Applications of Fluorescence Correlation Spectroscopy: Polydispersity Measurements

Fluorescence cross-correlation spectroscopy as a valuable tool to characterize cationic liposome-DNA nanoparticle assembly

The development of nonviral gene delivery vehicles for therapeutic applications requires methods capable of quantifying the association between the genes and their carrier counterparts. Here we investigate the potential of fluorescence cross-correlation spectroscopy (FCCS) to characterize and optimize the assembly of nonviral cationic liposome (CL)-DNA complexes based on a CL formulation consisting of the cationic lipid DOTAP and zwitterionic lipid DOPC. We use a DNA plasmid for lipoplex loading encoding the Oct4 gene, critically involved in reprogramming somatic cells into induced pluripotent stem cells. We demonstrate that FCCS is able to quantitatively determine the extent of the association between DNA and the liposomes and assess its loading capacity. We also establish that the cationic lipid fraction, being proportional to the liposome membrane charge density, as well as charge ratio between the CLs and anionic DNA play an important role in the degree of interaction between the liposomes and DNA.

Raman Spectroscopy characterization extracellular vesicles from bovine placenta and peripheral blood mononuclear cells

Placenta-derived extracellular vesicles (EVs) are involved in communication between the placenta and maternal immune cells possibly leading to a modulation of maternal T-cell signaling components. The ability to identify EVs in maternal blood may lead to the development of diagnostic and treatment tools for pregnancy complications. The objective of this work was to differentiate EVs from bovine placenta (trophoblast) and peripheral blood mononuclear cells (PBMC) by a label-free, non-invasive Raman spectroscopy technique. Extracellular vesicles were isolated by ultracentrifugation. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) were applied to verify the presence and the size distribution of EVs. Raman peaks at 728 cm^-1 (collagen) and 1573 cm^-1 (protein) were observed only in PBMC-derived EVs, while the peaks 702 cm^-1 (cholesterol) and 1553 cm^-1 (amide) appeared only in trophoblast-derived EVs. The discrimination of the Raman spectral fingerprints for both types of EVs from different animals was performed by principal component analysis (PCA) and linear discriminant analysis (LDA). The PCA and LDA results clearly segregated the spectral clusters between the two types of EVs. Moreover, the PBMC-derived EVs from different animals were indistinguishable, while the trophoblast-derived EVs from three placental samples of different gestational ages showed separate clusters. This study reports for the first time the Raman characteristic peaks for identification of PBMC and trophoblast-derived EVs. The development of this method also provides a potential tool for further studies investigating the causes and potential treatments for pregnancy complications.

Progress in Microfluidics-Based Exosome Separation and Detection Technologies for Diagnostic Applications

Exosomes are secreted by most cell types and circulate in body fluids. Recent studies have revealed that exosomes play a significant role in intercellular communication and are closely associated with the pathogenesis of disease. Therefore, exosomes are considered promising biomarkers for disease diagnosis. However, exosomes are always mixed with other components of body fluids. Consequently, separation methods for exosomes that allow high-purity and high-throughput separation with a high recovery rate and detection techniques for exosomes that are rapid, highly sensitive, highly specific, and have a low detection limit are indispensable for diagnostic applications. For decades, many exosome separation and detection techniques have been developed to achieve the aforementioned goals. However, in most cases, these two techniques are performed separately, which increases operation complexity, time consumption, and cost. The emergence of microfluidics offers a promising way to integrate exosome separation and detection functions into a single chip. Herein, an overview of conventional and microfluidics-based techniques for exosome separation and detection is presented. Moreover, the advantages and drawbacks of these techniques are compared.

