Theory of phase-modulation fluorescence spectroscopy for excited-state processes

Resonances at Fundamental and Harmonic Frequencies for Selective Imaging of Sine-Wave Illuminated Reversibly Photoactivatable Labels

We introduce HIGHLIGHT as a simple and general strategy to selectively image a reversibly photoactivatable fluorescent label associated with a given kinetics. The label is submitted to sine-wave illumination of large amplitude, which generates oscillations of its concentration and fluorescence at higher harmonic frequencies. For singularizing a label, HIGHLIGHT uses specific frequencies and mean light intensities associated with resonances of the amplitudes of concentration and fluorescence oscillations at harmonic frequencies. Several non-redundant resonant observables are simultaneously retrieved from a single experiment with phase-sensitive detection. HIGHLIGHT is used for selective imaging of four spectrally similar fluorescent proteins that had not been discriminated so far. Moreover, labels out of targeted locations can be discarded in an inhomogeneous spatial profile of illumination. HIGHLIGHT opens roads for simplified optical setups at reduced cost and easier maintenance.

Calcium imaging using fluorescence lifetimes and long-wavelength probes

We describe imaging of calcium concentrations using the long-wavelength Ca(2+) indicators, Calcium Green, Orange, and Crimson. The lifetimes of these probes were measured using the frequency-domain method and were found to increase from 50% to severalfold in response to calcium. The two-dimensional images of the calcium concentration were obtained using a new apparatus for fluorescence lifetime imaging (FLIM). We also describe procedures to correct for the position-dependent frequency response of the gain-modulated image intensifier used in the FLIM apparatus. Importantly, the FLIM method does not require the probe to display shifts in the excitation or emission spectra. Using the FLIM method, calcium imaging is possible using probes which display changes in lifetime in response to calcium. Consequently, calcium imaging is possible with excitation wavelengths ranging from 488 to as long as 620 nm, where autofluorescence and/or photochemical damage is minimal. These probes are also suitable for calcium measurements of single cells using lifetime-based flow cytometry.

Resolution of multiexponential spectral relaxation of Yt-base by global analysis of collisionally quenched samples

We measured the wavelength-dependent intensity decays of 4,9-dihydro-4,6-dimethyl-9-oxo-1H-imidazo-1,2a-purine (Yt-base) in propanol to determine the time-resolved emission spectra and rates of spectral relaxation. We found that resolution of the spectral relaxation times was dramatically improved by global analysis of the frequency-domain data with increasing amounts of the collisional quencher CCl4. Collisional quenching preferentially decreases the longer-lived relaxed component of the emission, thereby increasing the fractional contribution of the incompletely relaxed portion of the emission. The data could not be explained by a single spectral relaxation time, and at least two relaxation times are needed to describe the time-dependent emission center of gravity of Yt-base.

Quantitative multi-color FRET measurements by Fourier lifetime excitation-emission matrix spectroscopy

Förster resonant energy transfer (FRET) is extensively used to probe macromolecular interactions and conformation changes. The established FRET lifetime analysis method measures the FRET process through its effect on the donor lifetime. In this paper we present a method that directly probes the time-resolved FRET signal with frequency domain Fourier lifetime excitation-emission matrix (FLEEM) measurements. FLEEM separates fluorescent signals by their different phonon energy pathways from excitation to emission. The FRET process generates a unique signal channel that is initiated by donor excitation but ends with acceptor emission. Time-resolved analysis of the FRET EEM channel allows direct measurements on the FRET process, unaffected by free fluorophores that might be present in the sample. Together with time-resolved analysis on non-FRET channels, i.e. donor and acceptor EEM channels, time resolved EEM analysis allows precise quantification of FRET in the presence of free fluorophores. The method is extended to three-color FRET processes, where quantification with traditional methods remains challenging because of the significantly increased complexity in the three-way FRET interactions. We demonstrate the time-resolved EEM analysis method with quantification of three-color FRET in incompletely hybridized triple-labeled DNA oligonucleotides. Quantitative measurements of the three-color FRET process in triple-labeled dsDNA are obtained in the presence of free single-labeled ssDNA and double-labeled dsDNA. The results establish a quantification method for studying multi-color FRET between multiple macromolecules in biochemical equilibrium.

Red-shifted fluorescent proteins monitor enzymatic activity in live HT-1080 cells with fluorescence lifetime imaging microscopy (FLIM)

Membrane type 1 matrix metalloproteinase (MT1-MMP) is a membrane-tethered collagenase primarily involved in the mechanical destruction of extracellular matrix proteins. MT1-MMP has also been shown to be upregulated in several types of cancers. Many coordinated functions of MT1-MMP during migration and invasion remain to be determined. In this paper, live cells from the invasive cell line HT-1080 were imaged using an intracellular Förster resonance energy transfer-based biosensor specific for MT1-MMP; a substrate specific for MT1-MMP was hybridized with the mOrange2 and mCherry fluorescent proteins to form the Förster resonance energy transfer-based sensor. The configuration of the biosensor was determined with fluorescence lifetime-resolved imaging microscopy using both a polar plot-based analysis and a rapid data acquisition modality of fluorescence lifetime-resolved imaging microscopy known as phase suppression. Both configurations of the biosensor (with or without cleavage by MT1-MMP) were clearly resolvable in the same cell. Changes in the configuration of the MT1-MMP biosensor were observed primarily along the edge of the cell following the removal of the MMP inhibitor GM6001. The intensities highlighted by phase suppression correlated well with the fractional intensities derived from the polar plot.

