Photoselection of Luminescent Molecules in Anisotropic Media in the Case of Two-Photon Excitation. Part II. Experimental Studies of Hoechst 33342 in Stretched Poly(vinyl alcohol) Films

It was found by investigating dichroism and emission anisotropy in the case of one-and two-photon excitation of Hoechst 33342 [bis-benzimide,2,5'-bi-1H-benzimidazole, 2'-(4-ethoxyphenyl)-5-5(4-methyl-1-piperazinyl)] in stretched poly(vinyl alcohol) (PVA) films, that the absorption and fluorescence transition moments lie along the long molecular axis of the molecule studied. The slight deviation of the transition moment direction in fluorescence (about 8°) from that in absorption can be due to the incomplete linearity of the Hoechst molecule.

Fluorescence lifetime imaging of physiological free Cu(II) levels in live cells with a Cu(II)-selective carbonic anhydrase-based biosensor

Copper is a required trace element that plays key roles in a number of human enzymes, such that copper deficiency or genetic defects in copper transport lead to serious or fatal disease. Rae, et al., had famously predicted that free copper ion levels in the cell cytoplasm were extremely low, typically too low to be observable. We recently developed a variant of human apocarbonic anhydrase II for sensing metal ions that exhibits 25-fold better selectivity for Cu(II) over Zn(II) than the wild type protein, enabling us to accurately measure Cu(II) in the presence of ordinary cellular (picomolar) concentrations of free zinc. We inserted a fluorescent labeled Cu(II)-specific variant of human apocarbonic anhydrase into PC-12 cells and found that the levels are indeed extremely low (in the femtomolar range). We imaged the free Cu(II) levels in living cells by means of frequency-domain fluorescence lifetime microscopy. Implications of this finding are discussed.

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.

Review of fluorescence anisotropy decay analysis by frequency-domain fluorescence spectroscopy

This didactic paper summarizes the mathematical expressions needed for analysis of fluorescence anisotropy decays from polarized frequency-domain fluorescence data. The observed values are the phase angle difference between the polarized components of the emission and the modulated anisotropy, which is the ratio of the polarized and amplitude-modulated components of the emission. This procedure requires a separate measurement of the intensity decay of the total emission. The expressions are suitable for any number of exponential components in both the intensity decay and the anisotropy decay. The formalism is generalized for global analysis of anisotropy decays measured at different excitation wavelengths and for different intensity decay times as the result of quenching. Additionally, we describe the expressions required for associated anisotropy decays, that is, anisotropy decays where each correlation time is associated with a decay time present in the anisotropy decay. And finally, we present expressions appropriate for distributions of correlation times. This article should serve as a reference for researchers using frequency-domain fluorometry.

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.

Polarized emission from a rhenium metal-ligand complex

We report the first observation of polarized emission from a rhenium-phenanthroline complex, Re(CO)3(phen)Cl. Highly luminescent rhenium complexes are known, with quantum yields near 0.5 and lifetimes in excess of 10 μs. The detection of polarized emission suggests the use of rhenium complexes as probes of the hydrodynamics of large macromolecular complexes and for use in fluorescence polarization immunoassays with gated detection.

Fluorescence lifetime characterization of magnesium probes: Improvement of Mg(2+) dynamic range and sensitivity using phase-modulation fluorometry

We measured the Mg(2+)-dependent absorption spectra, emission spectra, quantum yields, and intensity decays of most presently available fluorescent magnesium probes. The lifetimes were found to be strongly Mg(2+) dependent for Mag-quin-1, Mag-quin-2, magnesium green, and magnesium orange and increased 2- to 10-fold upon binding of Mg(2+). The lifetimes of Mag-fura-2, Mag-fura-5, Mag-fura red, and Mag-indo-1 were similar in the presence and absence of Mg(2+). Detailed timeresolved measurements were carried out for Mag-quin-2 and magnesium green using phase-modulation fluorometry. Apparent dissociation constants (K d) were determined from the steady-state and time-resolved data. Their values were compared and discussed. Mg(2+) sensing is described using phase and modulation data measured at a single modulation frequency. Phase angle and modulation data showed the possibility of obtaining a wider Mg(2+)-sensitive range than available from intensity measurements. A significant expansion in the Mg(2+)-sensitive range was found for Mag-quin-2 using excitation wavelengths from 343 to 375 nm, where the apparentK d from the phase angle was found to vary from 0.3 to about 100 mM. Discrimination against Ca(2+) was also measured for Mag-quin-2 and magnesium green. Significant phototransformation and/or photode-composition, which affect the sensitivity to Mg(2+), were observed for Mag-quin-2 and magnesium green under intense and long illumination.

