Photophysical properties of 2-Phenylindole in poly (vinyl alcohol) film at room temperature. Enhanced phosphorescence anisotropy with direct triplet state excitation

We report the spectral properties of 2-Phenylindole (2PI) embedded in rigid poly (vinyl alcohol) (PVA) film. The 2PI in PVA film shows relatively strong and structured fluorescence with a maximum at 370 nm and surprisingly strong room temperature phosphorescence with an emission maximum of about 500 nm. The dye is highly immobilized in the polymer matrix, thus presenting high fluorescence anisotropy in an isotropic film of about 0.3 at room temperature. The 2-Phenylindole phosphorescence excited in the usual way through the electronic singlet state excitation (S₀ → S₁ absorption) results in a very low, near zero anisotropy. We now report that we can directly excite the dye to the triplet state T₁ and observe high phosphorescence anisotropy similar to the fluorescence anisotropy. The extinction coefficient for S₀ → T₁ absorption in the PVA matrix is unusually high- only about 3 orders of magnitude lower than S₀ → S₁ absorption. We consider this direct excitation to indole's triplet state a very significant finding that may lead to many practical applications. The unusually long-wavelength of excitation around 400 nm, much above typical UV absorption, results in a high phosphorescence anisotropy. This provides a new way to study rotational motion of larger biological objects in the microsecond time scale not accessible through typical fluorescence studies.

Linear dichroism and optical anisotropy of silver nanoprisms in polymer films

We present optical studies of two different size distributions of silver triangular nanoprisms, one with a dipole resonance at 520 nm and the other with a dipole resonance at 650 nm, placed in different media. Significant wavelength-dependent depolarization of scattered light from the silver nanoprisms suspended in water indicates strong interference of multiple surface plasmon resonant modes in the same particle. We use this depolarization as a probe of light scattering by the nanoprisms in a lipid solution due to the rejection of a polarized background scattering. Also, the silver nanoprisms were embedded in a polyvinyl alcohol polymer matrix and oriented by stretching the polymer/nanoprism nanocomposite films. We observe significantly increased linear dichroism in the region associated with the plasmonic in-plane dipole mode upon stretching. Additionally, there is a weaker linear dichroism in the region associated with out-of-plane modes, which vanish in the extinction spectrum of the stretched nanocomposite film.

Effect of a myosin regulatory light chain mutation K104E on actin-myosin interactions

Familial hypertrophic cardiomyopathy (FHC) is the most common cause of sudden cardiac death in young individuals. Molecular mechanisms underlying this disorder are largely unknown; this study aims at revealing how disruptions in actin-myosin interactions can play a role in this disorder. Cross-bridge (XB) kinetics and the degree of order were examined in contracting myofibrils from the ex vivo left ventricles of transgenic (Tg) mice expressing FHC regulatory light chain (RLC) mutation K104E. Because the degree of order and the kinetics are best studied when an individual XB makes a significant contribution to the overall signal, the number of observed XBs in an ex vivo ventricle was minimized to ∼20. Autofluorescence and photobleaching were minimized by labeling the myosin lever arm with a relatively long-lived red-emitting dye containing a chromophore system encapsulated in a cyclic macromolecule. Mutated XBs were significantly better ordered during steady-state contraction and during rigor, but the mutation had no effect on the degree of order in relaxed myofibrils. The K104E mutation increased the rate of XB binding to thin filaments and the rate of execution of the power stroke. The stopped-flow experiments revealed a significantly faster observed dissociation rate in Tg-K104E vs. Tg-wild-type (WT) myosin and a smaller second-order ATP-binding rate for the K104E compared with WT myosin. Collectively, our data indicate that the mutation-induced changes in the interaction of myosin with actin during the contraction-relaxation cycle may contribute to altered contractility and the development of FHC.

