Order in supported phospholipid monolayers detected by the dichroism of fluorescence excited with polarized evanescent illumination

Tuning fluorophore excitation in a total-internal-reflection-fluorescence microscopy

In a total-internal-reflection-fluorescence-microscopy method, there is anisotropy in the polarized evanescent wave. Since the evanescent wave is used as an excitation field, the mentioned anisotropy is a disadvantage in using the total-internal-reflection-fluorescence-microscopy technique. Therefore, by theoretical and analytical approaches, and based on the Fresnel coefficients, the effect of three dielectrics media on the anisotropy of the evanescent wave is investigated. Following that, a proper combination of the cover glass, oil immersion, and prism for both living and non-living samples is suggested that not only enhances the intensity of the evanescent wave, but also and importantly, decreases the essential anisotropy of the evanescent wave.

Angular Mapping of Protein Structure Using Nonlinear Optical Measurements

Proteins are inherently dynamic, flexible molecules that execute precise conformational changes to perform their functions, but existing techniques to directly measure relevant structural changes in solution at room temperature remain limited. Here, we demonstrate a structural technique using second-harmonic generation and two-photon fluorescence under single-laser excitation to map both the mean angular orientation and the distribution width of a probe at various sites throughout the protein with high sensitivity. Our work resolves distinct dihydrofolate reductase (DHFR) ligand-protein conformations, allows interrogation of regions unresolvable by other techniques, and reveals structural differences between DHFR and a point mutant (DHFR-G121V). The technique, angular mapping of protein structure, enables direct and rapid determination of previously unseen aspects of protein structure in a benchtop optical system.

Polarization-controlled TIRFM with focal drift and spatial field intensity correction

Total internal reflection fluorescence microscopy (TIRFM) is becoming an increasingly common methodology to narrow the illumination excitation thickness to study cellular process such as exocytosis, endocytosis, and membrane dynamics. It is also frequently used as a method to improve signal/noise in other techniques such as in vitro single-molecule imaging, stochastic optical reconstruction microscopy/photoactivated localization microscopy imaging, and fluorescence resonance energy transfer imaging. The unique illumination geometry of TIRFM also enables a distinct method to create an excitation field for selectively exciting fluorophores that are aligned either parallel or perpendicular to the optical axis. This selectivity has been used to study orientation of cell membranes and cellular proteins. Unfortunately, the coherent nature of laser light, the typical excitation source in TIRFM, often creates spatial interference fringes across the illuminated area. These fringes are particularly problematic when imaging large cellular areas or when accurate quantification is necessary. Methods have been developed to minimize these fringes by modulating the TIRFM field during a frame capture period; however, these approaches eliminate the possibility to simultaneously excite with a specific polarization. A new, to our knowledge, technique is presented, which compensates for spatial fringes while simultaneously permitting rapid image acquisition of both parallel and perpendicular excitation directions in ~25 ms. In addition, a back reflection detection scheme was developed that enables quick and accurate alignment of the excitation laser. The detector also facilitates focus drift compensation, a common problem in TIRFM due to the narrow excitation depth, particularly when imaging over long time courses or when using a perfusion flow chamber. The capabilities of this instrument were demonstrated by imaging membrane orientation using DiO on live cells and on lipid bilayers that were supported on a glass slide (supported lipid bilayer). The use of the approach to biological problems was illustrated by examining the temporal and spatial dynamics of exocytic vesicles.

Membrane order parameters for interdigitated lipid bilayers measured via polarized total-internal-reflection fluorescence microscopy

Incorporating ethanol in lipid membranes leads to changes in bilayer structure, including the formation of an interdigitated phase. We have used polarized total-internal-reflection fluorescence microscopy (pTIRFM) to measure the order parameter for Texas Red DHPE incorporated in the ethanol-induced interdigitated phase (LβI) formed from ternary lipid mixtures comprising dioleoylphosphatidylcholine, cholesterol and egg sphingomyelin or dipalmitoylphosphatidylcholine. These lipid mixtures have 3 co-existing phases in the presence of ethanol: liquid-ordered, liquid-disordered and LβI. pTIRFM using Texas Red DHPE shows a reversal in fluorescence contrast between the LβI phase and the surrounding disordered phase with changes in the polarization angle. The contrast reversal is due to changes in the orientation of the dye, and provides a rapid method to identify the LβI phase. The measured order parameters for the LβI phase are consistent with a highly ordered membrane environment, similar to a gel phase. An acyl-chain labeled BODIPY-FL-PC was also tested for pTIRFM studies of ethanol-treated bilayers; however, this probe is less useful since the order parameters of the interdigitated phase are consistent with orientations that are close to random, either due to local membrane disorder or to a mixture of extended and looping conformations in which the fluorophore is localized in the polar headgroup region of the bilayer. In summary, we demonstrate that order parameter measurements via pTIRFM using Texas Red-DHPE can rapidly identify the interdigitated phase in supported bilayers. We anticipate that this technique will aid further research in the effects of alcohols and other additives on membranes.

Imaging plasma membrane deformations with pTIRFM

To gain novel insights into the dynamics of exocytosis, our group focuses on the changes in lipid bilayer shape that must be precisely regulated during the fusion of vesicle and plasma membranes. These rapid and localized changes are achieved by dynamic interactions between lipids and specialized proteins that control membrane curvature. The absence of such interactions would not only have devastating consequences for vesicle fusion, but a host of other cellular functions that involve control of membrane shape. In recent years, the identity of a number of proteins with membrane-shaping properties has been determined. What remains missing is a roadmap of when, where, and how they act as fusion and content release progress.

