Interaction of the irisflorentin with bovine serum albumin: A fluorescence quenching study

Ratio-fluorescence detection of tert-butylhydroquinone based on non-conjugated polymer dots and gold nanoclusters

A novel ratiometric fluorescent probe based on non-conjugated polymer dots (NCPDs) and gold nanocluster (AuNCs) was constructed to determine tert-butylhydroquinone (TBHQ). The probe exhibited dual emission peaks at 480 nm and 630 nm under 370 nm excitation. The fluorescence of AuNCs was quenched by TBHQ due to strong electrostatic interactions, whereas the emission of NCPDs increased. The ratio of fluorescence intensity at 480 nm to 630 nm (F480 / F630) was monitored as analytical signal response. The probe have been utilized for the detection of TBHQ with good linear relationship in the range of 0.2 to 60 μg/mL. The limit of detection (LOD) and the limit of quantitation (LOQ) were 0.048 μg/mL and 0.159 μg/L, respectively. Three levels of spiked-in TBHQ concentrations were obtained with recovery rates from 80 % to 102 %. The present study provided an effective ratiometric fluorescence method for selective screening of TBHQ in food samples.

Molecular interactions and binding dynamics of Alpelisib with serum albumins: insights from multi-spectroscopic techniques and molecular docking

Alpelisib (ALP) is a potent anti-cancer drug showing promising activity against advanced breast cancers. Hence, profound understanding of its binding dynamics within the physiological system is vital. Herein, we have investigated interaction of ALP with human serum albumin (HSA) and bovine serum albumin (BSA) using spectroscopic techniques like absorption, fluorescence, time-resolved, synchronous and 3D-fluorescence, FRET, FT-IR, CD, and molecular docking studies. The intrinsic fluorescence of both BSA and HSA quenched significantly by ALP with an appreciable red shift in its emission maxima. Stern-Volmer analysis showed increase in Ksv with temperature indicating involvement of dynamic quenching process. This was further validated by no significant change in absorption spectrum of BSA and HSA (at 280 nm) upon ALP interaction, and by results of fluorescence time-resolved lifetime studies. ALP exhibited moderately strong binding affinity with BSA (of the order 106 M-1) and HSA (of the order 105 M-1), and the major forces accountable for stabilizing the interactions are hydrophobic forces. Competitive drug binding experiments and molecular docking suggested that ALP binds to site I in subdomain IIA of BSA and HSA. The Förster distance r was found to be less than 8 nm and 0.5 Ro < r < 1.5 Ro which suggests possible energy transfer between donors BSA/HSA and acceptor ALP. Synchronous and 3D-fluoresecnce, FT-IR and CD studies indicated that ALP induces conformational changes of BSA and HSA upon interaction.Communicated by Ramaswamy H. Sarma.

Interaction between roxarsone, an organic arsenic compound, with humic substances in the soil simulating environmental conditions

In environmental systems, the soil is a principal route of contamination by various potentially toxic species. Roxarsone (RX) is an arsenic (V) organic compound used to treat parasitic diseases and as an additive for animal fattening. When the animal excretes RX, the residues may lead to environmental contamination. Due to their physicochemical properties, the soil's humic substances (HS) are important in species distribution in the environment and are involved in various specific interaction/adsorption processes. Since RX, an arsenic (V) compound, is considered an emerging contaminant, its interaction with HS was evaluated in simulated environmental conditions. The HS-RX interaction was analyzed by monitoring intrinsic HS fluorescence intensity variations caused by complexation with RX, forming non-fluorescent supramolecular complexes that yielded a binding constant Kb (on the order of 103). The HS-RX interaction occurred through static quenching due to complex formation in the ground state, which was confirmed by spectrophotometry. The process was spontaneous (ΔG < 0), and the predominant interaction forces were van der Waals and hydrogen bonding (ΔH < 0 and ΔS < 0), with an electrostatic component evidenced by the influence of ionic strength in the interaction process. Structural changes in the HS were verified by synchronized and 3D fluorescence, with higher variation in the region referring to the protein-like fraction. In addition, metal ions (except ions Cu(II)) favored HS-RX interaction. When interacting with HS, the RX epitope was suggested by 1H NMR, which indicated that the entire molecule interacts with the superstructure. An enzyme inhibition assay verified the ability to reduce the alkaline phosphatase activity of free and complexed RX (RX-HS). Finally, this work revealed the main parameters associated with HS and RX interaction in simulated environmental conditions, thus, providing data that may help our understanding of the dynamics of organic arsenic-influenced soils.