The spatial distribution of actin and mechanical cycle of myosin are different in right and left ventricles of healthy mouse hearts

The contraction of the right ventricle (RV) expels blood into the pulmonary circulation, and the contraction of the left ventricle (LV) pumps blood into the systemic circulation through the aorta. The respective afterloads imposed on the LV and RV by aortic and pulmonary artery pressures create very different mechanical requirements for the two ventricles. Indeed, differences have been observed in the contractile performance between left and right ventricular myocytes in dilated cardiomyopathy, in congestive heart failure, and in energy usage and speed of contraction at light loads in healthy hearts. In spite of these functional differences, it is commonly believed that the right and left ventricular muscles are identical because there were no differences in stress development, twitch duration, work performance, or power among the RV and LV in dogs. This report shows that on a mesoscopic scale [when only a few molecules are studied (here three to six molecules of actin) in ex vivo ventricular myofibrils], the two ventricles in rigor differ in the degree of orientational disorder of actin within in filaments and during contraction in the kinetics of the cross-bridge cycle.

Long lived BSA Au clusters as a time gated intensity imaging probe

The work presented here reports the use of long lifetime (>1 μs) BSA Au clusters as a cellular/tissue, time gated, intensity imaging probe. By collecting the emission signal 50 ns post excitation, one can off-gate the intense auto-fluorescence background, thereby greatly enhancing the clarity/specificity in fluorescence imaging.

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.

Secondary emission influenced fluorescence decay of a homogeneous fluorophore solution

An analytical expression is found allowing the calculation of the secondary emission influenced fluorescence decay of a homogeneous fluorophore solution. Before starting the calculation one has to know the shape of the primary fluorescence decay of the fluorophore and the value of the parameter κ denoting the ratio of the secondary to primary steady-state fluorescence intensities. The method elaborated by Budó and Ketskeméty is recommended for evaluation of the parameter κ. The main importance of the obtained expression is that it can be used to recover parameters characterizing the fluorescence decay in the absence of secondary emission. The cases of monoexponential, biexponential and multiexponential primary fluorescence decays are discussed in detail.

Cross-bridge kinetics in myofibrils containing familial hypertrophic cardiomyopathy R58Q mutation in the regulatory light chain of myosin

Familial hypertrophic cardiomyopathy (FHC) is a heritable form of cardiac hypertrophy caused by single-point mutations in genes encoding sarcomeric proteins including ventricular myosin regulatory light chain (RLC). FHC often leads to malignant outcomes and sudden cardiac death. The FHC mutations are believed to alter the kinetics of the interaction between actin and myosin resulting in inefficient energy utilization and compromised function of the heart. We studied the effect of the FHC-linked R58Q-RLC mutation on the kinetics of transgenic (Tg)-R58Q cardiac myofibrils. Kinetics was determined from the rate of change of orientation of actin monomers during muscle contraction. Actin monomers change orientation because myosin cross-bridges deliver periodic force impulses to it. An individual impulse (but not time average of impulses) carries the information about the kinetics of actomyosin interaction. To observe individual impulses it was necessary to scale down the experiments to the level of a few molecules. A small population (∼4 molecules) was selected by using (deliberately) inefficient fluorescence labeling and observing fluorescent molecules by a confocal microscope. We show that the kinetic rates are significantly smaller in the contracting cardiac myofibrils from Tg-R58Q mice then in control Tg-wild type (WT). We also demonstrate a lower force per cross-section of muscle fiber in Tg-R58Q versus Tg-WT mice. We conclude that the R58Q mutation-induced decrease in cross-bridge kinetics underlines the mechanism by which Tg-R58Q fibers develop low force and thus compromise the ability of the mutated heart to efficiently pump blood.

Evidence for pre- and post-power stroke of cross-bridges of contracting skeletal myofibrils

We examined the orientational fluctuations of a small number of myosin molecules (approximately three) in working skeletal muscle myofibrils. Myosin light chain 1 (LC1) was labeled with a fluorescent dye and exchanged with the native LC1 of skeletal muscle myofibrils cross-linked with 1-ethyl-3-[3(dimethylamino) propyl] carbodiimide to prevent shortening. We observed a small volume within the A-band (∼10(-15) L) by confocal microscopy, and measured cyclic fluctuations in the orientation of the myosin neck (containing LC1) by recording the parallel and perpendicular components of fluorescent light emitted by the fluorescently labeled myosin LC1. Histograms of orientational fluctuations from fluorescent molecules in rigor were represented by a single Gaussian distribution. In contrast, histograms from contracting muscles were best fit by at least two Gaussians. These results provide direct evidence that cross-bridges in working skeletal muscle assume two distinct conformations, presumably corresponding to the pre- and post-power-stroke states.