Our understanding of the molecular events that enable membrane remodeling has historically been limited by a lack of analytical methods that are sensitive to membrane curvature or have the temporal resolution to track rapid changes. PTIRFM satisfies both of these criteria. We discuss how pTIRFM is implemented to visualize and interpret rapid, submicron changes in the orientation of chromaffin cell membranes during dense core vesicle (DCV) fusion. The chromaffin cells we use are isolated from bovine adrenal glands. The membrane is stained with a lipophilic carbocyanine dye,1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine, 4-chlorobenzenesulfonate, or diD. DiD intercalates in the membrane plane with a "fixed" orientation and is therefore sensitive to the polarization of the evanescent field. The diD-stained cell membrane is sequentially excited with orthogonal polarizations of a 561 nm laser (p-pol, s-pol). A 488 nm laser is used to visualize vesicle constituents and time the moment of fusion. Exocytosis is triggered by locally perfusing cells with a depolarizing KCl solution. Analysis is performed offline using custom-written software to understand how diD emission intensity changes relate to fusion pore dilation.

Changes in order parameters associated with ceramide-mediated membrane reorganization measured using pTIRFM

The enzymatic generation of ceramide has significant effects on the biophysical properties of lipid bilayers and can lead to the extensive reorganization of cell membranes. We have synthesized and characterized a headgroup-labeled fluorescent lipid probe (NBD-ceramide, NBD-Cer) and demonstrated that it can be used for polarized total internal reflection fluorescence microscopy experiments to probe changes in membrane order that result from ceramide incorporation. NBD-Cer measures significantly higher order parameters for the liquid-ordered (Lo) domains ([P2] = 0.40 ± 0.03) than for the liquid-disordered phase (Ld, fluid, [P2] = 0.22 ± 0.02) of phase-separated bilayers prepared from egg sphingomyelin, dioleolyphosphatidylcholine, and cholesterol mixtures. The probe also responds to changes in packing induced by the direct incorporation of ceramide or the variation in the ionic strength of the aqueous medium. Order parameter maps obtained after enzyme treatment of bilayers with coexisting Lo and Ld phases show two distinct types of behavior. In regions of high enzyme activity, the initial Lo/Ld domains are replaced by large, dark features that have high membrane order corroborating previous hypotheses that these are ceramide-enriched regions of the membrane. In areas of low enzyme activity, the size and shape of the Lo domains are conserved, but there is an increase in the order parameter for the initial Ld phase ([P2] = 0.30 ± 0.01). This is attributed to the incorporation of ceramide in the Lo domains with the concomitant expulsion of cholesterol into the surrounding fluid phase, increasing its order parameter.

Correlative optical and scanning probe microscopies for mapping interactions at membranes

Innovative approaches for real-time imaging on molecular-length scales are providing researchers with powerful strategies for characterizing molecular and cellular structures and dynamics. Combinatorial techniques that integrate two or more distinct imaging modalities are particularly compelling as they provide a means for overcoming the limitations of the individual modalities and, when applied simultaneously, enable the collection of rich multi-modal datasets. Almost since its inception, scanning probe microscopy has closely associated with optical microscopy. This is particularly evident in the fields of cellular and molecular biophysics where researchers are taking full advantage of these real-time, in situ, tools to acquire three-dimensional molecular-scale topographical images with nanometer resolution, while simultaneously characterizing their structure and interactions though conventional optical microscopy. The ability to apply mechanical or optical stimuli provides an additional experimental dimension that has shown tremendous promise for examining dynamic events on sub-cellular length scales. In this chapter, we describe recent efforts in developing these integrated platforms, the methodology for, and inherent challenges in, performing coupled imaging experiments, and the potential and future opportunities of these research tools for the fields of molecular and cellular biophysics with a specific emphasis on the application of these coupled approaches for the characterization of interactions occurring at membrane interfaces.

Single SNARE-mediated vesicle fusion observed in vitro by polarized TIRFM

Single-vesicle fusion assays in vitro are useful tools for examining mechanisms of membrane fusion at the molecular level mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). This approach allows the experimentalist to define the lipid and protein composition of the two fusing membranes and perform experiments under highly controlled conditions. In previous experiments, in which we reconstituted a SNARE acceptor complex into supported membranes and observed the docking and fusion of fluorescently labeled synaptobrevin proteoliposomes by total internal reflection fluorescence microscopy with millisecond time resolution, we were able to determine the optimal number of SNARE complexes needed for fast fusion. Here, we utilize this assay in combination with polarized total internal reflection fluorescence microscopy to investigate topology changes that vesicles undergo after the onset of fusion. The theory that describes the fluorescence intensity during the transformation of a single vesicle from a spherical particle to a flat membrane patch is developed and confirmed by experiments with three different fluorescent probes. Our results show that on average, the fusing vesicles flatten and merge into the planar membrane within 8 ms after fusion starts.