Plasmon-controlled fluorescence towards high-sensitivity optical sensing

Fluorescence spectroscopy is widely used in chemical and biological research. Until recently most of the fluorescence experiments have been performed in the far-field regime. By far-field we imply at least several wavelengths from the fluorescent probe molecule. In recent years there has been growing interest in the interactions of fluorophores with metallic surfaces or particles. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. The spectral properties of fluorophores can dramatically be altered by near-field interactions with the electron clouds present in metals. These interactions modify the emission in ways not seen in classical fluorescence experiments. Fluorophores in the excited state can create plasmons that radiate into the far-field and fluorophores in the ground state can interact with and be excited by surface plasmons. These reciprocal interactions suggest that the novel optical absorption and scattering properties of metallic nanostructures can be used to control the decay rates, location, and direction of fluorophore emission. We refer to these phenomena as plasmon-controlled fluorescence (PCF). An overview of the recent work on metal-fluorophore interactions is presented. Recent research combining plasmonics and fluorescence suggest that PCF could lead to new classes of experimental procedures, novel probes, bioassays, and devices.

Anisotropy decays of indole, melittin monomer and melittin tetramer by frequency-domain fluorometry and multi-wavelength global analysis

We used frequency-domain fluorescence spectroscopy to measure the fluorescence lifetime and anisotropy decays of indole in propylene glycol, and of the tryptophan emission of melittin monomer and tetramer in water solutions at 5 degrees C. We obtained an increase in resolution of the anisotropy decays by using multiple excitation wavelengths, chosen to provide a range of fundamental anisotropy values. The multi-excitation wavelength anisotropy decays were analyzed globally to recover a single set of correlation times with wavelength-dependent anisotropy amplitudes. Simulated data and kappaR2 surfaces are shown to reveal the effect of multi-wavelength data on the resolution of complex anisotropy decays. For both indole and melittin, the anisotropy decays are heterogeneous and require two correlation times to fit the frequency-domain data. For indole in propylene glycol at 5 degrees C we recovered correlation times of 0.59 and 4.10 ns, which appear to be characteristic of the rigid and asymmetric indole molecule. For melittin monomer the correlation times were 0.13 and 1.75 ns, and for melittin tetramer 0.12 and 3.96 ns. The shorter and longer correlation times of melittin are due to segmental motions and overall rotational diffusion of the polypeptide.

Fluctuation correlation spectroscopy and photon histogram analysis of light scattered by gold nanospheres

Fluorescence correlation spectroscopy (FCS) is a valuable tool in biological research. In recent years there has been growing interest in using light scattered from metallic colloids in place of organic fluorophores. Metallic colloids display optical cross sections for scattering that are orders of magnitude brighter than fluorophores. We used the FCS method to study the scattering properties of varying sizes of gold colloids 38-100 nm in diameter. The optical cross sections of the gold colloids increase rapidly with size, as can be seen by both the G(0) value of the autocorrelation function and the scattering intensity distributions. In mixtures of different size gold colloids the autocorrelation function is dominated by the larger (brighter) colloids, even when present at a small fractional population. We show that it is possible to detect one 100 nm gold colloid in the presence of 10(3)-10(4)smaller 39 nm diameter colloids. Because the scattering cross sections of colloids will increase with aggregation, we believe that FCS can be used to detect a small number of associated bio-labeled colloids in the presence of a much larger population of non-associated colloids.

New color chemosensors for monosaccharides based on azo dyes

Sugar detection is important for many applications. New developments in sugar signaling would provide new technologies to monitor glucose and other sugars. Azo dye 1 presents a new way to build molecular color sensors for monosaccharides. The boronic acid group is used as chelator group for monosaccharides and linked directly in resonance with the aromatic dye. Dye 1 shows a color change, from orange to purple, in the presence of sugar at neutral pH. [structure: see text]

Texture analysis of fluorescence lifetime images of AT- and GC-rich regions in nuclei

We used intensity and fluorescence lifetime microscopy (FLIM) of 3T3 nuclei to investigate the existence of AT-rich and GC-rich regions of the nuclear DNA. Hoechst 33258 (Ho) and 7-aminoactinomycin D (7-AAD) were used as fluorescence probes specific for AT and GC base pairs, respectively. YOYO-1 (Yo) was used as a dye that displays distinct fluorescence lifetimes when bound to AT or GC base pairs. We combined fluorescence imaging of Ho and 7-AAD with time-resolved measurements of Yo and took advantage of an additional information content of the time-resolved fluorescence. Because a single nucleus could not be stained and measured with all three dyes, we used texture analysis to compare the spatial distribution of AT-rich and GC-rich DNA in 100 nuclei in different phases of the cell cycle. The fluorescence intensity-based analysis of Ho- or 7-AAD-stained images indicates increased number and larger size of the DNA condensation centers in the G2/M-phases compared to G0/1-phases. The lifetime-based study of Yo-stained images suggests spatial separation of the AT- or GC-rich DNA regions in the G2/M-phase. Texture analysis of fluorescence intensity and lifetime images was used to quantitatively study the spatial change of condensation and separation of AT- and GC-rich DNA during the cell cycle.

