Binding of a monoclonal antibody and its Fab fragment to supported phospholipid monolayers measured by total internal reflection fluorescence microscopy

Inter-Leaflet Phospholipid Exchange Impacts the Ligand Density Available for Protein Binding at Supported Lipid Bilayers

Phospholipid bilayers formed at solid-liquid interfaces have garnered interest as mimics of cell membranes to model association reactions of proteins with lipid bilayer-tethered ligands. Despite the importance of understanding how ligand density in a lipid bilayer impacts the protein-ligand association response, relating the ligand-modified lipid fraction to the absolute density of solution-accessible ligands in a lipid bilayer remains a challenge in interfacial quantitative analysis. In this work, confocal Raman microscopy is employed to quantify the association of anti-biotin IgG with a small fraction of biotinylated lipids dispersed in either gel-phase or liquid-crystalline supported lipid bilayers deposited on the interior surfaces of wide-pore silica surfaces. We examine the question of whether inter-leaflet lipid translocation contributes to the population of solution-accessible biotin ligands on the distal leaflet of a supported lipid bilayer by comparing their protein accumulation response with ligands dispersed in lipid monolayers on nitrile-derivatized silica surfaces. The binding of the antibody to biotin ligands dispersed in gel-phase bilayers exhibited an equivalent biotin coverage response as the accumulation of IgG onto gel-phase monolayers, indicating that gel-phase bilayer symmetry was preserved. This result contrasts with the ∼60% greater anti-biotin capture observed at fluid-phase bilayers compared to fluid-phase monolayers prepared at equivalent biotin fractions. This enhanced protein capture is attributed to biotin-capped lipids being transferred from the surface-associated proximal leaflet of the bilayer to the solution-exposed distal leaflet by the inter-leaflet exchange or lipid flip-flop, a facile process in fluid-phase supported lipid bilayers. The results suggest caution in interpreting the results of quantitative studies of protein binding to lipid-tethered ligands dispersed in fluid-phase phospholipid bilayers.

Recent Advances in Fluorescence Recovery after Photobleaching for Decoupling Transport and Kinetics of Biomacromolecules in Cellular Physiology

Among the new molecular tools available to scientists and engineers, some of the most useful include fluorescently tagged biomolecules. Tools, such as green fluorescence protein (GFP), have been applied to perform semi-quantitative studies on biological signal transduction and cellular structural dynamics involved in the physiology of healthy and disease states. Such studies focus on drug pharmacokinetics, receptor-mediated endocytosis, nuclear mechanobiology, viral infections, and cancer metastasis. In 1976, fluorescence recovery after photobleaching (FRAP), which involves the monitoring of fluorescence emission recovery within a photobleached spot, was developed. FRAP allowed investigators to probe two-dimensional (2D) diffusion of fluorescently-labelled biomolecules. Since then, FRAP has been refined through the advancements of optics, charged-coupled-device (CCD) cameras, confocal microscopes, and molecular probes. FRAP is now a highly quantitative tool used for transport and kinetic studies in the cytosol, organelles, and membrane of a cell. In this work, the authors intend to provide a review of recent advances in FRAP. The authors include epifluorescence spot FRAP, total internal reflection (TIR)/FRAP, and confocal microscope-based FRAP. The underlying mathematical models are also described. Finally, our understanding of coupled transport and kinetics as determined by FRAP will be discussed and the potential for future advances suggested.

Measuring Binding Kinetics of Antibody-Conjugated Gold Nanoparticles with Intact Cells

Antibody-conjugated nanomaterials have attracted much attention because of their applications in nanomedicine and nanotheranostics, and amplification of detection signals. For many of these applications, the nanoconjugates must bind with a cell membrane receptor (antigen) specifically before entering the cells and reaching the final target, which is thus important but not well understood. Here, a plasmonic imaging study of the binding kinetics of antibody-conjugated gold nanoparticles with antigen-expressing cells is presented, and the results are compared with that of the nanoparticle-free antibody. It is found that the nanoconjugates can significantly affect the binding kinetics compared with free antibody molecules, depending on the density of the antibody conjugated on the nanoparticles, and expressing level of the antigen on the cell membrane. The results are analyzed in terms of a transition from monovalent binding model to a bivalent binding model when the conjugation density and expressing level increase. These findings help optimize the design of functional nanomaterials for drug delivery and correct interpretation of data obtained with nanoparticle signal amplification.

Unquenchable Surface Potential Dramatically Enhances Cu(2+) Binding to Phosphatidylserine Lipids

Herein, the apparent equilibrium dissociation constant, K(Dapp), between Cu(2+) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS), a negatively charged phospholipid, was measured as a function of PS concentrations in supported lipid bilayers (SLBs). The results indicated that K(Dapp) for Cu(2+) binding to PS-containing SLBs was enhanced by a factor of 17,000 from 110 nM to 6.4 pM as the PS density in the membrane was increased from 1.0 to 20 mol %. Although Cu(2+) bound bivalently to POPS at higher PS concentrations, this was not the dominant factor in increasing the binding affinity. Rather, the higher concentration of Cu(2+) within the double layer above the membrane was largely responsible for the tightening. Unlike the binding of other divalent metal ions such as Ca(2+) and Mg(2+) to PS, Cu(2+) binding does not alter the net negative charge on the membrane as the Cu(PS)2 complex forms. As such, the Cu(2+) concentration within the double layer region was greatly amplified relative to its concentration in bulk solution as the PS density was increased. This created a far larger enhancement to the apparent binding affinity than is observed by standard multivalent effects. These findings should help provide an understanding on the extent of Cu(2+)-PS binding in cell membranes, which may be relevant to biological processes such as amyloid-β peptide toxicity and lipid oxidation.