Observing cycling of a few cross-bridges during isometric contraction of skeletal muscle

During muscle contraction a myosin cross-bridge imparts periodic force impulses to actin. It is possible to visualize those impulses by observing a few molecules of actin or myosin. We have followed the time course of orientation change of a few actin molecules during isometric contraction by measuring parallel polarized intensity of its fluorescence. The orientation of actin reflects local bending of a thin filament and is different when a cross-bridge binds to, or is detached from, F-actin. The changes in orientation were characterized by periods of activity during which myosin cross-bridges interacted normally with actin, interspersed with periods of inactivity during which actin and myosin were unable to interact. The periods of activity lasted on average 1.2 +/- 0.4 s and were separated on average by 2.3 +/- 1.0 s. During active period, actin orientation oscillated between the two extreme values with the ON and OFF times of 0.4 +/- 0.2 and 0.7 +/- 0.4 s, respectively. When the contraction was induced by a low concentration of ATP both active and inactive times were longer and approximately equal. These results imply that cross-bridges interact with actin in bursts and suggest that during active period, on average 36% of cross-bridges are involved in force generation.

Single molecule immunoassay on plasmonic platforms

We examined the photophysical properties of the new near infrared (NIR) fluorescent label SeTau-665 on a plasmonic platform of self- assembled colloidal structures (SACS) of silver prepared on a semitransparent silver film. A SeTau-665 immunoassay was performed on this platform and a control glass slide. The fluorescence properties of this label substantially change due to plasmonic interactions. While the average brightness increase of SeTau 665 in ensemble measurements was about 70-fold, fluorescence enhancements up to four-hundred times were observed on certain "hot spots" for single molecule measurements. The intensity increase is strongly correlated with a simultaneous decrease in fluorescence lifetime in these "hot spots". The large increase in brightness allows the reduction of the excitation power resulting in a reduced background and increased photostability. The remarkable fluorescence enhancements observed for SeTau 665 on our plasmonic platform should allow to substantially improve single molecule detection and to reduce the detection limits in sensing devices.

Fluorescence lifetime of actin in the familial hypertrophic cardiomyopathy transgenic heart

Clinical studies have revealed that the D166V mutation in the ventricular myosin regulatory light chain (RLC) can cause a malignant phenotype of familial hypertrophic cardiomyopathy (FHC). It has been proposed that RLC induced FHC in the heart originates at the level of the myosin cross-bridge due to alterations in the rates of cross-bridge cycling. In this report, we examine whether the environment of an active cross-bridge in cardiac myofibrils from transgenic (Tg) mice is altered by the D166V mutation in RLC. The cross-bridge environment was monitored by tracking the fluorescence lifetime (tau) of Alexa488-phalloidin-labeled actin. The fluorescence lifetime is the average rate of decay of a fluorescent species from the excited state, which strongly depends on various environmental factors. We observed that the lifetime was high when cross-bridges were bound to actin and low when they were dissociated from it. The lifetime was measured every 50 ms from the center half of the I-band during 60 s of rigor, relaxation and contraction of muscle. We found no differences between lifetimes of Tg-WT and Tg-D166V muscle during rigor, relaxation and contraction. The duty ratio expressed as a fraction of time that cross-bridges spend attached to the thin filaments during isometric contraction was similar in Tg-WT and Tg-D166V muscles. Since independent measurements showed a large decrease in the cross-bridge turnover rate in Tg-D166V muscle compared to Tg-WT, the fact that the duty cycle remains constant suggests that the D166V mutation of RLC causes a decrease in the rate of cross-bridge attachment to actin.

Monolayers of silver nanoparticles decrease photobleaching: application to muscle myofibrils

