Fluorescence spectroscopic studies on binding of a flavonoid antioxidant quercetin to serum albumins

Spontaneous interactions between typical antibiotics and soil enzyme: Insights from multi-spectroscopic approaches, XPS technology, molecular modeling, and joint toxic actions

A large amount of antibiotics enters the soil environment and accumulates therein as individuals and mixtures, threatening the soil safety. However, there is little information regarding the influence of single and mixed antibiotics on key soil proteins at molecular level. In this study, setting sulfadiazine (SD) and tetracycline hydrochloride (TC) as the representative antibiotics, the interactions between these agents and α-amylase (an important hydrolase in soil carbon cycle) were investigated through multi-spectroscopic approaches, X-ray photoelectron spectrometry, and molecular modeling. It was found that both SD and TC spontaneously bound to α-amylase with 1:1 stoichiometry mainly via forming stable chemical bonds. The interactions altered the polarity of aromatic amino acids, protein backbone, secondary structure, hydrophobicity and activity of α-amylase. The SD-TC mixtures were designed based on the direct equipartition ray to comprehensively characterize the possible concentration distribution, and interactive effects indicated that the mixtures antagonistically impacted α-amylase. These findings reveal the binding characteristics between α-amylase and typical antibiotics, which probably influence the ecological functions of α-amylase in soil. This study clarifies the potential harm of antibiotics on soil functional enzyme, which is significant for the environmental risk assessment of antibiotics and their mixtures.

Probing the interaction of uranyl(VI) complex with bovine serum albumin via in-depth experimental and computational perspectives

Interaction aspects of uranyl(VI) complexes as well as the coordinated ONNO-donor ligand with bovine serum albumin (BSA) were investigated by the fluorescence spectroscopy and computational insights. Under optimal physiological condition, it was observed that there was significant decrease in fluorescence intensity of BSA upon interaction with uranyl(VI) complexes as well as the ligand. The mechanism of interaction between the uranyl(VI) complex and BSA protein was examined by fluorescence measurement. The Stern-Volmer constant, binding affinity, binding constant, standard free energy, and fluorescence lifetime decay profile of BSA in the absence as well as in the presence of uranyl(VI) complex were determined. Furthermore, the conformational binding of uranyl(VI) complexes with BSA protein was explored via molecular docking studies, and confirmed that there is a strong affinity between the Trp-213 residue in the binding pocket of sub-domain IIA and uranyl(VI) complex.

Digital Database of Absorption Spectra of Diverse Flavonoids Enables Structural Comparisons and Quantitative Evaluations

Flavonoids play diverse roles in plants, comprise a non-negligible fraction of net primary photosynthetic production, and impart beneficial effects in human health from a plant-based diet. Absorption spectroscopy is an essential tool for quantitation of flavonoids isolated from complex plant extracts. The absorption spectra of flavonoids typically consist of two major bands, band I (300-380 nm) and band II (240-295 nm), where the former engenders a yellow color; in some flavonoids the absorption tails to 400-450 nm. The absorption spectra of 177 flavonoids and analogues of natural or synthetic origin have been assembled, including molar absorption coefficients (109 from the literature, 68 measured here). The spectral data are in digital form and can be viewed and accessed at http://www.photochemcad.com. The database enables comparison of the absorption spectral features of 12 distinct types of flavonoids including flavan-3-ols (e.g., catechin, epigallocatechin), flavanones (e.g., hesperidin, naringin), 3-hydroxyflavanones (e.g., taxifolin, silybin), isoflavones (e.g., daidzein, genistein), flavones (e.g., diosmin, luteolin), and flavonols (e.g., fisetin, myricetin). The structural features that give rise to shifts in wavelength and intensity are delineated. The availability of digital absorption spectra for diverse flavonoids facilitates analysis and quantitation of these valuable plant secondary metabolites. Four examples are provided of calculations─multicomponent analysis, solar ultraviolet photoprotection, sun protection factor (SPF), and Förster resonance energy transfer (FRET)─for which the spectra and accompanying molar absorption coefficients are sine qua non.

