Measuring the forces that control protein interactions

Multispectroscopic Study of the Interaction of Chloramphenicol with Human Neuroglobin

The interaction between chloramphenicol (CHL) and neuroglobin (Ngb) has been investigated by using fluorescence, synchronous fluorescence, UV-Vis and circular dichroism (CD) spectroscopy. It has been found that CHL molecule can quench the intrinsic fluorescence of Ngb in a way of dynamic quenching mechanism, which was supported by UV-Vis spectral data. Their effective quenching constants (KSV) are2.2×104,2.6×104,and 3.1×104 L⋅mol−1at 298 K, 303 K, and 308 K, respectively. The enthalpy change (ΔH) and entropy change (ΔS) for this reaction are 26.42 kJ⋅mol−1and 171.7 J⋅K−1, respectively. It means that the hydrophobic interaction is the main intermolecular force of the interaction between CHL and Ngb. Synchronous fluorescence spectra showed that the microenvironment of tryptophan and tyrosine residues of Ngb has been changed slightly. The fluorescence quenching efficiency of CHL to tyrosine residues is a little bit more than that to tryptophan residues of Ngb. Furthermore, CD spectra indicated that CHL can induce the formation of α-helix of Ngb.

N-Glycosylation Alters Cadherin-Mediated Intercellular Binding Kinetics

These results present direct evidence that the N-glycosylation state of neural cadherin impacts the intrinsic kinetics of cadherin-mediated intercellular binding. Micropipette manipulation measurements quantified the effect of N-glycosylation mutations intercellular binding dynamics. The wild type protein exhibits a two-stage binding process in which a fast, initial binding step is followed by a short lag and second, slower transition to the final binding stage. Mutations that ablate N-glycosylation at three sites on the extracellular domains 2 and 3 (EC2-3) of neural cadherin alter this kinetic fingerprint. Glycosylation does not affect the affinities between the adhesive N-terminal domains, but instead modulates additional cadherin interactions, which govern the dynamics of intercellular binding. These results, together with prior findings that these hypo-glycosylation mutations increase the prevalence of cis dimers on cell membranes, suggest a binding mechanism in which initial adhesion is followed by additional cadherin interactions, which enhance binding but are modulated by N-glycosylation. Given that oncogene expression drives specific changes in N-glycosylation, these results provide insight into possible mechanisms altering cadherin function during tumor progression.

Biophysics of cadherin adhesion

Since the identification of cadherins and the publication of the first crystal structures, the mechanism of cadherin adhesion, and the underlying structural basis have been studied with a number of different experimental techniques, different classical cadherin subtypes, and cadherin fragments. Earlier studies based on biophysical measurements and structure determinations resulted in seemingly contradictory findings regarding cadherin adhesion. However, recent experimental data increasingly reveal parallels between structures, solution binding data, and adhesion-based biophysical measurements that are beginning to both reconcile apparent differences and generate a more comprehensive model of cadherin-mediated cell adhesion. This chapter summarizes the functional, structural, and biophysical findings relevant to cadherin junction assembly and adhesion. We emphasize emerging parallels between findings obtained with different experimental approaches. Although none of the current models accounts for all of the available experimental and structural data, this chapter discusses possible origins of apparent discrepancies, highlights remaining gaps in current knowledge, and proposes challenges for further study.

Binding of fullerol to human serum albumin: spectroscopic and electrochemical approach

