A study of the adsorption of dyes on bovine serum albumin by the method of polarization of fluorescence

Analytical Aspects of ANSA-BSA Association: A Thermodynamic and Conformational Approach

Many studies have demonstrated the manner in which ANS interacts with bovine serum albumin (BSA), although they are limited by the extremely low solubility of dye. The present study demonstrates the binding of ANSA dye with BSA, and since this dye can easily replace ANS, it not only simplifies research but also improves sensor accuracy for serum albumin. A combination of calorimetry and spectroscopy has been employed to establish the thermodynamic signatures associated with the interaction of ANSA with the protein and the consequent conformational changes in the latter. The results of differential scanning calorimetry reveal that when the concentration of ANSA in solution is increased, the thermal stability of the protein increases substantially. The fluorescence data demonstrated a decrease in the binding affinity of ANSA with the protein when pH increased but was unable to identify a change in the mode of interaction of the ligand. ITC has demonstrated that the mode of interaction between ANSA and the protein varies from a single set of binding sites at pH 5 and 7.4 to a sequential binding site at pH 10, emphasizing the potential relevance of protein conformational changes. TCSPC experiments suggested a dynamic type in the presence of ANSA. Molecular docking studies suggest that ANSA molecules are able to find ionic centers in the hydrophobic pockets of BSA. The findings further imply that given its ease of use in experiments, ANSA may be a useful probe for tracking the presence of serum albumin and partially folded protein states.

Design of Tracers in Fluorescence Polarization Assay for Extensive Application in Small Molecule Drug Discovery

Development of fluorescence polarization (FP) assays, especially in a competitive manner, is a potent and mature tool for measuring the binding affinities of small molecules. This approach is suitable for high-throughput screening (HTS) for initial ligands and is also applicable for further study of the structure-activity relationships (SARs) of candidate compounds for drug discovery. Buffer and tracer, especially rational design of the tracer, play a vital role in an FP assay system. In this perspective, we provided different kinds of approaches for tracer design based on successful cases in recent years. We classified these tracers by different types of ligands in tracers, including peptide, nucleic acid, natural product, and small molecule. To make this technology accessible for more targets, we briefly described the basic theory and workflow, followed by highlighting the design and application of typical FP tracers from a perspective of medicinal chemistry.

Complexity of the Role of Various Site-Specific and Selective Sudlow Binding Site Drugs in the Energetics and Stability of the Acridinedione Dye-Bovine Serum Albumin Complex: A Molecular Docking Approach

Molecular docking (Mol.Doc) techniques were employed to ascertain the binding affinity of two resorcinol-based acridinedione dyes (ADR1 and ADR2) with the widely studied globular protein Bovine Serum Albumin (BSA) in the presence of site-selective binding drugs by Autodock Vina 4.2 software. Docking of various feasible conformers of ADR1 dye with BSA was found to be energetically more favored than ADR2 dye, even though both these dyes differ in the 9th position of the basic dye structure. Analysis of dyes with BSA establishes the location of dye in all of the binding sites of BSA, predominantly through conventional and nonconventional hydrogen-bonding (HB) interactions. The coexistence of hydrophobic interactions resulted in the stability of various conformers generated. The introduction of site I and site II (Sudlow site binding drugs) into ADR1-BSA and ADR2-BSA complexes effectively destabilizes the dye-protein complex; however, the drugs do not displace ADR dyes completely from their selective binding domains. Site II binding drugs effectively destabilize the binding ability of the dye-protein complex rather than site I drugs. However, docking of site I drug 3-carboxyl-4-methyl-5-propyl-2-furanpropanic acid (CMPF) largely destabilizes the ADR1-protein complex, whereas indomethacin (INDO) enhances the binding affinity of the ADR2-protein complex. Interestingly, simultaneous docking of ADR dyes to the BSA-drug complex results in larger stability of the protein-drug complex through HB interactions rather than hydrophobic interactions. Both ADR1 and ADR2 dyes predominantly occupy the Sudlow binding sites of BSA, and the introduction of either site I or site II binding drugs does not displace the dye efficiently from the corresponding binding sites, rather the drugs are effectively displaced toward other binding domains apart from their specific site-binding domains of BSA. Through Mol.Doc techniques, we authenticate that the interactions in host-guest complex systems involving competing ligands are established in depth, wherein the dye as well as the amino acid (AA) moieties in BSA act as both HB donor and acceptor sites apart from several hydrophobic interactions coexisting toward the stability.