Interaction Characterization of a Tyrosine Kinase Inhibitor Erlotinib with a Model Transport Protein in the Presence of Quercetin: A Drug-Protein and Drug-Drug Interaction Investigation Using Multi-Spectroscopic and Computational Approaches

The interaction between erlotinib (ERL) and bovine serum albumin (BSA) was studied in the presence of quercetin (QUR), a flavonoid with antioxidant properties. Ligands bind to the transport protein BSA resulting in competition between different ligands and displacing a bound ligand, resulting in higher plasma concentrations. Therefore, various spectroscopic experiments were conducted in addition to in silico studies to evaluate the interaction behavior of the BSA-ERL system in the presence and absence of QUR. The quenching curve and binding constants values suggest competition between QUR and ERL to bind to BSA. The binding constant for the BSA-ERL system decreased from 2.07 × 10⁴ to 0.02 × 10² in the presence of QUR. The interaction of ERL with BSA at Site II is ruled out based on the site marker studies. The suggested Site on BSA for interaction with ERL is Site I. Stability of the BSA-ERL system was established with molecular dynamic simulation studies for both Site I and Site III interaction. In addition, the analysis can significantly help evaluate the effect of various quercetin-containing foods and supplements during the ERL-treatment regimen. In vitro binding evaluation provides a cheaper alternative approach to investigate ligand-protein interaction before clinical studies.

Binding interactions of cationic gemini surfactants with gold nanoparticles-conjugated bovine serum albumin: A FRET/NSET, spectroscopic, and docking study

This work demonstrates binding interactions of two cationic gemini surfactants, 12-4-12,2Br^- and 12-8-12,2Br^- with gold nanoparticles (AuNPs)-conjugated bovine serum albumin (BSA) presenting binding isotherms from specific binding to saturation binding regions of surfactants. The binding isotherm has been successfully constructed using Förster's resonance energy transfer (FRET) and nanometal surface energy transfer (NSET) parameters calculated based on fluorescence quenching of donor, tryptophan (Trp) residue by acceptor, AuNP. Energy transfer efficiency (E_T) changes due to alteration in the donor-acceptor distance when surfactants interact with bioconjugates. A solid reverse relationship between α-helix and β-turn contents of BSA-AuNPs-conjugates is noted while interacting with surfactants. 12-8-12,2Br^- shows stronger binding interactions with BSA-bioconjugates than 12-4-12,2Br^-. The effect of bioconjugation on secondary/tertiary structures of BSA in the absence and presence of a surfactant is studied through circular dichroism, fluorescence, and Fourier transform infrared spectroscopic measurements. Motional restrictions imposed by AuNPs on Trp residues of folded and unfolded BSA have been investigated using red edge emission shift (REES) measurements. Finally, the molecular docking results present the modes of interactions of 12-4-12,2Br^- and 12-8-12,2Br^-, and Au-nanoclusters (Au₉₂) with BSA. An approach to describe the binding isotherms of surfactants using AuNPs-bioconjugates as optical-based molecular ruler and possible effects of AuNPs on microenvironment and conformations of the protein is presented.

Influence of antioxidant flavonoids quercetin and rutin on the in-vitro binding of neratinib to human serum albumin

This study was designed to examine the interaction of neratinib (NRB) with human serum albumin (HSA) in presence of flavonoids quercetin and rutin. Both quercetin and rutin can compete with NRB to bind to HSA and displace NRB from its binding site. The interaction mechanism was studied with several spectroscopic techniques and molecular docking. Static fluorescence quenching mechanism was observed on interaction of HSA with NRB. van der Waals force and hydrogen bond were involved in the HSA-NRB interaction as per the results of thermodynamic parameters. Further, the conformational changes were observed in the HSA on its interaction with NRB. Interaction of NRB with HSA in presence of quercetin and rutin resulted in changes in the binding constants of HSA-NRB suggesting some impact on the binding of NRB in the presence of flavonoids.

Binding properties of sodium glucose co-transporter-2 inhibitor empagliflozin to human serum albumin: spectroscopic methods and computer simulations

Empagliflozin is an oral sodium glucose co-transporter-2 inhibitor for type 2 diabetes mellitus. The interaction between empagliflozin and human serum albumin (HSA) was investigated experimentally and theoretically. Fluorescence quenching and time-resolved fluorescence spectroscopy indicated that the quenching mechanism of empagliflozin and HSA was dynamic and that the effective binding constant at body temperature was 3.495 × 10³ M^-1. Thermodynamic parameters showed that hydrophobic forces were the major binding force in the interaction between empagliflozin and HSA. Circular dichroism, Fourier transform infrared, and 3 D fluorescence spectroscopy revealed that empagliflozin showed a slight change in secondary structure without changing the basic carbon framework of HSA. Site marker displacement experiments revealed that empagliflozin bound to site I of HSA, which was supported by molecular docking. Molecular dynamic simulations indicated that empagliflozin could bind to HSA stably. This study provided insights into the binding mechanism between empagliflozin and HSA.Communicated by Ramaswamy H. Sarma.

