nanomaterial. The various analytical applications of FP/FA sensor based on nanomaterial are discussed. We also provide perspectives on the current challenges and future prospects in the promising field.     

Recent progress in nanomaterial-enhanced fluorescence polarization/anisotropy sensors

As a promising signaling transduction approach, fluorescence polarization (FP)/fluorescence anisotropy (FA), provides a powerful quantitative tool for the rotational motion of fluorescently labeled molecules in chemical or biological homogeneous systems. Unlike fluorescence intensity, FP/FA is almost independent the concentration or quantum of fluorophores, but they are highly dependent on the size or molecular weight of the molecules or materials attached to fluorophores. Recently, significant progress in FP/FA was made, due to the introduction of some nanomaterials as FP/FA enhancers. The detection sensitivity is thus greatly improved by using nanomaterials as FP/FA enhancers, and nanomaterial-based FP/FA is currently used successfully in immunoassay, and analysis of protein, nucleic acid, small molecule and metal ion. Nanomaterial-based FP/FA provides a new kind of strategy to design fluorescent sensors and establishes innovative analytical methods. In this review, we summarize the scientific publications in the field of FP/FA sensor in recent five years, and first introduce the recent progress of FP/FA sensor based on nanomaterial. Subsequently, the various analytical applications of FP/FA based on nanomaterial are discussed. Finally, we provide perspectives on the current challenges and future prospects in this promising field.     

Energy migration in novel pH-triggered self-assembled beta-sheet ribbons

Energy migration between tryptophan residues has been experimentally demonstrated in self-assembled peptide tapes. Each peptide contains 11 amino acids with a Trp at position 6. The peptide self-assembly is pH-sensitive and forms amphiphilic tapes, which further stack in ribbons (double tapes) and fibrils in water depending on the concentration. Fluorescence spectra, quenching, and anisotropy experiments showed that when the pH is lowered from 9 to 2, the peptide self-assembly buries the tryptophan in a hydrophobic and restricted environment in the interior of stable ribbons as expected on the basis of the peptide design. These fluorescence data support directly and for the first time the presence of such ribbons which are characterized by a highly packed and stable hydrophobic interior. In common with Trp in many proteins, fluorescence lifetimes are nonexponential, but the average lifetime is shorter at low pH, possibly due to quenching with neighboring Phe residues. Unexpectedly, time-resolved fluorescence anisotropy does not change significantly with self-assembly when in water. In highly viscous sucrose-water mixtures, the anisotropy decay at low pH was largely unchanged compared to that in water, whereas at high pH, the anisotropy decay increased significantly. We concluded that depolarization at low pH was not due to rotational diffusion but mainly due to energy migration between adjacent tryptophan residues. This was supported by a master equation kinetic model of Trp-Trp energy migration, which showed that the simulated and experimental results are in good agreement, although on average only three Trp residues were visited before emission.     

Quantifying surface coverage of colloidal silica by a cationic peptide using a combined centrifugation/time-resolved fluorescence anisotropy approach

Recent experimental studies have shown that time-resolved fluorescence anisotropy (TRFA) is a promising methodology for in situ characterization of the surface modification of aqueous silica nanocolloids. Here we provide a more fundamental insight into the principle of this approach and discuss how the adsorption parameters for a cationic peptide, Lys-Trp-Lys (denoted using the standard shortform KWK), onto Ludox nanoparticles (NPs) are linked to the rotational dynamics of rhodamine 6G (R6G) dispersed in the KWK/Ludox mixture. First, the adsorption isotherm of KWK on hydrophilic controlled pore glass (CPG-3000) was obtained using the traditional centrifugation method, which provides the total molar amount of KWK per unit surface area of the silica. Assuming that both CPG and Ludox particles possess identical surface properties when suspended in the same aqueous buffer, both materials should also have identical adsorption properties. Thus, the adsorbed amount of KWK per unit area at a given total KWK concentration, as determined by the centrifugation method, can be plotted against the fractions of R6G anisotropy decay components at the same KWK concentration to relate the anisotropy components to the absolute surface coverage. Using this approach, it was determined that the concentration of KWK at which the CPG surface was saturated corresponded to the condition g = 0 in the R6G decay, where g is the fraction of the nondecaying anisotropy component. This condition means that there is no R6G bound to the fraction of Ludox NPs with a radius R > 2.5 nm at maximum KWK coverage, consistent with the adsorbed peptide forming a continuous layer on the Ludox surface. Hence, the g value obtained from TRFA analysis can be used to assess the absolute surface coverage of monolayer coatings on colloidal nanoparticles.     