Avidin functionalized maghemite nanoparticles and their application for recombinant human biotinyl-SERCA purification

We report on the surface characterization, functionalization, and application of stable water suspensions of novel surface active maghemite nanoparticles (SAMNs), characterized by a diameter of 11 ± 2 nm and possessing peculiar colloidal properties and surface interactions. These features permitted the acquisition of titration curves and aqueous UV-vis spectra and suggested a role played by surface under-coordinated iron atoms. This new class of nanoparticles was obtained through an easy, inexpensive, one-step, green procedure and functionalized with ligands of high biotechnological interest, such as biotin and avidin, by simple incubation in aqueous solution. Bound avidin was determined by measuring the disappearance of free avidin absorbance at 280 nm, as a function of increasing nanoparticle concentration, showing the presence of 10 ± 3 avidin molecules per nanoparticle. The biological activity of the SAMN@avidin complex was evaluated and the number of available biotin binding sites was determined, using biotinyl-fluorescein as a probe, showing that each bound avidin molecule is able to bind 2.8 ± 0.8 biotin molecules, confirming the maintenance of biological activity and excellent binding capacity of the SAMN@avidin complex. Furthermore a Langmuir isotherm model was used to describe the biomolecule specific monolayer adsorption onto the particle surface, and in the case of avidin, the maximum adsorption capacity was 100 ± 27 μg avidin/mg SAMN, whereas the binding constant is 45.18 μL μg(-1). The SAMN@avidin complex was characterized by UV-vis spectroscopy, quartz crystal microbalance, FTIR spectroscopy, and transmission electron microscopy. Finally, SAMN@avidin was applied for the large scale purification of recombinant biotinylated human sarco/endoplasmic reticulum Ca(2+)-ATPase (hSERCA-2a), expressed by Saccharomyces cerevisiae. The protein was magnetically purified, and about 500 μg of a 70% pure hSERCA-2a were recovered from 4 L of yeast culture, with a purification yield of 64%.

Quantitative analysis of affinity enhancement by noncovalently oligomeric ligands

Designed ligands that self-assemble noncovalently via an independent oligomerization domain have demonstrated enhancement in affinity for a variety of chemical and biological targets. To better understand the thermodynamic linkage between enhanced receptor binding and self-assembly, we have developed linkage models for the three commonly encountered types of noncovalently oligomeric ligands: homofunctional oligomeric ligands, heterodimeric ligands that target a single receptor, and bispecific ligands that crosslink noninteracting receptors. Expressions and numerical approaches for exact analysis as a function of total ligand concentrations are provided. We apply the linkage models to the binding data for two published noncovalently oligomeric ligands: one targeting a small molecule (phosphocholine) and the other targeting a soluble protein (tumor necrosis factor α). The linkage models provide a quantitative measure of the potential and realized enhancement in affinity that could inform and guide design optimization efforts, and they reveal physical insight that would elude model-free analysis. Incorporation of the linkage models, therefore, is expected to be valuable in the rational engineering of noncovalently oligomeric ligands.

Rifampicin-independent interactions between the pregnane X receptor ligand binding domain and peptide fragments of coactivator and corepressor proteins

The pregnane X receptor (PXR), a member of the nuclear receptor superfamily, regulates the expression of drug-metabolizing enzymes in a ligand-dependent manner. The conventional view of nuclear receptor action is that ligand binding enhances the receptor's affinity for coactivator proteins, while decreasing its affinity for corepressors. To date, however, no known rigorous biophysical studies have been conducted to investigate the interaction among PXR, its coregulators, and ligands. In this work, steady-state total internal reflection fluorescence microscopy (TIRFM) and total internal reflection with fluorescence recovery after photobleaching were used to measure the thermodynamics and kinetics of the interaction between the PXR ligand binding domain and a peptide fragment of the steroid receptor coactivator-1 (SRC-1) in the presence and absence of the established PXR agonist, rifampicin. Equilibrium dissociation and dissociation rate constants of ~5 μM and ~2 s(-1), respectively, were obtained in the presence and absence of rifampicin, indicating that the ligand does not enhance the affinity of the PXR and SRC-1 fragments. Additionally, TIRFM was used to examine the interaction between PXR and a peptide fragment of the corepressor protein, the silencing mediator for retinoid and thyroid receptors (SMRT). An equilibrium dissociation constant of ~70 μM was obtained for SMRT in the presence and absence of rifampicin. These results strongly suggest that the mechanism of ligand-dependent activation in PXR differs significantly from that seen in many other nuclear receptors.

Using covalent dimers of human carbonic anhydrase II to model bivalency in immunoglobulins

This paper describes the development of a new bivalent system comprising synthetic dimers of carbonic anhydrase linked chemically through thiol groups of cysteine residues introduced by site-directed mutagenesis. These compounds serve as models with which to study the interaction of bivalent proteins with ligands presented at the surface of mixed self-assembled monolayers (SAMs). Monovalent carbonic anhydrase (CA) binds to benzenesulfonamide ligands presented on the surface of the SAM with K(d)(surf) = 89 nM. The synthetic bivalent proteins--inspired by the structure of immunoglobulins--bind bivalently to the sulfonamide-functionalized SAMs with low nanomolar avidities (K(d)(avidity,surf) = 1-3 nM); this difference represents a ~50-fold enhancement of bivalent over monovalent association. The paper describes dimers of CA having (i) different lengths of the covalent linker that joined the two proteins and (ii) different points of attachment of the linker to the protein (either near the active site (C133) or distal to the active site (C185)). Comparison of the thermodynamics of their interactions with SAMs presenting arylsulfonamide groups demonstrated that varying the length of the linker between the molecules of CA had virtually no effect on the rate of association, or on the avidity of these dimers with ligand-presenting surfaces. Varying the point of attachment of the linker between monomeric CA's also had almost no effect on the avidity of the dimers, although changing the point of attachment affected the rates of binding and unbinding. These observations indicate that the avidities of these bivalent proteins, and by inference the avidities of structurally similar bivalent proteins such as IgG, are unexpectedly insensitive to the structure of the linker connecting them.

Measuring surface binding thermodynamics and kinetics by using total internal reflection with fluorescence correlation spectroscopy: practical considerations

The combination of total internal reflection illumination and fluorescence correlation spectroscopy (TIR-FCS) is an emerging method useful for, among a number of things, measuring the thermodynamic and kinetic parameters describing the reversible association of fluorescently labeled ligands in solution with immobilized, nonfluorescent surface binding sites. However, there are many parameters (both instrumental and intrinsic to the interaction of interest) that determine the nature of the acquired fluorescence fluctuation autocorrelation functions. In this work, we define criteria necessary for successful measurements and then systematically explore the parameter space to define conditions that meet the criteria. The work is intended to serve as a guide for experimental design, in other words, to provide a methodology to identify experimental conditions that will yield reliable values of the thermodynamic and kinetic parameters for a given interaction.