Navigating the Landscape of Tumor Extracellular Vesicle Heterogeneity

The last decade has seen a rapid expansion of interest in extracellular vesicles (EVs) released by cells and proposed to mediate intercellular communication in physiological and pathological conditions. Considering that the genetic content of EVs reflects that of their respective parent cell, many researchers have proposed EVs as a source of biomarkers in various diseases. So far, the question of heterogeneity in given EV samples is rarely addressed at the experimental level. Because of their relatively small size, EVs are difficult to reliably isolate and detect within a given sample. Consequently, standardized protocols that have been optimized for accurate characterization of EVs are lacking despite recent advancements in the field. Continuous improvements in pre-analytical parameters permit more efficient assessment of EVs, however, methods to more objectively distinguish EVs from background, and to interpret multiple single-EV parameters are lacking. Here, we review EV heterogeneity according to their origin, mode of release, membrane composition, organelle and biochemical content, and other factors. In doing so, we also provide an overview of currently available and potentially applicable methods for single EV analysis. Finally, we examine the latest findings from experiments that have analyzed the issue at the single EV level and discuss potential implications.

The Methods of Choice for Extracellular Vesicles (EVs) Characterization

In recent years, extracellular vesicles (EVs) have become a subject of intense study. These membrane-enclosed spherical structures are secreted by almost every cell type and are engaged in the transport of cellular content (cargo) from parental to target cells. The impact of EVs transfer has been observed in many vital cellular processes including cell-to-cell communication and immune response modulation; thus, a fast and precise characterization of EVs may be relevant for both scientific and diagnostic purposes. In this review, the most popular analytical techniques used in EVs studies are presented with the emphasis on exosomes and microvesicles characterization.

Aggregation kinetics and colloidal stability of functionalized nanoparticles

The functionalization of nanoparticles has primarily been used as a means to impart stability in nanoparticle suspensions. In most cases even the most advanced nanomaterials lose their function should suspensions aggregate and settle, but with the capping agents designed for specific solution chemistries, functionalized nanomaterials generally remain monodisperse in order to maintain their function. The importance of this cannot be underestimated in light of the growing use of functionalized nanomaterials for wide range of applications. Advanced functionalization schemes seek to exert fine control over suspension stability with small adjustments to a single, controllable variable. This review is specific to functionalized nanoparticles and highlights the synthesis and attachment of novel functionalization schemes whose design is meant to affect controllable aggregation. Some examples of these materials include stimulus responsive polymers for functionalization which rely on a bulk solution physicochemical threshold (temperature or pH) to transition from a stable (monodisperse) to aggregated state. Also discussed herein are the primary methods for measuring the kinetics of particle aggregation and theoretical descriptions of conventional and novel models which have demonstrated the most promise for the appropriate reduction of experimental data. Also highlighted are the additional factors that control nanoparticle stability such as the core composition, surface chemistry and solution condition. For completeness, a case study of gold nanoparticles functionalized using homologous block copolymers is discussed to demonstrate fine control over the aggregation state of this type of material.

High-throughput and label-free single nanoparticle sizing based on time-resolved on-chip microscopy

Sizing individual nanoparticles and dispersions of nanoparticles provides invaluable information in applications such as nanomaterial synthesis, air and water quality monitoring, virology, and medical diagnostics. Several conventional nanoparticle sizing approaches exist; however, there remains a lack of high-throughput approaches that are suitable for low-resource and field settings, i.e., methods that are cost-effective, portable, and can measure widely varying particle sizes and concentrations. Here we fill this gap using an unconventional approach that combines holographic on-chip microscopy with vapor-condensed nanolens self-assembly inside a cost-effective hand-held device. By using this approach and capturing time-resolved in situ images of the particles, we optimize the nanolens formation process, resulting in significant signal enhancement for the label-free detection and sizing of individual deeply subwavelength particles (smaller than λ/10) over a 30 mm(2) sample field-of-view, with an accuracy of ±11 nm. These time-resolved measurements are significantly more reliable than a single measurement at a given time, which was previously used only for nanoparticle detection without sizing. We experimentally demonstrate the sizing of individual nanoparticles as well as viruses, monodisperse samples, and complex polydisperse mixtures, where the sample concentrations can span ∼5 orders-of-magnitude and particle sizes can range from 40 nm to millimeter-scale. We believe that this high-throughput and label-free nanoparticle sizing platform, together with its cost-effective and hand-held interface, will make highly advanced nanoscopic measurements readily accessible to researchers in developing countries and even to citizen-scientists, and might especially be valuable for environmental and biomedical applications as well as for higher education and training programs.