Fluorescence Resonance Energy Transfer, Small-Angle Neutron Scattering, and Dynamic Light Scattering Study on Interactions of Gemini Surfactants Having Different Spacer Groups with Protein at Various Regions of Binding Isotherms

The binding interactions of three gemini surfactants having different spacer groups (12-4-12, 12-8-12, and 12-4(OH)-12) with a high concentration (150 μM) of bovine serum albumin (BSA) at various regions of binding isotherms have been studied by means of steady-state fluorescence and fluorescence anisotropy, time-correlated single-photon counting fluorescence of trans-2-[4-(dimethylamino)styryl]benzothiazole, small-angle neutron scattering (SANS), and dynamic light scattering (DLS) measurements. The fluorescence resonance energy transfer phenomenon between the twisted intramolecular charge transfer fluorescent molecule, trans-2-[4-(dimethylamino)styryl]benzothiazole as an acceptor, and tryptophan 213 (Trp-213) of BSA as a donor has been successfully used to probe the binding interactions of gemini surfactants with protein at all regions of binding isotherms. The increasing order of energy transfer efficiency at a higher concentration range of surfactants is 12-8-12 > 12-4-12 > 12-4(OH)-12. Stronger binding of micelles of gemini surfactant molecules having a comparatively more hydrophobic spacer group with the hydrophobic segments of the protein results in closer approach of trans-2-[4-(dimethylamino)styryl]benzothiazole molecules solubilized in micelles to Trp-213. The average excited-state lifetimes become shorter with a trend of increase in contribution from the fast component and decrease in contribution from the slow component to the decay with increasing concentration of a surfactant. The nonradiative rate constant of trans-2-[4-(dimethylamino)styryl]benzothiazole increases with increasing concentration of a surfactant because the average microenvironment around it in protein-surfactant aggregates is more polar as compared to that in native protein. SANS and DLS measurements were carried out for the study of the structural deformations in the protein, on enhancement of the concentration of the gemini surfactants. The necklace and bead model has been used for the analysis of SANS data for the protein-surfactant complexes. At a higher concentration range, 12-8-12 and 12-4-12 have a slightly smaller fractal dimension and a larger correlation length as compared to 12-4(OH)-12. DLS data show that the increasing order of hydrodynamic diameter for the complexes of protein with three gemini surfactants in their high concentration range is 12-4(OH)-12 < 12-4-12 < 12-8-12.

Visible-Light-Activated Quinolone Carbon-Monoxide-Releasing Molecule: Prodrug and Albumin-Assisted Delivery Enables Anticancer and Potent Anti-Inflammatory Effects

The delivery of controlled amounts of carbon monoxide (CO) to biological targets is of significant current interest. Very few CO-releasing compounds are currently known that can be rigorously controlled in terms of the location and amount of CO released. To address this deficiency, we report herein a new metal-free, visible-light-induced CO-releasing molecule (photoCORM) and its prodrug oxidized form, which offer new approaches to controlled, localized CO delivery. The new photoCORM, based on a 3-hydroxybenzo[ g]quinolone framework, releases 1 equiv of CO upon visible-light illumination under a variety of biologically relevant conditions. This nontoxic compound can be tracked prior to CO release using fluorescence microscopy and produces a nontoxic byproduct following CO release. An oxidized prodrug form of the photoCORM is reduced by cellular thiols, providing an approach toward activation in the reducing environment of cancer cells. Strong noncovalent affinity of the nonmetal photoCORM to albumin enables use of an albumin:photoCORM complex for targeted CO delivery to cancer cells. This approach produced cytotoxicity IC₅₀ values among the lowest reported to date for CO delivery to cancer cells by a photoCORM. This albumin:photoCORM complex is also the first CO delivery system to produce significant anti-inflammatory effects when introduced at nanomolar photoCORM concentration.