Single molecules observed by near-field scanning optical microscopy

Individual carbocyanine dye molecules in a sub-monolayer spread have been imaged with near-field scanning optical microscopy. Molecules can be repeatedly detected and spatially localized (to approximately lambda/50 where lambda is the wavelength of light) with a sensitivity of at least 0.005 molecules/(Hz)(1/2) and the orientation of each molecular dipole can be determined. This information is exploited to map the electric field distribution in the near-field aperture with molecular spatial resolution.

Peptide-induced domain formation in supported lipid bilayers: direct evidence by combined atomic force and polarized total internal reflection fluorescence microscopy

Direct visualization of the mechanism(s) by which peptides induce localized changes to the structure of membranes has high potential for enabling understanding of the structure-function relationship in antimicrobial and cell-penetrating peptides. We have applied a combined imaging strategy to track the interaction of a model antimicrobial peptide, PFWRIRIRR-amide, with bacterial membrane-mimetic supported phospholipid bilayers comprised of POPE/TOCL. Our in situ studies revealed rapid reorganization of the POPE/TOCL membrane into localized TOCL-rich domains with a concomitant change in the organization of the membranes themselves, as reflected by changes in fluorescent-membrane-probe order parameter, upon introduction of the peptide.

Localized topological changes of the plasma membrane upon exocytosis visualized by polarized TIRFM

Imaging of individual secretory granules reveals how exocytosis curves the membrane.

Total internal reflection fluorescence microscopy (TIRFM) images the plasma membrane–cytosol interface and has allowed insights into the behavior of individual secretory granules before and during exocytosis. Much less is known about the dynamics of the other partner in exocytosis, the plasma membrane. In this study, we report the implementation of a TIRFM-based polarization technique to detect rapid submicrometer changes in plasma membrane topology as a result of exocytosis. A theoretical analysis of the technique is presented together with image simulations of predicted topologies of the postfusion granule membrane–plasma membrane complex. Experiments on diI-stained bovine adrenal chromaffin cells using polarized TIRFM demonstrate rapid and varied submicrometer changes in plasma membrane topology at sites of exocytosis that occur immediately upon fusion. We provide direct evidence for a persistent curvature in the exocytotic region that is altered by inhibition of dynamin guanosine triphosphatase activity and is temporally distinct from endocytosis measured by VMAT2-pHluorin.

Total internal reflection with fluorescence correlation spectroscopy: Applications to substrate-supported planar membranes

In this paper, the conceptual basis and experimental design of total internal reflection with fluorescence correlation spectroscopy (TIR-FCS) is described. The few applications to date of TIR-FCS to supported membranes are discussed, in addition to a variety of applications not directly involving supported membranes. Methods related, but not technically equivalent, to TIR-FCS are also summarized. Future directions for TIR-FCS are outlined.

Probing membrane order and topography in supported lipid bilayers by combined polarized total internal reflection fluorescence-atomic force microscopy

Determining the local structure, dynamics, and conformational requirements for protein-protein and protein-lipid interactions in membranes is critical to understanding biological processes ranging from signaling to the translocating and membranolytic action of antimicrobial peptides. We report here the application of a combined polarized total internal reflection fluorescence microscopy-in situ atomic force microscopy platform. This platform's ability to image membrane orientational order was demonstrated on DOPC/DSPC/cholesterol model membranes containing the fluorescent membrane probe, DiI-C(20) or BODIPY-PC. Spatially resolved order parameters and fluorophore tilt angles extracted from the polarized total internal reflection fluorescence microscopy images were in good agreement with the topographical details resolved by in situ atomic force microscopy, portending use of this technique for high-resolution characterization of membrane domain structures and peptide-membrane interactions.

Combined scanning probe and total internal reflection fluorescence microscopy

Combining scanning probe and optical microscopy represents a powerful approach for investigating structure-function relationships and dynamics of biomolecules and biomolecular assemblies, often in situ and in real-time. This platform technology allows us to obtain three-dimensional images of individual molecules with nanometer resolution, while simultaneously characterizing their structure and interactions though complementary techniques such as optical microscopy and spectroscopy. We describe herein the practical strategies for the coupling of scanning probe and total internal reflection fluorescence microscopy along with challenges and the potential applications of such platforms, with a particular focus on their application to the study of biomolecular interactions at membrane surfaces.

Chapter 7: Total internal reflection fluorescence microscopy

Total internal reflection fluorescence microscopy (TIRFM), also known as evanescent wave microscopy, is used in a wide range of applications, particularly to view single molecules attached to planar surfaces and to study the position and dynamics of molecules and organelles in living culture cells near the contact regions with the glass coverslip. TIRFM selectively illuminates fluorophores only in a very thin (less than 100 nm deep) layer near the substrate, thereby avoiding excitation of fluorophores outside this subresolution optical section. This chapter reviews the history, current applications in cell biology and biochemistry, basic optical theory, combinations with numerous other optical and spectroscopic approaches, and a range of setup methods, both commercial and custom.

Combinatorial microscopy

By taking advantage of combinations of the many rich properties of photons, new forms of optical microscopy can now be used to visualize features of samples beyond thickness and density variations. We are now within reach of viewing the motions, orientations, binding kinetics and specific transient associations of previously 'submicroscopic' cellular structures and single molecules.