Multivalent ligand-receptor binding on supported lipid bilayers

Fluid supported lipid bilayers provide an excellent platform for studying multivalent protein-ligand interactions because the two-dimensional fluidity of the membrane allows for lateral rearrangement of ligands in order to optimize binding. Our laboratory has combined supported lipid bilayer-coated microfluidic platforms with total internal reflection fluorescence microscopy (TIRFM) to obtain equilibrium dissociation constant (K(D)) data for these systems. This high throughput, on-chip approach provides highly accurate thermodynamic information about multivalent binding events while requiring only very small sample volumes. Herein, we review some of the most salient findings from these studies. In particular, increasing ligand density on the membrane surface can provide a modest enhancement or attenuation of ligand-receptor binding depending upon whether the surface ligands interact strongly with each other. Such effects, however, lead to little more than one order of magnitude change in the apparent K(D) values. On the other hand, the lipophilicity and presentation of lipid bilayer-conjugated ligands can have a much greater impact. Indeed, changing the way a particular ligand is conjugated to the membrane can alter the apparent K(D) value by at least three orders of magnitude. Such a result speaks strongly to the role of ligand availability for multivalent ligand-receptor binding.

Impact of hapten presentation on antibody binding at lipid membrane interfaces

We report the effects of ligand presentation on the binding of aqueous proteins to solid supported lipid bilayers. Specifically, we show that the equilibrium dissociation constant can be strongly affected by ligand lipophilicity and linker length/structure. The apparent equilibrium dissociation constants (K(D)) were compared for two model systems, biotin/anti-biotin and 2,4-dinitrophenyl (DNP)/anti-DNP, in bulk solution and at model membrane surfaces. The binding constants in solution were obtained from fluorescence anisotropy measurements. The surface binding constants were determined by microfluidic techniques in conjunction with total internal reflection fluorescence microscopy. The results showed that the bulk solution equilibrium dissociation constants for anti-biotin and anti-DNP were almost identical, K(D)(bulk) = 1.7 +/- 0.2 nM vs. 2.9 +/- 0.1 nM. By contrast, the dissociation constant for anti-biotin antibody was three orders of magnitude tighter than for anti-DNP at a lipid membrane interface, K(D) = 3.6 +/- 1.1 nM vs. 2.0 +/- 0.2 microM. We postulate that the pronounced difference in surface binding constants for these two similar antibodies is due to differences in the ligands' relative lipophilicity, i.e., the more hydrophobic DNP molecules had a stronger interaction with the lipid bilayers, rendering them less available to incoming anti-DNP antibodies compared with the biotin/anti-biotin system. However, when membrane-bound biotin ligands were well screened by a poly(ethylene glycol) (PEG) polymer brush, the K(D) value for the anti-biotin antibody could also be weakened by three orders of magnitude, 2.4 +/- 1.1 microM. On the other hand, the dissociation constant for anti-DNP antibodies at a lipid interface could be significantly enhanced when DNP haptens were tethered to the end of very long hydrophilic PEG lipopolymers (K(D) = 21 +/- 10 nM) rather than presented on short lipid-conjugated tethers. These results demonstrate that ligand presentation strongly influences protein interactions with membrane-bound ligands.

The influence of rigid or flexible linkage between two ligands on the effective affinity and avidity for reversible interactions with bivalent receptors

Bivalent or polyvalent cooperative binding between ligand and receptor is much tighter and more efficient than monovalent binding of the same counterparts. Because of this, many biological processes involve polyvalent binding for realization of regulatory mechanisms. For this reason it is necessary to develop a general formalism for prediction of the relationship between the binding affinities of each ligand subunits, the length of the flexible linker between them and avidity of the interaction. Here, we consider an approach that is based on the description of the state of equilibrium for the reaction of mono- and multivalent ligand-receptor binding of algebraic equation systems. This approach allows the evaluation of the avidity of bivalent binding and to determine the concentrations of ligand-receptor complexes, which will be obtained at the equilibrium state. The analysis presented here may be useful in analysing the binding behaviour of a bivalent receptor and a ligand consisting of two subunits covalently connected with a rigid or flexible linker.

Engineering and rapid selection of a low-affinity glucose/galactose-binding protein for a glucose biosensor

Periplasmic expression screening is a selection technique used to enrich high-affinity proteins in Escherichia coli. We report using this screening method to rapidly select a mutated D-glucose/D-galactose-binding protein (GGBP) having low affinity to glucose. Wild-type GGBP has an equilibrium dissociation constant of 0.2 microM and mediates the transport of glucose within the periplasm of E. coli. The protein undergoes a large conformational change on binding glucose and, when labeled with an environmentally sensitive fluorophore, GGBP can relay glucose concentrations, making it of potential interest as a biosensor for diabetics. This use necessitates altering the glucose affinity of GGBP, bringing it into the physiologically relevant range for monitoring glucose in humans (1.7-33 mM). To accomplish this a focused library was constructed using structure-based site-saturation mutagenesis to randomize amino acids in the binding pocket of GGBP at or near direct H-bonding sites and screening the library within the bacterial periplasm. After selection, equilibrium dissociation constants were confirmed by glucose titration and fluorescence monitoring of purified mutants labeled site-specifically at E149C with the fluorophore IANBD (N,N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)ethylene-diamine). The screening identified a single mutation A213R that lowers GGBP glucose affinity 5000-fold to 1 mM. Computational modeling suggested the large decrease in affinity was accomplished by the arginine side chain perturbing H-bonding and increasing the entropic barrier to the closed conformation. Overall, these experiments demonstrate the ability of structure-based site-saturation mutagenesis and periplasmic expression screening to discover low-affinity GGBP mutants having potential utility for measuring glucose in humans.

GM1 clustering inhibits cholera toxin binding in supported phospholipid membranes

The present studies explore multivalent ligand-receptor interactions between pentameric cholera toxin B subunits (CTB) and the corresponding membrane ligand, ganglioside GM1. CTB binding was monitored on supported phospholipid bilayers coated on the walls and floors of microfluidic channels. Measurements were made by total internal reflection fluorescence microscopy (TIRFM). Apparent dissociation constants were extracted by fitting the binding data to both the Hill-Waud and Langmuir adsorption isotherm equations. Studies of the effect of ligand density on multivalent CTB-GM1 interactions revealed that binding weakened with increasing GM1 density from 0.02 mol % to 10.0 mol %. Such a result could be explained by the clustering of GM1 on the supported phospholipid membranes, which in turn inhibited the binding of CTB. Atomic force microscopy (AFM) experiments directly verified GM1 clustering within the supported POPC bilayers.