Application of PARAFAC to a two-component system exhibiting fluorescence resonance energy transfer: from theoretical prediction to experimental validation

One of the conventionally accepted requirements for parallel factor analysis (PARAFAC) to handle the fluorescence excitation emission matrices (EEMs) is the independence of each component's absorption and emission spectra in simple mixtures of fluorophores. EEMs of samples in which F?rster resonance energy transfer (FRET) occurs between fluorophores seem to fail to meet this requirement. A rigorous theoretical treatment of the steady-state kinetics in the present work indicates that the fluorescence in the presence of FRET, excited by relatively weak excitation light intensity, can be reasonably separated into additive contributions from three parts: donors, acceptors and FRET. This prediction is for the first time verified experimentally in sodium dodecyl sulfate micellar solutions containing biphenyl as the energy donor and 2,5-diphenyloxazole as the energy acceptor. The experimental EEMs were well fitted to three components as predicted. A well accepted diagnostic test called core consistency (CC), specifically designed for modeling simple mixtures of fluorophores with PARAFAC, was found to be negative for the 3-component model in the present study. The simultaneous occurrence of good model fit and significantly negative CC when modeling fluorophore mixtures by conventional PARAFAC would be indicative of the presence of physical/chemical processes (e.g., FRET) that deviate from the conventional working requirements for PARAFAC. The extent of FRET has been independently measured or calculated by three methods: 1) decrease in steady state fluorescence of donor; 2) lifetime measurements with population analysis; and 3) Poisson statistics based on PARAFAC-determined distribution constants. The results of the three methods are consistent. The normalized scores of the three components found by PARAFAC also agree to within a few percent with relative concentrations in aqueous and micelle phases determined from distribution constants for the solutions prepared with nine different combinations of total donor and acceptor concentrations. Our theoretical treatment also for the first time spells out in detail the relationship between the PARAFAC scores and concentrations of components, in terms of photophysical constants of the components and spectral shape factors.     

Picosecond time-resolved fluorescence study on solute-solvent interaction of 2-aminoquinoline in room-temperature ionic liquids: aromaticity of imidazolium-based ionic liquids

Time-resolved fluorescence spectra and fluorescence anisotropy decay of 2-aminoquinoline (2AQ) have been measured in eight room-temperature ionic liquids, including five imidazolium-based aromatic ionic liquids and three nonaromatic ionic liquids. The same experiments have also been carried out in several ordinary molecular liquids for comparison. The observed time-resolved fluorescence spectra indicate the formation of pi-pi aromatic complexes of 2AQ in some of the aromatic ionic liquids but not in the nonaromatic ionic liquids. The fluorescence anisotropy decay data show unusually slow rotational diffusion of 2AQ in the aromatic ionic liquids, suggesting the formation of solute-solvent complexes. The probe 2AQ molecule is likely to be incorporated in the possible local structure of ionic liquids, and hence the anisotropy decays only through the rotation of the whole local structure, making the apparent rotational diffusion of 2AQ slow. The rotational diffusion time decreases rapidly by adding a small amount of acetonitrile to the solution. This observation is interpreted in terms of the local structure formation in the aromatic ionic liquids and its destruction by acetonitrile. No unusual behavior upon addition of acetonitrile has been found for the nonaromatic ionic liquids. It is argued that the aromaticity of the imidazolium cation plays a key role in the local structure formation in imidazolium-based ionic liquids.     

荧光各向异性法快速测定荧光标记物对蛋白质的标记比

免疫荧光技术是免疫学检测的重要手段之一,该技术在病原微生物的早期诊断、自身免疫研究、抗原或抗体的免疫组化定位等方面都得到了广泛应用^[1].荧光色素对抗体(或抗原)标记比的测定是免疫荧光技术的重要部分.     

A new look at fluorescence depolarization and the dynamics of anisotropic rotational diffusion

Existing derivations of the time-dependent fluorescence anisotropy of an asymmetric molecule constitute a straightforward application of Favro's work on the rotational diffusion equation (RDE), and make no contribution to the elucidation of rotational dynamics as such. A new approach is developed, and the problem formulated in the parlance of conventional reaction kinetics by demonstrating, with the aid of the method of moments, that the RDE is completely equivalent to a set of ordinary linear differential equations, formally identical with those used to describe a first-order series-parallel reaction scheme.     