In situ fluorescent protein imaging with metal film-enhanced total internal reflection microscopy

Fluorescence detection of single molecules provides a means to investigate protein dynamics minus ambiguities introduced by ensemble averages of unsynchronized protein movement or of protein movement mimicking a local symmetry. For proteins in a biological assembly, taking advantage of the single molecule approach could require single protein isolation from within a high protein concentration milieu. Myosin cross-bridges in a muscle fiber are proteins attaining concentrations of approximately 120 muM, implying single myosin detection volume for this biological assembly is approximately 1 attoL (10(-18) L) provided that just 2% of the cross-bridges are fluorescently labeled. With total internal reflection microscopy (TIRM) an exponentially decaying electromagnetic field established on the surface of a glass-substrate/aqueous-sample interface defines a subdiffraction limit penetration depth into the sample that, when combined with confocal microscopy, permits image formation from approximately 3 attoL volumes. Demonstrated here is a variation of TIRM incorporating a nanometer scale metal film into the substrate/glass interface. Comparison of TIRM images from rhodamine-labeled cross-bridges in muscle fibers contacting simultaneously the bare glass and metal-coated interface show the metal film noticeably reduces both background fluorescence and the depth into the sample from which fluorescence is detected. High contrast metal film-enhanced TIRM images allow secondary label visualization in the muscle fibers, facilitating elucidation of Z-disk structure. Reduction of both background fluorescence and detection depth will enhance TIRM's usefulness for single molecule isolation within biological assemblies.

A method for determining the absorption ellipsoid of single conjugated polymer molecules and single luminescent nanoparticles

We propose a simple method for the measurement of the absorption ellipsoid of luminescent nanoparticles. The method is based on a combination of far-field and near-field polarized excitation in a wide-field fluorescence microscope and provides the orientation and axes ratio r of a rotationally symmetric ellipsoid. Potential applications of the method including the study of conjugated polymer conformations are discussed.

Determination of molecular anisotropy in thin films of discotic assemblies using attenuated total reflectance UV-visible spectroscopy

We report here an investigation of absorbance anisotropy in highly ordered, single bilayer (ca. 5.6 nm) Langmuir-Blodgett (LB) thin films of discotic liquid-crystalline phthalocyanines, using a recently introduced broad-band attenuated total reflectance (ATR) spectroscopic technique, capable of measuring dichroism in such films in the UV--visible optical region down to absorbances of ca. 0.003 absorbance units. On the basis of the ATR measurements of LB-deposited films, a thorough treatment was established to determine the ensemble average of the Cartesian components and the associated optical anisotropy of transition dipoles in the molecular film. In an effort to recover order parameters of molecular orientation, those results were interpreted with a circular dipole model, which is the expected model for the isolated molecule based on symmetry properties. We measured a strong dipole component normal to the film plane that cannot be explained in terms of a truly circular model, indicating that the molecular transition dipoles were perturbed upon aggregation. The utility of the experimental approach was further demonstrated by (a) investigating the effect of substrate modifiers (methyl- and phenyl-terminated silanes) on the ordering within the phthalocyanine film and (b) the effect of water immersion and re-annealing of the thin film on molecular ordering and optical anisotropy.

Self-assembly of single integral membrane proteins into soluble nanoscale phospholipid bilayers

One of the biggest challenges in pharmaceutical research is obtaining integral membrane proteins in a functional, solubilized, and monodisperse state that provides a native-like environment that maintains the spectrum of in vivo activities. Many of these integral membrane proteins are receptors, enzymes, or other macromolecular assemblies that are important drug targets. An example is the general class of proteins composed of seven-transmembrane segments (7-TM) as exemplified by the G-protein-coupled receptors. In this article, we describe a simple system for self-assembling bacteriorhodopsin, as a model protein containing 7-TM helices, with phospholipids to form a nanometer-scale soluble bilayer structure encircled by a 200 amino acid scaffold protein. The result is the single molecule incorporation of an integral membrane protein target into a soluble and monodisperse structure that allows the structural and functional tools of solution biochemistry to be applied.

Absolute surface coverage measurement using a vibrational overtone

Determination of absolute surface coverage with sub-monolayer sensitivity is demonstrated using evanescent-wave cavity ring-down spectroscopy (EW-CRDS) and conventional CRDS by employing conservation of the absolute integrated absorption intensity between gas and adsorbed phases. The first C-H stretching overtones of trichloroethylene (TCE), cis-dichloroethylene, and trans-dichloroethylene are probed using the idler of a seeded optical parametric amplifier having a 0.075 cm(-1) line width. Polarized absolute adsorbate spectra are obtained by EW-CRDS using a fused-silica monolithic folded resonator having a finesse of 28 500 at 6050 cm(-1), while absolute absorption cross sections for the gas-phase species are determined by conventional CRDS. A measure of the average transition moment orientation on the surface, which is utilized for the coverage determination, is derived from the polarization anisotropy of the surface spectra. Coverage measurement by EW-CRDS is compared to a mass-spectrometer-based surface-uptake technique, which we also employ for coverage measurements of TCE on thermally grown SiO(2) surfaces. To assess the potential for environmental sensing, we also compare EW-CRDS to optical waveguide techniques developed previously for TCE detection.

Determination of surface-bound-fluorophore orientation by goniometric fluorescence polarization: application to quantification of DNA-chip readouts

We present a polarized goniofluorimeter designed to measure the observation-angle and polarization-dependent intensity emitted by a group of surface-bound fluorescent molecules. We studied two types of surface bonding: In one case, dyes were adsorbed into the surface by spin coating, and in the other, dyes were covalently immobilized to DNA strands. Fluorescent dyes consisted of Cy3 and Alexa546. The substrate was a silicon wafer bearing a silicon dioxide layer. The different samples presented a wide panel of reproducible experimental behavior. By confronting experimental behavior with theory and simulation, we can explain these differences as directly linked to the mean orientation of fluorophores with respect to the surface.