Investigation of hexacyanoferrate(II)/(III) charge-dependent interactions with bovine and human serum albumins

In the present paper, the binding interactions of highly negative-charged ions, namely hexacyanoferrates(II/III), i.e. [Fe(CN)₆]^4- and [Fe(CN)₆]^3- with bovine and human serum albumins (BSA and HSA, respectively) have been studied for the first time in an aqueous solution (10 mM cacodylate buffer of pH 7.0) using steady-state fluorescence spectroscopy, isothermal titration calorimetry, and CD spectroscopy supported by molecular dynamics-based computational approaches. The Stern-Volmer equation as well as its modifications suggested that hexacyanoferrates(II/III) effectively quenched the intrinsic fluorescence of the albumins through a static mechanism. The proteins under study possess only one binding site on the surface capable of binding one mole of hexacyanoferrates(II/III) ions per one mole of albumin (HSA or BSA). The formation of albumin complexes is an enthalpy-driven process (|ΔH_ITC| > |TΔS_ITC|). The strength of the interactions depends mainly on the type of albumin, and changes as follows: BSA-K₃[Fe(CN)₆] ∼ BSA-K₄[Fe(CN)₆] > HSA-K₃[Fe(CN)₆] ∼ HSA-K₄[Fe(CN)₆]. Finally, potential binding sites of bovine and human serum albumins have been investigated and discussed based on a competitive fluorescence displacement assay (with warfarin and ibuprofen as site markers) and molecular dynamics simulations.

Analyzing protein conjugation reactions for antibody-drug conjugate synthesis using polarized excitation emission matrix spectroscopy

Antibody-drug conjugates (ADCs) are promising anticancer therapeutics, which offer important advantages compared to more classical therapies. There are a variety of ADC critical quality attributes (CQAs) such as the protein structure, aggregation, and drug-to-antibody ratio (DAR), which all impact on potency, stability, and toxicity. Production processes can destabilize antibodies via a variety of physical and chemical stresses, and or by increased aggregation after conjugation of hydrophobic drugs. Thus, a proper control strategy for handling, production, and storage is necessary to maintain CQA levels, which requires the use of in-process quality measurements to first identify, then understand, and control the variables which adversely affect ADC CQAs during manufacturing. Here, we show how polarized excitation emission matrix (pEEM) spectroscopy, a sensitive, nondestructive, and potentially fast technique, can be used for rapidly assessing aggregation and DAR in a single measurement. pEEM provides several sources of information for protein analysis: Rayleigh scatter for identifying aggregate/particle formation and fluorescence emission to assess chemical and structural changes induced by attachment of a linker and/or a small molecule drug payload. Here, we used a nontoxic ADC mimic (monoclonal antibody with linker molecule) to demonstrate efficacy of the measurement method. Emission changes caused via light absorption by the attached linker, allowed us to predict DAR with good accuracy using fluorescence signal from the final purified products (6% relative error of prediction [REP]) and also from unpurified alkylation intermediates (11% REP). pEEM changes could also be correlated with size (hydrodynamic radius, Rh ) and aggregate content parameters obtained from dynamic light scattering and size exclusion chromatography (SEC). For the starting material and purified product samples, pEEM correlated better with Rh (R2  = 0.99, 6% REP) than SEC determined aggregate content (18% REP). Combining both fluorescence and light scatter signals also enabled in-process size quantification (6% REP). Overall, combining polarized measurements with EEM and Rayleigh scatter provides a single measurement, multi-attribute test method for ADC manufacturing.