Phase differential enhancement of FLIM to distinguish FRET components of a biosensor for monitoring molecular activity of Membrane Type 1 Matrix Metalloproteinase in live cells

Fluorescence lifetime-resolved imaging microscopy (FLIM) has been used to monitor the enzymatic activity of a proteolytic enzyme, Membrane Type 1 Matrix Metalloproteinase (MT1-MMP), with a recently developed FRET-based biosensor in vitro and in live HeLa and HT1080 cells. MT1-MMP is a collagenaise that is involved in the destruction of extra-cellular matrix (ECM) proteins, as well as in various cellular functions including migration. The increased expression of MT1-MMP has been positively correlated with the invasive potential of tumor cells. However, the precise spatiotemporal activation patterns of MT1-MMP in live cells are still not well-established. The activity of MT1-MMP was examined with our biosensor in live cells. Imaging of live cells was performed with full-field frequency-domain FLIM. Image analysis was carried out both with polar plots and phase differential enhancement. Phase differential enhancement, which is similar to phase suppression, is shown to facilitate the differentiation between different conformations of the MT1-MMP biosensor in live cells when the lifetime differences are small. FLIM carried out in differential enhancement or phase suppression modes, requires only two acquired phase images, and permits rapid imaging of the activity of MT1-MMP in live cells.

Global analysis of dynamic fluorescence anisotropy by a polarized phasor approach

Recently, the graphical analysis of the fluorescence lifetime imaging using the phasor approach has been highlight, and a series of the reports have made it on the way for the applications by the nonprofessionals. In this paper, we put forward a similar theory validated by the experiments for the dynamic fluorescence anisotropy imaging. By subtracting the perpendicular component from the parallel one in the frequency-domain polarization measurement, we deduce a new analytical expression about the fluorescence joint time, and find that as much as the fluorophore is a single exponential decay and r∞ is equal to zero, △I(t) is a single exponential decay with the time constant X as well, and the center of its histograms is located on the semicircle in the polarized phasor plot. In the end, we conclude that the fluorescence joint time is the best parameter to weigh the fluorescence dynamics for the macromolecules.

Spectrally resolved fluorescent lifetime imaging

Placing an imaging spectrograph or related components capable of generating a spectrum between a microscope and the image intensifier of a conventional fluorescence lifetime imaging (FLIM) system creates a spectrally resolved FLIM (SFLIM). This arrangement provides a number of opportunities not readily available to conventional systems using bandpass filters. The examples include: simultaneous viewing of multiple fluorophores; tracking of both the donor and acceptor; and observation of a range of spectroscopic changes invisible to the conventional FLIM systems. In the frequency-domain implementation of the method, variation in the fractional contributions from different fluorophores along the wavelength dimension can behave as a surrogate for a frequency sweep or spatial variations while analysing fluorophore mixtures. This paper reviews the development of the SFLIM method, provides a theoretical and practical overview of frequency-domain SFLIM including: presentation of the data; manifestations of energy transfer; observation of multiple fluorophores; and the limits of single frequency methods.

Quantitative analysis of fluorescence lifetime imaging made easy

Fluorescence lifetime imaging is a valuable and versatile tool for the investigation of the molecular environment of fluorophores in living cells. It is ideally suited-and is therefore increasingly used-for the quantification of the occurrence of Förster Resonance Energy Transfer, a powerful microscopy method for the detection of subnanometer conformational changes, protein-protein interactions, and protein biochemical status. However, careful quantitative analysis is required for the correct and meaningful interpretation of fluorescence lifetime data. This can be a daunting task to the nonexpert user, and is the source for many avoidable errors and unsound interpretations. Digman and colleagues (Digman et al., 2007, Biophys. J. 94, L14-6) present an analysis technique that avoids data fitting in favor of a simple graphical polar data representation. In this "phasor" space, the physics of lifetime imaging becomes more intuitive and accessible also to the inexperienced user. The cumulated information from image pixels, even over different cells, describes patterns and trajectories that can be visually interpreted in physically meaningful ways. Its usefulness is demonstrated in the study of the dimerization of the uPAR receptor (Caiolfa et al., 2007, J. Cell Biol. 179, 1067-1082).

Optimizing frequency-domain fluorescence lifetime sensing for high-throughput applications: photon economy and acquisition speed

The signal-to-noise ratio of a measurement is determined by the photon economy of the detection technique and the available photons that are emitted by the sample. We investigate the efficiency of various frequency-domain lifetime detection techniques also in relation to time-domain detection. Nonlinear effects are discussed that are introduced by the use of image intensifiers and by fluorophore saturation. The efficiency of fluorescence lifetime imaging microscopy setups is connected to the speed of acquisition and thus to the imaging throughput. We report on the optimal conditions for balancing signal-to-noise ratio and acquisition speed in fluorescence lifetime sensing.

Analysis of data obtained from a frequency-multiplexed phase-modulation fluorometer using an autoregressive model

In order to derive plural values of fluorescence lifetimes simultaneously from a multi-component sample, we formulate a mathematical method for analyzing data obtained from a frequency-multiplexed phase-modulation fluorometer (FM-PMF) using an autoregressive (AR) model. Various parameter settings necessary for performing accurate data analysis based on the AR model are studied through numerical simulations. Measurement results of fluorescence lifetimes of real samples, 10 ppm quinine sulfate in 0.1 N H(2)SO(4), 10 ppm rhodamine 6G in ethanol, and their mixture with a volume ratio of 1:1, demonstrate that the proposed method works quite well.