Radiative decay engineering: biophysical and biomedical applications

Fluorescence spectroscopy is a widely used research tool in biochemistry and molecular biology. Fluorescence has also become the dominant method enabling the revolution in medical diagnostics, DNA sequencing, and genomics. To date all the fluorescence observables, including spectral shifts, anisotropies, quantum yields, and lifetimes, have all been utilized in basic and applied uses of fluorescence. In this forward-looking article we describe a new opportunity in fluorescence, radiative decay engineering (RDE). By RDE we mean modifying the emission of fluorophores or chromophores by increasing or decreasing their radiative decay rates. In most fluorescence experiments the radiative rates are not changed because these rates depend on the extinction coefficient of the fluorophore. This intrinsic rate is not changed by quenching and is only weakly dependent on environmental effects. Spectral changes are usually caused by changes in the nonradiative rates resulting from quenching or resonance energy transfer. These processes affect the emission by providing additional routes for decay of the excited states without emission. In contrast to the relatively constant radiative rates in free solution, it is known that the radiative rates can be modified by placing the fluorophores at suitable distances from metallic surfaces and particles. This Review summarizes results from the physics literature which demonstrate the effects of metallic surfaces, colloids, or islands on increasing or decreasing emissive rates, increasing the quantum yields of low quantum yield chromophores, decreasing the lifetimes, and directing the typically isotropic emission in specific directions. These effects are not due to reflection of the emitted photons, but rather as the result of the fluorophore dipole interacting with free electrons in the metal. These interactions change the intensity and temporal and spatial distribution of the radiation. We describe the unusual effects expected from increases in the radiative rates with reference to intrinsic and extrinsic biochemical fluorophores. For instance, the decreased lifetime can result in an effective increase in photostability. Proximity to nearby metallic surfaces can also increase the local field and modify the rate of excitation. We predict that the appropriate localization of fluorophores near particles can result in usefully high emission from "nonfluorescent" molecules and million-fold increases in the number of photons observable from each fluorophore. We also describe how RDE can be applied to medical testing and biotechnology. As one example we predict that nearby metal surfaces can be used to increase the low intrinsic quantum yields of nucleic acids and make unlabeled DNA detectable using its intrinsic metal-enhanced fluorescence.

Intrinsic fluorescence from DNA can be enhanced by metallic particles

High sensitivity detection of DNA is essential for genomics. The intrinsic fluorescence from DNA is very weak and almost all methods for detecting DNA rely on the use of extrinsic fluorescent probes. We show that the intrinsic emission from DNA can be enhanced many-fold by spatial proximity to silver island films. Silver islands are subwavelength size patches of metallic silver on an inert substrate. Time-resolved measurements show a decreased lifetime for the intrinsic DNA emission near the silver islands. These results of increased intensity and decreased lifetime indicate a metal-induced increase in the radiative rate decay of the DNA bases. The possibility of increased radiative decay rates for DNA bases and other fluorophores suggest a wide variety of DNA measurements and other biomedical assays based on metal-induced increases in the fluorescence quantum yield of weakly fluorescent substances.

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.

Frequency-domain fluorescence microscopy with the LED as a light source

We describe a frequency-domain lifetime fluorometer based on a microscope and a modulated light-emitting diode (LED) excitation source (370/460 nm), which operates in the frequency range 120 Hz--250 MHz. We collected multifrequency phase and modulation fluorescence responses from cellular areas as small as 10--15 microm in diameter. We also collected fluorescence lifetime data from cells stained by a lipophilic coumarin sensitized europium fluorophore, Coum-Eu, with a millisecond lifetime, and Ru(bpy)(2)phe-C(12),with microsecond lifetime. Nanosecond lifetimes from native nuclei stained with SYTO 14 and SYTO 16 probes were measured as well. We demonstrate that a simple LED excitation source can, for many applications, successfully replace complex and expensive laser systems, which have been used for cellular frequency-domain lifetime measurements. As the LEDs are very stable with low noise, it will be possible to image even smaller sample areas using brighter LEDs. With availability of modulated LEDs emitting at several wavelengths covering almost the entire visible spectrum it is easy to assemble a system for the fluorophore of choice. The ability to select an excitation source for a given fluorophore and low price make such an excitation source even more practical.

Fluorescence lifetime characterization of novel low-pH probes

The structures and functions of the cellular acidic compartments are strongly dependent on the pH gradients across vesicular membranes. Measurement and imaging of the vesicular pH require fluorophores with appropriate pK(a) values. In this report, we characterized the pH-dependent lifetime responses of a family of acidotropic probes, LysoSensors, to evaluate their usefulness to low-pH lifetime imaging. LysoSensors are cell-permeable weak bases that selectively accumulate in acidic vesicles after being protonated. They have higher quantum yields at lower pH ranges to allow visualization of the lysosomes. For LysoSensors DND-167, DND-189, and DND-153, raising the buffer pH increased the quenching effects of their basic side chains and substantially reduced their steady-state fluorescence and lifetimes. The apparent pK(a) values determined from their lifetime responses were shifted to near neutral values because of the dominant intensity contribution from their protonated species. One unique property of LysoSensor DND-189 is its nonmonotonic lifetime responses of the maxima occurring between pH 4 and 5. LysoSensor DND-192 did not show significant lifetime changes over a wide pH range. LysoSensor DND-160, which was the only excitation and emission ratiometric probe, showed significant pH-dependent lifetime changes as well as its spectral shifts. Its apparent pK(a) values determined from the lifetime responses were comparable to the lysosomal pH because of its bright basic form. Because of the pH-dependent absorption spectra, the apparent pK(a) values could be manipulated between 3 and 5 by changing the excitation and/or emission wavelengths. These results indicate that LysoSensor DND-160 is a promising probe for lifetime imaging to determine lysosomal pH.