Control of a redox reaction on lipid bilayer surfaces by membrane dipole potential

Nitro-2,1,3-benzoxadiazol-4-yl (NBD) group is a widely used, environment-sensitive fluorescent probe. The negatively charged dithionite rapidly reduces the accessible NBD-labeled lipids in liposomes to their corresponding nonfluorescent derivatives. In this study both the phospholipid headgroup and acyl chain NBD-labeled L-alpha-1,2-dipalmitoyl-sn-glycero-3-phospho-[N-(4-nitrobenz-2-oxa-1,3-diazole)-ethanolamine] (DPPN) and 1-acyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (NBD-PC), respectively, were employed. The correlation of both the rate coefficient k(1) of the redox reaction and the fluorescence properties of the two probes with the membrane dipole potential Psi in fluid dipalmitoylglycerophosphocholine (DPPC) liposomes is demonstrated. When Psi of the bilayer was varied (decreased by phloretin or increased by 6-ketocholestanol), the value for k1 decreased for both DPPN and NBD-PC with increasing Psi. For both fluorophores a positive correlation to Psi was evident for the relative fluorescence emission intensity (RFI, normalized to the emission of the fluorophore in a DPPC matrix). The relative changes in emission intensity as a function of Psi were approximately equal for both NBD derivatives. Changes similar to those caused by phloretin were seen when dihexadecylglycerophosphocholine (DHPC) was added to DPPC liposomes, in keeping with the lower dipole potential for the former lipid compound compared with DPPC. These effects of Psi on NBD fluorescence should be taken into account when interpreting data acquired using NBD-labeled lipids as fluorescent probes.

Monolithic folded resonator for evanescent wave cavity ringdown spectroscopy

An optical resonator is characterized that employs both ultrahigh-reflective coated surfaces and total internal reflection to enable cavity ringdown spectroscopy of surfaces, films, and liquids. The monolithic folded design possesses a polarization-independent finesse that allows polarization-dependent phenomena, such as molecular orientation, to be probed. Although a restricted bandwidth (~15% of the design wavelength) results from use of reflective coatings, the resonator provides high sensitivity and facile operation. A minimum detectable absorption of 2.2 x 10(-6) was obtained for single laser shots by use of multimode excitation at 530 nm with an excimer-pumped, pulsed dye laser.

Equilibrium, kinetics, diffusion and self-association of proteins at membrane surfaces: measurement by total internal reflection fluorescence microscopy

The equilibrium, kinetics, diffusion and self-association of proteins at membrane/solution interfaces may deviate substantially from these processes in bulk solution. A set of methods for examining these phenomena combines substrate-supported planar model membranes and the use of evanescent illumination with laser-based, quantitative fluorescence microscopy. Measurement of the steady-state, surface-associated fluorescence can be used to examine the thermodynamic properties of proteins at membranes. When combined with fluorescence photobleaching recovery, this technique provides information about membrane-binding kinetics; and when combined with fluorescence pattern photobleaching recovery, measurement of the translational diffusion coefficients of proteins weakly bound to membranes is possible. The use of polarized evanescent illumination can provide information about the orientation distributions of adsorbed fluorophores. Fluorescence correlation spectroscopy provides information about the self-association (e.g. dimerization) of membrane-associated proteins.

Cytoplasmic viscosity near the cell plasma membrane: translational diffusion of a small fluorescent solute measured by total internal reflection-fluorescence photobleaching recovery

Total internal reflection-fluorescence recovery after photobleaching (TIR-FRAP) was applied to measure solute translational diffusion in the aqueous phase of membrane-adjacent cytoplasm. TIR fluorescence excitation in aqueous solutions and fluorescently labeled cells was produced by laser illumination at a subcritical angle utilizing a quartz prism; microsecond-resolution FRAP was accomplished by acousto-optic modulators and electronic photomultiplier gating. A mathematical model was developed to determine solute diffusion coefficient from the time course of photobleaching recovery, bleach time, bleach intensity, and evanescent field penetration depth; the model included irreversible and reversible photobleaching processes, with triplet state diffusion. The validity and accuracy of TIR-FRAP measurements were first examined in aqueous fluorophore solutions. Diffusion coefficients for fluorescein isothiocyanate-dextrans (10-2000 kDa) determined by TIR-FRAP (recovery t1/2 0.5-2.2 ms) agreed with values measured by conventional spot photobleaching. Model predictions for the dependence of recovery curve shape on solution viscosity, bleach time, and bleach depth were validated experimentally using aqueous fluorescein solutions. To study solute diffusion in cytosol, MDCK epithelial cells were fluorescently labeled with the small solute 2',7'-bis-2-carboxyethyl-5-carboxyfluorescein-acetoxymethyl-ester (BCECF). A reversible photobleaching process (t1/2 approximately 0.5 ms) was identified that involved triplet-state relaxation and could be eliminated by triplet-state quenching with 100% oxygen. TIR-FRAP t1/2 values for irreversible BCECF bleaching, representing BCECF translational diffusion in the evanescent field, were in the range 2.2-4.8 ms (0.2-1 ms bleach times), yielding a BCECF diffusion coefficient 6-10-fold less than that in water. These results establish the theory and the first experimental application of TIR-FRAP to measure aqueous-phase solute diffusion, and indicate slowed translational diffusion of a small solute in membrane-adjacent cytosol.