Spectrally resolved frequency domain analysis of multi-fluorophore systems undergoing energy transfer

Complex systems of fluorophores undergoing energy transfer can exhibit a variety of anomalous lifetime behavior when probed with frequency domain methods. When presented in traditional apparent lifetime format the data from such systems can exhibit "nodal" behavior in which the computed lifetime approaches +/-infinity. The location of the nodes is system and frequency dependent. In addition, simpler systems, not undergoing energy transfer, show ill behavior in the region of zero lifetime (tau(m)) and long lifetime (tau(pi)) due to noise in typical measurements. Here, we systematically investigate systems of multiple fluorophores with and without energy transfer to provide insight into frequency domain investigations of complex systems of fluorophores. The results of simulations are compared to data collected from a multi-fluorophore system designed to exhibit fluorescence resonance energy transfer (FRET) using imaging spectroscopic fluorescence lifetime imaging microscopy (ISFLIM). The results are applicable to both cuvette and imaging arrangements.

Microspectroscopic fluorescence analysis with prism-based imaging spectrometers: review and current studies

BACKGROUND

Fluorescence imaging spectroscopy is a powerful but under-utilized tool. This article gives perspective on the use of imaging spectroscopy, and provides two examples of imaging spectroscopy done with a prism-based system. The intent is to give insight into the power of imaging spectroscopy when used in combination with other imaging techniques. In particular, studies of intact coral photobleaching and beads designed to show energy transfer are reported. In the bead study, spectroscopic lifetime imaging was performed at each photobleaching step.

RESULTS

Spectroscopic photobleaching of the hard coral, Montastrea annularis, revealed two spectral regions. A region in the red portion of the spectrum bleached rapidly while progressively increasing fluorescence was observed over a wide portion of the spectrum. This behavior is consistent with current theories for the role of fluorescent proteins in corals. Following a photobleaching study of beads designed to exhibit energy transfer with imaging spectroscopic fluorescence lifetime imaging microscopy (ISFLIM) allowed unambiguous assignment of fluorescence resonance energy transfer (FRET). The data in this experiment indicated that most of the commonly used markers of FRET would have been inconclusive. The ability of the ISFLIM to look at all regions of the spectrum, particularly the acceptor region, allowed FRET to be assigned.

CONCLUSIONS

Fluorescence imaging spectroscopy is a rapidly advancing technology, uniquely suited to the flexible detection of dyes over a wide range of wavelengths.

Analysis of heterogeneous fluorescence decays in proteins. Using fluorescence lifetime of 8-anilino-1-naphthalenesulfonate to probe apomyoglobin unfolding at equilibrium

The solvatochromic fluorescent dye 8-anilino-1-naphthalenesulfonate (ANS) is one of the popular probes of protein folding. Folding kinetics is tracked with ANS fluorescence intensity, usually interpreted as a reflection of protein structure-the hydrophobicity of the binding environments. Such simplistic view overlooks the complicated nature of ANS-protein complexes: the fluorescence characteristics are convoluted results of the ground state populational distribution of the probe-protein complex, the structural changes in the protein and the excited state photophysics of the probe. Understanding of the interplay of these aspects is crucial in accurate interpretation of the protein dynamics. In this work, the fluorescence decay of ANS complexed with apomyoglobin at different conformations denatured by pH is modeled. The fluorescence decay of the ANS-apomyoglobin complex contains information on not only apomyoglobin structure but also molecular populational distributions. The challenge in modeling fluorescence decay profiles originates from the convolution of heterogeneous binding and excited-state relaxation of the fluorescent probe. We analyzed frequency-domain fluorescence lifetime data of ANS-apomyoglobin with both maximum entropy methods (MEM) and nonlinear least squares methods (NLLS). MEM recovers a model of two expanding-and-merging lifetime distributions for ANS-apomyoglobin in the equilibrium transition from the native (N) through an intermediate (I-1) to the acid-unfolded state U(A). At pH 6.5 and above, when apomyoglobin is mostly populated at the N-state, ANS-apomyoglobin emits a predominant long-lifetime fluorescence from a relaxed charge transfer state S(1,CT) of ANS, and a short-lifetime fluorescence that is mainly from a nascent excited-state S(1,np) of ANS stabilized by the strong ANS-apomyoglobin interaction. Lowering the pH diminishes the contribution from the S(1,np) state. Meanwhile, more protein molecules become populated at the U(A) state, which exhibits a short lifetime that is not distinguishable from the S(1,np) state. At pH 3.4, when the population of the U(A) becomes significant, the short-lifetime fluorescence comes predominantly from ANS binding to the U(A). Further lowering the pH leads to more exposure of the bound ANS. The long lifetime shifts toward and finally merges with the short lifetime and becomes one broad distribution that stands for ANS binding to the U(A) below pH 2.4. The above expanding-and-merging model is consistent with F-statistic analysis of NLLS models. The consistency of this model with the knowledge from the literature, as well as the continuity of the decay parameters changing upon experimental conditions are also crucial in drawing the conclusions.

Fluorescence lifetime heterogeneity resolution in the frequency domain by lifetime moments analysis

Fluorescence lifetime imaging microscopy presents a powerful tool in biology and biophysics because it allows the investigation of the local environment of a fluorochrome in living cells in a quantitative manner. Furthermore, imaging Förster-type resonance energy transfer (FRET) by fluorescence lifetime imaging microscopy enables protein-protein interactions and intermolecular distances to be mapped under physiological conditions. Quantitative and precise data analysis methods are required to access the richness of information that is contained in FRET data on biological samples. Lifetime detection in the frequency-domain yields two lifetime estimations. The lifetime moments analysis (LiMA) provides a quantitative measure of the lifetime distribution broadness by exploiting the analytical relationship between the phase- and demodulation-lifetime estimations and relating them to the weighted average and variance of the lifetime distribution. The LiMA theoretical framework is validated by comparison with global analysis and by applying it to a constrained two-component FRET system using simulations and experiments. Furthermore, a novel LIMA-based error analysis and a more intuitive formalism for global analysis are presented. Finally, a new method to resolve a FRET system is proposed and experimentally applied to the investigation of protein-protein interactions.