The potential impact of human exposure to carbonaceous nanomaterials in the environment becomes a concerning issue. Here we report on the interaction of fullerol with human serum albumin (HSA) using spectroscopic and electrochemical methods. The water-soluble fullerene derivative (fullerol) was synthesized and characterized by IR, (1)H NMR, TG-DSC, XRD, HR-TEM, etc. The spectroscopic methods show that the fluorescence quenching of HSA by fullerol is the result of the formation of an HSA-fullerol complex. Binding parameters such as ΔG, ΔH and ΔS were calculated, and the quenching constant K(a) at different temperatures was determined using the modified Stern-Volmer equation. The electrochemical experiments further confirmed the conclusions. In addition, the influences of coexisting heavy metal ions have also been studied in the present system. The circular dichroism spectra (CD), 3D fluorescence spectra and FT-IR spectra results suggest that the secondary structure of HSA was changed by fullerol. Based on the site marker competitive experiments, we can predict the possible binding position of fullerol on the HSA was located at the site of sub domain II A. Furthermore, the distance r between donor (HSA) and acceptor (fullerol) was obtained according to the famous fluorescence resonance energy transfer (FRET) mechanism.

The investigation of the interaction between NCP-EDA and bovine serum albumin by spectroscopic approaches

The fluorescence and ultraviolet spectroscopies were explored to study the interaction between N-confused porphyrins-edaravone diad (NCP-EDA) and bovine serum albumin (BSA) under simulative physiological condition at different temperatures. The experimental results show that the fluorescence quenching mechanism between NCP-EDA and BSA is a combined quenching (dynamic and static quenching). The binding constants, binding sites and the corresponding thermodynamic parameters (ΔG, ΔH, and ΔS) of the interaction system were calculated at different temperatures. According to Förster non-radiation energy transfer theory, the binding distance between NCP-EDA and BSA was calculated to be 3.63 nm. In addition, the effect of NCP-EDA on the conformation of BSA was analyzed using synchronous fluorescence spectroscopy.

The basic property of Lys385 is important for potentiation of the human α1 glycine receptor by ethanol

Ethanol alters the function of several members of the Cys-loop ligand-gated ion channel superfamily. Recent studies have shown that the sensitivity of the α1 glycine receptor (GlyR) to ethanol can be affected by the state of G protein activation mediated by the interaction of Gβγ with intracellular amino acids in the GlyR. Here, we evaluated the physicochemical property of Lys385 that contributes to ethanol modulation by using mutagenesis, patch-clamp, and biochemical techniques. A conserved substitution (K385R) did not affect either the apparent glycine EC₅₀ (40 ± 1 versus 41 ± 0.5 μM) or the ethanol-induced potentiation (53 ± 5 versus 46 ± 5%) of the human α1 GlyR. On the other hand, replacement of this residue with glutamic acid (K385E), an acidic amino acid, reduced the potentiation of the GlyR to 10 ± 1%. Furthermore, mutations with a hydrophobic leucine (K385L), a hydrogen bond donor glutamine (K385Q), or a neutral residue (K385A) also reduced ethanol modulation. Finally, substitution by a large and hydrophobic residue (K385F) and deletion of 385 (Lys385_) reduced ethanol modulation to 10 ± 4 and 17 ± 0.4%, respectively. Experiments using dynamic cysteine substitution with a methanethiosulfonate reagent and homology modeling indicate that the basic property and the position of Lys385, probably because of its interaction with Gβγ, is critical for ethanol potentiation of the receptor.

Modeling nuclear blebs in a nucleoskeleton of independent filament networks

Correlations between altered nuclear shape and disease are empirically observed, but the causes of nuclear dysmorphisms are poorly understood. The nucleoskeleton, which provides the majority of the mechanical stability of the nucleus, is composed primarily of intermediate filaments of lamin proteins. The nucleoskeleton forms a mostly-planar network between the inner nuclear membrane and chromatin. It is unclear if blebs and larger scale changes in nuclear morphology are consequences of reorganization of the nucleoskeleton alone or of other cellular processes. To test this, we computationally recapitulate the lamina network using a mechanical network model created as a network of Hookean springs. A- and B-type lamin filaments were distributed over a spherical surface into distinct networks linked to one another by lamin-associated proteins. Iterative force-based adjustment of the network structure, together with a stochastically modified Bell model of bond breakage and formation, simulates nucleoskeleton reorganization with blebs. The rate of bleb retraction into the nucleus depends on both initial size of the bleb and number of networks being deformed. Our results show that induced blebs are more stable when only one filament component is deformed or when the networks have no interconnections. Also, the kinetics of retraction is influenced by the composition of the bleb. These results match with our experiments and others.