High-pressure homogenization: A potential technique for transforming insoluble pea protein isolates into soluble aggregates

High-pressure homogenization (HPH) is a technique that impacts the aggregation of globular proteins. In this study, the effect of HPH (at a pressure of 30/50 MPa for three cycles) was investigated on the aggregation states and functional properties of insoluble commercial pea protein isolates (CPPI). Results showed that HPH significantly improved the solubility, foaming and emulsifying capacity of CPPI. Samples treated at 50 MPa demonstrated better foaming and emulsifying capacity than that at 30 MPa. Surface hydrophobicity, intrinsic fluorescence, SDS-PAGE and FTIR analysis revealed that insoluble precipitates/aggregates (most legumins included) of CPPI were broken down and converted into soluble aggregates. Low-pressure HPH (30 MPa) can break non-covalent bonds (hydrophobic interactions), whereas higher pressure (50 MPa) can further break covalent bonds (SS). The study sheds light on the mechanism of disruption of insoluble CPPI under HPH and proposes a method to enhance their techno-functional properties for application in food formulations.

Lentil Protein and Tannic Acid Interaction Limits in Vitro Peptic Hydrolysis and Alters Peptidomic Profiles of the Proteins

In this study, the nature of lentil protein-tannic acid (LPTA) interaction and its effect on in vitro pepsin digestion were investigated. LPTA mixtures containing 1% w/v LP and 0.001-0.5% TA were prepared and characterized in terms of particle size, thermal properties, and secondary and tertiary structures. A 20-fold increase in particle size was observed in LPTA0.5% compared to LP control (without TA), indicating aggregation. Static quenching of tryptophan residues within the protein hydrophobic folds was observed. Increasing TA levels also enhanced protein thermal stability. Over 50% reduction in free amino groups of LPTA 0.5%, relative to LP, was observed after pepsin digestion. Cleavage specificity of pepsin and peptidomic profile of LP were modified by the presence of TA in LPTA 0.5%. This study showed that 0.5% w/v TA induced protein aggregation and reduced LP digestibility by hindering the accessibility of pepsin to the protein network, thus modifying the profile of released peptides.

Effect of Tetraphenylborate on Physicochemical Properties of Bovine Serum Albumin

The binding interactions of bovine serum albumin (BSA) with tetraphenylborate ions ([B(Ph)4]-) have been investigated by a set of experimental methods (isothermal titration calorimetry, steady-state fluorescence spectroscopy, differential scanning calorimetry and circular dichroism spectroscopy) and molecular dynamics-based computational approaches. Two sets of structurally distinctive binding sites in BSA were found under the experimental conditions (10 mM cacodylate buffer, pH 7, 298.15 K). The obtained results, supported by the competitive interactions experiments of SDS with [B(Ph)4]- for BSA, enabled us to find the potential binding sites in BSA. The first site is located in the subdomain I A of the protein and binds two [B(Ph)4]- ions (logK(ITC)1 = 7.09 ± 0.10; ΔG(ITC)1 = -9.67 ± 0.14 kcal mol-1; ΔH(ITC)1 = -3.14 ± 0.12 kcal mol-1; TΔS(ITC)1 = -6.53 kcal mol-1), whereas the second site is localized in the subdomain III A and binds five ions (logK(ITC)2 = 5.39 ± 0.06; ΔG(ITC)2 = -7.35 ± 0.09 kcal mol-1; ΔH(ITC)2 = 4.00 ± 0.14 kcal mol-1; TΔS(ITC)2 = 11.3 kcal mol-1). The formation of the {[B(Ph)4]-}-BSA complex results in an increase in the thermal stability of the alfa-helical content, correlating with the saturation of the particular BSA binding sites, thus hindering its thermal unfolding.

Effects of reductive inorganics and NOM on the formation of chlorite in the chlorine dioxide disinfection of drinking water