An Economical High-Throughput "FP-Tag" Assay for Screening Glycosyltransferase Inhibitors*

O-GlcNAc transferase (OGT) is involved in many cellular processes, and selective OGT inhibitors are valuable tools to investigate O-GlcNAcylation functions, and could potentially lead to therapeutics. However, high-throughput OGT assays that are suitable for large-scale HTS and can identify inhibitors targeting both acceptor, donor sites, and allosteric binding-sites are still lacking. Here, we report the development of a high-throughput "FP-Tag" OGT assay with bovine serum albumin (BSA) as a low-cost and superior "FP-Tag". With this assay, 2-methyleurotinone was identified as a low-micromolar OGT inhibitor. This type of assay with BSA as "FP-Tag" would find more applications with other glycosyltransferases.

The Acid-Base Equilibrium of Pyrazinoic Acid Drives the pH Dependence of Pyrazinamide-Induced Mycobacterium tuberculosis Growth Inhibition

Pyrazinamide, a first-line antibiotic used against Mycobacterium tuberculosis, has been shown to act in a pH-dependent manner in vitro. Why pyrazinamide, an antitubercle prodrug discovered more than 65 years ago, exhibits this pH-dependent activity was unclear. Upon entering mycobacterial cells, pyrazinamide is deamidated to pyrazinoate by an enzymatic process and exists in an acid-base equilibrium with pyrazinoic acid. Thus, the effects of total pyrazinoic acid (pyrazinoic acid + pyrazinoate) on M. tuberculosis growth, pH homeostasis, and proton motive force over a range of pH values found in host tissues were investigated. Although M. tuberculosis was able to maintain pH homeostasis over an external pH range of 7.0 to 5.5, total pyrazinoic acid induced growth inhibition increased as culture medium pH was decreased from 7.3 to 6.4. Consistent with growth inhibition, total pyrazinoic acid increased both acidification of the bacterial cytoplasm and dissipation of membrane potential as the environmental pH decreased when added to the bacterial suspensions. The results suggest pyrazinoic acid is the active form of the drug, which acts as an uncoupler of proton motive force, likely a protonophore, providing a mechanistic explanation for the pH dependence of the drug activity.

Competitive hydrogen bonding influences of fluorophore- urea-adenine system in water: Photophysical and photochemical approaches

Photophysical and photochemical investigation of photoinduced electron transfer (PET)-based acridinedione dye (ADR1) with urea in the presence of a nitrogenous base (adenine) were carried out in water. Urea suppresses the PET resulting in a fluorescence enhancement and the extent of binding is correlated and governed by the number of urea molecules surrounding the close vicinity of dye. On the contrary, adenine forms a true 1:2 complex with dye. Presence of adenine in dye-urea microenvironment results in the displacement of dye from the vicinity of urea molecules. The stability of dye-urea network in the presence of adenine reveals that the microenvironment of dye is governed and influenced by both urea and adenine. Introduction of adenine to dye-urea results in the formation of several hydrogen bonding assemblies that are competitive and influences the excited state characteristics of ADR1 dye. The micro assemblies comprise dye-urea (DU), dye-adenine (DA), urea-adenine (UA), urea-water (UW), urea-urea (UU), and adenine-water (AW) framework and the existence of several competitive hydrogen bonding results in a large variation in fluorescence properties of ADR1 dye. The presence of several assemblies also signifies that no confined phase selectively of DU or DA assemblies exist in any stoichiometric proportion in the aqueous phase. The binding constant, the variation in the fluorescence lifetime and its relative amplitude of DA in the presence of urea authenticate that the binding nature of dye-urea-adenine (DUA) is dependent on the several hydrogen bonding assemblies that coexist at any concentration. The extent of hydrogen bonding of DA is found to be entirely different from that of urea. Further, urea resulted in changes in the transient absorption peak of dye with a large variation in lifetime and shift of the transient absorption peaks. Fluorescence spectral techniques are used as an efficient tool in elucidating the binding nature of DU framework in the presence of non-fluorescent hydrogen-bonding solute like adenine.