Fluorescence Correlation Spectroscopy in Dilute Polymer Solutions: Effects of Molar Mass Dispersity and the Type of Fluorescent Labeling

Fluorescence correlation spectroscopy (FCS) has become an important tool in polymer science. Among various other applications the method is often applied to measure the hydrodynamic radius and the degree of fluorescent labeling of polymers in dilute solutions. Here we show that such measurements can be strongly affected by the molar mass dispersity of the studied polymers and the way of labeling. As model systems we used polystyrene and poly(methyl methacrylate) synthesized by atom transfer radical polymerization or free-radical polymerization. Thus, the polymers were either end-labeled bearing one fluorophore per chain or side-labeled with a number of fluorophores per chain proportional to the degree of polymerization.The experimentally measured autocorrelation curves were fitted with a newly derived theoretical model that uses the Schulz-Zimm distribution function to describe the dispersity in the degree of polymerization. For end-labeled polymers having a molecular weight distribution close to Schulz-Zimm, the fits yield values of the number-average degree of polymerization and the polydispersity index similar to those obtained by reference gel permeation chromatography. However, for the side-labeled polymers such fitting becomes unstable, especially for highly polydisperse systems. Brownian dynamic simulations showed that the effect is due to a mutual dependence between the fit parameters, namely, the polydispersity index and the number-average molecular weight. As a consequence, an increase of the polydispersity index can be easily misinterpreted as an increase of the molecular weight when the FCS autocorrelation curves are fitted with a standard single component model, as commonly done in the community.

3D plasmonic nanobowl platform for the study of exosomes in solution

Thin silver film coated nanobowl Surface Enhanced Raman Spectroscopy (SERS) substrates are used to capture exosomes in solution for SERS measurements that can provide biochemical analysis of intact and ruptured exosomes. Exosomes derived via Total Exosome Isolation Reagent (TEIR) as well as ultracentrifugation (UC) from the SKOV3 cell line were analyzed. Spectra of exosomes derived via TEIR are dominated by a signal characteristic for the TEIR kit that needs to be subtracted for all measurements. Differences in SERS spectra recorded at different times during the drying of the exosome solution are statistically analyzed with Principal Component Analysis (PCA). At the beginning of the drying process, SERS spectra of exosomes exhibit peaks characteristic for both lipids and proteins. Later on during the drying process, new SERS peaks develop, suggesting that the initially intact exosome ruptures over time. This time-dependent evolution of SERS peaks enables analysis of exosomal membrane contents and the contents inside the exosomes.

Fluorescence correlation spectroscopy directly monitors coalescence during nanoparticle preparation

Dual color fluorescence cross-correlation spectroscopy (DC FCCS) experiments were conducted to study the coalescence and aggregation during the formation of nanoparticles. To assess the generality of the method, three completely different processes were selected to prepare the nanoparticles. Polymeric nanoparticles were formed either by solvent evaporation from emulsion nanodroplets of polymer solutions or by miniemulsion polymerization. Inorganic nanocapsules were formed by polycondensation of alkoxysilanes at the interface of nanodroplets. In all cases, DC FCCS provided fast and unambiguous information about the occurrence of coalescence and thus a deeper insight into the mechanism of nanoparticle formation. In particular, it was found that coalescence played a minor role for the emulsion-solvent evaporation process and the miniemulsion polymerization, whereas substantial coalescence was detected during the formation of the inorganic nanocapsules. These findings demonstrate that DC FCCS is a powerful tool for monitoring nanoparticles genesis.

Determination of membrane protein transporter oligomerization in native tissue using spatial fluorescence intensity fluctuation analysis

Membrane transporter proteins exist in a complex dynamic equilibrium between various oligomeric states that include monomers, dimers, dimer of dimers and higher order oligomers. Given their sub-optical microscopic resolution size, the oligomerization state of membrane transporters is difficult to quantify without requiring tissue disruption and indirect biochemical methods. Here we present the application of a fluorescence measurement technique which combines fluorescence image moment analysis and spatial intensity distribution analysis (SpIDA) to determine the oligomerization state of membrane proteins in situ. As a model system we analyzed the oligomeric state(s) of the electrogenic sodium bicarbonate cotransporter NBCe1-A in cultured cells and in rat kidney. The approaches that we describe offer for the first time the ability to investigate the oligomeric state of membrane transporter proteins in their native state.

