Optical spectroscopic exploration of binding of Cochineal Red A with two homologous serum albumins

Structure and energetics of serum protein complex of tea adulterant dye Bismarck brown Y using experimental and computational methods

Tea, a widely consumed aromatic beverage, is often adulterated with dyes such as Bismarck brown Y (C.I. 21000) (BBY), Prussian blue, and Plumbago, which pose potential health risks. The objective of this study is to analyze how the food dye BBY interacts with serum protein, bovine serum albumin (BSA). This study investigated the BBY-BSA interaction at the molecular level. Fluorescence spectroscopy results showed that the quenching of BSA by BBY is carried out by dynamic quenching mechanism. The displacement assay and molecular docking studies revealed that BBY binds at the flavanone binding site of BSA with hydrophobic interactions. Circular Dichroism results indicate the structural stability of the protein upon BBY binding. Molecular dynamics simulations demonstrated the stability of the complex in a dynamic solvent system, and quantum mechanics calculations showed slight conformational changes of the diaminophenyl ring due to increased hydrophobic interaction. The energetics of gas phase optimized and stable MD structures of BBY indicated similar values which further confirmed that the conformational changes were minor, and it also exhibited a moderate binding with BSA as shown by the MM/PBSA results. This study enhances our understanding of the molecular-level interactions between BBY and BSA, emphasizing the critical role of hydrophobic interactions.

Probing the interaction between encapsulated ethoxyquin and its β-cyclodextrin inclusion complex with bovine serum albumin

Ethoxyquin (EQ) is a synthetic antioxidant that is derived from quinolines and found in many meat products. EQ is strictly regulated in feed due to its potential health implications. An investigation of the interaction mechanism between EQ and transporter protein before and after β-cyclodextrin (β-CD) encapsulation was conducted with the use of multi-spectroscopy, cyclic voltammetry, and molecular docking. EQ formed complexes with bovine serum albumin (BSA), and affected secondary structure and microenvironment polarity of BSA. However, at 298 K, EQ's fluorescence quenching constants decreased from (9.81 ± 0.05) × 103 L mol-1 to (4.94 ± 0.09) × 103 L mol-1, binding constants decreased from (10.28 ± 0.02) × 103 L mol-1 to (2.08 ± 0.07) × 103 L mol-1, after encapsulation in β-CD as well as the binding distance increased. β-CD contains part of EQ in its hydrophobic cavity, inhibiting its binding to BSA. β-CD inclusion complex prevented adverse effects of EQ on BSA conformation. However, β-CD encapsulation had no effect on EQ's antioxidant activity.

High-throughput screening and investigation of the inhibitory mechanism of α-glucosidase inhibitors in teas using an affinity selection-mass spectrometry method

An affinity selection-mass spectrometry method was applied for high-throughput screening of α-glucosidase (AGH) inhibitors from teas. Fourteen out of nineteen screened AGH inhibitor candidates were clustered as galloylated polyphenols (GPs). "AGH-GPs" interaction studies, including enzyme kinetics, fluorescence spectroscopy, circular dichroism, and molecular docking, jointly suggested that GPs noncompetitively inhibit AGH activity by interacting with amino acid residues near the active site of AGH and inducing changes in AGH secondary structure. Representative GPs and white tea extract (WTE) showed comparable AGH inhibition effects in Caco2 cells and postprandial hypoglycemic efficacy in diabetic mice as acarbose. The area under the curve of oral sucrose tolerance test was lower by 8.16%, 6.17%, and 7.37% than control group in 15 mg/kg EGCG, 15 mg/kg strictinin, and 150 mg/kg WTE group, respectively. Our study presents a high-efficiency approach to discover novel AGH inhibitors and elucidates a potential mechanism by which tea decreases diabetes risks.

