Resonance energy transfer between cysteine-34 and tryptophan-214 in human serum albumin. Distance measurements as a function of pH

Exploring the inner environment of protein hydrogels with fluorescence spectroscopy towards understanding their drug delivery capabilities

Time-resolved fluorescence have used to explore the inner surface and solvation dynamics within protein hydrogels assisting in rationalizing their drug binding and release capabilities.

Inhibitory kinetics and mechanism of rifampicin on α-glucosidase: Insights from spectroscopic and molecular docking analyses

α-Glucosidase is a critical enzyme associated with diabetes mellitus, and the inhibitors of the enzyme play important roles in the treatment of the disease. In this study, the inhibitory effect and mechanism of rifampicin on α-glucosidase were investigated by multispectroscopic methods along with molecular docking technique. The results showed that rifampicin inhibited α-glucosidase activity prominently (IC₅₀ = 135 ± 1.2 μM) in a reversible and competitive-type manner. The fluorescence intensity of α-glucosidase was quenched by rifampicin through forming rifampicin-α-glucosidase complex in a static procedure. And the formation of the rifampicin-α-glucosidase complex was driven spontaneously by hydrophobic forces and hydrogen bonds. The results obtained from molecular docking further indicated that hydrophobic forces were formed between rifampicin and amino acid residues Phe 173, Pro151, and hydrogen bonds were generated by the interactions of rifampicin with residues Ser 180, Asn 414, Gly160, and Gly161 of α-glucosidase. Moreover, it was found that the binding of rifampicin to α-glucosidase could alter the conformation of the enzyme to make it steady, and the binding distance was estimated to be 1.02 nm. Therefore, this study confirmed a novel α-glucosidase inhibitor and possibly contributed to the improvement of newfangled anti-diabetic agent.

A novel high-performance liquid chromatography-fluorescence analysis coupled with in situ degradation-derivatization technique for quantitation of organophosphorus thioester pesticide residues in tea

A novel high-performance liquid chromatography-fluorescence analysis in combination with in situ degradation-derivatization (ISD-D) technique was developed for simultaneous determination of seven organophosphorus thioester pesticides (OPTPs) in tea. The ISD-D technique was based on degradation of OPTPs by a nucleophilic substitution reaction between phenylbutane-1,2,3-trione-2-oxime and OPTPs, which can give thiol degradation products (DPs). The thiol DPs obtained were derivatized with the novel derivatization reagent N-(4-(carbazole-9-yl)-phenyl)-N-maleimide (NCPM) in a syringe. Attractively, NCPM itself did not fluoresce, whereas the derivatives of the thiol DPs fluoresced intensely, with excitation and emission maxima at 290 nm and 368 nm, respectively, which extraordinary reduced the background interference and increased the detection sensitivity for thiol DPs. Excellent linearity (R² > 0.995) for all OPTPs was achieved, with limits of detection and limits of quantitation ranging from 0.23 to 0.45 μg/kg and from 0.75 to 1.43 μg/kg, respectively. Satisfactory recoveries ranging from 90.5% to 96.0% were obtained for all OPTPs. The ISD-D technique provided a novel and sensitive strategy for quantitation of trace amounts of OPTPs in real samples. Graphical abstract ᅟ.

Photoacids as a new fluorescence tool for tracking structural transitions of proteins: following the concentration-induced transition of bovine serum albumin

Spectroscopy-based techniques for assessing structural transitions of proteins follow either an intramolecular chromophore, as in absorption-based circular dichroism (CD) or fluorescence-based tryptophan emission, or an intermolecular chromophore such as fluorescent probes. Here a new fluorescent probe method to probe the structural transition of proteins by photoacids is presented, which has a fundamentally different photo-physical origin to that of common fluorescent probes. Photoacids are molecules that release a proton upon photo-excitation. By following the steady-state and time-resolved emission of the protonated and de-protonated species of the photoacid we probe the environment of its binding site in bovine serum albumin (BSA) in a wide range of weight concentrations (0.001-8%). We found a unique concentration-induced structural transition of BSA at pH2 and at concentrations of >0.75%, which involves the exposure of its hydrophobic core to the solution. We confirm our results with the common tryptophan emission method, and show that the use of photoacids can result in a much more sensitive tool. We also show that common fluorescent probes and the CD methodologies have fundamental restrictions that limit their use in a concentration-dependent study. The use of photoacids is facile and requires only a fluorospectrometer (and preferably, but not mandatorily, a time-resolution emission system). The photoacid can be either non-covalently (as in this study) or covalently attached to the molecule, and can be readily employed to follow the local environment of numerous (bio-)systems.