Studying single molecules in a cell has the essential advantage that kinetic information is not averaged out. However, since fluorescence is faint, such studies require that the sample be illuminated with the intense light beam. This causes photodamage of labeled proteins and rapid photobleaching of the fluorophores. Here, we show that a substantial reduction of these types of photodamage can be achieved by imaging samples on coverslips coated with monolayers of silver nanoparticles. The mechanism responsible for this effect is the interaction of localized surface plasmon polaritons excited in the metallic nanoparticles with the transition dipoles of fluorophores of a sample. This leads to a significant enhancement of fluorescence and a decrease of fluorescence lifetime of a fluorophore. Enhancement of fluorescence leads to the reduction of photodamage, because the sample can be illuminated with a dim light, and decrease of fluorescence lifetime leads to reduction of photobleaching because the fluorophore spends less time in the excited state, where it is susceptible to oxygen attack. Fluorescence enhancement and reduction of photobleaching on rough metallic surfaces are usually accompanied by a loss of optical resolution due to refraction of light by particles. In the case of monolayers of silver nanoparticles, however, the surface is smooth and glossy. The fluorescence enhancement and the reduction of photobleaching are achieved without sacrificing the optical resolution of a microscope. Skeletal muscle myofibrils were used as an example, because they contain submicron structures conveniently used to define optical resolution. Small nanoparticles (diameter approximately 60 nm) did not cause loss of optical resolution, and they enhanced fluorescence approximately 500-fold and caused the appearance of a major picosecond component of lifetime decay. As a result, the sample photobleached approximately 20-fold more slowly than the sample on glass coverslips.

Metal Enhanced Fluorescence on Silicon Wafer Substrates

We report on the fluorescence enhancement induced by silver island film (SIF) deposited on a silicon wafer. The model immunoassay was studied on silvered and unsilvered wafers. The fluorescence brightness of Rhodamine Red X increased about 300% on the SIF, while the lifetime was reduced by several fold and the photostability increased substantially. We discuss potential uses of silicon wafer substrates in multiplex assays in which the fluorescence is enhanced due to the SIF, and the multiplexing is achieved by using micro transponders.

Fluorescence correlation spectroscopy in a reverse Kretchmann surface plasmon assisted microscope

Fluorescence Correlation Spectroscopy (FCS) demands a high rate of photon detection per molecule, low background, and large fluctuations of fluorescence associated with translational motion. The new approach presented here, Surface Plasmon Assisted Microscope (SPAM), meets these requirements by drastically limiting the observation volume. In this method, the observational layer is made so thin that fluctuations are mostly due to the axial motion of molecules. This is conveniently realized by placing a sample on a thin metal film and illuminating it with a laser beam through an aqueous medium. The excited fluorophores close to the surface couple (via near-field interactions) to surface plasmons in the metal. Propagated surface plasmons decouple on opposite side of the metal film as a far-field radiation and emit in directional manner. Fluorescence is collected with a high Numerical Aperture objective. A confocal aperture inserted in its conjugate image plane reduces lateral dimensions of the detection volume to a diffraction limit. The thickness of the detection layer is reduced further by metal quenching of excited fluorophores at a close proximity (about 30 nm) to the surface. We used a suspension of fluorescent microspheres to show that FCS-SPAM is an efficient method to measure molecular diffusion.

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.

Cross-bridge duty cycle in isometric contraction of skeletal myofibrils

During interaction of actin with myosin, cross-bridges impart mechanical impulses to thin filaments resulting in rotations of actin monomers. Impulses are delivered on the average every tc seconds. A cross-bridge spends a fraction of this time (ts) strongly attached to actin, during which it generates force. The "duty cycle" (DC), defined as the fraction of the total cross-bridge cycle that myosin spends attached to actin in a force generating state (ts/ tc), is small for cross-bridges acting against zero load, like freely shortening muscle, and increases as the load rises. Here we report, for the first time, an attempt to measure DC of a single cross-bridge in muscle. A single actin molecule in a half-sarcomere was labeled with fluorescent phalloidin. Its orientation was measured by monitoring intensity of the polarized TIRF images. Actin changed orientation when a cross-bridge bound to it. During isometric contraction, but not during rigor, actin orientation oscillated between two values, corresponding to the actin-bound and actin-free state of the cross-bridge. The average ts and tc were 3.4 and 6 s, respectively. These results suggest that, in isometrically working muscle, cross-bridges spend about half of the cycle time attached to actin. The fact that 1/ tc was much smaller than the ATPase rate suggests that the bulk of the energy of ATP hydrolysis is used for purposes other than performance of mechanical work.