Surface binding affinity measurements from order transitions of lipid membrane-coated colloidal particles

Lipid bilayers can be assembled onto the surface of colloidal silica particles to form a continuous and fluid supported membrane coating. In this configuration, the collective behavior of the colloidal dispersion is governed by interactions between particles and exhibits a sensitive dependency on chemical features of the membrane surface. Protein binding to membrane surface receptors can trigger macroscopic changes in the colloidal order, which provides a label-free readout of such binding events. Here, the relationship between order in the colloidal dispersion and the surface concentration of bound protein is characterized quantitatively in terms of the radial pair distribution function. Using parallel fluorescence measurements for comparison, we construct a scalar measure of the distribution function that exhibits linear proportionality with surface protein binding. This is used to determine binding affinity based only on observations of the colloidal distribution.

Controlling binding site densities on glass surfaces

The density of surface-immobilized ligands or binding sites is an important issue for the development of sensors, array- or chip-based assays, and single-molecule detection methods. The goal of this research is to control the binding site density of reactive ligands on surfaces by diluting surface amine groups in self-assembled and cross-linked monolayers on glass prepared from solutions containing very low concentrations of (3-aminopropyl)triethoxysilane (APTES) and much higher concentrations of (2-cyanoethyl)triethoxysilane. The surface amine sites are suitable for attaching labels and ligands by reaction with succinimidyl ester reagents. Labeling the amine sites with fluorescent molecules and imaging the single molecules with fluorescence microscopy provides a means of determining the density of amine sites on the surface, which were incorporated into the self-assembled monolayer with micrometer spacings in proportion to the concentration of APTES in the synthesis. Biotin ligands were also bound to these surface amine sites using a succinimidyl ester linker, and the immobilized biotin was then reacted with either streptavidin-conjugated gold colloid particles or fluorescently labeled neutravidin. Imaging of these samples yields consistent amine and biotin site coverages, indicating that quantitative control and chemical conversion of binding sites can be achieved at very low (<10(-7)) fractions of a monolayer.

Synthesis of thiol-reactive, long-wavelength fluorescent phenoxazine derivatives for biosensor applications

Two environmentally sensitive, long-wavelength fluorescent phenoxazine derivatives, INR and IANR, were synthesized with linkers for conjugation to the thiol group of cysteine in binding proteins. The linkers were designed based on the attachment sites at two different positions on the phenoxazine, which were chosen in order to study the orientation of the dye with respect to the binding protein. Conjugation of the dyes to the S337C maltose binding protein (MBP) mutant provided conjugates of these dyes that are capable of detecting maltose with different sensitivities. The dye INR gave a 3-fold (+200%) change in fluorescence intensity upon maltose binding when conjugated to S337C MBP with a binding constant (K(d)) of 435 microM. The fluorescence change for IANR was only 20% and the K(d) was 1.4 mM. Conformational analysis of the dyes by molecular modeling suggested that the linker in IANR imparted greater conformational freedom to the dye, resulting in little change in environment between the open and the closed-form conformations. The linker in INR, on the other hand, showed restricted motion, which placed the dye in different environments in the open and closed forms of the protein. Thus, design and placement of the linker play a critical role in the performance of these dyes as environmentally sensitive probes.

A long-wavelength fluorescent glucose biosensor based on bioconjugates of galactose/glucose binding protein and Nile Red derivatives

BACKGROUND

Fluorescent biosensors based on galactose/glucose binding protein (GGBP) and environmentally sensitive derivatives of the phenoxazine dye Nile Red are described. These biosensors are proposed as the sensing platform for a minimally invasive, continuous glucose monitoring system that can be implanted under the skin and read transdermally using an external fluorometer.

METHODS

To construct the biosensors, the thiol-reactive Nile Red derivatives INR and IANR were prepared and conjugated to GGBP proteins possessing cysteine mutations that were designed for optimal site-specific fluorophore attachment. The attachment sites were selected to maximize the local environment change for attached dyes between the bound and unbound conformations of GGBP.

RESULTS

Fluorescence responses at the selected cysteine sites of GGBP upon binding to glucose showed that the conjugates typically yielded fluorescence emission around 640-650 nm with up to 50% changes in fluorescence intensity. Conjugate E149C/A213C/L238S INR GGBP also displayed glucose binding in the human physiological range (K (D) = 7.4 mM).

CONCLUSIONS

The phenoxazine derivatives fluoresced at longer wavelengths (>600 nm) approaching the near-infrared spectral window, where interference from scattering and tissue absorbance are minimal. Ultimately, we expect that monitoring systems based on GGBP and longwavelength dyes will be implanted for up to 6 months and can be used to transmit information through the skin to an external monitor.

Dinitrophenyl ligand substrates and their application to immunosensors

We present an approach for the development of highly specific and sensitive antibody based biosensors by chemically tailoring the sensor surface with materials that control specific and nonspecific binding of biologically relevant molecules. As a model system we employed surface immobilized 2,4-dinitrophenyl (DNP)-ligands that bind specifically to anti-DNP antibodies. Self-assembling characteristics and minimization of the nonspecific interactions were used in the ligand design. The redox activity of the DNP-head group was used to calculate the surface density (coverage) of these assemblies using cyclic voltammetry. Quartz crystal microbalance (QCM) and impedance analysis were used to assess the ligand-antibody interaction and estimate the quantity of antibodies bound to the surface. The ligand surface density and the QCM data were useful in determining the sensitivity of our model system. A simple two-step kinetic model was shown to fit the experimental data.

Kinetic and Equilibrium Binding Analysis of Protein−Ligand Interactions at Poly(amidoamine) Dendrimer Monolayers

The interaction of streptavidin (SA) with a biotinylated surface has been of great interest in the development of an interfacial layer for protein immobilization based on self-assembled monolayers (SAMs) and polymeric layers. Here, we demonstrate the unique characteristics of protein-ligand interactions on dendrimer monolayers based on kinetic and equilibrium binding analyses. With amine-ended poly(amidoamine) dendrimers from the first (G1) to fourth (G4) generation, the formation of even, compact dendrimer monolayers on gold was confirmed using FT-IR spectroscopy and ellipsometry. For the SA-biotin interaction, quantitative analysis of bound SA using surface plasmon resonance showed that the saturation binding level of SA was fairly higher in all dendrimer layers when compared to other tested systems of 11-mercaptoundecylamine SAMs and a poly(L-lysine) layer. Kinetic studies revealed that the initial binding rate of SA up to the saturation level was 2-fold higher in all dendrimer layers than in the SAMs regardless of the surface density of functionalized biotin. Concurrently, the dendrimer layers led to much higher values of sticking probability, which is defined as the probability that the SA molecule adsorbs upon collision with a biotinylated surface, at a fixed SA coverage, and prolonged the significant levels around the maximum probability with increasing SA coverage. Plots of the saturation coverage of SA versus the SA concentration in solution showed that SA binding onto the biotinylated G1 and G3 layers fit to a Langmuir isotherm model. Taken together, faster binding of SA and highly ordered packing of the molecules seems to be achieved through typical properties of the dendrimer monolayers such as surface distribution of functionalized biotin, surface corrugation, and flexibility of highly branched larger dendrimers, which provides a guideline for the construction and analysis of an interfacial layer in biosensing applications.