Fluorescence detection of hyaluronidase

We labeled hyaluronan (HA) with two fluorophores, fluorescein amine and rhodamine B amine. These two fluorophores are suitable for a fluorescence (Foerster) resonance energy transfer (FRET) which results in a fluorescein quenching and an enhanced rhodamine emission. Such labeled HA (HA-FRET) is a potential sensor for HA degradation. We studied fluorescence properties of HA-FRET in the absence and presence of hyaluronidase enzyme (HA-ase). The time-resolved fluorescence measurements indicate more than 50% of FRET in the absence of HA-ase. In the presence of HA-ase FRET decreases with time, and relative fluorescence intensities of fluorescein and rhodamine shifts to fluorescein indicating a release of FRET. The kinetics of the digestion process of HA by HA-ase depends on the concentration of the enzyme. We demonstrate that simultaneous measurements of green and red emission of HA-FRET can be used in ratio metric detection of the HA-ase presence and activity. This in turn, can be utilized for the construction of a robust but reliable HA-ase sensing device.     

Installation/modulation of the emission response via click reaction

We have demonstrated the installation of a fluorescence property into a nonfluorescent precursor and modulation of an emission response of a pyrene fluorophore via click reaction. The synthesized fluorophores show different solvatochromicity and/or intramolecular charge transfer (ICT) feature as is revealed from the UV-visible, fluorescence photophysical properties of these fluorophores, and DFT/TDDFT calculation. We observed that some of the synthesized fluorophores showed purely ICT character while emission from some of them arose from the LE state. A structureless and solvent polarity-sensitive dual emission behavior was observed for one of the triazolylpyrene fluorophores that contains an electron-donating -NMe(2) substituent (fluorophore, 7a). Conversely, triazolylpyrene with an electron-withdrawing -CN group (fluorophore, 7b) showed a solvent polarity-independent vibronic emission. The effect of ICT on the photophysical properties of these fluorophores was studied by fluorescence emission spectra and DFT/TDDFT calculations. Fluorescence lifetimes were also measured in different solvents. All of our findings revealed the delicate interplay of structure and emission properties and thus having broader general utility. As the CT to LE intensity ratio can be employed as a sensing index, the dual emissive fluorophore can be utilized in designing the molecular recognition system too. We envisage that our investigation is of importance for the development of new fluorophores with predetermined photophysical properties that may find a wide range of applications in chemistry, biology, and material sciences.     

Luminescence behaviour of 5-hydroxyindole in different environments

Steady state fluorescence emission spectroscopic studies along with some lifetime measurements have been performed for 5-hydroxyindole (5HI) in different environments. 5HI merits particular attention, since it is the chromophoric moiety of the non-natural amino acid 5-hydroxytryptophan (5HT), which has come into significant, recent prominence as a novel intrinsic optical probe for protein structure, function and dynamics. Studies in representative homogeneous solvents and solvent-mixtures indicate that unlike other fluorophores of related interest like indole (I) and 7-azaindole (7AI), the fluorescence emission maximum (lambda(em)max) of 5HI is relatively insensitive to solvent polarity. This behaviour suggests the lack of appreciable solvent dipolar relaxation in 5HI, which is consistent with our low temperature (77 K) emission data. Notwithstanding such limitation, fluorescence anisotropy (r) and quenching studies are shown to be effective for exploring changes in the micro-environments of 5HI in sodium bis-(2-ethylhexyl)sulfosuccinate (AOT) reverse micellar assemblies (which serve as a biomembrane mimetic model system) with variation in water/surfactant molar ratio (w0).     

Compartmental modeling of the fluorescence anisotropy decay of a cylindrically symmetric brownian rotor: Identifiability analysis.

We present the results of the deterministic identifiability analysis based on similarity transformation for models of one-state excited-state events of cylindrically symmetric rotors in isotropic environments undergoing rotational diffusion described by Brownian reorientation. Such an analysis on error-free time-resolved fluorescence (anisotropy) data can reveal whether the parameters of the considered model can be determined. The fluorescence delta-response functions I(parallel)(t) and I(perpendicular)(t), for fluorescence polarized respectively parallel and perpendicular to the electric vector of linearly polarized excitation, are used to construct, in convenient matrix form, expressions of the sum S(t) = I(parallel)(t) + 2I(perpendicular)(t), the difference D(t) = I(parallel)(t) - I(perpendicular)(t), and the time-resolved fluorescence anisotropy r(t) = D(t)/S(t). The identifiability analysis of r(t) demonstrates that the rotational diffusion coefficients D(parallel) and D(perpendicular) for rotation respectively about and perpendicular to the symmetry axis can be uniquely resolved. However, the polar and azimuthal angles defining the absorption and emission transition moments in the molecular reference frame are not individually identifiable. Nevertheless, the difference between the polar angles of these transition moments is uniquely determined.