Decreasing photobleaching by silver island films: application to muscle

Recently it has become possible to study interactions between proteins at the level of single molecules. This requires collecting data from an extremely small volume, small enough to contain one molecule-typically of the order of attoliters (10(-18) L). Collection of data from such a small volume with sufficiently high signal-to-noise ratio requires that the rate of photon detection per molecule be high. This calls for a large illuminating light flux, which in turn leads to rapid photobleaching of the fluorophores that are labeling the proteins. To decrease photobleaching, we measured fluorescence from a sample placed on coverslips coated with silver island films (SIF). SIF reduce photobleaching because they enhance fluorescence brightness and significantly decrease fluorescence lifetime. Increase in the brightness effectively decreases photobleaching because illumination can be attenuated to obtain the same fluorescence intensity. Decrease of lifetime decreases photobleaching because short lifetime minimizes the probability of oxygen attack while the fluorophore is in the excited state. The decrease of photobleaching was demonstrated in skeletal muscle. Myofibrils were labeled lightly with rhodamine-phalloidin, placed on coverslips coated with SIF, illuminated by total internal reflection, and observed through a confocal aperture. We show that SIF causes the intensity of phalloidin fluorescence to increase 4-5 fold and its fluorescence lifetime to decrease on average 23-fold. As a consequence, the rate of photobleaching of four or five molecules of actin of a myofibril on Olympus coverslips coated with SIF decreased at least 30-fold in comparison with photobleaching on an uncoated coverslip. Significant decrease of photobleaching makes the measurement of signal from a single cross-bridge of contracting muscle feasible.

Application of surface plasmon coupled emission to study of muscle

Muscle contraction results from interactions between actin and myosin cross-bridges. Dynamics of this interaction may be quite different in contracting muscle than in vitro because of the molecular crowding. In addition, each cross-bridge of contracting muscle is in a different stage of its mechanochemical cycle, and so temporal measurements are time averages. To avoid complications related to crowding and averaging, it is necessary to follow time behavior of a single cross-bridge in muscle. To be able to do so, it is necessary to collect data from an extremely small volume (an attoliter, 10(-18) liter). We report here on a novel microscopic application of surface plasmon-coupled emission (SPCE), which provides such a volume in a live sample. Muscle is fluorescently labeled and placed on a coverslip coated with a thin layer of noble metal. The laser beam is incident at a surface plasmon resonance (SPR) angle, at which it penetrates the metal layer and illuminates muscle by evanescent wave. The volume from which fluorescence emanates is a product of two near-field factors: the depth of evanescent wave excitation and a distance-dependent coupling of excited fluorophores to the surface plasmons. The fluorescence is quenched at the metal interface (up to approximately 10 nm), which further limits the thickness of the fluorescent volume to approximately 50 nm. The fluorescence is detected through a confocal aperture, which limits the lateral dimensions of the detection volume to approximately 200 nm. The resulting volume is approximately 2 x 10(-18) liter. The method is particularly sensitive to rotational motions because of the strong dependence of the plasmon coupling on the orientation of excited transition dipole. We show that by using a high-numerical-aperture objective (1.65) and high-refractive-index coverslips coated with gold, it is possible to follow rotational motion of 12 actin molecules in muscle with millisecond time resolution.

Fluorescence correlation spectroscopy in surface plasmon coupled emission microscope

Study of dynamics of single molecules by Fluorescence Correlation Spectroscopy (FCS) requires that the rate of photon detection per molecule be high, that the background be low, and that there be a large change in fluorescent signal associated with change in a position of a molecule. FCS applied to microscopic Surface Plasmon Coupled Emission (SPCE) suggests a powerful method to meet those requirements. In this method, the observational volume is made shallow by placing a sample on a thin metal film and illuminating it with the laser beam at Surface Plasmon Resonance (SPR) angle through high numerical aperture objective. The illuminating light excites surface plasmons in the metal film that produce an evanescent wave on the aqueous side of the interface. The thickness of the detection volume is a product of evanescent wave penetration depth and distance-dependent fluorescence coupling to surface plasmons. It is further reduced by a metal quenching of excited fluorophores at a close proximity (below 10 nm) to a surface. The fluorescent light is emitted through the metal film only at an SPCE angle. Objective collects emitted light, and a confocal aperture inserted in its conjugate image plane reduces lateral dimensions of the detection volume to a fraction of a micrometer. By using diffusion of fluorescent microspheres, we show that SPCE-FCS is an efficient method to measure molecular diffusion and that on gold surface the height of the detection volume is ~35 nm.