Diketo modification of curcumin affects its interaction with human serum albumin

Curcumin isoxazole (CI) and Curcumin pyrazole (CP), the diketo modified derivatives of Curcumin (CU) are metabolically more stable and are being explored for pharmacological properties. One of the requirements in such activities is their interaction with circulatory proteins like human serum albumin (HSA). To understand this, the interactions of CI and CP with HSA have been investigated employing absorption and fluorescence spectroscopy and the results are compared with that of CU. The respective binding constants of CP, CI and CU with HSA were estimated to be 9.3×10⁵, 8.4×10⁵ and 2.5×10⁵M^-1, which decreased with increasing salt concentration in the medium. The extent of decrease in the binding constant was the highest in CP followed by CI and CU. This revealed that along with hydrophobic interaction other binding modes like electrostatic interactions operate between CP/CI/CU with HSA. Fluorescence quenching studies of HSA with these compounds suggested that both static and dynamic quenching mechanisms operate, where the contribution of static quenching is higher for CP and CI than that for CU. From fluorescence resonance energy transfer studies, the binding site of CU, CI and CP was found to be in domain IIA of HSA. CU was found to bind in closer proximity with Trp214 as compared to CI and CP and the same was responsible for efficient energy transfer and the same was also established by fluorescence anisotropy measurements. Furthermore docking simulation complemented the experimental observation, where both electrostatic as well as hydrophobic interactions were indicated between HSA and CP, CI and CU. This study is useful in designing more stable CU derivatives having suitable binding properties with proteins like HSA.

Development of an eco-friendly approach based on dispersive liquid–liquid microextraction for the quantitative determination of quercetin inNasturtium officinale,Apium graveolens,Spinacia oleracea,Brassica oleracea var. sabellica, and food samples

A fast, sensitive, inexpensive and environment-friendly dispersive liquid–liquid microextraction technique was developed based on solidification of floating organic drops.

Evaluation of the binding of four anti-tumor Casiopeínas® to human serum albumin

The metal complexes designated by Casiopeínas® are mixed-ligand Cu^II-compounds some of them having promising antineoplastic properties. We report studies of binding of Cu(glycinato)(4,7-dimethyl-1,10-phenanthroline) (Cas-II-Gly (1)), Cu(acetylacetonato)(4,7-dimethyl-1,10-phenanthroline) (Cas-III-Ea (2)), Cu(glycinato)(4,4'-dimethyl-2,2'-bipyridine) (Cas-IV-Gly (3)) and Cu(acetylacetonato)(4,4'-dimethyl-2,2'-bipyridine) (Cas-III-ia (4)) to human serum albumin (HSA) by circular dichroism (CD), Electron paramagnetic resonance (EPR) and fluorescence spectroscopy. The results indicate that HSA may bind up to three molecules of the tested Casiopeínas. This is confirmed by inductively coupled plasma - atomic absorption spectroscopy measurements of samples of HSA-Casiopeínas after passing by adequate size-exclusion columns. The binding of Cas-II-Gly to HSA was also confirmed by MALDI-TOF mass spectrometric experiments. In the physiological range of concentrations the Casiopeínas form 1:1 adducts with HSA, with conditional binding constants of ca. 1×10⁹ (1), 4×10⁷ (2), 1×10⁶ (3) and 2×10⁵ (4), values determined from the CD spectra measured, and the fluorescence emission spectra indicates that the binding takes place close to the Trp214 residue. Overall, the data confirm that these Casiopeínas may bind to HSA and may be transported in blood serum by this protein; this might allow some selective tumor targeting, particularly in the case of Cas-II-Gly. In this work we also discuss aspects associated to the reliability of the frequently used methodologies to determine binding constants based on the measurement of fluorescence emission spectra of solutions containing low concentrations of proteins such as HSA and BSA, by titrations with solutions of metal complexes.

Investigation of the binding and simultaneous quantifications of propanil and bromoxynil herbicide concentrations in human serum albumin

This study reported the use of UV-visible and fluorescence spectroscopy and partial-least-square (PLS) multivariate regression for accurate and simultaneous quantifications of two widely used herbicides, propanil, 3',4'-dichloropropionanilide (PPL) and bromoxynil, 3,5-dibromo-4-hydroxybenzonitrile (BXL) in human serum albumin (HSA) at physiological conditions. The binding affinity and thermodynamic properties of PPL-HSA and BXL-HSA complexes were also investigated. Partial-least-square (PLS) regression was used to collate the variability in the absorption or emission spectra of PPL-HSA and BXL-HSA complexes with PPL and/or BXL concentrations in HSA samples. The binding constants of 7.66× 10⁸ M^-1 for PPL-HSA and 4.88× 10⁶ M^-1 for BXL-HSA complexes were calculated at physiological conditions (temperature, 310 K; pH 7.4). Thermodynamic parameter values: enthalpy (ΔH) (13.99 kJ mol^-1), entropy (ΔS) (0.078 kJ mol^-1 K^-1), and Gibbs free energy (ΔG) (-10.19 kJ mol^-1) were determined for PPL-HSA complexation at physiological conditions. However, differences in thermodynamic property values of: ΔH (-214.3 kJ mol^-1), ΔS (-0.563 kJ mol^-1 K^-1), and ΔG (-39.70 kJ mol^-1) were observed for BXL-HSA complexes. The binding constants and negative ΔG values indicated strong binding affinity and thermodynamically favorability of PPL-HSA and BXL-HSA complex formation. Results of the PLS regression calibration showed good linearity (R² ≥ 0.998289), high sensitivity, and impressive low limit-of-detections (LODs) of 1.38× 10^-8 M for PPL and 1.68× 10^-8 M for BXL that are comparable and/or lower than many previously reported LODs for herbicide and pesticide analyses. Most importantly, PLS regression is capable of simultaneous quantifications of PPL and BXL concentrations in HSA samples with good accuracy and low errors of 3.66%. UV-visible spectrophotometers and spectrofluorometers are fairly inexpensive, easy to use, and are readily available in almost every laboratory, making this protocol excellent and affordable for routine analysis of weed/pest control chemical residues in humans. The results of this study are significant and remarkable that will provide critical insight into the binding mechanism of herbicide toxicity in humans and non-target organisms, which are of special interest in the area of biomedical study, environmental risk assessment, and ecotoxicology.