Effect of the fluorescent probes ThT and ANS on the mature amyloid fibrils

Fluorescent probes thioflavin T (ThT) and 1-anilino-8-naphthalene sulfonate (ANS) are widely used to study amyloid fibrils that accumulate in the body of patients with serious diseases, such as Alzheimer’s, Parkinson’s, prion diseases, etc. However, the possible effect of these probes on amyloid fibrils is not well understood. In this work, we investigated the photophysical characteristics, structure, and morphology of mature amyloid fibrils formed from two model proteins, insulin and lysozyme, in the presence of ThT and ANS. It turned out that ANS affects the secondary structure of amyloids (shown for fibrils formed from insulin and lysozyme) and their fibers clusterization (valid for lysozyme fibrils), while ThT has no such effects. These results confirm the differences in the mechanisms of these dyes interaction with amyloid fibrils. Observed effect of ANS was explained by the electrostatic interactions between the dye molecule and cationic groups of amyloid-forming proteins (unlike hydrophobic binding of ThT) that induce amyloids conformational changes. This interaction leads to weakening repulsion between positive charges of amyloid fibrils and can promote their clusterization. It was shown that when fibrillogenesis conditions and, consequently, fibrils structure is changing, as well as during defragmentation of amyloids by ultrasonication, the influence of ANS to amyloids does not change, which indicates the universality of the detected effects. Based on the obtained results, it was concluded that ANS should be used cautiously for the study of amyloid fibrils, since this fluorescence probe have a direct effect on the object of study.

Competitive Binding Study Revealing the Influence of Fluorophore Labels on Biomolecular Interactions

Fluorescence methods are important tools in modern biology. Direct labeling of biomolecules with a fluorophore might, however, change interaction surfaces. Here, we introduce a competitive binding assay in combination with fluorescence correlation spectroscopy that reports binding affinities of both labeled and unlabeled biomolecules to their binding target. We investigated how fluorophore labels at different positions of a DNA oligonucleotide affect hybridization to a complementary oligonucleotide and found dissociation constants varying within 2 orders of magnitude. We next demonstrated that placing a fluorophore label at position Leu280 in the protein ligand internalin B does not alter the binding affinity to the MET receptor tyrosine kinase, compared to unlabeled internalin B. Our approach is simple to implement and can be applied to investigate the influence of fluorophore labels in a large variety of biomolecular interactions.

Interlocking Enzymes in Graphene-Coated Cellulose Paper for Increased Enzymatic Efficiency

A simple method for interlocking glucose oxidase and horseradish peroxidase in a network of cellulose fibers coated with bovine serum albumin (BSA)-exfoliated graphene (biographene) is reported here. The resulting paper reactor is inexpensive and stable. Biographene is expected to function as an electron shuttle, making the reaction between the enzyme and the substrate more efficient, and this hypothesis is examined here. The BSA used to separate the sheets of graphene provides extra carboxylic acid groups and primary amines to help interlock the enzymes and the graphene in between the fibers. The decrease in entropy associated with interlocking the enzymes on a solid support is likely responsible for the increase in enzymatic stability/activity observed. Each cellulose disk contained 5.2mg of enzyme per gram of paper and 93% of the enzyme is retained after washing for 0.5-2h. This simple methodology provides a low cost, effective approach for achieving high enzymatic activity and good loadings on a benign, versatile support.

A Simple Theoretical Model for Fluorescence Polarization Binding Assay Development

Fluorescence polarization is a screening technology that is radioactivity free, homogeneous, and ratiometric. The signal measured with this technology is a weighted value of free and bound ligand. As a consequence, saturation curves are accessible only after calculation of the corresponding concentrations of free and bound ligand. To make this technology more accessible to assay development, the authors propose a simple mathematical model that predicts fluorescence polarization values from ligand and receptor total concentrations, depending on the corresponding dissociation constant. This model was validated using data of Bodipy-NDP-αMSH binding to MC5, obtained after either ligand saturation of a receptor preparation or, conversely, receptor saturation of a ligand solution. These experimental data were also used to calculate the actual concentration of free and bound ligand and receptor and to obtain pharmacological constants by Scatchard analysis. A general method is proposed, which facilitates the design of fluorescence polarization binding assays by relying on the representation of theoretical polarization values. This approach is illustrated by the application to 2 systems of very different affinities.