Functionally different agonists induce distinct conformations in the G protein coupling domain of the beta 2 adrenergic receptor

G protein-coupled receptors represent the largest class of drug discovery targets. Drugs that activate G protein-coupled receptors are classified as either agonists or partial agonists. To study the mechanism whereby these different classes of activating ligands modulate receptor function, we directly monitored ligand-induced conformational changes in the G protein-coupling domain of the beta(2) adrenergic receptor. Fluorescence lifetime analysis of a reporter fluorophore covalently attached to this domain revealed that, in the absence of ligands, this domain oscillates around a single detectable conformation. Binding to an antagonist does not change this conformation but does reduce the flexibility of the domain. However, when the beta(2) adrenergic receptor is bound to a full agonist, the G protein coupling domain exists in two distinct conformations. Moreover, the conformations induced by a full agonist can be distinguished from those induced by partial agonists. These results provide new insight into the structural consequence of antagonist binding and the basis of agonism and partial agonism.

On the effect of sodium dodecyl sulfate on the structure of beta-galactosidase from Escherichia coli. A fluorescence study

An understanding of the structure-function relationship of proteins under different chemical-physical conditions is of fundamental importance for an understanding of their structure and function in cells. In this paper we report the effects of sodium dodecyl sulfate and temperature on the structure of beta-galactosidase from Escherichia coli, as monitored by fluorescence spectroscopy. The structure of the protein was studied in the temperature range of 10-60 degrees C in the absence and presence of sodium dodecyl sulfate by frequency-domain measurement of the intrinsic fluorescence intensity and anisotropy decays. The time-resolved fluorescence data in the absence of SDS indicated that at 10 degrees C the tryptophanyl emission decays were well described by a three exponential decays model, and that the temperature increase resulted in shortening of the long-lived component with little change in the short- and middle-lived components. The addition of SDS to the protein solution also affected the long-lived component. The effects of the detergent and temperature on the enzyme structure were also investigated by means of quenching experiments and anisotropy decays. The obtained results showed that the presence of SDS confers more flexibility to the protein structure, and suggest a strict relation between enzyme activity and protein flexibility.

Fluorescence properties of albumin blue 633 and 670 in plasma and whole blood

We have determined the fluorescence characteristics of two long wavelength dyes, albumin blue 633 (AB633) and 670 (AB670), in plasma and blood to evaluate the possibility of making direct fluorescence sensing measurements in blood. Using binding and lifetime measurements we were also able to show that these dyes bind selectively to human serum albumin (HSA) in plasma and blood. By measuring changes in the mean lifetime of AB670 with changes in the HSA concentration, we showed that lifetime-based sensing can be used to monitor HSA concentrations using these albumin blue dyes. Anisotropy measurements for AB633 and AB670 in plasma and blood revealed high anisotropy values for these dyes in these media. Exploiting these high anisotropies, we were also able to determine HSA concentrations in plasma and blood mimics using changes in AB670 anisotropy with HSA concentration. These results show that, apart from being able to make fluorescence measurements directly in plasma and blood, it is possible to sense directly for specific plasma/blood components using fluorescent probes that bind preferentially to them.

Optical determination of glutamine using a genetically engineered protein

We have developed a reagentless optical biosensor for glutamine based on the Escherichia coli glutamine binding protein (GlnBP). Site-directed mutagenesis was performed to engineer single cysteine mutants which were covalently modified with environmentally sensitive sulfhydryl-reactive probes. The fluorescence emission of acrylodan and 2-(4'-(iodoacetamido)anilino)naphthalene-6-sulfonic acid (IAANS) attached to GlnBP mutant S179C was shown to decrease 65 and 35%, respectively, upon titration with increasing amounts of glutamine (0 to 6.4 microM; K(Dapp) 160 nM). No significant changes in the fluorescence intensity were observed for the structurally similar amino acids glutamate, asparagine, and arginine. Time-resolved intensity decays showed a 2.4-fold decrease in mean lifetime for GlnBP S179C-acrylodan upon the addition of glutamine, indicating the possibility of a lifetime-based assay. Anisotropy decay measurements for GlnBPS179C-acrylodan showed a 13-ns rotational correlation time in the ligand-free state, whereas multiple correlation times were assigned in the glutamine-bound conformation. The decrease in fluorescence intensity of S179C-acrylodan was adapted to polarization sensing of glutamine. The engineered GlnBP is a first step toward the development of a nonenzymatic biosensor capable of determining glutamine concentrations in cell cultures.