Analyses of polycyclic aromatic hydrocarbon (PAH) and chiral-PAH analogues-methyl-β-cyclodextrin guest-host inclusion complexes by fluorescence spectrophotometry and multivariate regression analysis

The negative health impact of polycyclic aromatic hydrocarbons (PAHs) and differences in pharmacological activity of enantiomers of chiral molecules in humans highlights the need for analysis of PAHs and their chiral analogue molecules in humans. Herein, the first use of cyclodextrin guest-host inclusion complexation, fluorescence spectrophotometry, and chemometric approach to PAH (anthracene) and chiral-PAH analogue derivatives (1-(9-anthryl)-2,2,2-triflouroethanol (TFE)) analyses are reported. The binding constants (K_b), stoichiometry (n), and thermodynamic properties (Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS)) of anthracene and enantiomers of TFE-methyl-β-cyclodextrin (Me-β-CD) guest-host complexes were also determined. Chemometric partial-least-square (PLS) regression analysis of emission spectra data of Me-β-CD-guest-host inclusion complexes was used for the determination of anthracene and TFE enantiomer concentrations in Me-β-CD-guest-host inclusion complex samples. The values of calculated K_b and negative ΔG suggest the thermodynamic favorability of anthracene-Me-β-CD and enantiomeric of TFE-Me-β-CD inclusion complexation reactions. However, anthracene-Me-β-CD and enantiomer TFE-Me-β-CD inclusion complexations showed notable differences in the binding affinity behaviors and thermodynamic properties. The PLS regression analysis resulted in square-correlation-coefficients of 0.997530 or better and a low LOD of 3.81×10^-7M for anthracene and 3.48×10^-8M for TFE enantiomers at physiological conditions. Most importantly, PLS regression accurately determined the anthracene and TFE enantiomer concentrations with an average low error of 2.31% for anthracene, 4.44% for R-TFE and 3.60% for S-TFE. The results of the study are highly significant because of its high sensitivity and accuracy for analysis of PAH and chiral PAH analogue derivatives without the need of an expensive chiral column, enantiomeric resolution, or use of a polarized light.

Spectroscopic study of 3-Hydroxyflavone - protein interaction in lipidic bi-layers immobilized on silver nanoparticles

The interaction of 3-Hydroxyflavone with serum proteins (BSA and HSA) in lecithin lipidic bi-layers (PC) immobilized on silver nanoparticles (SNPs), was studied by fluorescence and Raman spectroscopy. BSA secondary structure was quantified with a deconvolution algorithm, showing a decrease in α-helix structure when lipids were added to the solution. The effect of temperature on the rate of the excited-state intra-molecular proton transfer and on the dual fluorescence emission of 3-HF in the HSA/PC/SNPs systems was discussed. Evaluation of the antioxidant activity of 3-HF in HSA/PC/SNPs systems was also studied. The antioxidant activity of 3-HF decreased in the presence of SNPs. The results are discussed with relevance to the secondary structure of proteins and of the 3-HF based nano-systems to a topical formulation useful in the oxidative stress process.

Properties of a flavonol-based photoCORM in aqueous buffered solutions: influence of metal ions, surfactants and proteins on visible light-induced CO release

A flavonol-based photoCORM exhibits reliable visible light-induced CO release in aqueous buffer environments containing constituents of relevance to biological environments.