Fluorescence techniques in analysis of protein-ligand interactions

Fluorescence spectroscopy may serve as a universal tool for the study of protein-ligand interactions. Applications of fluorometry have made use of various aspects of fluorescence such as intensity, emission and excitation spectra, lifetime, quantum yield, polarization state, and anisotropy, as well as energy transfer and other electronic phenomena. An experimentalist has to consider each of these characteristics carefully, frequently in combination with each other, for the analysis of protein-ligand complexes and for the determination of binding constants. Most of the available techniques are of a rather general nature and a wealth of possibilities exists for their utilization. In this chapter we will provide a short survey of selected techniques that can be used for measuring binding constants and probing protein-ligand interactions. Basic principles and phenomena are discussed followed by experimental considerations and examples of binding constant determination. Emphasis is placed on steady-state techniques that employ the use of intrinsic protein fluorescence, labeled ligands, as well as anisotropy and resonance energy transfer.

An improved method for removal of azo dye orange II from textile effluent using albumin as sorbent

Azo dyes are generally resistant to biodegradation due to their complex structures. Acid orange II is one of the most widely used dyes in the textile industry. The influence of bovine serum albumin (BSA) in different concentrations, pH, and time of contact on Orange II was investigated using kinetics and adsorption-isotherm experiments. The results showed that the maximum colour removed from dye/albumin was 99.50% and that a stable dye-protein complex had been formed at pH 3.5 and in a proportion of 1:3 (v/v), respectively. The synthetic effluent did not show toxicity to the microcrustacean Artemia salina, and showed a CL₅₀ equal to 97 µg/mL to azo dye orange II. Additionally, the methodology was effective in removing the maximum of orange II using BSA by adsorption at pH 3.5 which mainly attracted ions to the azo dye during the adsorption process. This suggests that this form of treatment is economical and easy to use which potentially could lead to bovine serum albumin being used as a sorbent for azo dyes.

A novel application of the fluorescent dye bis-ANS for labeling neurons in acute brain slices

The cell-impermeant oligomer-(e.g. beta-amyloid-, or tubulin-) specific fluorescent dye, bis-ANS (4,4'-bis-1-anilinonaphtalene-8-sulfonate), was successfully used for labeling mechanically damaged but still viable neuron bodies, neurites and neurite cross sections in acute brain slices. Acute hippocampal brain slices of rats were co-stained with bis-ANS and the cell-impermeant, DNA-specific dye propidium iodide (PI) and were then analyzed using fluorescence and confocal microscopes. Both the neuron bodies and the neurites were found to exhibit increased fluorescence intensities, suggesting that using this method they can be detected more easily. In addition, bis-ANS showed good region - but not cell specific co-localization with the neuron-specific fluorescent dye Dil (1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). These two dyes label different neuronal structures: Dil binds specifically to intact cell membranes while bis-ANS can enter cells with compromised cell membranes and then stain the microtubules in the cytoplasm. For a quick (10min) staining of acute brain slices with bis-ANS both HEPES and NaHCO(3) were needed in order to achieve high signal intensity. Labeling with bis-ANS fluorescent dye is an easy method for imaging the neuronal structures on the surface of acute brain slices.

Serum albumins as differential receptors for the discrimination of fatty acids and oils

Using fluorescent indicators and several serum albumins, a sensing system for fatty acids was developed. Fatty acids were able to be differentiated based on their carbon chain length and degree of unsaturation. A sensing ensemble was then applied to study complex mixtures of triglycerides, namely, edible oils. The oils, with different fatty acid compositions, were successfully differentiated using principal component analysis.

Extrinsic fluorescent dyes as tools for protein characterization

Noncovalent, extrinsic fluorescent dyes are applied in various fields of protein analysis, e.g. to characterize folding intermediates, measure surface hydrophobicity, and detect aggregation or fibrillation. The main underlying mechanisms, which explain the fluorescence properties of many extrinsic dyes, are solvent relaxation processes and (twisted) intramolecular charge transfer reactions, which are affected by the environment and by interactions of the dyes with proteins. In recent time, the use of extrinsic fluorescent dyes such as ANS, Bis-ANS, Nile Red, Thioflavin T and others has increased, because of their versatility, sensitivity and suitability for high-throughput screening. The intention of this review is to give an overview of available extrinsic dyes, explain their spectral properties, and show illustrative examples of their various applications in protein characterization.