Intercalation of the daphnetin–Cu(ii) complex with calf thymus DNA

The daphnetin–Cu(ii) complex binds to the A–T bases region of ctDNA and causes cleavage of plasmid DNA.

Characterization of the binding of paylean and DNA by fluorescence, UV spectroscopy and molecular docking techniques

The interaction of paylean (PL) with calf thymus DNA (ctDNA) was investigated using fluorescence spectroscopy, UV absorption, melting studies, ionic strength, viscosity experiments and molecular docking under simulated physiological conditions. Values for the binding constant Ka between PL and DNA were 5.11 × 10(3) , 2.74 × 10(3) and 1.74 × 10(3)  L mol(-1) at 19, 29 and 39°C respectively. DNA quenched the intrinsic fluorescence of PL via a static quenching procedure as shown from Stern-Volmer plots. The relative viscosity and the melting temperature of DNA were basically unchanged in the presence of PL. The fluorescence intensity of PL-DNA decreased with increasing ionic strength. The value of Ka for PL with double-stranded DNA (dsDNA) was larger than that for PL with single-stranded DNA (ssDNA). All the results revealed that the binding mode was groove binding, and molecular docking further indicated that PL was preferentially bonded to A-T-rich regions of DNA. The values for ΔH, ΔS and ΔG suggested that van der Waals forces or hydrogen bonding might be the main acting forces between PL and DNA. The binding distance was determined to be 3.37 nm based on the theory of Förster energy transference, which indicated that a non-radiation energy transfer process occurred. Copyright © 2015 John Wiley & Sons, Ltd.

Binding studies of terbutaline sulfate to calf thymus DNA using multispectroscopic and molecular docking techniques

The interaction of terbutaline sulfate (TS) with calf thymus DNA (ctDNA) were investigated by fluorescence quenching, UV-vis absorption, viscosity measurements, ionic strength effect, DNA melting experiments and molecular docking. The binding constants (Ka) of TS to ctDNA were determined as 4.92×10(4), 1.26×10(4) and 1.16×10(4) L mol(-1) at 17, 27 and 37 °C, respectively. Stern-Volmer plots suggested that the quenching of fluorescence of TS by ctDNA was a static quenching. The absorption spectra of TS with ctDNA revealed a slight blue shift and hyperchromic effect. The relative viscosity ctDNA was hardly changed by TS, and melting temperature varied slightly. For the system of TS-ctDNA, the intensity of fluorescence decreased with the increase of ionic strength. Also, the Ka for TS-double stranded DNA (dsDNA) was clearly weaker than that for TS-single stranded DNA (ssDNA). All these results revealed that the binding mode of TS with ctDNA should be groove binding. The enthalpy change and entropy change suggested that van der Waals force or hydrogen bonds was a main binding force between TS and ctDNA. Furthermore, the quantum yield of TS was measured by comparing with the standard solution. Based on the Förster energy transference theory (FRET), the binding distance between the acceptor and donor was calculated. Molecular docking showed that TS was a minor groove binder of ctDNA and preferentially bound to A-T rich regions.