Interaction of monosulfonate tetraphenyl porphyrin (H2TPPS1) with plant-esterase: determination of the binding mechanism by spectroscopic methods

The interaction of monosulfonate tetraphenyl porphyrin (H(2)TPPS(1)) with plant-esterase was investigated using fluorescence and UV-vis absorption spectroscopy. Fluorescence quenching, from which the binding parameters were evaluated, revealed that the quenching of the esterase by H(2)TPPS(1) resulted from the formation of a dye-esterase complex. According to the modified Stern-Volmer equation, the effective quenching constants (K(a)) between H(2)TPPS(1) and plant-esterase at four different temperatures (297 K, 300 K, 303 K, and 306 K) were obtained to be 14.132×10(5), 5.734×10(5), 2.907×10(5), and 2.291×10(5) M(-1), respectively. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) for the reaction were calculated to be -181.67 kJ M(-1) and -0.49 kJ M(-1)K(-1), indicating that van der Waals force and hydrogen bonds were the dominant intermolecular force in stabilizing the complex. Site marker competitive experiments showed that the binding of H(2)TPPS(1) to plant-esterase primarily took place in the active site. The binding distance (r) was obtained to be 5.99 nm according to Förster theory of non-radioactive energy transfer. The conformation of plant-esterase was investigated by synchronous fluorescence and UV-vis absorption spectroscopy, and the results confirmed some micro-environmental and conformational changes of plant-esterase molecules.

Interaction of cinnamic acid derivatives with serum albumins: a fluorescence spectroscopic study

Cinnamic acid (CA) derivatives are known to possess broad therapeutic applications including anti-tumor activity. The present study was designed to determine the underlying mechanism and thermodynamic parameters for the binding of two CA based intramolecular charge transfer (ICT) fluorescent probes, namely, 4-(dimethylamino) cinnamic acid (DMACA) and trans-ethyl p-(dimethylamino) cinnamate (EDAC), with albumins by fluorescence spectroscopy. Stern-Volmer analysis of the tryptophan fluorescence quenching data in presence of the added ligand reveals fluorescence quenching constant (κ(q)), Stern-Volmer constant (K(SV)) and also the ligand-protein association constant (K(a)). The thermodynamic parameters like enthalpy (ΔH) and entropy (ΔS) change corresponding to the ligand binding process were also estimated. The results show that the ligands bind into the sub-domain IIA of the proteins in 1:1 stoichiometry with an apparent binding constant value in the range of 10(4) dm(3) mol(-1). In both the cases, the spontaneous ligand binding to the proteins occur through entropy driven mechanism, although the interaction of DMACA is relatively stronger in comparison with EDAC. The temperature dependence of the binding constant indicates the induced change in protein secondary structure.

Engineered biological entities for drug delivery and gene therapy protein nanoparticles

The development of genetic engineering techniques has speeded up the growth of the biotechnological industry, resulting in a significant increase in the number of recombinant protein products on the market. The deep knowledge of protein function, structure, biological interactions, and the possibility to design new polypeptides with desired biological activities have been the main factors involved in the increase of intensive research and preclinical and clinical approaches. Consequently, new biological entities with added value for innovative medicines such as increased stability, improved targeting, and reduced toxicity, among others have been obtained. Proteins are complex nanoparticles with sizes ranging from a few nanometers to a few hundred nanometers when complex supramolecular interactions occur, as for example, in viral capsids. However, even though protein production is a delicate process that imposes the use of sophisticated analytical methods and negative secondary effects have been detected in some cases as immune and inflammatory reactions, the great potential of biodegradable and tunable protein nanoparticles indicates that protein-based biotechnological products are expected to increase in the years to come.