Fluorescence correlation spectroscopy: an efficient tool for measuring size, size-distribution and polydispersity of microemulsion droplets in solution

Fluorescence correlation spectroscopy (FCS) is an ideal tool for measuring molecular diffusion and size under extremely dilute conditions. However, the power of FCS has not been utilized to its best to measure diffusion and size parameters of complex chemical systems. Here, we apply FCS to measure the size, and, most importantly, the size distribution and polydispersity of a supramolecular nanostructure (i.e., microemulsion droplets, MEDs) in dilute solution. It is shown how the refractive index mismatch of a solution can be corrected in FCS to obtain accurate size parameters of particles, bypassing the optical matching problem of light scattering techniques that are used often for particle-size measurements. We studied the MEDs of 13 different W(0) values from 2 to 50 prepared in a ternary mixture of water, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and isooctane, with sulforhodamine-B as a fluorescent marker. We find that, near the optical matching point of MEDs, the dynamic light scattering (DLS) measurements underestimate the droplet sizes while FCS estimates the accurate ones. A Gaussian distribution model (GDM) and a maximum-entropy-based FCS data fitting model (MEMFCS) are used to analyze the fluorescence correlation curves that unfold Gaussian-type size distributions of MEDs in solution. We find the droplet size varies linearly with W(0) up to ~20, but beyond this W(0) value, the size variation deviates from this linearity. To explain nonlinear variation of droplet size for W(0) values beyond ~20, we invoke a model (the coated-droplet model) that incorporates the size polydispersity of the droplets.

Optical and non-optical methods for detection and characterization of microparticles and exosomes

Microparticles and exosomes are cell-derived microvesicles present in body fluids that play a role in coagulation, inflammation, cellular homeostasis and survival, intercellular communication, and transport. Despite increasing scientific and clinical interest, no standard procedures are available for the isolation, detection and characterization of microparticles and exosomes, because their size is below the reach of conventional detection methods. Our objective is to give an overview of currently available and potentially applicable methods for optical and non-optical determination of the size, concentration, morphology, biochemical composition and cellular origin of microparticles and exosomes. The working principle of all methods is briefly discussed, as well as their applications and limitations based on the underlying physical parameters of the technique. For most methods, the expected size distribution for a given microvesicle population is determined. The explanations of the physical background and the outcomes of our calculations provide insights into the capabilities of each method and make a comparison possible between the discussed methods. In conclusion, several (combinations of) methods can detect clinically relevant properties of microparticles and exosomes. These methods should be further explored and validated by comparing measurement results so that accurate, reliable and fast solutions come within reach.

Deposition of TiO2 nanoparticles onto silica measured using a quartz crystal microbalance with dissipation monitoring

Titanium dioxide (TiO2) nanoparticles introduced into subsurface environments may lead to contamination of drinking water supplies and can act as colloidal carriers for sorbed contaminants. A model laboratory system was used to examine the influence of water chemistry on the physicochemical properties of TiO2 nanoparticles and their deposition. Deposition rates of TiO2 particles onto a silica surface were measured over a broad range of solution conditions (pH and ionic strength) using a quartz crystal microbalance with energy dissipation monitoring (QCM-D). Higher particle deposition rates were observed under favorable interaction conditions (i.e., in the presence of attractive electrostatic interactions) in comparison to unfavorable deposition conditions where electrostatic repulsion dominates particle-surface interactions. Nanoparticle sizes were characterized by fluorescence correlation spectroscopy (FCS), dynamic light scattering (DLS), and atomic force microscopy (AFM). These analyses confirmed the nanoscale of the system under study as well as the presence of TiO2 aggregates in some cases. TiO2 deposition behavior onto silica measured using QCM-D was generally found to be in qualitative agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability.