Ligand-induced Dimer-Tetramer Transition during the Activation of the Cell Surface Epidermal Growth Factor Receptor-A Multidimensional Microscopy Analysis

The epidermal growth factor receptor (EGFR) is a member of the erbB tyrosine kinase family of receptors. For many years it has been believed that receptor activation occurs via a monomer-dimer transition that is associated with a conformational change to activate the kinase. However, little is known about the quaternary state of the receptor at normal levels of expression (<10(5) receptors/cell). We employed multidimensional microscopy techniques to gain insight into the state of association of the human EGFR, in the absence and presence of ligand, on the surface of intact BaF/3 cells (50,000 receptors/cell). Image correlation microscopy of an EGFR-enhanced green fluorescent protein chimera was used to establish an average degree of aggregation on the submicron scale of 2.2 receptors/cluster in the absence of ligand increasing to 3.7 receptors/cluster in the presence of ligand. Energy transfer measurements between mixtures of fluorescein isothiocyanate-EGF and Alexa 555-EGF were performed using fluorescence lifetime imaging microscopy as a function of the donor: acceptor labeling ratio to gain insight into the spatial disposition of EGFR ligand binding sites on the nanometer scale. In the context of a two-state Förster resonance energy transfer (FRET)/non-FRET model, the data are consistent with a minimum transfer efficiency of 75% in the FRET population. The microscopy data are related to biophysical data on the EGFR in the A431 cell line and the three-dimensional structure of the ligated EGFR extracellular domain. In the context of a monomer-dimer-oligomer model, the biophysical data are consistent with a significant fraction of ligated EGFR tetramers comprising two dimers juxtaposed in a side-by-side (or slightly staggered) arrangement. Our data are consistent with a specific higher order association of the ligand-bound EGFR on the nanometer scale and indicate the existence of distinct signaling entities beyond the level of the EGFR dimer which could play an important role in receptor transactivation.

Phase-modulation fluorometer using a dynode-voltage burst-modulated photomultiplier tube

We propose a new scheme for a phase-modulation fluorometer (PMF) in which a photomultiplier tube (PMT) is used as a photo detector whose gain is modulated sinusoidally with a burst signal of period T and duty ratio 0.5. The carrier wave of the burst modulation signal is synchronized with an incident fluorescence signal. In order to modulate the gain of the PMT, one of the dynodes in the PMT was deeply biased and the burst signal was superimposed. Because the fluorescence signal is converted to a direct current (dc) signal by the PMT internal modulation, we can make the value of the load resistance of the PMT relatively large under the condition tau < or = T/2, where tau is a time constant of a low-pass filter attached to the output of the PMT. The proposed scheme brings about advantages in sensitivity and noise immunity in detecting weak fluorescence in comparison with those of the conventional PMF. The combined technique of the burst modulation of the gain of the PMT and the alternating current (ac) signal detection alleviates the influence of the background light.

Correlation between lifetime heterogeneity and kinetics heterogeneity during chlorophyll fluorescence induction in leaves: 2. Multi-frequency phase and modulation analysis evidences a loosely connected PSII pigment-protein complex

We report the first direct decomposition of the fluorescence lifetime heterogeneity during multiphasic fluorescence induction in dark-adapted leaves by multi-frequency phase and modulation fluorometry (PMF). A very fast component, assigned to photosystem I (PSI), was found to be constant in lifetime and yield, whereas the two slow components, which are strongly affected by the closure of the reaction centers by light, were assigned to PSII. Based on a modified "reversible radical pair" kinetic model with three compartments, we showed that a loosely connected pigment complex, which is assumed to be the CP47 complex, plays a specific role with respect to the structure and function of the PSII: (i) it explains the heterogeneity of PSII fluorescence lifetime as a compartmentation of excitation energy in the antenna, (ii) it is the site of a conformational change in the first second of illumination, and (iii) it is involved in the mechanisms of nonphotochemical quenching (NPQ). On the basis of the multi-frequency PMF analysis, we reconciled two apparently antagonistic aspects of chlorophyll a fluorescence in vivo: it is heterogeneous with respect to the kinetic structure (several lifetime components) and homogeneous with respect to average quantities (quasi-linear mean tau-Phi relationship).

Dynamic fluorescence anisotropy imaging microscopy in the frequency domain (rFLIM)

We describe a novel variant of fluorescence lifetime imaging microscopy (FLIM), denoted anisotropy-FLIM or rFLIM, which enables the wide-field measurement of the anisotropy decay of fluorophores on a pixel-by-pixel basis. We adapted existing frequency-domain FLIM technology for rFLIM by introducing linear polarizers in the excitation and emission paths. The phase delay and intensity ratios (AC and DC) between the polarized components of the fluorescence signal are recorded, leading to estimations of rotational correlation times and limiting anisotropies. Theory is developed that allows all the parameters of the hindered rotator model to be extracted from measurements carried out at a single modulation frequency. Two-dimensional image detection with a sensitive CCD camera provides wide-field imaging of dynamic depolarization with parallel interrogation of different compartments of a complex biological structure such as a cell. The concepts and technique of rFLIM are illustrated with a fluorophore-solvent (fluorescein-glycerol) system as a model for isotropic rotational dynamics and with bacteria expressing enhanced green fluorescent protein (EGFP) exhibiting depolarization due to homotransfer of electronic excitation energy (emFRET). The frequency-domain formalism was extended to cover the phenomenon of emFRET and yielded data consistent with a concentration depolarization mechanism resulting from the high intracellular concentration of EGFP. These investigations establish rFLIM as a powerful tool for cellular imaging based on rotational dynamics and molecular proximity.