Comparative study of the effects of the structurally similar flavonoids quercetin and taxifolin on the therapeutic behavior of alprazolam

After a meal rich in plant products, dietary flavonoids can be detected in plasma as serum albumin bound conjugates. Flavonoid–albumin binding is expected to control the bioavailability of drugs. In this study, the binding of alprazolam (ALP) and human serum albumin (HSA) has been investigated in the absence and presence of two flavonoids with similar structures, quercetin (QUER) and taxifolin (TAX), by means of fluorescence spectroscopy, chemometrics, and molecular dynamics simulation. Our results show that ALP has the ability to quench the intrinsic fluorescence of HSA. This quenching is affected by flavonoids. The presence of QUER and TAX decreased the quenching constants, binding constants, and equilibrium constants associated with ALP binding to HSA. The effect of ALP and both flavonoids on the conformation of HSA was analyzed using synchronous fluorescence spectroscopy. Our results indicate a conformational change of HSA with the addition of ligands. The molecular dynamics study makes an important contribution to understanding the effect of the binding of ALP, QUER, and TAX on conformational changes of HSA and modification of its tertiary structure in the absence and presence of flavonoids. All of these results may have relevant consequences in rationalizing the interferences of common food and drugs.

Investigation on the interaction between isorhamnetin and bovine liver catalase by spectroscopic techniques under different pH conditions

The binding of isorhamnetin to bovine liver catalase (BLC) was first investigated at 302, 310 and 318 K at pH 7.4 using spectroscopic methods including fluorescence spectra, circular dichroism (CD) and UV-vis absorption. Spectrophotometric observations are rationalized mainly in terms of a static quenching process. The binding constants and binding sites were evaluated by fluorescence quenching methods. Enzymatic activity of BLC in the absence and presence of isorhamnetin was measured using a UV/vis spectrophotometer. The result revealed that the binding of isorhamnetin to BLC led to a reduction in the activity of BLC. The positive entropy change and enthalpy change indicated that the interaction of isorhamnetin with BLC was mainly driven by hydrophobic forces. The distance r between the donor (BLC) and acceptor (isorhamnetin) was estimated to be 2.99 nm according to fluorescence resonance energy transfer. Fluorescence, synchronous fluorescence, and CD spectra showed no obvious change in the conformation of BLC upon the binding of isorhamnetin. In addition, the influence of pH on the binding of isorhamnetin to BLC was investigated and the binding ability of the drug to BLC deceased under other pH conditions (pH 9.0, 6.5, 5.0, 3.5, or 2.0) as compared with that at pH 7.4. Copyright © 2016 John Wiley & Sons, Ltd.

Caffeine and sulfadiazine interact differently with human serum albumin: A combined fluorescence and molecular docking study

The interaction and binding behavior of the well-known drug sulfadiazine (SDZ) and psychoactive stimulant caffeine (CAF) with human serum albumin (HSA) was monitored by in vitro fluorescence titration and molecular docking calculations under physiological condition. The quenching of protein fluorescence on addition of CAF is due to the formation of protein-drug complex in the ground state; whereas in case of SDZ, the experimental results were explained on the basis of sphere of action model. Although both these compounds bind preferentially in Sudlow's site 1 of the protein, the association constant is approximately two fold higher in case of SDZ (∼4.0×10(4)M(-1)) in comparison with CAF (∼9.3×10(2)M(-1)) and correlates well with physico-chemical properties like pKa and lipophilicity of the drugs. Temperature dependent fluorescence study reveals that both SDZ and CAF bind spontaneously with HSA. However, the binding of SDZ with the protein is mainly governed by the hydrophobic forces in contrast with that of CAF; where, the interaction is best explained in terms of electrostatic mechanism. Molecular docking calculation predicts the binding of these drugs in different location of sub-domain IIA in the protein structure.

Structural and electronic determinants of flavonoid binding to human serum albumin: an extensive ligand-based study

The most prominent features responsible for binding of flavonoid aglycones to the IIA region of human serum albumin (HSA) were determined based onin vitrofluorescence measurements and density functional theory calculations.