Shield, Anchor, and Adhesive Roles of Methylene Blue in Tyrosinase Adsorbed on Carbon Felt for a Flow Injection Amperometric Enzyme Biosensor for Phenolic Substrates and Inhibitors

Methylene blue (MB) acted as a stabilizer for preventing surface-induced denaturation of tyrosinase (TYR) adsorbed on a carbon felt (CF) surface, which is based on shield and anchor roles preventing the unfavorable conformational change of TYR on the hydrophobic CF surface. Furthermore, MB acted as an effective adhesive for TYR immobilization on CF. The resulting TYR and MB coadsorbed CF (TYR/MB-CF) worked as an excellent working electrode unit in an electrochemical detector in a flow injection amperometric biosensor, which allowed highly sensitive consecutive determination of not only TYR substrates but also competitive inhibitors. Simultaneous adsorption of TYR and MB from their mixed solution was much useful as compared with step-wise separated adsorption of TYR on the MB-adsorbed CF, which suggests that the binding interaction of MB with TYR in the solution phase is important for this phenomenon. Fluorescence and UV-vis spectroscopy revealed that not only electrostatic forces between the cationic MB and anionic amino acid residues of TYR but also hydrophobic interactions via the phenothiazine ring of MB play a principal binding driving force of MB with TYR at the surface of the TYR molecules. Synchronous fluorescence, three-dimensional fluorescence, and circular dichroism (CD) spectroscopy clarified that the conformation and the secondary structure of TYR slightly changed upon the MB binding, implying that MB binding leads to the modification of the original intramolecular bonding in part.

Investigation of the separate and simultaneous bindings of warfarin and fenofibrate to bovine serum albumin

Lipid-lowering drugs are often taken with anticoagulant drugs in hyperlipidemia patients. Fenofibrate (FNBT) and warfarin (WAR) are common clinical lipid-lowering drugs and anticoagulant drugs, respectively. A study of binding affinity, binding force, binding distance, and binding sites was performed to determine the interaction mechanism between drugs and carrier proteins (bovine serum albumin, BSA), as well as their effects on BSA conformation. Both FNBT and WAR can form complexes with BSA by van der Waals force and hydrogen bonds. WAR had a stronger fluorescence quenching effect on BSA, a stronger binding affinity, and greater effects on BSA conformation than FNBT. According to fluorescence spectroscopy and cyclic voltammetry, co-administration of drugs decreased one drug's binding constant to BSA and increased its binding distance. This suggested that each drug's binding to BSA was disturbed by each other, as well as each drug's binding ability to BSA was altered by the other. It was demonstrated that co-administration of drugs had greater effects on the secondary structure of BSA and microenvironment polarity surrounding amino acid residues, using multiple spectroscopy techniques, such as ultraviolet spectroscopy, Fourier transform infrared spectroscopy, and synchronous fluorescence spectroscopy.

Interaction mechanism of benzophenone-type UV filters on bovine serum albumin: Insights from structure-affinity relationship

Benzophenone (BP)-type UV filters can cause structural changes of carrier protein in plasma. The binding process of five BP-type UV filters with bovine serum albumin (BSA) was investigated by multiple characterization methods, along with their structure-affinity relationship involving the structure of the five BP-type UV filters and their binding affinity for BSA. The BP-type UV filters investigated bound to BSA spontaneously, and altered conformation of BSA. The binding constants and number of binding sites between BP-type UV filters and BSA were 10³-10⁶ M^-1 and 0.82-1.26, respectively. These BP-type UV filters and BSA interacted with the same binding forces and went through the similar binding process, suggesting that the benzophenone skeleton structure was primarily responsible for the BP-type UV filters and BSA binding, and changes in the structure of the BSA. The BP-type UV filters with hydroxyl substituent (BP-1 and BP-9) and non-polar molecules (BP-6) had a high affinity for binding BSA and had a greater impact on BSA conformation.

Alterations in the conformation and function of human serum albumin induced by the binding of methyl hydrogen phthalate

Phthalate esters (PAEs) are widely used as plasticizer components in production. Methyl hydrogen phthalate (MHP) is a metabolite of dimethyl phthalate (DMP, a kind of PAEs), and its toxic residues accumulate in the nature and can enter the human body. Here, the interaction between MHP and human serum albumin (HSA) was probed by using multi-spectral, computer simulations, and biochemical techniques. The results showed that MHP was spontaneously embedded in site I of HSA to form a complex by H-bonds and van der Waals forces (ΔH < 0, ΔS < 0). The binding constant (K_a) of the HSA-MHP system was 1.136 ± 0.026 × 10⁴ M^-1 (298 K). The combination of MHP produced conformational variations of HSA, as shown by the 3D fluorescence spectrum, CD spectra, and molecular dynamics simulation. Additionally, molecular docking indicated that MHP was surrounded by multiple residues, such as Lys199, Leu203, Phe206, and Trp214. Specifically, Lys199 and Trp214 exerted a crucial effect on the interaction of HSA and MHP. The residues with important energy contribution were mostly located in site I. The ASA values of the aromatic amino acids of HSA changed after combining with MHP. The R_g and SASA values of HSA increased after adding MHP, suggesting that the structure of HSA was less compact. Moreover, the esterase-like activity of HSA increased after adding MHP to HSA, indicating that MHP may disturb the normal physiological activities in the human body. This study was helpful to understand the biological function of MHP and provided some insights for its side effect in the human body.