Synthesis and Characterization of a Fluorescence Probe of the Phase Transition and Dynamic Properties of Membranes

We describe the synthesis and characterization of a new fluorescence probe whose emission spectra, anisotropies, and wavelength-dependent decay times are highly sensitive to the phase state of phospholipid vesicles. This probe is 6-palmitoyl-2-[[2-(trimethylammonio)ethyl]methylamino] naphthalene chloride (Patman). The emission maximum of Patman shifts from 425 to 470 nm at the bilayer transition temperatures. The spectral properties of Patman reveal nanosecond time-dependent spectral shifts, which are the result of membrane relaxation around the excited state of Patman. The apparent fluorescence lifetimes of Patman are strongly dependent upon the emission wavelength, and the fluorescence phase and modulation data prove that the spectral shifts are due to an excited-state process, and not ground-state heterogeneity. As expected, the fluorescence anisotropies reflect the phase transitions of the bilayers. In addition, the anisotropies are dependent upon the emission wavelength because the duration of the excited state varies across the emission spectrum. The different apparent lifetimes across the emission spectrum allow the relaxed and unrelaxed emission spectra to be resolved by phase-sensitive detection of fluorescence. Also, the emission spectra of Patman show marked shifts to longer wavelengths as the excitation wavelength is increased. These red-edge excitation shifts are sensitive to the temperature and phase state of the bilayers.

Phase-fluorometry study on dielectric relaxation of acrylodan-labeled human serum albumin

Dielectric relaxation (DR) of acrylodan-labeled human serum albumin (HSA/AC) was studied by phase-fluorometry. A non-monoexponential behavior of both the total fluorescence--and the DR decays has been found. The protein environment of the fluorescent marker shows DR times ranging from the pico to nanosecond timescale. In fluorescence emission decays measured on the red side of the fluorescence spectrum a time constant (<10 ps) affected by a negative preexponential was found supporting the existence of DR of the excited states.

Dynamics of ANS binding to tuna apomyoglobin measured with fluorescence correlation spectroscopy

The dynamics of the binding reaction of ANS to native and partly folded (molten globule) tuna and horse apomyoglobins has been investigated by fluorescence correlation spectroscopy and frequency domain fluorometry. The reaction rate has been measured as a function of apomyoglobin and ANS concentrations, pH, and temperature. Examination of the autocorrelation functions shows that the reaction rate is fast enough to be observed in tuna apomyoglobin, whereas the reaction rate in horse apomyoglobin is on the same time scale as diffusion through the volume or longer. Specifically, for tuna apomyoglobin at pH 7 and room temperature the on rate is 2200 microM(-1) s(-1) and the off rate is 5900 s(-1), in comparison with k(on) = 640 microM(-1) s(-1) and k(off) = 560 s(-1) for horse myoglobin as measured previously. The independence of the reaction rate from the ANS concentration indicates that the reaction rate is dominated by the off rate. The temperature dependence of the on-rate shows that this rate is diffusion limited. The temperature dependence of the off rates analyzed by Arrhenius and Ferry models indicates that the off rate depends on the dynamics of the protein. The differences between horse and tuna apomyoglobins in the ANS binding rate can be explained in terms of the three-dimensional apoprotein structures obtained by energy minimization after heme removal starting from crystallographic coordinates. The comparison of the calculated apomyoglobin surfaces shows a 15% smaller cavity for tuna apomyoglobin. Furthermore, a negative charge (D44) is present in the heme cavity of tuna apomyoglobin that could decrease the strength of ANS binding. At pH 5 the fluorescence lifetime distribution of ANS-apomyoglobin is bimodal, suggesting the presence of an additional binding site in the protein. The binding rates determined by FCS under these conditions show that the protein is either in the open configuration or is more flexible, making it much easier to bind. At pH 3, the protein is in a partially denatured state with multiple potential binding sites for ANS molecule, and the interpretation of the autocorrelation function is not possible by simple models. This conclusion is consistent with the broad distribution of ANS fluorescence lifetimes observed in frequency domain measurements.

Long-wavelength long-lifetime luminophores

We describe a new approach to making luminophores that display long emission wavelengths, long decay times, and high quantum yields. These luminophores are covalently linked pairs with a long-lifetime resonance-energy-transfer donor and a long-wavelength acceptor. The donor was a ruthenium (Ru) metal-ligand complex. The acceptor was the Texas Red. The donor and acceptor were covalently linked by polyproline spacers. The long-lifetime donor results in a long-lived component in the acceptor decay, which is due to RET. Importantly, the quantum yield of the luminophores approaches that of the higher quantum yield acceptor, rather than the lower quantum yield typical of metal-ligand complexes. The emission maxima and decay time of such tandem luminophores can be readily adjusted by selection of the donor, acceptor, and distance between them. Luminophores with these useful spectral properties can also be donor-acceptor pairs brought into close proximity by some biochemical association reaction. Luminophores with long-wavelength emission and long lifetimes can have numerous applications in biophysics, clinical diagnostics, DNA analysis, and drug discovery.