Determination of the orientation distribution of adsorbed fluorophores using TIRF. I. Theory

The spectroscopic technique total internal reflection fluorescence can be used for determination of the orientation of adsorbed fluorescent molecules. The underlying theory is presented in general terms and elaborated in detail for the case that the fluorescent group is a porphyrin ring. It is shown that order parameters of the orientation distribution can be obtained if both the fluorescence intensity and its polarization are measured as functions of the polarization of the incident laser beam. From these order parameters an approximation of the orientation distribution can be derived by the maximum-entropy method.

Cell volume measured by total internal reflection microfluorimetry: application to water and solute transport in cells transfected with water channel homologs

Total internal reflection (TIR) microfluorimetry was established as a method to measure continuously the volume of adherent cells and applied to measure membrane permeabilities in cells transfected with water channel homologs. Cytosol was labeled with the membrane-impermeant fluorophore calcein. Fluorescence was excited by the TIR evanescent field in a thin section of cytosol (approximately 150 nm) adjacent to the cell-substrate interface. Because cytosolic fluorophore number per cell remains constant, the TIR fluorescence signal should be inversely related to cell volume. For small volume changes in Sf-9 and LLC-PK1 cells, relative TIR fluorescence was nearly equal to inverse relative cell volume; deviations from the ideal were modeled theoretically. To measure plasma membrane osmotic water permeability, Pf, the time course of osmotically induced cell volume change was inferred from the TIR fluorescence signal. LLC-PK1 cells expressing the CHIP28 water channel had an HgCl2-sensitive, threefold increase in Pf compared to nontransfected cells (Pf = 0.0043 cm/s at 10 degrees C). Solute permeability was measured from the TIR fluorescence time course in response to solute gradients. Glycerol permeability in Sf-9 cells expressing the water channel homolog GLIP was (1.3 +/- 0.2) x 10(-5) cm/s (22 degrees C), greater than that of (0.36 +/- 0.04) x 10(-5) cm/s (n = 4, p < 0.05) for control cells, indicating functional expression of GLIP. Water and urea permeabilities were similar in GLIP-expressing and control cells. The TIR method should be applicable to the study of water and solute permeabilities and cell volume regulation in cells of arbitrary shape and size.

Total internal reflection fluorescence microscopy: application to substrate-supported planar membranes

The use of total internal reflection illumination in fluorescence microscopy (TIRFM) is reviewed with emphasis on application to fluorescent macromolecules that specifically and reversibly bind to planar model membranes supported on glass or quartz substrates. Several methods for characterizing macromolecular motion and organization are discussed: the measurement of equilibrium binding curves to obtain values for equilibrium binding constants; the measurement of fluorescence photobleaching recovery curves to obtain values of kinetic rate constants and surface diffusion coefficients; and the measurement of fluorescence intensities as a function of the evanescent field polarization to characterize orientational order. Applications to cell-substrate contact regions are summarized and future directions of TIRFM are outlined.

Cytoplasmic viscosity near the cell plasma membrane: measurement by evanescent field frequency-domain microfluorimetry

The purpose of this study was to determine whether the unique physical milieu just beneath the cell plasma membrane influences the rheology of fluid-phase cytoplasm. Cytoplasmic viscosity was evaluated from the picosecond rotation of the small fluorophore 2',7'-bis-(2-carboxyethyl)-5-carboxyfluorescein (BCECF) by parallel-acquisition Fourier transform microfluorimetry (Fushimi and Verkman, 1991). Information about viscosity within < 200 nm of cell plasma membranes was obtained by selective excitation of fluorophores in an evanescent field created by total internal reflection (TIR) of impulse-modulated s-plane-polarized laser illumination (488 nm) at a glass-aqueous interface. Measurements of fluorescence lifetime and time-resolved anisotropy were carried out in solutions containing fluorescein or BCECF at known viscosities, and monolayers of BCECF-labeled Swiss 3T3 fibroblasts and Madin-Darby canine kidney (MDCK) cells. Specific concerns associated with time-resolved fluorescence measurements in the evanescent field were examined theoretically and/or experimentally, including variations in lifetime due to fluorophore proximity to the interface, and the use of the s and p polarized excitation. In fluorescein solutions excited with s-plane polarized light, there was a 5-10% decrease in fluorescein lifetime with TIR compared to trans (subcritical) illumination, but no change in rotational correlation time (approximately 98 ps/cP). Intracellular BCECF had a single lifetime of 3.7 +/- 0.1 ns near the cell plasma membrane. Apparent fluid-phase viscosity near the cell plasma membrane was 1.1 +/- 0.2 cP (fibroblast) and 1.0 +/- 0.2 cP (MDCK), not significantly different from the viscosity measured in bulk cytoplasm far from the plasma membrane. The results establish the methodology for time-resolved microfluorimetric measurement of polarization in the evanescent field and demonstrate that the cell plasma membrane has little effect on the fluid-phase viscosity of adjacent cytoplasm.