Anti-Apoptotic Gene Delivery with cyclo-(d-Trp-Tyr) Peptide Nanotube via Eye Drop Following Corneal Epithelial Debridement

Corneal keratocyte apoptosis triggered by cornel debridement is one mechanism of corneal disorders. In this study, the feasibility of cyclo-(d-Trp-Tyr) peptide nanotubes (PNTs) as carriers of caspase 3 silence shRNA delivery was assessed. A model of epithelial injury by epithelial debridement was applied to investigate the feasibility of PNTs as gene delivery carriers on corneal injury. First, the PNTs were found within 2 μm in length and 300 nm in width by an atomic force microscope and confocal laser microscope system. Plasmid DNAs were observed to be associated with PNTs by atomic force microscope and confocal laser scanning microscope. The plasmids were associated with tyrosine of PNTs with a binding constant of 2.7 × 108 M-1. The stability of plasmid DNA with PNTs against the DNase was found at 60 min. Using thioflavin T pre-stained PNTs on the corneal eye drop delivery, the distribution of PNTs was in the epithelial and stroma regions. After corneal debridement, the rhodamine-labeled plasmid DNA and thioflavin T pre-stained PNTs were also delivered and could be observed in the stroma of cornea. PNTs complexed with anti-apoptotic plasmid caspase 3 silencing shRNA eye drop delivery decreased 41% of caspase 3 activity after the first dose by caspase 3 activity and Western blot analysis.

Spectroscopic study on binding of gentisic acid to bovine serum albumin

The interaction of (gentisic acid) GA with (bovine serum albumin) BSA has been studied by different spectroscopic techniques. GA is a monoanionic specie at the working pH of 7.4, it was determined by combining UV-Vis absorption spectroscopy and theoretical calculations. A set of fluorescence quenching experiments at different temperatures was carried out employing the native fluorescence of BSA. A Stern-Volmer constant (KSV) of (2.07±0.12)×10(4) mol(-1) L and a binding constant (Ka) of (8.47±4.39)×10(3) were determined at 310 K. The static quenching caused by the BSA-GA complex formation seems to play a significant role in the overall quenching process. A single binding site on BSA for GA was observed. ΔH=-55.6±0.2 kJ mol(-1) and ΔS=-104.3±0.6 J mol(-1) K(-1) were determined in a set of experiments on the dependence of Ka with the temperature. The binding process is, therefore, spontaneous and enthalpy-driven. Van der Waals forces and hydrogen bonds could also play the major role in the binding mode. The secondary structure changes of BSA in the absence and presence of GA were studied by FTIR and UV-Vis absorption spectroscopy.

Evaluation of a quercetin–gadolinium complex as an efficient positive contrast enhancer for magnetic resonance imaging

The quercetin–gadolinium complex showing superior contrast than the commercially used gadopentetate dimeglumine.

A new antitumoral Heteroarylaminothieno[3,2-b]pyridine derivative: its incorporation into liposomes and interaction with proteins monitored by fluorescence

The fluorescence properties of the new potent antitumoral methyl 3-amino-6-(benzo[d]thiazol-2-ylamino)thieno[3,2-b]pyridine-2-carboxylate in solution and when encapsulated in several different nanoliposome formulations were investigated. The compound exhibits very reasonable fluorescence quantum yields and a solvent sensitive emission in several polar and non-polar media, despite not being fluorescent in protic solvents. Fluorescence anisotropy measurements of the compound incorporated into liposomes revealed that this thienopyridine derivative can be carried in the hydrophobic region of the lipid membrane. Liposome formulations including this antitumor compound are nanometric in size, with a diameter lower than 130 nm and generally low polydispersity, and are promising for future drug delivery developments. The interaction of the compound with bovine serum albumin (BSA) and the multidrug resistance protein MDR1 was monitored by FRET, the compound acting as an energy acceptor. It was observed that the drug had a lower interaction with the MDR1 protein than with the native form of BSA, which is an important result regarding applications of this antitumoral drug.

Size and molecular flexibility affect the binding of ellagitannins to bovine serum albumin

Binding to bovine serum albumin of monomeric (vescalagin and pedunculagin) and dimeric ellagitannins (roburin A, oenothein B, and gemin A) was investigated by isothermal titration calorimetry and fluorescence spectroscopy, which indicated two types of binding sites. Stronger and more specific sites exhibited affinity constants, K1, of 10(4)-10(6) M(-1) and stoichiometries, n1, of 2-13 and dominated at low tannin concentrations. Weaker and less-specific binding sites had K2 constants of 10(3)-10(5) M(-1) and stoichiometries, n2, of 16-30 and dominated at higher tannin concentrations. Binding to stronger sites appeared to be dependent on tannin flexibility and the presence of free galloyl groups. Positive entropies for all but gemin A indicated that hydrophobic interactions dominated during complexation. This was supported by an exponential relationship between the affinity, K1, and the modeled hydrophobic accessible surface area and by a linear relationship between K1 and the Stern-Volmer quenching constant, K(SV).