Minimization of detection volume by surface-plasmon-coupled emission

We report theoretical predictions and experimental observations of the reduced detection volume with the use of surface-plasmon-coupled emission (SPCE). The effective fluorescence volume (detection volume) in SPCE experiments depends on two near-field factors: the depth of evanescent wave excitation and a distance-dependent coupling of excited fluorophores to the surface plasmons. With direct excitation of the sample (reverse Kretschmann excitation) the detection volume is restricted only by the distance-dependent coupling of the excitation to the surface plasmons. However, with the excitation through the glass prism at surface plasmon resonance angle (Kretschmann configuration), the detection volume is a product of evanescent wave penetration depth and distance-dependent coupling. In addition, the detection volume is further reduced by a metal quenching of excited fluorophores at a close proximity (below 10nm). The height of the detected volume size is 40-70nm, depending on the orientation of the excited dipoles. We show that, by using the Kretschmann configuration in a microscope with a high-numerical-aperture objective (1.45) together with confocal detection, the detection volume can be reduced to 1-2attoL. The strong dependence of the coupling to the surface plasmons on the orientation of excited dipoles can be used to study the small conformational changes of macromolecules.

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.

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.

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.

Calcium-induced flexibility changes in the troponin C-troponin I complex

The contraction of vertebrate striated muscle is modulated by Ca(2+) binding to the regulatory protein troponin C (TnC). Ca(2+) binding causes conformational changes in TnC which alter its interaction with the inhibitory protein troponin I (TnI), initiating the regulatory process. We have used the frequency domain method of fluorescence resonance energy transfer (FRET) to measure distances and distance distributions between specific sites in the TnC-TnI complex in the presence and absence of Ca(2+) or Mg(2+). Using sequences based on rabbit skeletal muscle proteins, we prepared functional, binary complexes of wild-type TnC and a TnI mutant which contains no Cys residues and a single Trp residue at position 106 within the TnI inhibitory region. We used TnI Trp-106 as the FRET donor, and we introduced energy acceptor groups into TnC by labeling at Met-25 with dansyl aziridine or at Cys-98 with N-(iodoacetyl)-N'-(1-sulfo-5-naphthyl)ethylenediamine. Our distance distribution measurements indicate that the TnC-TnI complex is relatively rigid in the absence of Ca(2+), but becomes much more flexible when Ca(2+) binds to regulatory sites in TnC. This increased flexibility may be propagated to the whole thin filament, helping to release the inhibition of actomyosin ATPase activity and allowing the muscle to contract. This is the first report of distance distributions between TnC and TnI in their binary complex.

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.

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.

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 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.

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.

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.

Three-photon excitation of N-acetyl-L-tyrosinamide

We observed emission from the tyrosine derivative N-acetyl-L-tyrosinamide (NATyrA) when excited with the fundamental output of a femtosecond Ti:Sapphire laser from 780 to 855 nm. The dependence on incident laser power indicates a three-photon process. The emission spectra and intensity decay in glycerol-water (30:70) at 5 degrees C were found to be identical for one- and three-photon excitation. Also the excitation spectrum of three-photon-induced fluorescence of NATyrA corresponds to the one-photon excitation spectrum. The time-zero or fundamental anisotropy spectrum was reconstructed from the frequency-domain anisotropy decays. The three-photon anisotropies are similar or larger than the one-photon anisotropies. These three-photon anisotropies are surprising given the near zero values known for tyrosine with two-photon excitation. The observations indicate that one- and three-photon excitation directly populates the same singlet excited states(s). However, the origin of the anisotropies with multi-photon excitation of tyrosine remain unclear and unpredictable.

Fluorescence spectral properties of the anticancer drug topotecan by steady-state and frequency domain fluorometry with one-photon and multi-photon excitation

Topotecan is an antitumor agent with activity against a variety of cancers. We examined the steady-state and time-resolved fluorescence spectral properties of topotecan with one- and two-photon excitation. Topotecan was found to display a high two-photon cross section near 20 GM for wavelengths within the fundamental output of a Ti:sapphire laser, 800-880 nm. In frozen solution the anisotropies of topotecan are near the theoretical maxima for one-photon and two-photon excitation with colinear electronic transitions. The intensity and anisotropy decays of topotecan fluorescence were found to be homogeneous (single exponentials) in phosphate-buffered saline and propylene glycol. The steady-state and time-resolved data indicate that topotecan binds to a double-helical DNA oligomer d(AT)10 resulting in increased anisotropies and multiexponential intensity and anisotropy decays. Subnanosecond components in the anisotropy decay of the DNA-topotecan complex suggest loose binding of the drug to DNA. Loose binding of topotecan to DNA is also revealed by accessibility of topotecan to collisional quenching by iodide.