The binding of novel two-color fluorescence probe FA to serum albumins of different species

The novel two-color ratiometric fluorescence probe FA belonging to a class of 3-hydroxychromone dyes was applied for characterization of binding sites in serum albumins obtained from seven species (bovine, dog, horse, human, pig, rabbit and sheep). On strong and highly specific FA binding to the same location in protein structure, the species-dependent differences were observed in positions of absorption maxima, positions of two fluorescence emission bands and the intensity ratios between them. They were analyzed by multiparametric algorithm that allowed a detailed characterization of probe-binding sites and were characterized by very low polarity, high electronic polarizability and different extent of intermolecular hydrogen bonding. The species-dependent differences were also observed in time-resolved fluorescence emission decays. Fluorescence competition experiments with the drug warfarin, suggested the location of FA binding site within or in proximity to Domain IIA.

Fluorescence polarization/anisotropy approaches to study protein-ligand interactions: effects of errors and uncertainties

Fluorescence techniques are widely used in the study of protein-ligand interactions because of their inherent sensitivity, and the fact that they can be implemented at true equilibrium conditions. Fluorescence polarization/anisotropy methodologies, in particular, are now extensively utilized in biotechnology and clinical chemistry. In this chapter, we shall discuss both theoretical and practical aspects of polarization/anisotropy methods. We shall also focus attention on considerations of errors and uncertainties in such measurements, and how these uncertainties affect the ultimate estimation of ligand-protein dissociation constants.

Novel two-color fluorescence probe with extreme specificity to bovine serum albumin

We report on strong, highly specific and stochiometric binding to bovine serum albumin of novel fluorescence probe FA, 2-(6-diethylaminobenzo[b]furan-2-yl)-3-hydroxychromone, that exhibits a very characteristic two-band fluorescence spectrum. Both absorption band and two fluorescence bands of FA are very sensitive to non-covalent interactions in the immediate environment of the probe. Multiparametric analysis of this spectroscopic information allows us to conclude that the binding site is characterized by very low polarity, high extent of screening from aqueous environment and unusually high electronic polarizability. The latter suggests the proximal location of probe FA to the aromatic amino acid residues in the binding site. The new probe can be proposed for the study of interaction of ligands and drugs of different nature with serum albumins.

Conformational changes in albumin molecule: a new response to pathological process

A new fluorescent test developed at the Institute of Physicochemical Medicine (Ministry of Health of the Russian Federation) allows to determine not only blood concentration of albumin, but also to evaluate the state of its molecule. The test is feasible and enables express-analysis of the plasma and serum without fractionation and other preliminary procedures. This test reveals abnormalities in albumin molecule caused by toxic metabolites or conformational changes. Many diseases are accompanied by conformational changes in albumin, while its concentration often remains unchanged. Changes in albumin conformation can serve a diagnostic and prognostic criterion in some pathologies.

The trace and subgrouping of lymphocyte activation by dynamic fluorescence intensity and polarization measurements

Cell activation involves conformational changes of cytosolic enzymes, and/or their regulatory proteins, as well as intracellular matrix re-organization. In this work, these changes were monitored by dynamic measurements of fluorescence polarization in single cells incubated with or without phytohaemagglutinin (PHA), using the Cellscan mark S (CS-S) cytometer. This instrument and the procedure used proved to be a powerful tool for distinguishing subpopulations of cells. Grouping of cells by their staining rates (the time rate of change of the fluorescence intensity) yielded three major subgroups. For each subgroup, the fluorescence depolarization (FDP) induced by the incubation with PHA was measured. The kinetics of the subgroups indicate that the major FDP is contributed by the cells with the lowest staining rate. This FDP is approximately 1.5 times greater than that of a bulk population. It is believed that the analysis of kinetic probing might yield an important and more sensitive method for functional marking of subgroups of cells by their response characteristics.

The interaction of polycyclic hydrocarbons and nucleic acids

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Characterization of monomeric 4-aminobutyrate aminotransferase at low pH

4-Aminobutyrate aminotransferase undergoes a reversible process of association/dissociation at low pH. At pH 5.0, monomeric species exist predominantly in solution as revealed by FPLC and time-dependent emission anisotropy measurements. The observed rotational correlation time at pH 5.0, phi obs = 25 ns, corresponds to a compact spherical unit of 52 kDa. An increase in the net charge of the macromolecule at pH 5.0 is responsible for destabilization of the dimeric structure, (WEL approximately 41.84 kJ/mol), but the dissociation of the protein does not perturb the secondary structure as revealed by CD measurements. The fluorescent probe 1-anilinonaphthalene-8-sulfonate (ANS), bound to hydrophobic sites of the enzyme, was used to monitor the kinetics of protein dissociation by stopped-flow spectroscopy. The dissociation of the dimeric structure at pH 5.0 was characterized by a relaxation time of 18 ms. The rate of association of monomeric subunits at pH 7.0 was too fast to be detected in the stopped-flow instrument. These observations have some bearing on the mechanism of reconstitution of dimeric structures of 4-aminobutyrate aminotransferase in the cell.