Cholesterol modulated antibody binding in supported lipid membranes as determined by total internal reflectance microscopy on a microfabricated high-throughput glass chip

A high-throughput microfabricated all-glass microchip, lipid biochip, was created and used to measure fluorescently tagged antibody binding to dinitrophenol (DNP) haptens in planar supported phospholipid/cholesterol lipid bilayers as a function of cholesterol-to-lipid molar ratio (X(CHOL)). Multiple parallel microchannels etched in the lipid biochip allowed simultaneous measurement of antibody binding to hapten-containing and hapten-free lipid bilayers, for a range of aqueous antibody concentrations. Specific and nonspecific antibody binding to the supported lipid bilayers was determined from the internally calibrated intensity of the surface fluorescence using total internal reflectance fluorescence (TIRF) microscopy. The TIRF intensity data of the specific antibody binding were fitted to the Langmuir isotherm and Hill equation models to determine the apparent dissociation constant K(d), the maximum fluorescence parameter F(infinity), and binding cooperativity n. As X(CHOL) increased from 0 to 0.50, K(d) exhibited a minimum of approximately 4 microM and n reached a maximum of approximately 2.2 at X(CHOL) approximately 0.20. However, F(infinity) appeared to be insensitive to the cholesterol content. The nonspecific binding fraction (NS), defined as the ratio of the TIRF intensity for hapten-free bilayers to that with hapten, showed a minimum of approximately 0.08 also at X(CHOL) approximately 0.20. The results suggest that cholesterol regulates the specific binding affinity and cooperativity, as well as suppresses nonspecific binding of aqueous antibody to a planar supported lipid bilayer surface at an optimal cholesterol content of X(CHOL) approximately 0.20. Interestingly, for X(CHOL) approximately 0.40, NS reached a maximum of approximately 0.57, suggesting significant packing defects in the lipid bilayer surface, possibly as a result of lipid domain formation as predicted by the lipid superlattice model. We conclude that cholesterol plays a significant role in regulating both specific and nonspecific antibody/antigen binding events on the lipid bilayer surface and that our lipid biochip represents a new and useful high-resolution microfluidic device for measuring lipid/protein surface binding activities in a parallel and high-throughput fashion.

Mass spectrometric analysis of affinity-captured proteins on a dendrimer-based immunosensing surface: investigation of on-chip proteolytic digestion

The monolayer of fourth-generation poly(amidoamine) dendrimers was adopted to construct the immunoaffinity surface of an antibody layer. The antibody layer as a bait on the dendrimer monolayer was found to result in high binding capacity of antigenic proteins and a reliable detection. The affinity-captured protein at the immunosensing surface was subjected to direct on-chip tryptic digestion, and the resulting proteolytic peptides were analyzed by using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The performance of the on-chip digestion procedure was investigated with respect to the ratio of trypsin to protein, digestion time, composition of a reaction buffer, and the amount of affinity-captured protein on a surface. Addition of a water-miscible organic solvent to a reaction buffer had no significant effect on the digestion efficiency under the optimized digestion conditions. The on-chip digestion method identified the affinity-captured bovine serum albumin (BSA), lysozyme, and ferritin at the level of around 100 fmol. Interestingly, the detected number of peptide hits through the on-chip digestion was almost similar regardless of the amount of captured protein ranging from low- to high-femtomole levels, whereas the efficiency of in-solution digestion decreased significantly as the amount of protein decreased to low-femtomole levels. The structural alignment of the peptide fragments from on-chip-digested BSA revealed that the limited exterior of the captured protein is subjected to attack by trypsin. The established detection procedures enabled the identification of BSA in the biological mixtures at the level of 0.1 ng/mL. The use of antibodies against the proteins involved in the metabolic pathway of L-threonine in Escherichia coli also led to discrimination of the respective target proteins from cell lysates.

Investigations of bivalent antibody binding on fluid-supported phospholipid membranes: the effect of hapten density

Investigations of ligand-receptor binding between bivalent antibodies and membrane-bound ligands are presented. The purpose of these studies was to explore binding as a function of hapten density in a two-dimensionally fluid environment. A novel microfluidic strategy in conjunction with total internal reflection fluorescence microscopy was designed to achieve this. The method allowed binding curves to be acquired with excellent signal-to-noise ratios while using only minute quantities of protein solution. The specific system investigated was the interaction between anti-DNP antibodies and phospholipid membranes containing DNP-conjugated lipids. Binding curves for ligand densities ranging from 0.1 to 5.0 mol % were obtained. Two individual dissociation constants could be extracted from the data corresponding to the two sequential binding events. The first dissociation constant, K(D1), was 2.46 x 10(-)(5) M, while the second was K(D2) = 1.37 x 10(-)(8) mol/m(2). This corresponded to a positively cooperative binding effect with an entropic difference between the two events of 62.3 +/- 2.7 J/(mol.K). Furthermore, the percentage of monovalently and bivalently bound protein was determined at each ligand density.

Using bifunctional polymers presenting vancomycin and fluorescein groups to direct anti-fluorescein antibodies to self-assembled monolayers presenting d-alanine-d-alanine groups

This paper describes the synthesis of bifunctional polyacrylamides containing pendant vancomycin (Van) and fluorescein groups, and the use of these polymers to direct antibodies against fluorescein to self-assembled monolayers (SAMs) presenting d-alanine-d-alanine (dAdA) groups. These polymers bind biospecifically to these SAMs via interactions between the dAdA and Van groups and serve as a molecular bridge between the anti-fluorescein antibodies and the SAM. The binding events were characterized using surface plasmon resonance spectroscopy and fluorescence microscopy. The paper demonstrates that polyvalent, biospecific, noncovalent interactions between a polymer and a surface can be used to tailor the properties of the surface in molecular recognition. It also represents a first step toward the design of polymers that direct arbitrarily chosen antibodies to the surfaces of cells.