Interaction study of pioglitazone with albumin by fluorescence spectroscopy and molecular docking

Pioglitazone is a medicine of thiazolidinedione (TZD) class with hypoglycemic (antihyperglycemic, antidiabetic) action. Pioglitazone binding to human serum albumin (HSA) was investigated at different temperatures (290, 300 and 310 K) by fluorescence spectroscopic method. Molecular docking study was also carried out besides the experiments. Experimental results revealed that pioglitazone have an ability to quench the intrinsic fluorescence of HSA tryptophan through a static quenching procedure. The binding constant was determined using Stern-Volmer modified equation and energy transfer mechanisms of quenching were discussed. Thermodynamic parameters were also calculated according to enthalpy changes dependence on different temperatures. According to the theoretical and experimental results, hydrogen bonding was found to play a major role in the interaction of pioglitazone with HSA.

An atomic force microscopy method for the detection of binding forces between bacteria and a lipid bilayer containing higher order gangliosides

We developed an atomic force microscopy (AFM) method to determine the binding forces between a model cell wall plasma membrane and Vibrio cholerae. V. cholerae cells were covalently attached to AFM probes and forces were determined against a lipid bilayer containing sialic acid (N-acetylneuraminic acid) molecules as well as several control surfaces.

Structural relationship and binding mechanisms of five flavonoids with bovine serum albumin

Flavonoids are structurally diverse and the most ubiquitous groups of dietary polyphenols distributed in various fruits and vegetables. In this study, the interaction between five flavonoids, namely formononetin-7-O-β-D-glucoside, calycosin- 7-O-β-D-glucoside, calycosin, rutin, and quercetin, and bovine serum albumin (BSA) was investigated by fluorescence and UV-vis absorbance spectroscopy. In the discussion, it was proved that the fluorescence quenching of BSA by flavonoids was a result of the formation of a flavonoid–BSA complex. Fluorescence quenching constants were determined using the Stern-Volmer and Lineweaver-Burk equations to provide a measure of the binding affinity between the flavonoids and BSA. The binding constants ranked in the order quercetin > rutin > calycosin > calycosin-7-O-β-D-glucoside ≈ formononetin-7-O-β-D-glucoside. The results of thermodynamic parameters ΔG, ΔH, and ΔS at different temperatures indicated that the hydrophobic interaction played a major role in flavonoid–BSA association. The distance r between BSA and acceptor flavonoids was also obtained according to Förster’s theory of non-radiative energy transfer.

Synthesis and characterization of polymer eight-coordinate (enH2)[YIII(pdta)(H2O)](2)·10H2O as well as the interaction of [YIII(pdta)(H2O)]2(2-) with BSA

The eight-coordinate (enH2)[YIII(pdta)(H2O)](2)·10H2O (en=ethylenediamine and H4pdta=1,3-propylenediamine-N,N,N',N'-tetraacetic acid) was synthesized, meanwhile its molecular and crystal structures were determined by single-crystal X-ray diffraction technology. The interaction between [Y(III)(pdta)(H2O)]2(2-) and bovine serum albumin (BSA) was investigated by UV-vis and fluorescence spectra. The results indicate that [YIII(pdta)(H2O)]2(2-) quenched effectively the intrinsic fluorescence of BSA via a static quenching process with the binding constant (Ka) of the order of 10(4). Meanwhile, the binding and damaging sites to BSA molecules were also estimated by synchronous fluorescence. Results indicate that the hydrophobic environments around Trp and Tyr residues were all slightly changed. The thermodynamic parameters (ΔG=-25.20 kJ mol(-1), ΔH=-26.57 kJ mol(-1) and ΔS=-4.58 J mol(-1) K(-1)) showed that the reaction was spontaneous and exothermic. What is more, both ΔH and ΔS were negative values indicated that hydrogen bond and Van der Waals forces were the predominant intermolecular forces between [YIII(pdta)(H2O)]2(2-) and BSA.