On the possibility of long-wavelength long-lifetime high-quantum-yield luminophores

We describe an approach to creating a new class of luminophores which display both long wavelength emissions exceeding 600 nm and long lifetimes. These luminophores are based on resonance energy transfer (RET) from a long lifetime donor to a short lifetime but long wavelength acceptor. We demonstrated the possibility of obtaining these desirable spectral properties using donors and acceptors noncovalently bound to DNA. The donor was a ruthenium (Ru) metal-ligand complex in which one of the diimine ligands intercalated into double-helix DNA. The acceptors were either nile blue, TOTO-3, or TO-PRO-3. Upon binding of the acceptor to donor-labeled DNA, we found that the acceptor quantum yield was remarkably enhanced so that the wavelength-integrated intensities of the donor and acceptor bound to DNA were many-fold greater than the intensity of the donor and acceptor alone when separately bound to DNA. The origin of this effect is efficient energy transfer from the donor. Under these conditions the effective overall quantum yield approaches that of the acceptor. Importantly, the increased quantum yield can be obtained while maintaining usefully long apparent acceptor lifetimes of 30 to 80 ns. The effect of an increased quantum yield from a low quantum yield donor may find use in assays to detect macromolecular binding interactions. These results suggest the synthesis of covalently linked donor-acceptor pairs with the desirable spectral properties of long wavelength emission, high quantum yield, and moderately long lifetimes for gated detection.

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.

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.

Time-resolved spectral observations of cadmium-enriched cadmium sulfide nanoparticles and the effects of DNA oligomer binding

We measured the steady-state and time-resolved fluorescence spectral properties of cadmium-enriched nanoparticles (CdS-Cd2+). These particles displayed two emission maxima, at 460 and 580 nm. The emission spectra were independent of excitation wavelength. Surprisingly, the intensity decays were strongly dependent on the observation wavelength, with longer decay times being observed at longer wavelengths. The mean lifetime increased from 150 to 370 ns as the emission wavelength was increased from 460 to 650 nm. The wavelength-dependent lifetimes were used to construct the time-resolved emission spectra, which showed a growth of the long-wavelength emission at longer times, and decay-associated spectra, which showed the longer wavelength emission associated with the longer decay time. These nanoparticles displayed anisotropy values as high as 0.35, depending on the excitation and emission wavelengths. Such high anisotropies are unexpected for presumably spherical nanoparticles. The anisotropy decayed with two correlation times near 5 and 370 ns, with the larger value probably due to overall rotational diffusion of the nanoparticles. Addition of a 32-base pair oligomer selectively quenched the 460-nm emission, with less quenching being observed at longer wavelengths. The time-resolved intensity decays were minimally affected by the DNA, suggesting a static quenching mechanism. The wavelength-selected quenching shown by the nanoparticles may make them useful for DNA analysis.

Measurement of the Intensity of Long-Lifetime Luminophores in the Presence of Background Signals Using Phase-Modulation Fluorometry

We describe a method to correct for background fluorescence and to measure the intensity of long-lifetime probes using phase-modulation fluorometry. The theoretically predicted and simulated data were supported by two experiments. The fractional contribution of background fluorescence in the sample was determined by measurement of phase angle and/or modulation at single modulation frequency. In certain experimental situations, where the mean decay times of the background and the long lifetime probe are widely separated, determination of background signals in the sample does not require a blank sample or information about the nature of intensity decay of the background. Hence, phase-modulation fluorometry can be used to directly determine the intensity of the long-lifetime probe in the presence of an unknown short-lifetime background. We also discuss the effects of ambient light (indefinitely long lifetime) and scattered excitation light (zero lifetime) on phase-modulation measurements.

Two-photon excitation of dioxane: time-resolved measurements of excited state complex formation with water

We observed emission from the non-aromatic hydrocarbon 1,4-dioxane upon illumination with ps pulses at 380 nm. The emission intensity depended quadratically on incident power at 380 nm, indicating a two-photon process. In the absence of water the intensity decay was close to a single exponential, but displayed some evidence of an excited state process. In the presence of 1% water the emission spectra shifted dramatically to long wavelength. Water also resulted in wavelength-dependent intensity decays with negative pre-exponential factors on the long wavelength side of the emission, demonstrating the presence of an excited state reaction. At this water concentration the results are consistent with a two-state model due to emission from dioxane and a dioxane and a dioxane-water complex.

Analysis of fluorescence lifetime and quenching of FITC-conjugated antibodies on cells by phase-sensitive flow cytometry

Fluorescent antibodies are often used to measure the number of receptor sites on cells. The quantitative estimate of the number of receptor sites using this procedure assumes that the fluorescence intensity on a cell is proportional to the number of bound antibodies. Quenching may invalidate this assumption. For many fluorophores, intermolecular interactions and energy transfer between molecules in close proximity to one another results in self-quenching. This effect can occur in antibody probes with a high fluorochrome to protein (F/P) ratio. It can also occur due to close proximity antibodies relative to one another on a highly labeled cell surface. Since self-quenching is accompanied by a change in the fluorescence decay and a decrease in the fluorescence lifetime, it may be conveniently identified using fluorescence lifetime spectroscopy. In this paper we apply the phase-sensitive detection method to investigate the impact of self-quenching on fluorescence lifetimes by flow cytometry, using a model system consisting of FITC conjugated anti-mouse Thy1.2 antibodies bound to murine thymus cells. We show that in addition to the expected variation of lifetimes as a function of F/P ratio of the probes, the fluorescence lifetime diminishes also as a function of antibody labeling concentration on the cell surface. This is consistent with self-quenching effects expected at high densities of FITC molecules.