Chlorine dioxide (ClO₂) disinfection usually does not produce halogenated disinfection by-products, but the formation of the inorganic by-product chlorite (ClO₂^-) is a serious consideration. In this study, the ClO₂^- formation rule in the ClO₂ disinfection of drinking water was investigated in the presence of three representative reductive inorganics and four natural organic matters (NOMs), respectively. Fe²⁺ and S^2- mainly reduced ClO₂ to ClO₂^- at low concentrations. When ClO₂ was consumed, the ClO₂^- would be further reduced by Fe²⁺ and S^2-, leading to the decrease of ClO₂^-. The reaction efficiency of Mn²⁺ with ClO₂ was lower than that of Fe²⁺ and S^2-. It might be the case that MnO₂ generated by the reaction between Mn²⁺ and ClO₂ had adsorption and catalytic oxidation on Mn²⁺. However, Mn²⁺ would not reduce ClO₂^-. Among the four NOMs, humic acid and fulvic acid reacted with ClO₂ actively, followed by bovine serum albumin, while sodium alginate had almost no reaction with ClO₂. The maximum ClO₂^- yields of reductive inorganics (70%) was higher than that of NOM (around 60%). The lower the concentration of reductive substances, the more ClO₂^- could be produced by per unit concentration of reductive substances. The results of the actual water samples showed that both reductive inorganics and NOM played an important role in the formation of ClO₂^- in disinfection.

Ultra-sensitive and selective determination of a phenolic food additive using protein capped gold nanoclusters: a dual in-line fluorometric and colorimetric sensing probe

The application of red luminescence BSA-AuNCs towards the selective determination of food additive tert-butylhydroquinone (TBHQ) was demonstrated by both fluorometric and colorimetric methods.

Interactions of insulin with tragacanthic acid biopolymer: Experimental and computational study

Alternative methods for insulin delivery instead of subcutaneous injection in diabetic patients is of great essential, and biocompatible polymers are one of the most efficient vehicles for this purpose. This research aims to investigate the capability of tragacanthic acid (TA) to bind insulin and release it under physiological conditions without alteration in the structure and conformation of insulin. Interactions between TA and insulin were studied using spectroscopic techniques and computational modeling by docking and molecular dynamics simulations. Our results demonstrate an entropy-driven spontaneous interaction between insulin and TA, where hydrogen bonds act as the main enthalpic contribution. According to our findings, the weak interaction between insulin and TA provides the basis for efficient capture and appropriate release of insulin by TA as a potential part of the insulin delivery system. In conclusion, tragacanth acid can be a proper candidate for insulin delivery.

Spectrofluorimetric analysis of the binding of a target molecule to serum albumin: tricky aspects and tips

Protein binding heavily modulates drug activity. Therefore, the binding features need to be elucidated when chemistry researchers study new molecules (metal complexes) to be used as drugs. This paper concerns the experimental and data treatment aspects of the mechanistic analysis of the binding to a fluorescent protein (the golden standard serum albumin) by using direct fluorescence titrations. Fluorescence data are not rarely only qualitatively used, neglecting further treatments which could offer a precious detailed picture of the behavior of the drug. We aim to spread a mechanistic approach, discussing the critical aspects for correctly designing the experiments and treating the data. The researcher may confirm adduct formation and evaluate binding constants (Stern-Volmer K_SV or other types of K). Also, we discuss here, with the help of literature examples, the correct use of temperature dependence of K to extract thermodynamic parameters, comment on enthalpy-entropy compensation, together with the use of synchronous spectra and exchange experiments to gain information on the binding type and site. We think that this tutorial/critical synopsis can be of help for the increasing community dealing with these experiments, which are valuable but often much more tricky than it might appear at first sight.

Human Serum Albumin in the Presence of AGuIX Nanoagents: Structure Stabilisation without Direct Interaction

The gadolinium-based nanoagent named AGuIX^® is a unique radiosensitizer and contrast agent which improves the performance of radiotherapy and medical imaging. Currently tested in clinical trials, AGuIX^® is administrated to patients via intravenous injection. The presence of nanoparticles in the blood stream may induce harmful effects due to undesired interactions with blood components. Thus, there is an emerging need to understand the impact of these nanoagents when meeting blood proteins. In this work, the influence of nanoagents on the structure and stability of the most abundant blood protein, human serum albumin, is presented. Synchrotron radiation circular dichroism showed that AGuIX^® does not bind to the protein, even at the high ratio of 45 nanoparticles per protein at 3 mg/L. However, it increases the stability of the albumin. Isothermal thermodynamic calorimetry and fluorescence emission spectroscopy demonstrated that the effect is due to preferential hydration processes. Thus, this study confirms that intravenous injection of AGuIX^® presents limited risks of perturbing the blood stream. In a wider view, the methodology developed in this work may be applied to rapidly evaluate the impact and risk of other nano-products that could come into contact with the bloodstream.