Quantitative measurements of recombinant HIV surface glycoprotein 120 binding to several glycosphingolipids expressed in planar supported lipid bilayers

The interaction of recombinant HIV-1 surface glycoprotein gp120 (rgp120) with natural isolates of lactosylceramide (LacCer), glucosylceramide (GlcCer), and galactosylceramide (GalCer) has been quantitatively measured under equilibrium conditions using total internal reflection fluorescence (TIRF) spectroscopy. The binding affinity (K(a)) of rgp120 to these glycosphingolipids (GSLs), reconstituted at 5 mol % in supported planar lipid bilayers composed of 95 mol % POPC, is ca. 10(6) M(-1) for dissolved rgp120 concentrations greater than 25 nM. In contrast, at concentrations of rgp120 between 0.2 and 15 nM, rgp120 does not bind significantly to LacCer and GlcCer, but has a high affinity for GalCer with a measured K(a) value of 1.6 x 10(9) M(-1). However, protein surface coverage measurements show that this strong binding process accounts for very little of the total protein adsorbed over the entire concentration range studied. At a protein concentration of ca. 20 nM, the surface coverage is only 3% of that achieved at apparent saturation (i.e., when the protein concentration is ca. 220 nM). Thus the "high affinity" binding sites comprise only a small fraction of the total number of binding sites. Several other variables were investigated. Rgp120 binding behavior at membranes doped with alpha-hydroxygalactosylceramide (alpha-GalCer) was very similar to that observed with GalCer, showing that the presence/absence of an alpha-hydroxy moiety does not significantly affect galactosylceramide recognition. Phase segregation of GalCer, which occurs when the mole fraction of this GSL in a POPC bilayer exceeds ca. 0.1, was also investigated and showed no effect on binding affinity at low rgp120 concentrations. To investigate the influence of fatty acid chain length, GSLs with monodisperse C(18) and C(24) chain lengths, both with and without an alpha-hydroxy moiety, were synthesized, and their binding affinity to rgp120 was examined. Relative to the natural isolates (which contain a mixture of chain lengths), minimal differences were observed; thus among the compounds tested, fatty acid chain length does not affect GSL recognition. The results of this work should aid efforts to design anti-HIV-1 agents based on membrane-tethered, carbohydrate-based receptors for rgp120.

Total internal reflection with fluorescence correlation spectroscopy: combined surface reaction and solution diffusion

Total internal reflection with fluorescence correlation spectroscopy (TIR-FCS) is a method for measuring the surface association/dissociation rates and absolute densities of fluorescent molecules at the interface of solution and a planar substrate. This method can also report the apparent diffusion coefficient and absolute concentration of fluorescent molecules very close to the surface. An expression for the fluorescence fluctuation autocorrelation function in the absence of contributions from diffusion through the evanescent wave, in solution, has been published previously (N. L. Thompson, T. P. Burghardt, and D. Axelrod. 1981, Biophys. J. 33:435-454). This work describes the nature of the TIR-FCS autocorrelation function when both surface association/dissociation kinetics and diffusion through the evanescent wave contribute to the fluorescence fluctuations. The fluorescence fluctuation autocorrelation function depends in general on the kinetic association and dissociation rate constants, the surface site density, the concentration of fluorescent molecules in solution, the solution diffusion coefficient, and the depth of the evanescent field. Both general and approximate expressions are presented.

Lipid rafts reconstituted in model membranes

One key tenet of the raft hypothesis is that the formation of glycosphingolipid- and cholesterol-rich lipid domains can be driven solely by characteristic lipid-lipid interactions, suggesting that rafts ought to form in model membranes composed of appropriate lipids. In fact, domains with raft-like properties were found to coexist with fluid lipid regions in both planar supported lipid layers and in giant unilamellar vesicles (GUVs) formed from 1) equimolar mixtures of phospholipid-cholesterol-sphingomyelin or 2) natural lipids extracted from brush border membranes that are rich in sphingomyelin and cholesterol. Employing headgroup-labeled fluorescent phospholipid analogs in planar supported lipid layers, domains typically several microns in diameter were observed by fluorescence microscopy at room temperature (24 degrees C) whereas non-raft mixtures (PC-cholesterol) appeared homogeneous. Both raft and non-raft domains were fluid-like, although diffusion was slower in raft domains, and the probe could exchange between the two phases. Consistent with the raft hypothesis, GM1, a glycosphingolipid (GSL), was highly enriched in the more ordered domains and resistant to detergent extraction, which disrupted the GSL-depleted phase. To exclude the possibility that the domain structure was an artifact caused by the lipid layer support, GUVs were formed from the synthetic and natural lipid mixtures, in which the probe, LAURDAN, was incorporated. The emission spectrum of LAURDAN was examined by two-photon fluorescence microscopy, which allowed identification of regions with high or low order of lipid acyl chain alignment. In GUVs formed from the raft lipid mixture or from brush border membrane lipids an array of more ordered and less ordered domains that were in register in both monolayers could reversibly be formed and disrupted upon cooling and heating. Overall, the notion that in biomembranes selected lipids could laterally aggregate to form more ordered, detergent-resistant lipid rafts into which glycosphingolipids partition is strongly supported by this study.

Fabrication of phospholipid bilayer-coated microchannels for on-chip immunoassays

Herein we describe a new class of microfluidic immunoassays based upon solid supported lipid bilayers. Two-dimensionally fluid bilayer material, which can accommodate multivalent binding between surface-bound ligands and aqueous receptors, was coated on the surface of poly(dimethylsiloxane) microchannels. The bilayers contained dinitrophenyl (DNP)-conjugated lipids for binding with bivalent anti-DNP antibodies. Twelve independent data points of surface coverage versus bulk protein concentration could be made simultaneously by forming a linear array of channels and flowing fluorescently labeled antibodies into them. This enabled an entire binding curve to be obtained in a single experiment. The measured apparent binding constant for the DNP/anti-DNP system was 1.8 microM. The methodology for performing heterogeneous assays developed here not only produces rapid results but also requires much less protein than traditional procedures and eliminates some standard sources of experimental error.