Enzyme fluorescence as a sensing tool: new perspectives in biotechnology

The technology for fluorescence protein-sensing is advancing rapidly owing to the continued introduction of new concepts, new fluorophores, and proteins engineered for sensing-specific analytes. Concerns about the reversibility and selectivity of engineered proteins are being addressed by developing biosensors that are based on the utilisation of coenzyme-depleted enzymes. Such biomolecules do not consume the substrate and can exhibit conformational changes upon the binding of the analyte, which can be easily detected as fluorescence change. In addition, concerns about the stability of biosensors can be overcome by using thermostable enzymes isolated from thermophilic microorganisms. Finally, the development of new techniques such as polarization-based sensing, anisotropy-based sensing and lifetime-based sensing, all of which can be accomplished with light-emitting diodes as the light source, is prompting the design of a new class of specific and stable biosensors, as has occurred with blood glucose measurement. These biosensors represent a valid alternative to the conventional clinical chemistry diagnostics.

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.

A FRET-based sensor reveals large ATP hydrolysis-induced conformational changes and three distinct states of the molecular motor myosin

The molecular motor myosin is proposed to bind to actin and swing its light-chain binding region through a large angle to produce an approximately 10 nm step in motion coupled to changes in the nucleotide state at the active site. To date, however, direct dynamic measurements have largely failed to show changes of that magnitude. Here, we use a cysteine engineering approach to create a high resolution, FRET-based sensor that reports a large, approximately 70 degree nucleotide-dependent angle change of the light-chain binding region. The combination of steady-state and time-resolved fluorescence resonance energy transfer measurements unexpectedly reveals two distinct prestroke states. The measurements also show that bound Mg.ADP.Pi, and not bound Mg.ATP, induces the myosin to adopt the prestroke states.

A thermophilic apoglucose dehydrogenase as nonconsuming glucose sensor

Blood glucose is a clinically important analytes for diabetic health care. In this preliminary report we describe a protein biosensor for d-glucose based on a thermostable glucose dehydrogenase. The glucose dehydrogenase was noncovalently labeled with 8-anilino-1-naphthalene sulfonic acid (ANS). The ANS-labeled enzyme displayed an approximate 25% decrease in emission intensity upon binding glucose. This decrease can be used to measure the glucose concentration. Our results suggest that enzymes which use glucose as their substrate can be used as reversible and nonconsuming glucose sensors in the absence of required cofactors. Moreover, the possibility of using inactive apoenzymes for a reversible sensor greatly expands the range of proteins which can be used as sensors, not only for glucose, but for a wide variety of biochemically relevant analytes.

Thermodynamics and dynamics of histidine-binding protein, the water-soluble receptor of histidine permease. Implications for the transport of high and low affinity ligands

The bacterial histidine permease is a model system for ABC transporters (traffic ATPases). The water-soluble receptor of this permease, HisJ, binds L-histidine and L-arginine (tightly) and L-lysine and L-ornithine (less tightly) in the periplasm, interacts with the membrane-bound complex (HisQMP2) and induces its ATPase activity, which results in ligand translocation. HisJ is a two-domain protein; in the absence of ligand, the cleft between two domains is open and binding of substrate stabilizes the closed conformation. Surprisingly, various liganded HisJ forms display substantial differences in their physicochemical characteristics and capacity to induce the ATPase. This is due to either different effects of the individual ligands on the respective closed structures, or to different equilibria being reached for each ligand between the open liganded form and the closed liganded form [Wolf, A. , Lee, K.C., Kirsch, J.F. & Ames, G.F.-L. (1996) J. Biol. Chem. 271, 21243-21250]. In this work, time-resolved measurements of the decay of intrinsic HisJ fluorescence and of the decay of the anisotropy of the fluorescence, as well as the analysis of the steady-state near UV CD and fluorescence spectra, rule out the model in which the differences between liganded complexes reflect different equilibria. The decay of the anisotropy of the fluorescence shows that liganded complexes differ dramatically in their large-scale conformational dynamics. Differential scanning calorimetry (DSC) curves for the HisJ thermal unfolding are well described by a scheme of equilibrium two-state unfolding of two independent domains, which can be ascribed to the two-domain structure of HisJ. This is true both for apo-HisJ at various pH values, and for HisJ in the presence of its ligands at varying concentrations, at pH 8.3. The DSC and structural data suggest that all ligands interact more extensively with the larger domain. A qualitative model for the HisJ conformational dynamics employing the idea of a twisting movement of the domains is proposed, which explains the difference in the efficacy of the ATPase induction by the various liganded HisJ forms.

Novel fluorescence sensing methods for high throughput screening

We describe two new methods of fluorescence sensing for use in high throughput screening (HTS). Modulation sensing transforms analyte-dependent intensity changes into a change in the low-frequency modulation signal. Polarization sensing transforms an intensity change into a change in polarization. Both methods are internally calibrated by using a reference film immediately adjacent to the sample, which can be readily located on the HTS plate or on a nearby optical component and provides an intensity or polarization reference. Modulation sensing and polarization sensing were both shown useful for measurements of fluorophore concentrations, pH, or calcium concentrations in the wells of HTS plates. Sensing with a reference film provides the opportunity to internally reference HTS measurements without the need for additions to the sample. This approach can provide standardization for assays performed at different times.