Measurement of restricted rotational diffusion of fluorescent lipids in supported planar phospholipid monolayers using angle-dependent polarized fluorescence photobleaching recovery

A theory describing the shapes of polarized fluorescence photobleaching recovery (PFPR) curves for a population of fluorophores undergoing restricted rotational diffusion in two-dimensional systems such as planar membranes has been developed. In this model, restricted rotational diffusion of the fluorophores is described by using reflective boundary conditions, in which the fluorophores are assumed to diffuse freely but only within an angular space of width 2 omega. The magnitude and apparent rate of the PFPR postbleach fluorescence curves are a function of both omega and the angle between the bleaching and observation beam polarizations psi. It is shown that estimates of the degree of rotational restriction omega may be obtained from changes in the psi-dependent postbleach fluorescence intensities. Using angle-dependent PFPR, slow rotational reorientations of the fluorescent lipid analogue 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine in distearoylphosphatidylcholine Langmuir-Blodgett monolayers deposited on octadecyltrichlorosilane-treated fused quartz were measured. As theoretically predicted for a rotationally restricted fluorophore population, both the initial F psi (0) and final F psi (infinity) postbleach fluorescence intensities varied as a function of psi, and no measurable change in the postbleach fluorescence intensities was observed for psi = 45 degrees. Using the theory for restricted rotational motion, the psi-dependent variations of the final fluorescence intensities F psi (infinity) obtained at two bleaching intensities gave an average apparent omega approximately 52 degrees. However, to adequately fit the F psi (0) data, inclusion of the theoretical effects of rapid (faster than the duration of the photobleaching pulse) fluorophore dynamics was also required. Best fits of the F psi (0) and F psi (infinity) data were obtained when the fluorophores were assumed to rapidly wobble within a cone of semiangle delta approximately 30 degrees-50 degrees while slowly rotating within an angular space defined by semiangle omega approximately 35 degrees-60 degrees. Subsequent analysis of the time- and psi-dependent changes in the post-bleach fluorescence curves F psi (t) gave apparent diffusion coefficients ranging from D approximately 10(-3) s-1 to 4 x 10(-2) s-1.

Evanescent interference patterns for fluorescence microscopy

The increasing experimental use of total internal reflection/fluorescence photobleaching recovery has motivated a theoretical study of the spatial intensity profiles generated by two interfering evanescent waves. The interference patterns generated by evanescent waves differ considerably from those generated by plane waves in a homogenous medium because evanescent waves are not transverse and because the evanescent propagation number depends on the incidence angle of the totally internally reflected light. The periodicity and contrast of the evanescent interference patterns under various conditions are calculated; these parameters depend on the intensities, polarizations, and incidence angles of the two incident beams, as well as the refractive indices of the two media that form the planar interface where total internal reflection occurs. The derived intensity profiles are used to develop expressions for the shapes of fluorescence photobleaching recovery curves when evanescent interference patterns are used for fluorescence excitation and bleaching. The calculations also suggest that colliding beam experiments may confirm theoretically predicted evanescent field polarizations.

Binding and mobility of anti-dinitrophenyl monoclonal antibodies on fluid-like, Langmuir-Blodgett phospholipid monolayers containing dinitrophenyl-conjugated phospholipids

The association of a fluorescently labelled anti-dinitrophenyl monoclonal antibody (ANO2) with Langmuir-Blodgett monolayers composed of three different binary mixtures of phosphatidylcholine and dinitrophenyl-conjugated phosphatidylethanolamine has been characterized. Quantitative fluorescence microscopy measurements demonstrated that measurable amounts of antibodies bound to the monolayers only at high molar fractions of dinitrophenyl-conjugated lipid (greater than or equal to 5 mol%). Fluorescence pattern photobleaching recovery measurements showed that the apparent translational diffusion coefficients and mobile fractions of a fluorescent lipid were high for all monolayer compositions and that the antibody translational mobility was measurable but slow and depended on the two-dimensional antibody density. The results demonstrate that the ANO2-binding characteristics of Langmuir-Blodgett monolayers containing dinitrophenyl-conjugated phospholipids are substantially different from those of similar model systems but that the ANO2 antibodies, when bound, display similar diffusive behavior.

Slow rotational mobilities of antibodies and lipids associated with substrate-supported phospholipid monolayers as measured by polarized fluorescence photobleaching recovery

Polarized fluorescence photobleaching recovery has been used to monitor slow rotational motions of a fluorescently-labeled anti-dinitrophenyl mouse IgGl monoclonal antibody (ANO2) specifically bound to substrate-supported monolayers composed of a mixture of distearoylphosphatidylcholine (DSPC) and dinitrophenyldioleoylphosphatidylethanolamine (DNP-DOPE). ANO2 antibodies were labeled with a new bifunctional carbocyanine fluorophore that has two amino-reactive groups; steady-state fluorescence anisotropy data confirmed the expected result that the ANO2-conjugated bifunctional probe had less independent flexibility than ANO2-conjugated unifunctional fluorescence labels. Rotational mobilities were also measured for the fluorescent lipid 1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine (dil) in DSPC and in mixed DSPC/DNP-DOPE monolayers in the presence and absence of unlabeled ANO2 antibodies. The apparent rotational correlation time and fractional mobility of ANO2 on supported monolayers were approximately 70 and approximately 0.3 s, respectively. These measured parameters of rotational mobility did not depend on the ANO2 surface density or on kinetic factors, but addition of unlabeled polyclonal anti-(mouse IgG) antibodies significantly decreased the apparent mobile fraction. The measured fluorescence recovery curves for dil were consistent with two fluorophore populations with rotational correlation times of approximately 4 and approximately 100 s and a population of immobile fluorescent lipid. No difference in fluorescence recovery and decay curves was measured for dil in DSPC monolayers, DSPC/DNP-DOPE monolayers, and DSPC/DNP-DOPE monolayers treated with unlabeled ANO2 antibodies.