Carbon quantum dots (CQDs) modified ZnO/CdS nanoparticles based fluorescence sensor for highly selective and sensitive detection of Fe(III)

A simple and fast spectrofluorimetric method coupled with carbon quantum dots (CQDs) modified ZnO/CdS nanoparticles was developed for the detection of Ferric iron (Fe(III)). The fluorescence of CQDs/ZnO/CdS NPs was effectively quenched by Fe(III) due to the strong interaction between the CQDs/ZnO/CdS NPs and Fe(III). In addition, the detection limit of Fe(III) was about 1.72×10^-7M. The effect of foreign ions on the fluorescence intensity of CQDs/ZnO/CdS NPs showed that the interference response in detecting of Fe(III) ions was low. Moreover, the quenching of Fe(III) and CQDs/ZnO/CdS NPs was discussed to be a static quenching procedure, which was proved by quenching constant K_SV and fluorescence lifetime τ. The study of thermodynamics showed that the values of entropy change (ΔS) and enthalpy change (ΔH) were both positive, and the value of free energy (ΔG) was negative, which implied that the weak interaction of the molecular between CQDs/ZnO/CdS NPs and Fe(III) was hydrophobic force, and the quenching process was endothermic and spontaneous.

Probing the interaction of iron complex containing N3S2 macrocyclic ligand with bovine serum albumin using spectroscopic techniques

The interaction of bovine serum albumin (BSA) with seven-coordination iron (II) complex containing sulfur-based macrocyclic ligand was investigated by means of UV/vis absorption spectroscopy and fluorescence quenching technique. The accurate fluorescence spectra are obtained by using Inner filter effect (IFE) correction. The apparent association constant, k_app, the number of binding sites, n, and the apparent binding constant K_SV were found to be 0.95 × 10³ M^-1, 0.96, and 6.13 × 10⁴ M^-1, respectively. It found that BSA molecules are adsorbed on the surface of iron (II) complex by electrostatic interaction. The quenching mechanism is discussed involving energy transfer from BSA to iron (II) complex.

Fabrication of a novel high-performance leather waste-based composite retention aid

In this study, a novel biomass composite retention aid was developed by using collagen hydrolysate (CH) extracted from collagen waste as the starting material, glutaraldehyde as the organic crosslinking agent and polymeric aluminum chloride (PAC) as the inorganic modifying agent. The as-prepared retention aids were characterized by gel chromatography, hydrodynamic diameter, zeta potential, transmission electron microscope (TEM), ultraviolet-visible adsorption spectra (UV-Vis), Fourier infrared spectrometer (FT-IR), and X-ray photoelectron spectroscopy (XPS). The results indicated that glutaraldehyde increased the molecular size of CH (i.e., CCH) through the crosslinking reaction between the aldehyde group of glutaraldehyde and the primary amine group of CH. Subsequently, the PAC further increased cationic charge density and molecular size of CCH (i.e., PAC–CCH) by the coordination interaction and self-assembly, thereby endowing PAC–CCH with better charge neutralization and bridging flocculation abilities. Compared to CH, CCH and PAC, the PAC–CCH exhibited excellent retention and drainage performances, and the best retention rate was greater than 85% at the dosage of 0.6 wt%. Our experimental results suggest that collagen wastes have a great potential to produce novel high-performance retention aids.

A novel biomass composite retention aid was developed by using collagen hydrolysate extracted from collagen waste as starting material, glutaraldehyde as organic crosslinking agent and polymeric aluminum chloride as inorganic modifying agent.