Pyruvate kinase from the thermophilic eubacterium Bacillus acidocaldarius as probe to monitor the sodium concentrations in the blood

We describe the isolation and characterization of a pyruvate kinase from the thermophilic eubacterium Bacillus acidocaldarius. This protein appears to be a tetramer composed of four 55-kDa subunits. The intrinsic tryptophan fluorescence of this protein is quenched by approximately 20% upon binding sodium, which occurs with a dissociation constant near 15 mM. Importantly, the intrinsic fluorescence of this pyruvate kinase does not appear to be affected by potassium, magnesium, and calcium at the concentrations found in whole blood. It appears that this pyruvate kinase can provide the basis for a selective protein sensor for sodium with minimal interference from other cations.

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.

End-to-end diffusion on the microsecond timescale measured with resonance energy transfer from a long-lifetime rhenium metal-ligand complex

We measured the end-to-end diffusion coefficient of an alkyl chain-linked donor-acceptor pair using the time-resolved frequency-domain decay of the donor. The donor was a rhenium metal-ligand complex with a mean decay time ranging from 2.1 to 7.9 microseconds in the absence of the Texas red acceptor. The decay time was used to measure the donor-to-acceptor distance distribution and the mutual diffusion coefficient. Using this long lifetime donor, it was easily possible to determine a diffusion coefficient near 2 x 10(-8) cm2/s and diffusion coefficients as low as 1.3 x 10(-9) cm2/s were measurable. Such long lifetime donors should be valuable for measuring the flexing of peptides on the microsecond timescale, domain motions of proteins and lateral diffusion in membranes. The availability of microsecond decay time luminophores now allows luminescence spectroscopy to be useful generally for studies of microsecond dynamics of biological macromolecules.

Background suppression in frequency-domain fluorometry

Gated detection is often used in time-domain measurements of long-lived fluorophores for suppression of interfering short-lived autofluorescence. However, no direct method has been available for gated detection and background suppression when using frequency-domain fluorometry. We describe a direct method for real-time suppression of autofluorescence in frequency-domain fluorometry. The method uses a gated detector and the sample is excited by a pulsed train. The detector is gated on following each excitation pulse after a suitable time delay for decay of the prompt autofluorescence. Under the same experimental conditions a detectable reference signal is obtained by using a long lifetime standard with a known decay time. Because the sample and reference signals are measured under identical excitation, gating and instrumental conditions, the data can be analyzed as usual for frequency-domain data without further processing. We show by simulations that this method can be used to resolve single and multiexponential decays in the presence of short lifetime autofluorescence.

Donor fluorescence decay analysis for energy transfer in double-helical DNA with various acceptor concentrations

We studied fluorescence resonance energy transfer between donors and acceptors bound to double-helical DNA. The donor Hoechst 33258 binds to the minor groove of DNA and the acceptor propidium iodide (PI) is an intercalator. The time-resolved donor decays were measured in the frequency domain. The donor decays were consistent with a random 1-dimensional distribution of acceptors. The decays were analyzed in terms of three 1-dimensional models: a random continuous acceptor distribution; acceptors placed on discrete lattice sites; and a cylindrical model with the acceptor in the center, and the donors on a cylinder surface. The data were well described by all three models. Interpretation in terms of continuous distribution of acceptors revealed a minimum donor to acceptor distance of 13 A, which is 3 bp from the center of Hoechst 33252. These results suggest that PI is excluded from the 4 bp covered by Hoechst 33252 when it is bound to the minor groove of DNA.

Donor fluorescence decay analysis for energy transfer in double-helical DNA with various acceptor concentrations

We studied fluorescence resonance energy transfer between donors and acceptors bound to double-helical DNA. The donor Hoechst 33258 binds to the minor groove of DNA and the acceptor propidium iodide (PI) is an intercalator. The time-resolved donor decays were measured in the frequency domain. The donor decays were consistent with a random 1-dimensional distribution of acceptors. The decays were analyzed in terms of three 1-dimensional models: a random continuous acceptor distribution; acceptors placed on discrete lattice sites; and a cylindrical model with the acceptor in the center, and the donors on a cylinder surface. The data were well described by all three models. Interpretation in terms of continuous distribution of acceptors revealed a minimum donor to acceptor distance of 13 A, which is 3 bp from the center of Hoechst 33252. These results suggest that PI is excluded from the 4 bp covered by Hoechst 33252 when it is bound to the minor groove of DNA.

Synthesis and characterization of a sulfhydryl-reactive rhenium metal-ligand complex

We describe the synthesis and spectral characterization of a rhenium metal-ligand complex. This complex reacts with sulfhydryl groups via an iodoacetamide side chain on the phenanthroline ligand and displays a high limiting anisotropy near 0.35 when excited at 442 nm. When covalently linked to human serum albumin, this complex displayed a mean decay time of about 1 micros. This decay time is appropriate for measuring rotational correlation times on the microsecond time scale as may occur for large macromolecular complexes.