Orientation of adsorbed cytochrome c as a function of the electrical potential of the interface studied by total internal reflection fluorescence

A method for determination of the orientation of adsorbed structure-stable proteins using Total Internal Reflection Fluorescence is outlined. The theory has been elaborated for orientation studies on adsorbed free base cytochrome c, of which the prophyrin can be used as an intrinsic fluorescent label. The ratio of fluorescence intensities at two polarization modes of the incident light (the transverse magnetic and the transverse electric polarization mode, respectively) gives a relation between the orientation angles of the porphyrin relative to the interface. As an illustration of the theory, experimental results on the adsorption of cytochrome c at an optically transparent SnO2 film electrode are presented. It is concluded that the orientation of the molecules can only be affected by the interfacial potential during the process of adsorption, but, once adsorbed, the orientation cannot be changed anymore by variation of the potential.

Chemistry and biology of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-labeled lipids: fluorescent probes of biological and model membranes

Lipids that are covalently labeled with the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group are widely used as fluorescent analogues of native lipids in model and biological membranes to study a variety of processes. The fluorescent NBD group may be attached either to the polar or the apolar regions of a wide variety of lipid molecules. Synthetic routes for preparing the lipids, and spectroscopic and ionization properties of these probes are reviewed in this report. The orientation of various NBD-labeled lipids in membranes, as indicated by the location of the NBD group, is also discussed. The NBD group is uncharged at neutral pH in membranes, but loops up to the surface if attached to acyl chains of phospholipids. These lipids find applications in a variety of membrane-related studies which include membrane fusion, lipid motion and dynamics, organization of lipids and proteins in membranes, intracellular lipid transfer, and bilayer to hexagonal phase transition in liposomes. Use of NBD-labeled lipids as analogues of natural lipids is critically evaluated.

Multilayer adsorption of lysozyme on a hydrophobic substrate

Macromolecular adsorption is known to occur as a complex process, often in a series of steps. Several models are discussed in the literature which describe the microscopic structure of the adsorbate. In the present study we investigated the adsorption of hen egg white lysozyme on alkylated silicon oxide surfaces. A combination of fluorescence excitation in the evanescent field and fluorescence recovery after photobleaching allowed us to measure the amount of adsorbed fluorescent lysozyme and the equilibrium exchange kinetics with molecules in solution. We found that a model with at least three classes of adsorbed molecules is necessary to describe the experimental results. A first layer is formed by the molecules which adsorb within a short time after the beginning of the incubation. These molecules make up approximately 65% of the final coverage. They are quasi-irreversibly adsorbed and do not measurably exchange with bulk molecules within one day even at temperatures up to 55 degrees C. A second layer, which reaches equilibrium only after several hours of incubation, shows a pronounced exchange with bulk molecules. The on-off kinetics show a distinct temperature dependence from which an activation barrier of delta E approximately 22 kcal/mol is derived. A third layer of molecules that exchange rapidly with the bulk can be seen to comprise approximately 10% of the total coverage. The exchange rate is on the order of fractions of a second. The binding of the latter two classes of adsorbed molecules is exothermic. From the temperature dependence of the coverage, the binding enthalpy of the slowly exchanging layer was estimated to be delta Hads approximately 3.8 kcal/mol. The second and third class of molecules remain enzymatically active as a muramidase, which was tested by the lysis of the cell walls of Micrococcus lysodeiktikus. The molecules in the first layer, on the other hand, showed no enzymatic activity.

Interaction of antibodies with Fc receptors in substrate-supported planar membranes measured by total internal reflection fluorescence microscopy

A procedure for constructing substrate-supported planar membranes using membrane fragments isolated from the macrophage-related cell line J774A.1 is described. Total internal reflection (TIR) fluorescence microscopy is employed to demonstrate that fluorescently labeled Fab fragments of a monoclonal antibody (2.4G2) with specificity for a murine macrophage cell-surface receptor for IgG (moFc gamma RII) bind to the planar model membranes. These measurements show that the planar membranes contain moFc gamma RII and yield a value for the association constant of 2.4G2 Fab fragments with moFc gamma RII equal to (9.6 +/- 0.4) x 10(8) M-1 and indicate that the surface density of reconstituted moFc gamma RII is approximately 50 molecules/microns 2. In addition, TIR fluorescence microscopy is used to investigate the Fc-mediated competition of unlabeled, polyclonal murine IgG with labeled 2.4G2 Fab fragments for moFc gamma RII in the planar membranes. These measurements indicate that the reconstituted moFc gamma RII recognized by 2.4G2 Fab fragments also retains the ability to bind murine IgG Fc regions and yield a value for the association constant of polyclonal murine IgG with moFc gamma RII equal to (1-5) x 10(5) M-1. This work represents one of the first applications of TIR fluorescence microscopy to specific ligand-receptor interactions.