Frequency-domain lifetime measurements and sensing in highly scattering media

For clinical chemistry, or for non-invasive sensing through skin, it is often necessary to obtain quantitative information in highly scattering media. We describe two simple methods for fluorescence lifetime measurements in highly scattering media, and in particular in an intralipid suspension. Lifetimes can be measured using an intensity decay law that accounts for the time delays and pulse-broadening effects of multiple light-scattering events in the intralipid. Alternatively, the phase and modulation measurements can be performed relative.to a reference fluorophore with a known lifetime. These approaches provide reliable lifetime data for a pH-sensitive fluorophore contained within a microeuvette 4 mm under the surface of the intralipid suspension. Fluorescence lifetime-based sensing is now recognized as a valuable methodology in clinical and analytical chemistry, and the possibility of lifetime-based sensing in turbid media has been demonstrated.

Effect of Fluorescence Quenching by Stimulated Emission on the Spectral Properties of a Solvent-Sensitive Fluorophore

We examined the emission spectra and wavelength-dependent anisotropies of the solvent-sensitive fluorophore 4-(dimethylamino)-4′-cyanostilbene (DCS) under condition of fluorescence quenching by stimulated emission. The sample was illuminated with a train of 10 ps pulses at 285 nm, and a train of stimulating pulses at 570 nm which were delayed by a time t d relative to the excitation. Stimulated emission of DCS was demonstrated to occur by observation of gain in the long-wavelength beam. Illumination on the long-wavelength side of the emission spectrum with the long-wavelength time-delayed pulses resulted in a blue shift of the emission spectrum, and a progressive decrease of the emission anisotropy as the observation wavelength increased toward the stimulating wavelength. The spectral shifts and wavelength-dependent anisotropies of DCS were more pronounced in more viscous solvents where spectral relaxation is incomplete during the excited state lifetime. Light quenching of DCS in a low-viscosity solvent revealed no spectral shifts or wavelength-dependent anisotropies. Control measurements using acridine orange, which is relatively insensitive to solvent polarity, did not show any spectral shift or wavelength-dependent anisotropy with light quenching. The data for DCS can be explained by the presence of a time-dependent spectral shift and wavelength-selective quenching of the longer wavelength emission. In this model the relaxed state is formed following excitation of the unrelaxed state, and the relaxed state is preferentially quenched by long-wavelength illumination. Comparison of the data with model calculations indicates the presence of at least two spectral relaxation times. These results demonstrate that light quenching by stimulated emission acts selectively based on overlap of the stimulating wavelength with the emission spectrum. Observation of the emission spectrum in the presence of time-delayed and power-controlled long-wavelength pulses can be used to study time-dependent excited state processes.

Simultaneous dual-frequency phase-sensitive flow cytometric measurements for rapid identification of heterogeneous fluorescence decays in fluorochrome-labeled cells and particles

In frequency-domain lifetime spectroscopy, the apparent fluorescence lifetimes obtained from phase-shift measurements are independent of modulation frequency only in the special case of a single exponential fluorescence decay. For heterogeneous fluorescence decay, the apparent fluorescence lifetimes measured by the phase-shift methods are functions of the modulation frequency. This modulation-frequency dependent property of apparent fluorescence lifetimes may be used to identify heterogeneous fluorescence decays by measuring lifetimes at multiple frequencies. In this article we explore the requirements and experimental design considerations for making such measurements in flow. We report a phase-sensitive flow cytometric method that allows one to probe the excited state-lifetimes of labeled cells by using multiple simultaneous modulation frequencies. Application of this method is demonstrated by measuring fluorescence lifetimes of labeled cells at two frequencies simultaneously, using a continuous-wave, dual-frequency modulated excitation in flow. The dual-frequency method presented herein can be used to rapidly identify heterogeneity in the fluorescence decay on a cell-by-cell basis in real time. Information on the nature of the fluorescence decay is important in biological measurements because it can provide insight into intermolecular interactions at the subcellular level.

Possibility of simultaneously measuring low and high calcium concentrations using Fura-2 and lifetime-based sensing

We characterized the fluorescence probe Fura-2 for calcium measurements using frequency-domain phase-modulation fluorometry. By the use of different excitation wavelengths from 345 to 380 nm, the apparent calcium dissociation constants can be altered from 41 nM to 1.92 microM Ca2+. This change in apparent Kd results from changes in the relative extent of excitation of the calcium-bound and calcium-free forms, and the excitation wavelength-dependent contribution of each form to the intensity decay. These results indicate that lifetime-based measurements with Fura-2 can be used for imaging of calcium over a wide range of concentrations. An additional favorable feature of Fura-2 is that the calcium-free form can be almost exclusively excited at wavelength of 390 nm or longer, and can thus be used as a reference providing the lifetime in the absence of calcium, without removing the calcium. Additionally, exposure of Fura-2 to intense illumination shifts but does not distort the frequency response. For cellular imaging, these favorable properties of Fura-2 may allow calibration of the calcium concentrations without the use of ionophores.

A lifetime-based optical CO2 gas sensor with blue or red excitation and stokes or anti-stokes detection

We describe the fabrication and characterization of an optical CO2 sensor based on the change in fluorescence lifetimes due to fluorescence resonance energy transfer from a pH-insensitive donor, sulforhodamine 101, to a pH-sensitive acceptor, either m-cresol purple or thymol blue, entrapped in an ethyl cellulose film. A phase transfer agent allows incorporation of the dyes and water into the film, while providing an initially basic environment for the acceptor. Diffusion of CO2 into the water entrapped in the film produced carbonic acid, causing a pH-dependent decrease in the spectral overlap of the acceptor absorbance with the donor emission, and decreased energy transfer, resulting in increased SR101 donor lifetimes. The lifetime changes were detected as a change in the phase of the emission, relative to the modulated excitation, and were insensitive to excitation intensities and emission signal levels. In addition to an externally modulated 442-nm light source, we excited the sensor with a directly modulated 635-nm laser diode and detected the anti-Stokes emission. The CO2 sensor is not fragile and can provide stable readings for weeks. The use of fluorescence resonance energy transfer, along with the simple entrainment procedure, allows facile change of the CO2 response range through change of the acceptor dye and the use of laser diode excitation sources.