Binding of several benzodiazepines to bovine serum albumin: Fluorescence study

The interactions of lorazepam, oxazepam and bromazepam with bovine serum albumin (BSA) were studied by fluorescence spectrometry. The Stern-Volmer quenching constants and corresponding thermodynamic parameters DeltaH, DeltaG and DeltaS were calculated. The binding constants and the number of binding sites were also investigated. The distances between the donor (BSA) and the acceptors (benzodiazepines) were obtained according to fluorescence resonance energy transfer and conformational changes of BSA were observed from synchronous fluorescence spectra.

Sampling protein form and function with the atomic force microscope

To study the structure, function, and interactions of proteins, a plethora of techniques is available. Many techniques sample such parameters in non-physiological environments (e.g. in air, ice, or vacuum). Atomic force microscopy (AFM), however, is a powerful biophysical technique that can probe these parameters under physiological buffer conditions. With the atomic force microscope operating under such conditions, it is possible to obtain images of biological structures without requiring labeling and to follow dynamic processes in real time. Furthermore, by operating in force spectroscopy mode, it can probe intramolecular interactions and binding strengths. In structural biology, it has proven its ability to image proteins and protein conformational changes at submolecular resolution, and in proteomics, it is developing as a tool to map surface proteomes and to study protein function by force spectroscopy methods. The power of AFM to combine studies of protein form and protein function enables bridging various research fields to come to a comprehensive, molecular level picture of biological processes. We review the use of AFM imaging and force spectroscopy techniques and discuss the major advances of these experiments in further understanding form and function of proteins at the nanoscale in physiologically relevant environments.

Spatially pathogenic forms of tau detected in Alzheimer's disease brain tissue by fluorescence lifetime-based Förster resonance energy transfer

In tauopathies including Alzheimer's disease (AD) tau molecules have lost their normal spatial distance to each other and appear in oligomeric or aggregated forms. Conventional immunostaining methods allow detection of abnormally phosphorylated or conformationally altered aggregated tau proteins, but fail to visualize oligomeric forms of tau. Here we show that tau molecules that lost their normal spatial localization can be detected on a subcellular level in postmortem central nervous system (CNS) tissue sections of AD patients by fluorescence lifetime-based Förster resonance energy transfer (FRET). Paraffin sections were co-immunostained with two tau-specific monoclonal antibodies recognizing the same epitope, but labeled with distinct fluorescence dyes suitable for spatial resolution at a nanometer scale by lifetime-based FRET. A FRET signal was detected in neuritic plaques and neurofibrillary tangles of CNS tissue sections of AD patients, showing associated tau proteins typically reflecting either fibrillary, oligomeric or aggregated tau. The 'pretangle-like' structures within the neuronal perikarya did not contain spatially pathogenic forms of tau accordingly to this method. Data demonstrate that fluorescence lifetime-based FRET can be applied to human brain tissue sections to detect pathogenic forms of tau molecules that lost their normal spatial distance.

Spectroscopic investigation on the toxic interactions of Ni2+ with bovine hemoglobin

The toxic interaction between Ni(2+) and bovine hemoglobin (BHb) was investigated using fluorescence spectroscopy, synchronous fluorescence spectroscopy, ultraviolet-visible (UV-vis) absorption spectroscopy and circular dichroism spectroscopy (CD) under simulated physiological conditions. The experimental results showed that both dynamic and static quenching occurred simultaneously when Ni(2+) quenched the fluorescence of BHb. The binding site number n, apparent binding constant K(a) and corresponding thermodynamic parameters were measured at different temperatures. There was formation of Ni-BHb complex, but the binding between Ni(2+) and BHb was not strong. The process of the formation of Ni-BHb complex was a spontaneous interaction procedure in which electrostatic interaction played a major role. In addition, UV-vis and CD results showed that the addition of Ni(2+) changed the conformation of BHb.