In vitro studies on the behavior of salmeterol xinafoate and its interaction with calf thymus DNA by multi-spectroscopic techniques

The salmeterol xinafoate (SX) binding to calf thymus DNA in vitro was explored by fluorescence, resonance light scattering (RLS), UV-vis absorption, as well as viscometry, ionic strength effect and DNA melting techniques. It was found that SX could bind to DNA weakly, and the binding constants (Ka) were determined as 8.52×10(3), 8.31×10(3) and 6.14×10(3) L mol(-1) at 18, 28 and 38°C respectively. When bound to DNA, SX showed fluorescence quenching in the fluorescence spectra and hyperchromic effect in the absorption spectra. Stern-Volmer plots revealed that the quenching of fluorescence of SX by DNA was a static quenching. Furthermore, the relative viscosity and melting temperature of DNA solution were hardly influenced by SX, while the fluorescence intensity of SX-DNA was observed to decrease with the increasing ionic strength of system. Also, the binding constant between SX and double stranded DNA (dsDNA) was much weaker than that between SX and single stranded DNA (ssDNA). All these results suggested that the binding mode of SX to DNA should be groove binding. The obtained thermodynamic parameters indicated that electrostatic force might play a predominant role in SX binding to DNA. The quantum yield (φ) of SX was measured as 0.13 using comparative method. Based on the Förster resonance energy transfer theory (FRET), the binding distance (r0) between the acceptor and donor was calculated as 4.10 nm.

Binding characteristics of salbutamol with DNA by spectral methods

Salbutamol interacting with deoxyribonucleic acid (DNA) was examined by fluorescence, UV absorption, viscosity measurements, and DNA melting techniques. The binding constants and binding sites were obtained at different temperatures by fluorescence quenching. The Stern-Volmer plots showed that the quenching of fluorescence of salbutamol by DNA was a static quenching. To probe the binding mode, various analytical methods were performed and the results were as follows: hyperchromic effect was shown in the absorption spectra of salbutamol upon addition of DNA; there was no appreciable increase in melting temperature of DNA when salbutamol was presented in DNA solution; the fluorescence intensity of salbutamol-DNA decrease with the increasing ionic strength; the relative viscosity of DNA did not change in the presence of salbutamol; the binding constant of salbutamol with double strand DNA (dsDNA) was much higher than that of it with single strand DNA (ssDNA). All these results indicated that the binding mode of salbutamol to DNA should be groove binding. The thermodynamic parameters suggested that hydrogen bond or van der Waals force might play an important role in salbutamol binding to DNA. According to the Förster energy transference theory, the binding distance between the acceptor and donor was 3.70 nm.

Effect of pH on the interaction of vitamin B12 with bovine serum albumin by spectroscopic approaches

The interaction mechanism between vitamin B12 (B12, cyanocobalamin) and bovine serum albumin (BSA) has been investigated by fluorescence, synchronous fluorescence, ultraviolet-vis (UV) absorbance, and three-dimensional fluorescence. The intrinsic fluorescence of BSA was strongly quenched by the addition of B12 in different pH buffer solutions (pH 2.5, 3.5, 5.0, 7.4, and 9.0) and spectroscopic observations are mainly rationalized in terms of a static quenching process at lower concentration of B12 (C(B12)/C(BSA)<5) and a combined quenching process at higher concentration of B12 (C(B12)/C(BSA)>5). The structural characteristics of B12 and BSA were probed, and their binding affinities were determined under different pH conditions. The results indicated that the binding abilities of B12 to BSA in the acidic and basic pH regions (pH 2.5, 3.5, 5.0, and 9.0) were lower than that at simulating physiological condition (pH 7.4). In addition, the efficiency of energy transfer from tryptophan fluorescence to B12 was found to depend on the binding distance r between the donor and acceptor calculated using Förster's theory. The effect of B12 on the conformation of BSA was analyzed using UV, synchronous fluorescence and three-dimensional fluorescence under different pH conditions. These results showed that the binding of B12 to BSA causes apparent change in the secondary and tertiary structures of BSA.