Photophysics of Cy3-encapsulated calcium phosphate nanoparticles

Progress toward clinical application of biodegradable fluorescent calcium phosphate (CP) nanoparticles as a bioimaging agent requires detailed knowledge of chromophore interaction with CP. As readouts of this cargo-matrix interaction, we determined the principle photophysical properties of Cy3 encapsulated in CP nanparticles (CPNPs) using steady-state and time-resolved fluorescence spectroscopy. Fluorescence correlation spectroscopy (FCS)-determined diffusion coefficients and associated hydrodynamic radii confirmed the presence of highly monodisperse CPNPs with radii ranging from 7 to 10 nm. Single CP nanoparticles were 20 times brighter than free dye molecules because of a CP-induced 5-fold increase in quantum efficiency and encapsulation of four dye molecules per particle. Solvatochromic shifts resulting from hydrogen bonding between free dye and solvent or restricted intramolecular mobility by solvent viscosity were absent when Cy3 was encapsulated in CP. Encapsulation-mediated increases in radiative decay rates and decreases in nonradiative decay rates resulting in longer fluorescence lifetimes of Cy3 were attributed to solvent and CP-related local refractive indices and restricted flexibility of dye by rigid CP. Enhanced brightness of CPNPs enabled imaging of single nanoparticles under epifluorescence using both standard and total internal reflection fluorescence (TIRF) modes with camera exposure times on the order of tens of milliseconds. These enhanced photophysical properties together with excellent biocompatibility make CPNPs ideal for bioimaging applications ranging from single-molecule tracking to in vivo tumor detection and offer the possibility of timed codelivery of drugs to control cell function.

Fluorescence correlation spectroscopy and its application to the characterization of molecular properties and interactions

Fluorescence correlation spectroscopy (FCS) utilizes temporal fluctuations in fluorescence emission to extract quantitative measures of inter- or intramolecular dynamics or molecular motions of probe molecules, which occur on submicrosecond to second timescales. In typical experiments, one can readily obtain the probe's diffusion coefficient and concentration from small volumes of sample. Recent FCS applications have yielded information on interactions of the probe with changing or structured solvent, binding with other molecules, photophysical or conformational changes in the probe, polymerization, and other changes in the dynamics of the probe. In cross-correlation mode FCS promises to attract more applications as the technique can monitor interactions in a system with two or more probes with different fluorophores.

Combining small angle neutron scattering (SANS) and fluorescence correlation spectroscopy (FCS) measurements to relate diffusion in agarose gels to structure

Small angle neutron scattering (SANS) and fluorescence correlation spectroscopy (FCS) measurements were carried out on agarose hydrogels to link their microscopic structure to the diffusivity of solutes at different scales. SANS allowed for the determination of the distribution of void volumes within the gels. They were shown to be compatible with a random network of cylindrical fibers as described by the Ogston model. FCS measured solute diffusivity in spaces similar in size to the void volumes, and thus, the results reflected the gel heterogeneity. Solute diffusivity was predicted by modeling the gel as microscopic geometrical cells. Variations in the diffusivity of solutes of different sizes could be predicted from the structural parameters of the gel using theory, taking into account obstruction by cylindrical cells and solute hydrodynamics. Prediction of the FCS autocorrelation functions for solutes from a cell model demonstrated a lack of sensitivity of this technique for multicomponent analysis.