Polarization sensing with visual detection

We describe a new approach to fluorescence sensing which relies on visual determination the polarization. The sensing device consists of a fluorescent probe, which changes intensity in responses to the analyte, and an oriented fluorescent film, which is not affected by the analyte. An emission filter is selected to observe the emission from both the film and the sensing fluorophore. Changes in the probe intensity result in changes in the polarization of the combined emission from the sensor and reference. The degree of polarization can be detected visually using a dual polarizer with adjacent sections oriented orthogonally to each other. The emission passing through the dual polarizer is viewed with a second analyzing polarizer. This analyzer is rotated manually to yield equal intensities from both sides of the dual polarizer. This approach was used to measure the concentration of RhB in intralipid and to measure pH using 6-carboxyfluorescein. The analyzer angle is typically accurate to 1 degree, providing pH values accurate to +/- 0.1 pH unit at the midpoint of the titration curve. We also describe a method of visual polarization sensing that does not require an oriented film and that can use the same fluorophore for the sample and reference. These approaches to visual sensing are generic and can be applied to a wide variety of analytes for which fluorescent probes are available. Importantly, the devices are simple, with the only electronic component being the light source.

Anisotropy-based sensing with reference fluorophores

We describe a new approach to fluorescence sensing based on measurements of steady-state anisotropies in the presence of reference fluorophores with known anisotropies. The basic concept is that the anisotropy of a mixture reflects a weighted average of the anisotropies of the emitting species. By use of reference fluorophores the starting anisotropy can be near zero, or near 0.9 for oriented films which contain the reference fluorophore. Changing intensities of the analyte result in changes in anisotropy. A wide dynamic range of anisotropies is available because of the freedom to select high or low starting values. Anisotropy-based sensing was demonstrated for pH using 6-carboxyfluorescein and for protein affinity or immunoassay using an oriented film with high anisotropy and a protein labeled with a metal-ligand complex. The latter measurements were performed with a simple light-emitting diode excitation source without an excitation polarizer. The sensitive range of the assay can be adjusted by changing the intensity of the reference fluorophore. Anisotropy-based sensing can have numerous applications in clinical and analytical chemistry.

Glucose sensor for low-cost lifetime-based sensing using a genetically engineered protein

We describe a glucose sensor based on a mutant glucose/galactose binding protein (GGBP) and phase-modulation fluorometry. The GGBP from Escherichia coli was mutated to contain a single cysteine residue at position 26. When labeled with a sulfhydryl-reactive probe 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid, the labeled protein displayed a twofold decrease in intensity in response to glucose, with a dissociation constant near 1 microM glucose. The ANS-labeled protein displayed only a modest change in lifetime, precluding lifetime-based sensing of glucose. A modulation sensor was created by combining ANS26-GGBP with a long-lifetime ruthenium (Ru) metal-ligand complex on the surface of the cuvette. Binding of glucose changed the relative intensity of ANS26-GGBP and the Ru complex, resulting in a dramatic change in modulation at a low frequency of 2.1 MHz. Modulation measurements at 2.1 MHz were shown to accurately determine the glucose concentration. These results suggest an approach to glucose sensing with simple devices.

High Throughput Screening with Multiphoton Excitation

Fluorescence detection is extensively used in high throughput screening. In HTS there is a continuous migration toward higher density plates and smaller sample volumes. In the present report we describe the advantages of two-photon or multiphoton excitation for HTS. Multiphoton excitation (MPE) is the simultaneous absorption of two long-wavelength photons to excite the lowest singlet state of the fluorophore. MPE is typically accomplished with short but high-intensity laser pulses, which allows simultaneous absorption of two or more photons. The intensity of the multiphoton-induced fluorescence is proportional to the square, cube, or higher power of the instantneous photon flux. Consequently, two-photon or multiphoton excitation only occurs at the focal point of the incident beam. This property of two-photon excitation allows the excited volume to be very small and to be localized in the center of each well in the HTS plate. We show that two-photon-induced fluorescence of fluorescein can be reliably measured in microwell plates. We also show the use of 6-carboxy fluorescein as a pH probe with two-photon excitation, and measure 4'-6-diamidino-2-phenylindole (DAPI) binding and two-photon-induced fluorescence. In further studies we measure the time-dependent intensity decays of DAPI bound to DNA and of calcium-dependent fluorophores. Finally, we demonstrate the possibility of three-photon excitation of several fluorophores, including indole, in the HTS plate. These results suggest that MPE can be used in high-density multiwell plates.