New advances in the study on the interaction of [Cr(phen)2(dppz)]3+ complex with biological models; association to transporting proteins

The present study reports a detailed investigation with the interaction of [Cr(phen)(2)(dppz)](3+) with serum albumins, the key protein for the transport of drugs in the blood plasma, which allows us to understand further the role of [Cr(phen)(2)(dppz)](3+) as sensitizer in Photodynamic Therapy (PDT). Chromium(III) complex [Cr(phen)(2)(dppz)](3+), (dppz = dipyridophenazine and phen=1,10-phenanthroline), where dppz is a planar bidentate ligand with an extended π system, has been found to bind strongly with bovine and human serum albumins (BSA and HSA) with an intrinsic binding constants, K(b), of (1.7±0.3)×10(5) M(-1) and (2.2±0.3)×10(5) M(-1) at 295K, respectively. The interactions of serum albumins with [Cr(phen)(2)(dppz)](3+) were assessed employing fluorescence spectroscopy, circular dichroism and UV-vis absorption spectroscopy. The serum albumins-[Cr(phen)(2)(dppz)](3+) interactions caused conformational changes with the loss of helical stability of the protein and local perturbation in the domain IIA binding pocket. The relative fluorescence intensity of the albumin (BSA or HSA) bound to the Cr(III) complex decreased, suggesting that perturbation around the Trp 214 residue took place. The analysis of the thermodynamic parameters ΔG, ΔH, ΔS indicated that the hydrophobic interactions played a major role in both BSA-Cr(III) and HSA-Cr(III) association processes. The binding distances and transfer efficiencies for BSA-Cr(III) and HSA-Cr(III) binding reactions were calculated according to the Föster theory of non-radiation energy transfer. All these experimental results suggests that [Cr(phen)(2)(dppz)](3+) binds to serum albumins, by which these proteins could act as carriers of this complex for further applications in PDT.

Investigation on the binding of TNS to centrin, an EF-hand protein

The interaction between 2-p-toluidinylnaphthalene-6-sulfonate (TNS) and ciliate Euplotes Octocarinatus centrin (Cen) has been studied by fluorescence spectroscopy. The binding constants of TNS with Cen were measured at different temperature in the 0.01M Hepes, pH 7.4. The binding process is exothermic and involves a positive entropy change. The negative value of enthalpy predominately contributes to the negative free energy of binding between TNS and Cen. The salt (KCl) increases the association constant of TNS and Cen. These results and resonance light scattering experiment suggest that the binding force between TNS and Cen is hydrophobic. The distance (r) between TNS and tryptophan of mutant G115W, which sheds more insight into the binding of TNS to Cen, was determined as 4.85nm based on Förster non-radiative energy transfer theory.

Unfolding and refolding of bovine serum albumin at acid pH: ultrasound and structural studies

Serum albumin is the most abundant protein in the circulatory system. The ability of albumins to undergo a reversible conformational transition, observed with changes in pH, is conserved in distantly related species, suggesting for it a major physiological role possibly related to the transport of small molecules including drugs. We have followed changes of bovine serum albumin (BSA) in volume by densimetry and in adiabatic compressibility during its conformational transition from pH 7-2, using ultrasound measurements. In parallel, circular dichroism was measured. The volume and adiabatic compressibility decrease from pH 4 to 2. The change in ellipticity shows a decrease over the same pH range from 70% to 40% of its alpha-helix content. Sorbitol, at concentrations from 0 to 2 M, led to the progressive restoration of BSA volume and compressibility values, as well as a substantial recovery of its original alpha-helix content. This finding implies that the compressibility variation observed reflects the conformational changes during the transition. The mutual interactions of the mechanical properties and structural features of BSA reported here are important in biotechnology for research in material sciences and for the design and the development of new, tailor-made drug carriers.

Spectroscopic characterization of effective components anthraquinones in Chinese medicinal herbs binding with serum albumins

The interactions of serum albumins such as human serum albumin (HSA) and bovine serum albumin (BSA) with emodin, rhein, aloe-emodin and aloin were assessed employing fluorescence quenching and absorption spectroscopic techniques. The results obtained revealed that there are relatively strong binding affinity for the four anthraquinones with HSA and BSA and the binding constants for the interactions of anthraquinones with HSA or BSA at 20 degrees C were obtained. Anthraquinone-albumin interactions were studied at different temperatures and in the presence of some metal ions. And the competition binding of anthraquinones with serum albumins was also discussed. The Stern-Volmer curves suggested that the quenching occurring in the reactions was the static quenching process. The binding distances and transfer efficiencies for each binding reactions were calculated according to the Föster theory of non-radiation energy transfer. Using thermodynamic equations, the main action forces of these reactions were also obtained. The reasons of the different binding affinities for different anthraquinone-albumin reactions were probed from the point of view of molecular structures.