Stern-volmer in reverse: 2:1 stoichiometry of the cytochrome c-cytochrome c peroxidase electron-transfer complex

A reverse protocol for measurements of molecular binding and reactivity by excited-state quenching has been developed in which the quencher, held at a fixed concentration, is titrated by a photoexcitable probe molecule whose decay is monitored. The binding stoichiometries, affinities, and reactivities of the electron-transfer complexes between cytochrome c (Cc) and cytochrome c peroxidase (CcP) were determined over a wide range of ionic strengths (4.5 to 118 millimolar) by the study of photoinduced electron-transfer quenching of the triplet excited state of zinc-substituted Cc (ZnCc) by Fe3+CcP. The 2:1 stoichiometry seen for the binding of Cc to CcP at low ionic strength persists at the physiologically relevant ionic strengths and likely has functional significance. Analysis of the stoichiometric binding and rate constants confirms that one surface domain of CcP binds Cc with a high affinity but with poor electron-transfer quenching of triplet-state ZnCc, whereas a second binds weakly but with a high rate of electron-transfer quenching.

Thermodynamics of interaction of a fluorescent DNA oligomer with the anti-tumour drug netropsin

Fluorescence spectroscopy was used to study the interaction between the minor-groove-binding drug netropsin and the self-complementary oligonucleotide d(CTGAnPTTCAG)2 containing the fluorescent base analogue 2-aminopurine (nP). The binding of netropsin to this oligonucleotide causes strong quenching of the 2-aminopurine fluorescence, observed by steady-state as well as time-resolved spectroscopy. From fluorescence titrations, binding isotherms were recorded and evaluated. The parameters showed one netropsin binding site/oligonucleotide duplex and an association constant of about 10(5) M-1 at 25 degrees C, 3-4 orders of magnitude weaker than for an exclusive adenine/thymine host sequence. From the temperature dependence of the association constant the thermodynamic parameters were obtained as delta G = -29 kJ/mol, delta H = -12 kJ/mol and delta S = +55 J.mol-1.K-1 at 25 degrees C. These parameters resemble those of the interaction of poly[(dG-dC).(dG-dC)] with netropsin, indicating a mainly entropy-driven reaction. The amino group of 2-aminopurine, like that of guanine, resides in the minor groove of DNA. Therefore the relatively weak binding of netropsin to d(CTGAnPTTCAG)2 is probably related to partial blockage of the tight fit of netropsin into the preferred minor groove of an exclusive adenine/thymine host sequence.

Fluorescent probes for measuring the binding constants and distances between the metal ions bound to Escherichia coli glutamine synthetase

TNS, 2-p-toluidinylnaphthalene-6-sulfonate, has been used as a fluorescent probe to determine the binding constants of metal ions to the two binding sites of Escherichia coli glutamine synthetase (GS). TNS fluorescence is enhanced dramatically when bound to proteins due to its high quantum yield resulting from its interactions with hydrophobic regions in proteins. The fluorescence energy transfer from a hydrophobic tryptophan residue of GS to TNS has been detected as an excitation band centered at 280 nm. Therefore, TNS is believed to be bound to a hydrophobic site on the GS surface other than the active site and is located near a hydrophobic Trp residue of GS. GS binds lanthanide ions [Ln(III)] more tightly than either Mn(II) or Mg(II), and the binding constants of several lanthanide ions were determined to be in the range (2.1-4.6) x 10(10) and (1.4-3.0) x 10(8) M-1 to the two metal binding sites of GS, respectively. The intermetal distances between the two metal binding sites of GS were also determined by measuring the efficiencies of energy transfer from Tb(III) to other Ln(III) ions. The intermetal distances of Tb(III)-Ho(III) and Tb(III)-Nd(III) were 7.9 and 6.8 A, respectively.