A surface plasmon resonance method for detecting multiple modes of DNA-ligand interactions

A simple and general surface plasmon resonance (SPR) based method has been developed to detect and quantitate binding of low molecular weight compounds (200-1,200 Da) to double stranded DNA. Several compounds were chosen to probe three different modes of binding interactions, intercalation, minor groove binding and electrostatic interactions. Ethidium bromide (MW 390 Da), a probe of intercalative binding, was tested by plotting the steady state SPR responses measured on a DNA modified surface versus ethidium bromide concentration. The best fit of the binding isotherm gave a Keq of 1.8 x 105 M-1. Co-solvents such as DMSO are often used in activity assays to increase the solubility of poorly water-soluble drugs. The effect of DMSO on the ethidium bromide/DNA interaction was also tested by measuring binding in the presence of 0, 1 and 5%, DMSO. No effect on the measured Keq was observed at these DMSO concentrations. The binding of actinomycin (MW 1,255 Da), an antibiotic known to bind DNA through intercalation and minor groove binding, was also tested. The Keq estimated from the steady state responses on a DNA surface was 1.9 x 106 M-1. DAPI (MW 350 Da) (4',6-diamidino-2-phenylindole) a fluorescent probe which binds the minor groove of DNA was also tested and gave a Keq of 1.8 x 106 M-1 measured by SPR. Finally, spermine (MW 202) a compound known to bind DNA through ionic interactions gave the weakest Keq of 1.7 x 104 M-1. All the Keq values measured by SPR and reported for these compounds were in good agreement with literature values measured by other techniques.

Rebinding of IgE Fabs at haptenated planar membranes: measurement by total internal reflection with fluorescence photobleaching recovery

In previous work, a general analytical theory for ligand rebinding at cell surfaces was developed for a reversible bimolecular reaction between ligands in solution and receptors on a membrane surface [Lagerholm, B. C., and Thompson, N. L. (1998) Biophys. J. 74, 1215-1228]. This theory can be used to predict theoretical forms for data obtained by using total internal reflection with fluorescence photobleaching recovery (TIR-FPR) [Thompson, N. L., Burghardt, T. P., and Axelrod, D. (1981) Biophys. J. 33, 435-454]. Thus, one method by which the rebinding theory can be tested is to use TIR-FPR. In the work described herein, the reversible kinetics of mouse monoclonal anti-dinitrophenyl (DNP) IgE Fabs at substrate-supported planar membranes composed of 25 mol % DNP-conjugated phosphatidylethanolamine and 75 mol % dipalmitoylphosphatidylcholine have been examined by using TIR-FPR. Data were obtained as a function of the Fab solution concentration. Higher Fab concentrations reduce rebinding (and increase the fluorescence recovery rate) because different Fab molecules compete for the same surface-binding sites. Data were also obtained for solutions containing different volume fractions of glycerol. In these measurements, higher glycerol concentrations increase rebinding (and decrease the fluorescence recovery rate) because the solution viscosity is increased and the Fab diffusion coefficient in solution is decreased. The TIR-FPR data were quantitatively compared with theoretical predictions which follow from the general theory for rebinding at the membrane surface. The data were consistent with the theoretical predictions and, therefore, provide experimental verification of the previously developed theory.

Molecular dynamics simulations of supported phospholipid/alkanethiol bilayers on a gold(111) surface

Molecular dynamics simulations have been used to investigate the structure of hybrid bilayers (HB) formed by dipalmitoylphosphatidylcholine (DPPC) lipid monolayers adsorbed on a hydrophobic alkanethiol self-assembled monolayer (SAM). The HB system was studied at 20 degrees C and 60 degrees C, and the results were compared with recent neutron reflectivity measurements (Meuse, C. W., S. Krueger, C. F. Majkrzak, J. A. Dura, J. Fu, J. T. Connor, and A. L. Plant. 1998. Biophys. J. 74:1388) and previous simulations of hydrated multilamellar bilayers (MLB) of DPPC (Tu, K., D. J. Tobias, and M. L. Klein. 1995. Biophys. J. 69:2558; and 1996. 70:595). The overall structures of the HBs are in very good agreement with experiment. The structure of the SAM monolayer is hardly perturbed by the presence of the DPPC overlayer. The DPPC layer presents characteristics very similar to the MLB gel phase at low temperature and to the liquid crystal phase at high temperature. Subtle changes have been found for the lipid/water interface of the HBs compared to the MLBs. The average phosphatidylcholine headgroup orientation is less disordered, and this produces changes in the electric properties of the HB lipid/water interface. These changes are attributed to the fact that the aqueous environment of the lipids in these unilamellar films is different from that of MLB stacks. Finally, examination of the intramolecular and whole-molecule dynamics of the DPPC molecules in the fluid phase HB and MLB membranes revealed that the reorientations of the upper part of the acyl chains (near the acyl ester linkage) are slower, the single molecule protrusions are slightly damped, and the lateral rattling motions are significantly reduced in the HB compared with the MLB.

Model and simulation of multivalent binding to fixed ligands

A model to quantitate the principal aspects of multivalent binding was developed. It describes the random distribution of an immobilized component (the ligand) taking into account local densities. The binding of a bivalent molecule (the analyte) to the ligand is described as occurring in two steps, the second of which is driven by the local concentration of neighboring ligands. The model was used to simulate the kinetics of bivalent binding in surface plasmon resonance biosensors such as BIAcore. The simulations are compared with measured data. The simulation quantitates the influence of bivalent binding on the sensor signal, as a function of ligand density, analyte concentration, and binding site distance. Such simulations will be helpful for understanding and designing experiments to assess avidity effects as well as for developing molecules with high avidity. Furthermore, they help to analyze the inherent complexity in seemingly simple sensorgrams.

Incorporation and Antibody Recognition of a Lipid-Anchored Membrane Protein in Supported Lipid Layers

The formation of supported lipid layers incorporating promastigote surface protease (PSP), a glycosylphosphatidylinositol-anchored protein, is investigated using surface plasmon resonance. Both hydrophilic and hydrophobic substrates are used for the formation of lipid layers, and results are consistent with the formation of lipid bilayers and monolayers, respectively. Specific antibody binding to layers containing PSP is observed, whereas nonspecific binding of the antibody to the surface is effectively suppressed by the phosphatidylcholine lipid layer. Phosphatidylinositol-specific phospholipase C is used to remove the lipid moieties from the membrane-incorporated PSP, releasing it into solution in a hydrophilic form and demonstrating that a large fraction of the protein is anchored in the lipid layer via the lipid moieties. Copyright 1997 Academic Press. Copyright 1997Academic Press

Binding of the soluble, truncated form of an Fc receptor (mouse Fc gamma RII) to membrane-bound IgG as measured by total internal reflection fluorescence microscopy