Modulation sensing of fluorophores in tissue: a new approach to drug compliance monitoring

We describe a method to detect the presence of fluorophores in scattering media, including intralipid suspensions and chicken muscle covered with skin. The fluorophores were rhodamine 800 (Rh800) and indocyanine green (IcG), both of which can be excited at long wavelengths where there is minimal absorption by tissues. These fluorophores were dissolved in intralipid or in chicken muscle under skin. A method to approximate the fluorophore concentration in such samples was developed using a long lifetime reference fluorophore in a polymer film placed immediately on the illuminated surface of the sample. Because of the long lifetime of the reference film, the modulation of its emission at low frequencies near 2 MHz is near zero. Since the lifetime of Rh800 and IcG are below 2 ns the modulation of the combined emission is a measure of the intensity of the fluorophore (Rh800 or IcG) relative to the long lifetime reference. Using this method we were able to measure the concentration-dependent intensities of Rh800 and IcG in an intralipid suspension. Additionally, micromolar concentrations of these probes could be detected in chicken muscles, even when the muscle was covered with a layer of chicken skin. The presence of an India ink absorber in the intralipid had only a moderate effect on the modulation values. We suggest the use of this transdermal detection of long-wavelength fluorophores as a noninvasive method to monitor patient compliance when taking medicines used for treatment of chronic diseases such as AIDS or tuberculosis. © 1999 Society of Photo-Optical Instrumentation Engineers.

Polarization-based sensing of glucose using an oriented reference film

We describe a new approach to glucose sensing using polarization measurements in the presence of a stretch-oriented reference film. The method relies on measurement of the polarized emission from the reference film and of a fluorophore which changes intensity in response to glucose. A glucose-sensitive fluorescent signal was provided by the glucose/galactose binding protein from E. coli. This protein was labeled with an environmentally sensitive fluorophore at a single genetically inserted cysteine residue, and displayed decreased fluorescence upon glucose binding. Using the protein and the reference film we observed glucose-sensitive polarization values for micromolar glucose concentrations. This method of polarization-based sensing is generic and can be used for any sensing fluorophore which displays a change in intensity. In principle, one can construct simple and economical devices for this type of glucose measurement. © 1999 Society of Photo-Optical Instrumentation Engineers.

The fluorescence emission of the apo-glucose oxidase from Aspergillus niger as probe to estimate glucose concentrations

We developed a new method of glucose sensing using an inactive form of glucose oxidase from Aspergillus niger. Glucose oxidase was rendered inactive by removal of the FAD cofactor. The resulting apo-glucose oxidase still binds glucose as observed from a decrease in its intrinsic tryptophan fluorescence. 8-Anilino-1-naphthalenesulfonic acid (ANS) was found to bind spontaneously to apo-glucose oxidase as seen from an enhancement of the ANS fluorescence. The steady state intensity of the bound ANS decreased 25% upon binding of glucose, and the mean lifetime of the bound ANS decreased about 40%. These spectral changes occurred with a midpoint from 10 to 20 mM glucose, which is comparable to the K(D) of holo-glucose oxidase. These results suggest that apo-glucose oxidase can be used as a reversible nonconsuming sensor for glucose.

The beta-glycosidase from the hyperthermophilic archaeon Sulfolobus solfataricus: enzyme activity and conformational dynamics at temperatures above 100 degrees C

Enzymes from thermophilic organisms are stable and active at temperatures which rapidly denature mesophilic proteins. However, there is not yet a complete understanding of the structural basis of their thermostability and thermoactivity since for each protein there seems to exist special networks of interactions that make it stable under the desired conditions. Here we have investigated the activity and conformational dynamics above 100 degrees C of the beta-glycosidase isolated from the hyperthermophilic archaeon Sulfolobus solfataricus. This has been made possible using a special stainless steel optical pressure cell which allowed us to perform enzyme assays and fluorescence measurements up to 160 degrees C without boiling the sample. The beta-glycosidase from S. solfataricus showed maximal activity at 125 degrees C. The time-resolved fluorescence studies showed that the intrinsic tryptophanyl fluorescence emission of the protein was represented by a bimodal distribution with Lorential shape and that temperature strongly affected the protein conformational dynamics. Remarkably, the tryptophan emission reveals that the indolic residues remain shielded from the solvent even at 125 degrees C, as shown by shielding from quenching and restricted tryptophan solubility. The relationship between enzyme activity and protein structural dynamics is discussed.