ADVANCES IN FLUORESCENCE SPECTROSCOPY: MULTI-PHOTON EXCITATION, ENGINEERED PROTEINS, MODULATION SENSING AND MICROSECOND RHENIUM METAL-LIGAND COMPLEXES

The technology and applications of fluorescence spectroscopy are rapidly advancing. In this overview presentation we summarize some recent developments from this laboratory. Two and three-photon excitation have been observed for a wide variety of intrinsic and extrinsic fluorophores, including tryptophan, tyrosine, DNA stains, membrane probes, and even alkanes. It has been possible to observe multi-photon excitation of biopolymers without obvious photochemical or photo-thermal effects. Although not de-scribed in our lecture, another area of increasing interest is the use of engineered proteins for chemical and clinical sensing. We show results for the glucose-galactose binding protein from E. coli. The labeled protein shows spectral changes in response to micromolar concentrations of glucose. This protein was used with a novel sensing method based on the modulated emission of the labeled proteins and a long lifetime reference fluorophore. And finally, we describe a recently developed rhenium complex which displays a lifetime near 3 µs in oxygenated aqueous solution. Such long life-time probes allow detection of microsecond dynamic processes, bypassing the usual nanosecond timescale limit of fluorescence. The result of these developments in protein engineering, sensing methods, and metal-ligand probe chemistry will be the increased use of fluorescence in clinical chemistry and point-of-care analyses.

Low-frequency modulation sensors using nanosecond fluorophores

We describe a new approach to fluorescence sensing based on a mixture of fluorophores, one of which is sensitive to the desired analyte. If a long-lifetime analyte-insensitive fluorophore is mixed with a short-lifetime analyte-sensitive fluorophore, the modulation of the emission at conveniently low frequencies becomes equal to the fractional fluorescence intensity of the sensing fluorophore. Under these conditions, the modulation can be used to determine the analyte concentration. This can be used with any fluorophore that changes intensity in response to analyte and does not require the sensing fluorophore to display a change in lifetime. The feasibility of modulation-based sensing was demonstrated using mixtures of 6-carboxyfluorescein and [Ru 2,2'-(bipyridyl)3]2+ as a pH sensor and of the calcium probe Fluo-3 and [Ru 2,2'-(bipyridyl)3]2+ as a calcium sensor.

Spatially localized ballistic two-photon excitation in scattering media

We describe spatially localized two-photon excitation in scattering media. Using femtosecond pulses at 770 nm from a Ti: Sapphire laser, we were able to excite fluorophores in capillary tubes under up to 1.5 mm of 0.5% intralipid. Displacement of the laser beam relative to the embedded samples indicates that highly localized excitation was possible with two-photon excitation, whereas one-photon excitation resulted in loss of spatial resolution due to excitation by the diffusely scattered photons. These results indicate that two-photon excitation in the scattering solution is due only to the ballistic photons, a result confirmed by frequency-domain time-resolved measurements. Selective excitation of adjacent embedded samples was found possible for two but not one-photon excitation.

Fluorescence anisotropy controlled by light quenching

We demonstrated that fluorescence anisotropy can be effectively decreased or increased in the presence of light quenching, depending on relative polarizations of excitation and quenching pulses. For parallel light quenching, anisotropy decreases to 0.103 and z-axis symmetry is preserved. In the presence of perpendicular light quenching, the steady-state anisotropy of a pyridine-2-glycerol solution increases from 0.368 for an unquenched sample to 0.484 for a quenched one. We show that the angular distribution of transition moments loses z-axis symmetry in the presence of perpendicular light quenching. In these cases we used more general definitions of anisotropy. Induced by light quenching, anisotropy can be applied in both steady-state and time-resolved measurements. In particular, the systems with low or no anisotropy can be investigated with the proposed technique.