Single-walled tubulin ring polymers

An unusual class of nanoscopic, ring-shaped, single-walled biopolymers arises when alphabeta-tubulin is mixed with certain small peptides obtained from various marine organisms and cyanobacteria. The single-ring structures, whose mean molecular weight depends on the specific peptide added to the reaction mixture, usually have sharp mass distributions corresponding, e.g., to rings containing eight tubulin dimers (when the added peptide is cryptophycin) and 14 dimers (e.g., with dolastatin). Although the ring-forming peptides have been shown to possess antimitotic properties when tested with cultured eukaryotic cells (and thus have generated considerable interest as possible agents to be used in the treatment of cancer), it is not our intention to extensively discuss the potential pharmacological properties of the peptides. Rather, we will review the polymeric structures that form and illustrate how certain physical techniques can be used to characterize their properties and interactions. The nanoscopic size and particular geometry of the individual rings make them appropriate targets for scattering and hydrodynamic techniques that provide details about their structure in solution, but it is necessary to relate measured data to postulated structures by nontrivial, albeit straight-forward, mathematical, and computational means. We will discuss how this is done when one uses such methods as small angle neutron scattering, dynamic light scattering, fluorescence correlation spectroscopy, and sedimentation velocity measurements. Moreover, we show that, by using several techniques, one can eliminate degeneracy to provide better discrimination between model structures.

Measurement of monomer-oligomer distributions via fluorescence moment image analysis

We present higher-order moment analysis of fluorescence intensity fluctuations from individual laser scanning microscopy images applied to study monomer-oligomer distributions. We demonstrate that the number densities and brightness ratios of a mixed population of monomers and oligomers can be determined by analyzing higher-order moments of the fluorescence intensity fluctuations from individual images for specific ranges of densities and particle brightness ratios. Computer simulations and experiments with fluorescent microspheres and cells were performed to illustrate the detection limits and accuracy of this statistical approach. The simulation results show that the concentration of the dimer or oligomer population should be less than or equal to the monomeric concentration for the method to provide accurate results, and that the upper density detection limit of the population of monomers is one order-of-magnitude higher than the concentration of the oligomers. We implemented this technique to resolve two populations of fluorescent microspheres with different brightness ratios and we also applied the moment-analysis method to examine the distribution of aggregation states of PDGF-beta receptors in human fibroblast cells. The method was able to resolve a tetrameric population of the PDGF-beta receptors relative to the background distribution of nonspecifically bound fluorophore.

Encapsulation of Fluorescent Molecules by Functionalized Polymeric Nanocontainers: Investigation by Confocal Fluorescence Imaging and Fluorescence Correlation Spectroscopy

Nanocontainers (NCs) were prepared from amphiphilic triblock copolymers, having an average molecular weight of around 8000 g/mol, by using previously published preparation methods consisting of dispersing the polymer in an aqueous buffer solution containing molecules for encapsulation. A small molecular weight fluorophore, sulforhodamine B, as well as the fluorescent protein avidin labeled with Alexa 488 were encapsulated, and the resulting nanocontainers were characterized using fluorescence correlation spectroscopy (FCS) and fluorescence cross-correlation spectroscopy (FCCS). Nanocontainer size determination by FCS is very robust and compares well with results obtained from photon correlation spectroscopy: the measured diameters of the polymeric nanocontainers vary between 140 and 172 nm. Encapsulation of fluorescent molecules was determined by evaluating the molecular brightness of nanocontainers with an encapsulated fluorescently labeled protein (avidin-Alexa 488). Results indicate that the number of encapsulated avidin-Alexa 488 molecules corresponds well with the initial concentration of the fluorescently labeled protein and the encapsulated volume. A nanocontainer binding assay was developed using biotinylated fluorescently labeled nanocontainers. Binding of biotinylated nanocontainers to fluorescently labeled streptavidin was followed by fluorescence cross-correlation spectroscopy. The intrinsic dissociation constant, K(d), of labeled streptavidin to the ligand-modified nanocontainers is 1.7 +/- 0.4 x 10(-8) M, and about 1921 +/- 357 molecules of labeled streptavidin are bound to each nanocontainer.

Flow profiling of a surface-acoustic-wave nanopump

The flow profile in a capillary gap and the pumping efficiency of an acoustic micropump employing surface acoustic waves is investigated both experimentally and theoretically. Ultrasonic surface waves on a piezoelectric substrate strongly couple to a thin liquid layer and generate a quadrupolar streaming pattern within the fluid. We use fluorescence correlation spectroscopy and fluorescence microscopy as complementary tools to investigate the resulting flow profile. The velocity was found to depend on the applied power approximately linearly and to decrease with the inverse third power of the distance from the ultrasound generator on the chip. The found properties reveal acoustic streaming as a promising tool for the controlled agitation during microarray hybridization.