Total internal reflection fluorescence microscopy has been used to investigate the binding of the soluble extracellular domain of mouse Fc gamma RII (sFc gamma RII) to an anti-trinitrophenyl monoclonal mouse IgG2b (GK14.1) specifically bound to substrate-supported planar membranes composed of dipalmitoylphosphatidylcholine (DPPC) and trinitrophenylaminocaproyldipalmitoylphosphatidylethanolamine (TNP-cap-DPPE). The equilibrium dissociation constants for sFc gamma RII at GK14.1-coated TNP-cap-DPPE/DPPC planar membranes containing 0.5-25 mol% TNP-cap-DPPE were approximately 1 microM. Total internal reflection with fluorescence photobleaching recovery was used to examine the dissociation kinetics. The fluorescence recovery curves were better described as a sum of two exponentials rather than by one exponential; the rates and fractional recoveries were approximately 1 s-1 (65%) and approximately 0.1 s-1 (35%). The similarity between the values of these equilibrium and kinetic parameters to those previously measured for the binding of IgG in solution to intact mouse Fc gamma RII reconstituted into planar membranes suggests that conformational changes which may occur when IgG is constrained to a membrane surface do not significantly affect the equilibrium or kinetics of IgG-mouse Fc gamma RII binding. The stoichiometry of sFc gamma RII-GK14.1 binding was 1:4, indicating that a significant fraction of the membrane-bound antibodies were not accessible for receptor binding. Possible mechanisms that might underlay the observed heterogeneity in sFc gamma RII-IgG binding kinetics are discussed.

Synthetic peptides from mouse Fc receptor (MoFc gamma RII) that alter the binding of IgG to MoFc gamma RII

Fc receptors are transmembrane proteins, found on the surfaces of immune cells, that aid in the removal of foreign pathogens by binding to antibody-coated targets via the Fc regions of the antibodies. Using peptides synthesized on pins, overlapping dodecapeptides (170) were synthesized to cover the extracellular region of the mouse Fc receptor for IgG, moFc gamma RII. The peptides were screened for antibody binding activity by using multivalent immune complexes composed of anti-dinitrophenyl monoclonal mouse IgG1 (ANO6) and dinitrophenyl conjugated to human serum albumin (DNP-HSA). Assays were also carried out with an anti-moFc gamma RII monoclonal rat IgG (2.4G2). The peptides that interacted with these antibodies prompted the synthesis of two soluble peptides: peptide A [Fc gamma RII-(108-119), RCHSWRNKLLNRamide] and peptide B [Fc gamma RII-(153-165), CKGSLGRTLHQSKamide]. Monomeric S-alkylated (A, B), homodimeric (AA, BB), heterodimeric (AB), and scrambled homodimeric (CC, DD) forms of these peptides were synthesized and examined for their ability to inhibit immune-complex binding to immobilized soluble Fc gamma RII. Peptides AA and CC completely inhibited immune-complex binding while each of the other peptides partially inhibited binding (AB, 80%; A, 80%; BB, 65%; DD, 64%; B, 52%). The pair of monomeric moFc gamma RII peptides and the set of five dimeric peptides showed the same increase in binding inhibition with increasing net positive charge per residue. These results suggest that the Fc region of IgG binds to the solvent-exposed B/C and F/G loops of the moFc gamma RII receptor through predominantly electrostatic forces.

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.

Surface engineering: optimization of antigen presentation in self-assembled monolayers

The formation of self-assembled monolayers (SAMs) on gold surfaces containing an antigenic peptide (NANP)6 and HS(CH2)11OH, and the specific binding of a monoclonal antibody to these layers were investigated by surface plasmon resonance (SPR). Peptides were synthesized by solid-state phase synthesis and were linked either to cysteine or to an alkyl-thiol to allow covalent attachment to gold. The content of the peptide in the SAMs was systematically varied, and the binding properties of the monoclonal antibody were compared with those measured by microcalorimetry in solution. At a critical peptide concentration in the SAM an optimal antibody binding and complete surface coverage was attained. At lower peptide concentrations, the amount of adsorbed antibody decreased; at higher peptide concentrations, the binding constant decreased. These effects can be explained if the accessibility of the antigenic epitopes depends on the peptide density. Addition of free antigen induced the desorption of bound antibodies and allowed accurate measurements of the dissociation rate constant. Binding constants obtained from steady-state measurements and from measurements of the kinetic rate constants were compared.

Imaging fluorescence correlation spectroscopy: nonuniform IgE distributions on planar membranes

Fluorescence correlation spectroscopy is useful for detecting and characterizing molecular clusters that are smaller than or approximately equal to optical resolution in size. Here, we report the development of an approach in which the pixel-to-pixel fluorescence fluctuations from a single fluorescence image are spatially autocorrelated. In these measurements, tetramethylrhodamine-labeled, anti-trinitrophenyl IgE antibodies were specifically bound to substrate-supported planar membranes composed of trinitrophenyl-aminocaproyldipalmitoylphosphatidylethanolamine and dipalmitoylphosphatidylcholine. The antibody-coated membranes were illuminated with the evanescent field from a totally internally reflected laser beam, and the fluorescence arising from the IgE-coated membranes was recorded with a cooled CCD camera. The image was corrected for the elliptical Gaussian shape of the evanescent illumination after background subtraction. The spatial autocorrelation functions of the resulting images generated two useful parameters: the extrapolated initial values, which were related to the average cluster intensity and density; and the correlation distances, which were related to the average cluster size. These parameters varied with the IgE density, and unlabeled polyclonal anti-IgE enhanced the nonuniform IgE distributions. The autocorrelation functions calculated from images of planar membranes containing fluorescently labeled lipids rather than bound, labeled IgE demonstrated that the spatial nonuniformities were prominent only in the presence of IgE. Fluorescent beads were used to demonstrate the principles and the methods.

Theory for measuring bivalent surface binding kinetics using total internal reflection with fluorescence photobleaching recovery

Total internal reflection with fluorescence photobleaching recovery (TIR-FPR) is a method for experimentally examining coupled diffusion and reaction kinetics at surfaces. In a previous work (Thompson et al. 1981. Biophys. J. 33:435-454), a theoretical basis for interpreting TIR-FPR data was described for monovalent ligands that undergo a reversible reaction with monovalent surface sites in a single step. Here, the theory for TIR-FPR has been extended to two different surface binding mechanisms that involve sequential, bivalent surface attachment. Methods for obtaining the intrinsic surface association and dissociation kinetic rates from measured fluorescence photobleaching recovery curves are described. The new theory should be applicable to the association of bivalent protein ligands such as antibodies with supported planar model membranes.