Interaction of human serum albumin with bendroflumethiazide studied by fluorescence spectroscopy

The interactions between bendroflumethiazide (BFTZ) and human serum albumin (HSA) have been studied by fluorescence spectroscopy. Binding constants for drug attachment to the various binding sites of HSA have been measured at different temperatures in physiological buffer solution. The effect of metal ions on BFTZ interaction with HSA was also investigated. The thermodynamic parameters, DeltaH and DeltaS, have been calculated to be 49.28kJmol(-1)>0, and 258.83Jmol(-1)K(-1)>0, respectively. The distance between HSA and BFTZ, r, was determined to be 1.47nm based on Förster's non-radiative energy transfer theory. The experimental results reveal that BFTZ has a strong ability to quench the intrinsic fluorescence of HSA through a static quenching mechanism. Furthermore, the binding constants between BFTZ and HSA are remarkably independent of temperature, and decrease in the presence of various ions, usually by about 30-55%. Hydrophobic interaction occurs between BFTZ and the sub-domain II A of HSA.

Conformational transitions of the three recombinant domains of human serum albumin depending on pH

Human serum albumin (HSA) is a protein of 66.5 kDa that is composed of three homologous domains, each of which displays specific structural and functional characteristics. HSA is known to undergo different pH-dependent structural transitions, the N-F and F-E transitions in the acid pH region and the N-B transition at slightly alkaline pH. In order to elucidate the structural behavior of the recombinant HSA domains as stand-alone proteins and to investigate the molecular and structural origins of the pH-induced conformational changes of the intact molecule, we have employed fluorescence and circular dichroic methods. Here we provide evidence that the loosening of the HSA structure in the N-F transition takes place primarily in HSA-DOM III and that HSA-DOM I undergoes a structural rearrangement with only minor changes in secondary structure, whereas HSA-DOM II transforms to a molten globule-like state as the pH is reduced. In the pH region of the N-B transition of HSA, HSA-DOM I and HSA-DOM II experience a tertiary structural isomerization, whereas with HSA-DOM III no alterations in tertiary structure are observed, as judged from near-UV CD and fluorescence measurements.

Unfolding of acrylodan-labeled human serum albumin probed by steady-state and time-resolved fluorescence methods

Steady-state and time-resolved fluorescence spectroscopy was used to follow the local and global changes in structure and dynamics during chemical and thermal denaturation of unlabeled human serum albumin (HSA) and HSA with an acrylodan moiety bound to Cys34. Acrylodan fluorescence was monitored to obtain information about unfolding processes in domain I, and the emission of the Trp residue at position 214 was used to examine domain II. In addition, Trp-to-acrylodan resonance energy transfer was examined to probe interdomain spatial relationships during unfolding. Increasing the temperature to less than 50 degrees C or adding less than 1.0 M GdHCl resulted in an initial, reversible separation of domains I and II. Denaturation by heating to 70 degrees C or by adding 2.0 M GdHCl resulted in irreversible unfolding of domain II. Further denaturation of HSA by either method resulted in irreversible unfolding of domain I. These results clearly demonstrate that HSA unfolds by a pathway involving at least three distinct steps. The low detection limits and high information content of dual probe fluorescence should allow this technique to be used to study the unfolding behavior of entrapped or immobilized HSA.

Protein binding investigation by difference circular dichroism: native and acetylated human serum albumins

A modified albumin was prepared by selective reaction of Lys199 with acetyl salicylic acid. Protein binding investigation was carried out on native and modified proteins by difference circular dichroism (delta CD). Acetylation of Lys199 reduces significantly the effects of the interaction between drugs in the stereoselective HSA binding at specific binding areas.