Calibration of measurements in vivo of fluorescein in the cornea

Quenching of fluorescence of fluorescein is not observed with broad field fluorophotometers. Fluorophotometric equipment which measures the fluorescence in a tiny spot has, however, been reported to underestimate the molarity of fluorescein in the rabbit corneal stroma by as much as a factor of two. In this experiment, quenching was measured in the rabbit cornea with two scanning fluorophotometers. The quenching was measured by four different techniques: (1) by elution of fluorescein, (2) by elution of albumin, (3) by polarization of fluorescence, and (4) by spectrofluorophotometry. It was estimated by all four methods that quenching in the living rabbit cornea with these instruments is approximately 20%. Taken together, the four experiments suggest that the quenching of fluorescence of fluorescein can be explained entirely on the basis of the interaction of fluorescein and albumin in the stroma.

The fluorescent indicator pyranine is suitable for measuring stromal and cameral pH in vivo

Three pH-sensitive fluorescent indicators (fluorescein, bis-carboxyethyl-carboxyfluorescein and pyranine) were studied to determine their usefulness as probes for the living cornea and anterior chamber. Of the three dyes, pyranine had properties which render it almost ideally suited for this purpose. Adequate concentrations are reached in the cornea and anterior chamber after topical administration; measurements can be made by fluorophotometry for many hours. The pH is calculated by measuring the ratio of fluorescent intensity at two excitation wavelengths, I463/I404, a measurement which is dependent on pH but independent of the concentration of the fluorophore and other variables which can alter the intensity. In the rabbit eye, pyranine in the cornea and anterior chamber was observed to undergo easily measurable changes in fluorescent ratios associated with lid closure and contact lens wear, indicating its sensitivity to mild changes in pH.

Thermodynamics of the binding of salicylate to human serum albumin: evidence of non-competition with imidazole

The thermodynamic characteristics of the binding of salicylate to human serum albumin have been studied using a technique based on the variation of the quantum yield of fluorescence of salicylate when it binds to the protein. The binding constants, number of sites and the values of delta G degrees, delta H degrees and delta S degrees were determined. The results are consistent with a model that proposes two equal and independent types of binding site with a predominantly ionic interaction and an important hydrophobic contribution in one of the sites. The technique was also used to demonstrate that imidazole and salicylate (that can be found simultaneously in plasma following administration of imidazole-2-hydroxybenzoate) do not compete for the same binding sites on the protein.

Interaction of ellipticine and an indolo[2,3b]-quinoxaline derivative with DNA and synthetic polynucleotides

The non-covalent DNA interaction of the anticancer drug ellipticine (Scheme I, 1a) as well as an indolo[2,3-b]-quinoxaline derivative (Scheme I, 3b) with a dimethylaminoethyl side chain has been studied by light absorption, linear dichroism (LD) and fluorescence. Compound 3b (Scheme I) has antitumorigenic as well as antiviral activity. Both compounds bind to DNA or synthetic polynucleotides such as poly(dA-dT).(dA-dT) and poly(dG-dC).(dG-dC) by intercalation. In contrast to ellipticine, compound 3b (Scheme I) exhibits a significant binding specificity for alternating AT sequences. Its fluorescence is strongly enhanced in AT sequences and quenched in GC sequences. Fluorescence titrations evaluated as Scatchard plots show that both ellipticine and compound 3b (Scheme I) bind to the nucleic acids according to a non-cooperative neighbor exclusion model.

Myoplasmic binding of fura-2 investigated by steady-state fluorescence and absorbance measurements

Binding of the fluorescent Ca2+ indicator dye fura-2 by intracellular constituents has been investigated by steady-state optical measurements. Fura-2's (a) fluorescence intensity, (b) fluorescence emission anisotropy, (c) fluorescence emission spectrum, and (d) absorbance spectra were measured in glass capillary tubes containing solutions of purified myoplasmic proteins; properties b and c were also measured in frog skeletal muscle fibers microinjected with fura-2. The results indicate that more than half, and possibly as much as 85%, of fura-2 molecules in myoplasm are in a protein-bound form, and that the binding changes many properties of the dye. For example, in vitro characterization of the Ca2+-dye reaction indicates that when fura-2 is bound to aldolase (a large and abundant myoplasmic protein), the dissociation constant of the dye for Ca2+ is three- to fourfold larger than that measured in the absence of protein. The problems raised by intracellular binding of fura-2 to cytoplasmic proteins may well apply to cells other than skeletal muscle fibers.