Evaluation of anthocyanins in Aronia melanocarpa/BSA binding by spectroscopic studies

The interaction between Anthocyanins in Aronia melanocarpa (AMA) and bovine serum albumin (BSA) were studied in this paper by multispectral technology, such as fluorescence quenching titration, circular dichroism (CD) spectroscopy and Fourier transform infrared spectroscopy (FTIR). The results of the fluorescence titration revealed that AMA could strongly quench the intrinsic fluorescence of BSA by static quenching. The apparent binding constants K_SV and number of binding sites n of AMA with BSA were obtained by fluorescence quenching method. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS), were calculated to be 18.45 kJ mol⁻¹ > 0 and 149.72 J mol⁻¹ K⁻¹ > 0, respectively, which indicated that the interaction of AMA with BSA was driven mainly by hydrophobic forces. The binding process was a spontaneous process of Gibbs free energy change. Based on Förster’s non-radiative energy transfer theory, the distance r between the donor (BSA) and the receptor (AMA) was calculated to be 3.88 nm. Their conformations were analyzed using infrared spectroscopy and CD. The results of multispectral technology showed that the binding of AMA to BSA induced the conformational change of BSA.

Binding of naringin and naringenin with hen egg white lysozyme: A spectroscopic investigation and molecular docking study

The interactions of naringenin (NG) and naringin (NR) with Hen Egg White Lysozyme (HEWL) in aqueous medium have been investigated using UV-vis spectroscopy, steady-state fluorescence, circular dichroism (CD), Fourier Transform infrared spectroscopy (FT-IR) and molecular docking analyses. Both NG and NR can quench the intrinsic fluorescence of HEWL via static quenching mechanism. At 300K, the value of binding constant (K_b) of HEWL-NG complex (5.596±0.063×10⁴M^-1) was found to be greater than that of HEWL-NR complex (3.404±0.407×10⁴M^-1). The negative ΔG° values in cases of both the complexes specify the spontaneous binding. The binding distance between the donor (HEWL) and acceptor (NG/NR) was estimated using the Försters theory and the possibility of non-radiative energy transfer from HEWL to NG/NR was observed. The presence of metal ions (Ca²⁺, Cu²⁺ and Fe²⁺) decreased the binding affinity of NG/NR towards HEWL. Synchronous fluorescence studies indicate the change in Trp micro-environment due to the incorporation of NG/NR into HEWL. CD and FT-IR studies indicated that the α-helicity of the HEWL was slightly enhanced due to ligand binding. NG and NR inhibited the enzymatic activity of HEWL and exhibited their affinity for the active site of HEWL. Molecular docking studies revealed that both NG and NR bind in the close vicinity of Trp 62 and Trp 63 residues which is vital for the catalytic activity.

Effect of heating and ionic strength on the interaction of bovine serum albumin and the antinutrients tannic and phytic acids, and its influence on in vitro protein digestibility

Bioavailability of food nutrients can be reduced in the presence of antinutrients such as phytates and tannins. This work aimed to study bovine serum albumin binding to phytic acid and tannic acid, and its influence on in vitro protein digestibility. The effect of autoclaving and boiling on protein digestibility and the microstructure of complexes was also evaluated. Results showed that high ionic strength promotes greater affinity between tannic acid and bovine serum albumin, and decreases in vitro protein digestibility. For phytic acid and bovine serum albumin, the opposite behavior is observed because interactions are governed by electrostatic forces. A rise in temperature above that causing denaturation of the protein favors its interaction with phytic acid, and disfavors that with tannic acid, probably due to different protein binding site exposure. For both antinutrients, heating treatment increased protein hydrolysis, the size of complexes and their fragility.

Spectroscopic analysis and docking simulation on the recognition and binding of TEM-1 β-lactamase with β-lactam antibiotics

The interaction between TEM-1 β-lactamase and antibiotics is very important in the hydrolysis of antibiotics. In the present study, the recognition and binding of TEM-1 β-lactamase with three β-lactam antibiotics, including penicillin G, cefalexin and cefoxitin, was investigated by fluorescence and ultraviolet-visible absorption spectra in combination with molecular docking in the temperature range of 278-288 K and under simulated physiological conditions. The results demonstrated that the fluorescence emissions of TEM-1 β-lactamase were extinguished by static quenching and the energy of TEM-1 β-lactamase was transferred in a non-radioactive manner. The binding of TEM-1 β-lactamase with the three antibiotics was a spontaneously exothermic process, with binding constants of 1.41×10⁷, 7.81×10⁶ and 5.43×10⁴ at 278 K. Furthermore, binding was driven by enthalpy change and the binding forces between them were mainly hydrogen bonding and Van der Waals forces. A TEM-1 β-lactamase only bound with one antibiotic at a time and the binding capacity between them was closely associated with the functional groups and flexibility in the antibiotics. In addition, a conformational change occurred in the TEM-1 β-lactamases when they bound with the three antibiotics and TEM-1 β-lactamase-antibiotic complexes were formed. The present study provided an insight into the recognition and binding of TEM-1 β-lactamase with β-lactam antibiotics, which may be helpful for designing a novel substrate for TEM-1 β-lactamase and developing novel antibiotics that are resistant to the enzyme.