Polarization sensing of fluorophores in tissues for drug compliance monitoring

We used a new method, polarization sensing, to monitor the concentration of the fluorophore rhodamine 800 in an intralipid suspension and in chicken tissue. Rhodamine 800 (Rh800) could be excited at 648 nm using a laser pointer. We developed a simple device for measuring the combined emission from a highly polarized reference film and the unpolarized or orthogonally polarized emission of Rh800 from the scattering intralipid or tissue. The concentration of Rh800 in this medium was revealed by large changes in the polarization (P) with values of P ranging from 0.8 to -0.9. It is possible to vary the sensitive Rh800 concentration range by variation of the detected emission wavelengths, orientation of the excitation polarizer, or fluorophore concentration in the reference film. Polarization sensing of fluorophores in tissue requires only steady-state detection, and can be accomplished with simple and/or portable electronics. Such devices may find use in electronic detection of ingested medicines based on transdermal detection of nontoxic long-wavelength fluorophores.

beta-glycosidase from the hyperthermophilic archaeon Sulfolobus solfataricus: structure and activity in the presence of alcohols

beta-Glycosidase from the extreme thermophilic archaeon Sulfolobus solfataricus is a tetrameric protein with a molecular mass of 240 kDa, stable in the presence of detergents, and with a maximal activity at temperatures above 95 degrees C. Understanding the structure-activity relationships of the enzyme under different conditions is of fundamental importance for both theoretical and applicative purposes. In this paper we report the effect of methanol, ethanol, 1-propanol, and 1-butanol on the activity of S. solfataricus beta-glycosidase expressed in Escherichia coli. The alcohols stimulated the enzyme activity, with 1-butanol producing its maximum effect at a lower concentration than the other alcohols. The structure of the enzyme was studied in the presence of 1-butanol by circular dichroism, and Fourier-transform infrared and fluorescence spectroscopies. Circular dichroism and steady-state fluorescence measurements revealed that at low temperatures the presence of the alcohol produced no significant changes in the tertiary structure of the enzyme. However, time-resolved fluorescence data showed that the alcohol modifies the protein microenvironment, leading to a more flexible enzyme structure, which is probably responsible for the enhanced enzymatic activity.

Resonance energy transfer study using a rhenium metal-ligand lipid conjugate as the donor in a model membrane

We measured steady state and time-resolved resonance energy transfer between donors and acceptors in model membranes. The donor was a long lifetime rhenium-lipid complex, which displayed a mean lifetime of 1 microsecond and lifetime components as long as 3 microseconds in the labeled DOPC membranes. The transfer efficiencies were found to be substantially larger than those predicted without consideration of lateral diffusion. The larger transfer efficiencies are consistent with a mutual lateral diffusion coefficient in the membrane near 2 x 10(-8) cm2/s. These results demonstrate that lateral diffusion in membranes can be detected with microsecond lipid probes.

Synthesis and spectral characterization of a long-lifetime osmium (II) metal-ligand complex: a conjugatable red dye for applications in biophysics

There is a need for luminescent probes, which display both long excitation and emission wavelengths and long decay times. We synthesized and characterized an osmium metal-ligand complex which displays a mean decay time of over 100 ns when bound to proteins. [Os(1,10-phenanthroline)2(5-amino-1,10-phenanthroline)[(PF6)2 can be excited at wavelengths up to 650 nm, and displays an emission maximum near 700 nm. The probe displays a modest but useful maximum fundamental anisotropy near 0.1 for 488-nm excitation, and thus convenient when using an argon ion laser. [Os(phen)2(aphen)](PF6)2 is readily activated to the isothiocyanate for coupling to proteins. When covalently linked to bovine serum albumin the intensity decay is moderately heterogeneous with a mean decay time of 145 ns. The anisotropy decay of the labeled protein displays a correlation time near 40 ns. This relatively long lifetime luminophores can be useful as a biophysical probe or in clinical applications such as fluorescence polarization immunoassays.

Sensing of carbon dioxide by a decrease in photoinduced electron transfer quenching

We described a new approach to sensing of carbon dioxide based on photoinduced electron transfer (PET) quenching. Fluorophores like naphthalene and anthracene are known to be quenched by unprotonated amines by the PET mechanism. We examined the fluorescence spectral properties of two amine-containing fluorophores, 1-naphthylmetylamine (NMA) and 9-ethanolaminomethylanthracene (EAA). When dissolved in an organic solvent, both fluorophores displayed increased intensity when equilibrated with gaseous carbon dioxide. In the case of NMA, we found that the mean lifetime increased with increasing partial pressures of CO(2). The intensity and lifetime changes of NMA are completely reversible when CO(2) is removed by purging with argon. Our results are consistent with decreased quenching by the covalently linked amino groups when CO(2) is dissolved in the solution. At present, we are not certain whether the increased intensity is due to protonation of the amino groups or to carbamate formation. In either event, these results suggest that CO(2) can be detected directly using amine-containing fluorophores without the use of bicarbonate and a pH-sensitive fluorophore.

Polarization-based oxygen sensor

A new approach to oxygen sensing based on the luminescence polarization observed from a novel type of sensor is described. The oxygen sensor consists of an oxygen-sensitive silicone film containing tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) chloride [Ru(dpp)3Cl2] and an oxygen-insensitive film of Styryl 7 in poly(vinyl alcohol). Polarizers are used to select orthogonally polarized emission components from Ru(dpp)3Cl2 and Styryl 7. The polarization of the combined emission was found to be highly sensitive to the partial pressure of oxygen. This method of polarization sensing is generic and can be used with any fluorophore which displays an analyte-dependent change in intensity.