Hydration potential of lysozyme: protein dehydration using a single microparticle technique

For biological molecules in aqueous solution, the hydration pressure as a function of distance from the molecular surface represents a very short-range repulsive pressure that limits atom-atom contact, opposing the attractive van der Waals pressure. Whereas the separation distance for molecules that easily arrange into ordered arrays (e.g., lipids, DNA, collagen fibers) can be determined from x-ray diffraction, many globular proteins are not as easily structured. Using a new micropipette technique, spherical, glassified protein microbeads can be made that allow determination of protein hydration as a function of the water activity (a(w)) in a surrounding medium (decanol). By adjusting a(w) of the dehydration medium, the final protein concentration of the solid microbead is controlled, and ranges from 700 to 1150 mg/mL. By controlling a(w) (and thus the osmotic pressure) around lysozyme, the repulsive pressure was determined as a function of distance between each globular, ellipsoid protein. For separation distances, d, between 2.5 and 9 A, the repulsive decay length was 1.7 A and the pressure extrapolated to d = 0 was 2.2 x 10(8) N/m(2), indicating that the hydration pressure for lysozyme is similar to other biological interfaces such as phospholipid bilayers.

Interaction of 2-(9H-purin-6-ylamino)-4-(methylthio) butanoic acid with human serum albumin: fluorescence and modeling studies

This study was designed to investigate the interaction between (S)-2-(9H-purin-6-ylamino)-4-(methylthio) butanoic acid (PYMBA) and human serum albumin (HSA) using fluorescence spectroscopy. The combination of UV absorption and molecular docking under simulative physiological conditions was also applied to comprehensively understand the binding mechanism of PYMBA to HSA. Fluorescence data indicated that PYMBA has a strong ability to quench the intrinsic fluorescence of HSA. The binding constants (K) at different temperatures, thermodynamic parameters including enthalpy change (DeltaH) and entropy change (DeltaS) of PYMBA-HSA were correlated to the relevant fluorescence data, which suggested that the hydrophobic force played a very important role for the PYMBA binding to the HSA. The experimental results were in agreement with the results obtained via a molecular docking study. The effects of other ions on the binding constants were also studied.

Spectroscopic studies on the binding of cobalt(II) 1,10-phenanthroline complex to bovine serum albumin

The binding interaction of the cobalt(II) 1,10-phenanthroline complex (Co(phen) (3) (2+) , phen = 1,10-phenanthroline) with bovine serum albumin (BSA) was investigated by fluorescence spectroscopy combined with UV-Vis absorption and circular dichroism measurements under simulative physiological conditions. The experiment results showed that the fluorescence intensity of BSA was dramatically decreased owing to the formation of Co(phen) (3) (2+) -BSA complex. The corresponding association constants (K (a)) between Co(phen) (3) (2+) and BSA at four different temperatures were calculated according to the modified Stern-Volmer equation. The enthalpy change (DeltaH degrees ) and entropy change (DeltaS degrees ) were calculated to be -2.73 kJ mol(-1) and 82.27 J mol(-1) K(-1), respectively, which suggested that electrostatic interaction and hydrophobic force played major roles in stabilizing the Co(phen) (3) (2+) -BSA complex. Site marker competitive experiments indicated that the binding of Co(phen) (3) (2+) to BSA primarily took place in site I of BSA. A value of 4.11 nm for the average distance r between Co(phen) (3) (2+) (acceptor) and tryptophan residues of BSA (donor) was derived from Förster's energy transfer theory. The conformational investigation showed that the presence of Co(phen) (3) (2+) resulted in the change of BSA secondary structure and induced the slight unfolding of the polypeptides of protein, which confirmed the microenvironment and conformational changes of BSA molecules.