Characterization of molecular structures of theaflavins and the interactions with bovine serum albumin

In this study, theaflavins (TF1, TF2A, TF2B and TF3) were prepared from black tea and their interaction with bovine serum albumin (BSA) was explored by fluorescence and CD spectroscopy. The results showed that the structures of theaflavins exhibited significant effects on the binding/quenching process, and the binding affinity increased with the increase of molecular weight of theaflavins and the presence of galloyl moiety. The quenching effects showed a sequence as TF3 > TF2A > TF2B > TF1, demonstrating the important role of the galloyl moiety on the C-3 position of theaflavins. CD spectra indicated that TF3 in high concentration could change the skeleton structure of BSA and induce the unfolding of BSA secondary structure. The present results provide a new perspective for better understanding of the likely physiological fate of theaflavins and help to control the functional characteristics of food.

Fluorescence quenching study on the interaction of ferroferric oxide nanoparticles with bovine serum albumin

Fluorescence quenching was used to study the potential interaction mechanism of Bovine serum albumin (BSA) with either hydrophilic ferroferric oxide (Fe₃O₄) nanoparticles (NPs) or hydrophobic Fe₃O₄ NPs. The experimental results indicated the mechanism between BSA and hydrophilic Fe₃O₄ NPs was static quenching and the one between BSA and hydrophobic Fe₃O₄ NPs was dynamic process that was drove by Förster's resonance energy transfer (FRET). And the binding parameters for the interaction of BSA with either hydrophilic or hydrophobic Fe₃O₄ NPs were calculated by using the fluorescence quenching measurement. The binding constant (K_A) values of hydrophilic Fe₃O₄ NPs were 8518.73±23.35 (at 298K), 1190.31±15.41 (at 306K) and 321.97±8.57 (at 313K), respectively. The thermodynamic analysis implied that the intermolecular forces between BSA and hydrophilic Fe₃O₄ NPs were Van der Waals interaction or hydrogen bond, because the values of ΔH and ΔS between them were negative. While the one of BSA and hydrophobic Fe₃O₄ NPs involved hydrophobic forces, owing to the positive ΔH and ΔS between them. But they were all enthalpy-driven and exothermic, since their ΔG values were all negative. Synchronous fluorescence spectroscopy suggested that the conformation of tryptophan residue of BSA was changed in the presence of hydrophilic Fe₃O₄ NPs or hydrophobic Fe₃O₄ NPs, because the position of the maximum emission wavelength had a discernible red shift.

Enhancement of the binding affinity of methylene blue to site I in human serum albumin by cupric and ferric ions

In this work, the binding characteristics of methylene blue (MB) to human serum albumin (HSA) and the influence of Cu(2+) and Fe(3+) on the binding affinity of MB to HSA were investigated using fluorescence, absorption, circular dichroism (CD) spectroscopy and molecular modelling. The results of competitive binding experiments using the site probes ketoprofen and ibuprofen as specific markers suggested that MB was located in site I within sub-domain IIA of HSA. The molecular modelling results agreed with the results of competitive site marker experiments and the results of CD spectra indicated that the interaction between MB and HSA caused the conformational changes in HSA. The binding affinity of MB to HSA was enhanced but to a different extent in the presence of Cu(2+) and Fe(3+), respectively, which indicated that the influence of different metal ions varied. Enhancement of the binding affinity of MB to HSA in the presence of Cu(2+) is due to the formation of Cu(2+)-HSA complex leading to the conformational changes in HSA, whereas in the presence of Fe(3+), enhancement of the binding affinity is due to the greater stability of the Fe(3+)-HSA-MB complex compared with the MB-HSA complex.