Stability of drug-induced tubulin rings by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) was applied to investigate the stability of tubulin rings that result from the interaction of alpha beta-tubulin dimers with three vinca domain-binding peptides--cryptophycin 1, hemiasterlin, and dolastatin 10. These peptides inhibit tubulin polymerization into microtubules and, instead, induce the formation of single-walled tubulin rings of 23.8 nm mean diameter for cryptophycin and 44.6 nm mean diameter for hemiasterlin and dolastatin, as revealed by electron microscopy on micromolar drug-tubulin samples. However, the hydrodynamic diameter and the apparent number of fluorescent particles, determined from analysis of FCS measurements obtained from nanomolar drug-tubulin samples, indicate variation in the stability of the rings depending on the drug and the tubulin concentration. Cryptophycin-tubulin rings appear to be the most stable even with tubulin concentration as low as 1 nM, whereas hemiasterlin-tubulin rings are the least, depolymerizing even at relatively high concentrations (100 nM). In contrast, the dolastatin-tubulin rings demonstrate an intermediate level of stability, depolymerizing significantly only at tubulin concentrations below 10 nM. We also compare the stability results with those of cytotoxicity measurements taken on several cell lines and note a rough correlation between the cytotoxicity of the drugs in cell cultures and the stability of the corresponding drug-induced rings.

Diffusion coefficients of humic substances in agarose gel and in water

Measurements of the diffusion coefficients of five different humic substances (HS) have been performed in water and in agarose hydrogels at several pH values (in the range of 3-10) and gel concentrations (in the range of 0.7-3% w/w). Fluorescence correlation spectroscopy (FCS) and classical diffusion cells were used in parallel to probe diffusion over both microscopic and mesoscopic distance scales. In general, agreement between the techniques was reasonable, which indicated that local nonhomogenities in the gel did not play an important role. Diffusion coefficients (D) in the gel were generally in the range of 0.9-2.5 x 10(-10) m2 s(-1) but were generally only 10-20% lower than in solution. At low pH values, one of the studied humic substances (a peat humic acid, PPHA) formed large aggregates that could not penetrate into the gel and therefore could not be defined by a single D value. The observed decreases of D in the gel for other HS were too large to be explained by the tortuousity and obstructive effects of the gel alone. D decreased slightly with increasing gel concentration and increased slightly with pH. Because modifications of D due to pH were similar in both the gel and the free solution, it is unlikely that complexation with the gel was greatly influenced by the pH. Rather, the main effect that appeared to decrease the diffusive flux in gels was likely small increases in the hydrodynamic radii of the humic macromolecules. An anomalous diffusion model was used to describe the FCS data in the gel. The characteristic exponent determined by fitting the autocorrelation functions with this model decreased only slightly (from 0.96 to 0.90) with increasing gel concentration providing support that HS complexation with the gel fibers was not very important. The results have important implications for our understanding of the fate and behavior of the HS and their associated pollutants and for interpreting metal speciation data obtained using gel-covered analytical sensors.

Application of Fluorescence Correlation Spectroscopy: A Study of Flocculation of Rigid Rod-like Biopolymer (Schizophyllan) and Colloidal Particles