A comparison study on the interaction of hyperoside and bovine serum albumin with Tachiya model and Stern-Volmer equation

The interaction between hyperoside and bovine serum albumin (BSA) was examined by fluorescence spectroscopy at 298, 304, and 310K. The spectroscopic data were analyzed using Tachiya model and Stern-Volmer equation to determine the binding sites and apparent binding constant between hyperoside and BSA. For Tachiya model, both binding sites and apparent binding constants increased with the increasing of temperature, whereas for Stern-Volmer equation, the corresponding binding constants decreased as temperature increasing and the binding sites were independent of temperature. The positive sign of enthalpy change (DeltaH) and entropy change (DeltaS) suggested that hydrophobic forces played a major role in the interaction. Synchronous fluorescence spectra indicated that the conformation of protein was perturbed by the interaction of hyperoside with BSA. Moreover, the presence of metal ion affected the hyperoside-BSA binding.

Dependence of T cell antigen recognition on T cell receptor-peptide MHC confinement time

T cell receptor (TCR) binding to diverse peptide-major histocompatibility complex (pMHC) ligands results in various degrees of T cell activation. Here we analyze which binding properties of the TCR-pMHC interaction are responsible for this variation in pMHC activation potency. We have analyzed activation of the 1G4 cytotoxic T lymphocyte clone by cognate pMHC variants and performed thorough correlation analysis of T cell activation with 1G4 TCR-pMHC binding properties measured in solution. We found that both the on rate (k_on) and off rate (k_off) contribute to activation potency. Based on our results, we propose a model in which rapid TCR rebinding to the same pMHC after chemical dissociation increases the effective half-life or “confinement time” of a TCR-pMHC interaction. This confinement time model clarifies the role of k_on in T cell activation and reconciles apparently contradictory reports on the role of TCR-pMHC binding kinetics and affinity in T cell activation.

Characterization of the interaction between 8-bromoadenosine with human serum albumin and its analytical application

This study was designed to examine the interaction of 8-bromoadenosine with human serum albumin (HSA) by fluorescence spectroscopy in combination with molecular modeling under simulative physiological conditions. The results of fluorescence measurements indicate that 8-bromoadenosine has a strong ability to quench the intrinsic fluorescence of HSA through static quenching procedure. The binding constants (K) at different temperatures and thermodynamic parameters, enthalpy changes (DeltaH) and entropy changes (DeltaS) were calculated according to the fluorescence data. The results showed that the hydrophobic force played the major role in the binding of 8-bromoadenosine to HSA. The fluorescence experimental results were in agreement with the results obtained by molecular modeling study. The effects of some normal positive and negative ions on the binding constants were also discussed. Moreover, the synchronous fluorescence technique was used to characterize the interaction of 8-bromoadenosine to HSA and successfully applied to determine the total proteins in human serum, urine and saliva samples at room temperature under the optimum conditions with a wide linear range and satisfactory results.

Binding interaction of quinclorac with bovine serum albumin: a biophysical study

Quinclorac (QUC) is a new class of highly selective auxin herbicides. The interaction between QUC and bovine serum albumin (BSA) was investigated by fluorescence spectroscopy, synchronous fluorescence, three-dimensional fluorescence, CD spectroscopy and UV-vis absorption spectroscopy under simulative physiological condition. It was proved that the probable quenching mechanism of BSA by quinclorac was dynamic quenching. The Stern-Volmer quenching model has been successfully applied and the activation energy of the interaction as much as 8.03 kJ mol(-1), corresponding thermodynamic parameters DeltaH(theta), DeltaS(theta) and DeltaG(theta) were calculated. The results indicated that the acting forces between QUC and BSA were mainly hydrogen bonding and van der Waals forces. According to the Förster non-radiation energy transfer theory, the average binding distance between donor (BSA) and acceptor (QUC) was obtained (r=3.12 nm). The alterations of protein secondary structure in the presence of QUC were confirmed by the evidences from three-dimensional fluorescence, synchronous fluorescence and CD spectroscopy. Furthermore, the site marker competitive experiments indicated that the binding of QUC to BSA primarily took place in Sudlow site I.