The existence of conformationally labile (preformed) drug binding sites in human serum albumin as evidenced by optical rotation measurements

The ability of certain drugs to induce conformational changes in human serum albumin has been examined by differential optical rotation measurements at 233 nm. At drug:protein molar ratios ([D]/[P]) of unity, the optical rotation increased, decreased or remained the same depending on the drug used. The change in the optical rotatory dispersion (ORD) signal was investigated as a function of the drug concentration. Drug-protein interactions were relatively specific. There exists at least one, and possibly more, stable preformed high affinity sites for the binding of drugs to albumin. At low [D]/[P] ratios, the ORD titration curves suggest that the high affinity sites are conformationally labile and that the albumin molecule is flexible.

Association of proteins in acidic solutions--a case study with beta-globulin

The investigation of the effect of acid pH on the structure of beta-globulin indicated several transitions as a function of pH. Upon reducing the pH from 7.0, the beta-globulin molecule underwent an expansion due to hydration up to pH 5.0, and a further increase in H+ concentration resulted in unfolding. This is a single step cooperative denaturation as indicated by the viscosity profile. At extreme acid pH values (below pH 2.0) the protein associates or folds to a different conformational motif as shown by blue shift of ultraviolet fluorescence emission maximum and decrease in reduced viscosity values by more than 30% due to an entropically driven hydrophobic interaction. The conformational analysis of beta-globulin showed a decrease up to pH 3.0, followed by an increase in the ordered structure at low pH values indicating that the low pH values stabilized this new conformation. These results are discussed in view of the molten globule structure of proteins.

Ionization of tyrosine residues in human serum albumin and in its complexes with bilirubin and laurate

Spectrophotometric titration of human serum albumin indicates that ionization of the 18 tyrosine residues takes place between pH 9 and 12.7. A Hill plot indicates that protons dissociate co-operatively from tyrosine residues, in pure albumin between pH 11.0 and 11.4 with a Hill coefficient 1.7, and in the bilirubin-albumin complex between pH 11.2 and 11.7 with a Hill coefficient 1.6. With a stopped-flow technique it is shown that about seven of the tyrosines ionize fast, with rate constants well above 10(2) s-1, when pH is suddenly changed from near neutral to pH 11.76. Further residues ionize slowly, with rate constants around 10(2) s-1 or less. The N-form of albumin (pH 6) contains one more fast ionizing tyrosine than the B-form of albumin (pH 10). Binding of bilirubin or laurate to the albumin molecule (molar ratio 1:1) transforms one to three of the fast ionizing tyrosines to slowly ionizing.

Conformational changes in human serum albumin studied by fluorescence and absorption spectroscopy. Distance measurements as a function of pH and fatty acids

pH- and fatty acid-induced conformational changes in human serum albumin were investigated by fluorescence-energy transfer, determining the distance between Trp-214 and bound bilirubin at 25 degrees C. This distance changes significantly with the pH, being 2.52 +/- 0.01 nm at pH 6, 2.31 +/- 0.04 nm at pH 9, 2.13 +/- 0.07 nm at pH 11.0 and 2.77 nm at pH 11.9. The influence of different fatty acids on the distance was also determined. At pH 7.4 medium-chain fatty acids seem to increase this distance, whereas long-chain fatty acids, at low concentrations, decrease the distance between the two chromophores. The contraction of the protein carrying long-chain saturated fatty acids is even more pronounced at pH 9.

New fluorescence of nonenzymatically glucosylated human serum albumin

Glucosylated human serum albumin (G-HSA) obtained under incubation with glucose at 37 degrees C for 8 days showed a new fluorescence with a maximum at 430 nm, resulting in quenching of the fluorescence of only one tryptophan residue on HSA. The quantum yield of new fluorescence is 0.024 at 25 degrees C. The analysis of the excitation spectra allowed us to conclude the absence of energy transfer. In G-HSA, non-disulfide cross-linking hexamer was confirmed by SDS-PAGE.