Evaluation of the number of binding sites in proteins from their intrinsic fluorescence: limitations and pitfalls

Changes in the intrinsic protein fluorescence with the additive concentration provide one of the most employed methodologies for the evaluation of the binding constant and the number of binding sites. In the last years, more than 175 studies have been published where the double logarithmic plot shown below is used toward determining the number of equivalent binding sites (n). Log [(F° - F)/F] = log K + n log [Q0 ]. However, the value of n evaluated by this procedure is unrelated to the number of equivalent binding sites; rather it represents the stoichiometry of the binding step. The confusion on the meaning of n arises upon assuming that the binding process is represented by the forward and backward elementary steps shown below, implying that binding of the n solutes takes place simultaneously, i.e. there are no intermediate species. nQ + P ⇆ Qn P. The conclusion that n is unrelated to the number of equivalent binding sites is supported by the fact that in all the systems considered (99% of them) n values are close to one and much smaller than those obtained by ultrafiltration. It is then remarkable, the profusion of publications in peer-reviewed, specialized journals including a conceptual error that confuses Hill's coefficient and/or the stoichiometry of the binding step with the number of independent binding sites. Here, we discuss the origin of this common misconception and provide alternative methods to determine the number of binding sites.

Spectroscopic analysis of the interaction between thiazolo[2,3-b]pyrimidine analogues and bovine serum albumin

The interaction between thiazolo[2,3-b]pyrimidine (TZPM) analogues and bovine serum albumin (BSA) was investigated by fluorescence spectroscopy and UV-Vis spectroscopy at two different temperatures (299 and 307K) under imitated physiological conditions. The results indicate that both static quenching and dynamic quenching contribute to the fluorescence quenching of BSA by TZPM. The binding constant (K(a)) and binding sites (n) were calculated from the obtained spectra. Based on the Förster non-radiation energy transfer theory, the average binding distance between BSA and TZPM was estimated. The synchronous fluorescence spectra indicate that the conformation of BSA has been changed. The comparison of binding potency of TZPM and BSA suggests that the substituents on the benzene ring enhance the binding affinity of TZPM and BSA. We investigated the possible sub-domains on BSA that bind TZPM by displacement experiments. Furthermore, to explore the effect of molecular structure on the binding, a study on quantitative structure-property relationship (QSPR) was performed, the quantitative relationship equation of R(0), r and K(a) were obtained. We observed that R(0), r and K(a) between BSA and TZPM is connected with the margin of the highest and the lowest occupied orbital energy (ΔE), dipole moment (μ), Molar Volume (V(m)), Mole Mass (M).

Oral gene delivery with cyclo-(D-Trp-Tyr) peptide nanotubes

The feasibility of cyclo-(D-Trp-Tyr) peptide nanotubes (PNTs) as oral gene delivery carriers was investigated in nude mice with eight 40 μg doses of pCMV-lacZ in 2 days at 3 h intervals. The association between DNA and PNTs, the DNase I stability of PNTs-associated DNA, and in vitro permeability of DNA were estimated. The results showed that the cyclo-(D-Trp-Tyr) PNTs self-associated at concentrations above 0.01 mg/mL. Plasmid DNA associated with PNTs with a binding constant of 3.2 × 10(8) M(-1) calculated by a fluorescence quenching assay. PNTs were able to protect DNA from DNase I, acid, and bile digestion for 50 min, 60 min, and 180 min, respectively. The in vitro duodenal apparent permeability coefficient of pCMV-lacZ calculated from a steady state flux was increased from 49.2 ± 21.6 × 10(-10) cm/s of naked DNA to 395.6 ± 142.2 × 10(-10) cm/s of pCMV-lacZ/PNT formulation. The permeation of pCMV-lacZ formulated with PNTs was found in an energy-dependent process. Furthermore, β-galatosidase (β-Gal) activity in tissues was quantitatively assessed using chlorophenol red-β-D-galactopyranoside (CPRG) and was significantly increased by 41% in the kidneys at 48 h and by 49, 63, and 46% in the stomach, duodenum, and liver, respectively, at 72 h after the first dose of oral delivery of pCMV-lacZ/PNT formulation. The organs with β-Gal activity were confirmed for the presence of pCMV-lacZ DNA with Southern blotting analysis and intracellular tracing the TM-rhodamine-labeled DNA and the presence of mRNA by reverse transcription-real time quantitative PCR (RT-qPCR). Another plasmid (pCMV-hRluc) encoding Renilla reniformis luciferase was used to confirm the results. An increased hRluc mRNA and luciferase in stomach, duodenum, liver, and kidney were detected by RT-qPCR, ex vivo bioluminescence imaging, luciferase activity quantification, and immunostaining, respectively.