The flocculation between the rod-like biopolymer Schizophyllan and two types of colloidal particles (latex with diameter 40 nm and alumina with diameter 60 nm) has been investigated by means of fluorescence correlation spectroscopy (FCS). The concentration ratio of Schizophyllan/particle q was varied in the range 0.1 approximately 20. Under conditions of pH about 5.7, 1 mmol.L(-1) NaCl, and room temperature (22+/-0.5 degrees C), the particles are strongly charged (alumina particles positively charged, latex negatively), while Schizophyllan is neutral. We observed that Schizophyllan chains flocculate with both types of particles, which suggests that the charge neutralization does not play a decisive role in these interactions. The ratio of fluorescence intensity of one floc over that of one particle, Q(f)/Q(p), and the corresponding hydrodynamic radius (r(h)) of the flocs have been measured. For a Schizophyllan-latex system, Q(f)/Q(p) reached a maximum value of 5 for q=3 indicating that the flocs contained five particles on average. The corresponding value of r(h) was r(h)=455 nm. The flocculation kinetic of latex particles with Schizophyllan was too fast to be measurable by FCS. For the Schizophyllan-alumina system, Q(f)/Q(p) was stable at about 1 in the whole studied range of q but r(h) increased with q suggesting that many Schizophyllan chains are adsorbed on individual particles. The flocculation kinetic of this system was studied by FCS and the obtained results were compatible with those of photon correlation spectroscopy.

Applications of fluorescence correlation spectroscopy: measurement of size-mass relationship of native and denatured schizophyllan

Diffusion dynamics of a polysaccharide, schizophyllan has been studied by fluorescence correlation spectroscopy (FCS). Several different sizes of nondenatured and denatured schizophyllan have been labeled with rhodamine 6G in borate buffer. The length of the nondenatured schizophyllan was calculated from FCS data by using the Broersma's relationship for rod-like macromolecules. The obtained length was close to that obtained by atomic force microscopy (AFM) measurements. Denatured schizophyllan possesses a random coil conformation. Its hydrodynamic radius R(h) was measured by FCS. The relationship between R(h) and the molecular mass M has been studied and the scaling relationship R(h)--M(0.59) has been obtained, which is in agreement with the random coil model with excluded volume effect. The persistence length q(denat) of the denatured schizophyllan was determined by Hearst's relationship, to be equal to 5.16 +/- 0.75 (nm). The work demonstrates the utility of FCS method for dynamics investigations of biopolymers especially in diluted regime (concentration lower than 10(-8)M could be measured) where other techniques could not be used.

Noise on Fluorescence Correlation Spectroscopy

The time dependence of the noise and the signal-to-noise (SN) ratio of the fluorescence correlation spectroscopy (FCS) autocorrelation function is obtained from replica measurements of standard dextran solutions. The noise dependence on the delay time is fitted by a hyperbolic function with two fitting parameters. The dependence of these parameters on concentration, fluorescence intensity, and accumulation time is obtained experimentally. The behavior of SN at zero delay time agrees well with the theoretical predictions reported in the literature. The obtained data are useful for the quantitative evaluation of the FCS data fits, as well as for simulation of the FCS autocorrelation functions. Copyright 2001 Academic Press.

Probing ligand protein binding equilibria with fluorescence fluctuation spectroscopy

We examine the binding of fluorescent ligands to proteins by analyzing the fluctuation amplitude g(0) of fluorescence fluctuation experiments. The normalized variance g(0) depends on the molecular brightness and the concentration of each species in the sample. Thus a single g(0) measurement is not sufficient to resolve individual species. Titration of the ligand with protein establishes the link between molecular brightness and concentration by fitting g(0) to a binding model and allows the separation of species. We first apply g(0) analysis to binary dye mixtures with brightness ratios of 2 and 4 to demonstrate the feasibility of this technique. Next we consider the influence of binding on the fluctuation amplitude g(0). The dissociation coefficient, the molecular brightness ratio, and the stochiometry of binding strongly influence the fluctuation amplitude. We show that proteins with a single binding site can be clearly differentiated from proteins with two independent binding sites. The binding of fluorescein-labeled digoxigenin to a high-affinity anti-digoxin antibody was studied experimentally. A global analysis of the fluctuation amplitude and the fluorescence intensity not only recovered the dissociation coefficient and the number of binding sites, but also revealed the molecular heterogeneity of the hapten-antibody complex. Two species were used to model the molecular heterogeneity. We confirmed the molecular heterogeneity independently by fluorescence lifetime experiments, which gave fractional populations and molecular brightness values that were virtually identical to those of the g(0) analysis. The identification and characterization of molecular heterogeneity have far-reaching consequences for many biomolecular systems. We point out the important role fluctuation experiments may have in this area of research.