Fluorescence lifetime measurements in a flow cytometer by amplitude demodulation using digital data acquisition technique

We have developed a method for fluorescence lifetime measurements in a flow cytometer based upon the amplitude demodulation of the fluorescence signals using digital data acquisition techniques. Amplitude demodulation is one of the two methods by which excited state lifetimes may be investigated in the frequency domain. The other method involves the phase-shift measurements. In frequency-domain measurement techniques, the amplitude-demodulation and phase-shift data serve mutually complementary roles to enhance the analytical capabilities of the measurements. The purpose of having amplitude demodulation measurement capability is to obtain information that supplements, rather than replaces, that obtained by the phase-shift method alone. Application of amplitude demodulation measurements has been widely explored in static, cuvette-based, frequency domain systems. However, due to time dependence of the amplitude of the modulated fluorescence signal in a flow cytometer, the amplitude demodulation measurements in flow turns out to be more complicated than similar measurements in a static system. The goal of the present work is to explore the problems involved in amplitude demodulation measurements in flow (using digital method), through detailed theoretical modeling and use the model to develop a practical method that can be incorporated into a flow cytometer to measure amplitude modulation lifetimes. We experimentally verify the amplitude demodulation measurement capability of this method using fluorescent microspheres. The experimental measurements show good agreement with static frequency-domain measurements on microspheres in bulk suspensions.

Calcium-dependent fluorescence lifetimes of Indo-1 for one- and two-photon excitation of fluorescence

We characterized the fluorescence intensity decays of Indo-1, which is commonly used as an emission wavelength-ratiometric calcium probe. The apparent lifetime of the long-wavelength side of the emission of Indo-1 is dependent on Ca2+. This long-wavelength emission displays the characteristics of an excited-state reaction, that is, a negative preexponential component in the multiexponential analysis. The emission spectra and lifetime of Indo-1 appear to be identical for one-photon and two-photon excitation at 351 and 702 mn, respectively, suggesting that the relative one- and two-photon cross sections are similar for the calcium-free and calcium-bound forms of Indo-1. Also, the two-photon cross section of Indo-1 is relatively high, about 4 x 10(-49) cm4 s/photon molecule at 690 nm for both the calcium-free and calcium-bound forms. Hence, Indo-1 can be used for calcium imaging based on one- or two-photon excitation, using either emission wavelength ratios or lifetime imaging methods.

Optical measurements of pH using fluorescence lifetimes and phase-modulation fluorometry

We measured the pH-dependent fluorescence decay times of the seminaphthofluoresceins (SNA-FL), seminaphthorhodafluors (SNARF), and BCE-CF using phase-modulation fluorometry. The phase and modulation values were found to be strongly pH-dependent in the physiological pH range, over the easily accessible range of light modulation frequencies from 10 to 300 MHz, making these probes useful as lifetime-based pH sensors. The phase and modulation values are dependent on excitation and emission wavelength as well as pH. This dependence allows the range of pH sensitivity to be chosen by selection of the wavelength(s) and enables increased precision of the pH measurements by use of phase and/or modulation measurements at several wavelengths. These probes can be excited using a a green He-Ne laser at 543 nm, which allows their use in low cost instrumentation. Phase and modulation measurements are especially suitable for sensing applications because they are insensitive to the changes in signal intensity that result from photobleaching, probe washout, and/or light losses.

Fluorescence lifetime imaging of calcium using Quin-2

We describe the use of a new imaging technology, fluorescence lifetime imaging (FLIM), for the imaging of the calcium concentrations based on the fluorescence lifetime of a calcium indicator. The fluorescence lifetime of Quin-2 is shown to be highly sensitive to [Ca2+]. We create two-dimensional lifetime images using the phase shift and modulation of the Quin-2 in response to intensity-modulated light. The two-dimensional phase and modulation values are obtained using a gain-modulated image intensifier and a slow-scan CCD camera. The lifetime values in the 2D image were verified using standard frequency-domain measurements. Importantly, the FLIM method does not require the probe to display shifts in the excitation or emission spectra, which may allow Ca2+ imaging using other Ca2+ probes not in current widespread use due to the lack of spectral shifts. Fluorescence lifetime imaging can be superior to stationary (steady-state) imaging because lifetimes are independent of the local probe concentration and/or intensity, and should thus be widely applicable to chemical imaging using fluorescence microscopy.

Resolution of multicomponent fluorescence emission using frequency-dependent phase angle and modulation spectra

We describe a new fluorescence method that allows the resolution of both the decay times and emission spectra of mixtures of fluorophores. This method is completely general and does not require any assumptions or knowledge of the decay times or emission spectra of the individual fluorophores. We use the phase angle spectra and modulation spectra of the mixture, measured over a range of suitable light modulation frequencies and emission wavelengths. These data are analyzed by nonlinear least-squares analysis to recover the emission spectra and the associated decay times. The principle of the method and the nature of the data are illustrated by using two-component mixtures with increasing spectral overlap. We then demonstrate the recovery of minor components, of structure emission spectra, and of a three-component mixture with completed overlapping emission spectra. And finally, we describe the resolution of a two-component mixture with decay times of 0.8 and 1.4 ns using modulation frequencies up to 774 MHz.