Characteristics and structure of a soy protein isolate-lutein nanocomplex produced via high-pressure homogenization

BACKGROUND

In recent years, nanocarriers for transporting active substances have attracted attention. This study was to explore the soy protein isolate (SPI) after high-pressure homogenization (HPH) (0, 30, 60, 90 and 120 MPa) as potential lutein carriers.

RESULTS

The load amount (LA) and encapsulation efficiency (EE) of the SPI-lutein nanocomplexes at a homogenization pressure of 60 MPa were the highest (2.32 mg mL^-1 and 92.85%, respectively), and the average particle size and ζ-potential of the SPI-lutein nanocomplexes were 192.1 nm and -30.06 mV, respectively. The DPPH (2,2-diphenyl-1-picrylhydrazyl) and hydroxyl-antioxidant activities of the complex increased from 12.4% and 23.3% to 52.7% and 61.07%, respectively, after the protein was treated with HPH. The surface hydrophobicity of the SPI and the SPI-lutein nanocomplexes increased with increasing homogenization pressure treatment. Fourier transform-infrared spectrophotometry analyses suggested that the homogenization treatments resulted in partial unfolding of the protein molecules, and the addition of lutein can also lead to the change of protein secondary structure. The fluorescence emission of SPI was quenched by lutein through the static quenching mechanism. Fluorescence experiments revealed that SPI and lutein had the strongest binding ability through hydrophobic interaction at a homogenization pressure of 60 MPa.

CONCLUSION

After HPH, the combination of SPI and lutein was beneficial, and the stability of lutein also improved after the combination. This study is conducive to expanding the application of soybean protein in the food industry. © 2022 Society of Chemical Industry.

Emerging Albumin-Binding Anticancer Drugs for Tumor-Targeted Drug Delivery: Current Understandings and Clinical Translation

Albumin has shown remarkable promise as a natural drug carrier by improving pharmacokinetic (PK) profiles of anticancer drugs for tumor-targeted delivery. The exogenous or endogenous albumin enhances the circulatory half-lives of anticancer drugs and passively target the tumors by the enhanced permeability and retention (EPR) effect. Thus, the albumin-based drug delivery leads to a potent antitumor efficacy in various preclinical models, and several candidates have been evaluated clinically. The most successful example is Abraxane, an exogenous human serum albumin (HSA)-bound paclitaxel formulation approved by the FDA and used to treat locally advanced or metastatic tumors. However, additional clinical translation of exogenous albumin formulations has not been approved to date because of their unexpectedly low delivery efficiency, which can increase the risk of systemic toxicity. To overcome these limitations, several prodrugs binding endogenous albumin covalently have been investigated owing to distinct advantages for a safe and more effective drug delivery. In this review, we give account of the different albumin-based drug delivery systems, from laboratory investigations to clinical applications, and their potential challenges, and the outlook for clinical translation is discussed. In addition, recent advances and progress of albumin-binding drugs to move more closely to the clinical settings are outlined.

Protein binding and cytotoxic activities of monomeric and dimeric oxido-vanadium(V) salan complexes: Exploring the solution behavior of monoalkoxido-bound oxido-vanadium(V) complex

Three ONNO donor tetradentate diamino bis(phenolato) "salan" ligands, N, N'-dimethyl-N, N'-bis-(5-chloro-2-hydroxy-3-methyl-benzyl)-1,2-diaminoethane (H₂L¹), N, N'-dimethyl-N, N'-bis-(5-chloro-2-hydroxy-3-isopropyl-6-methyl-benzyl)-1,2-diamino-ethane (H₂L²) and N, N'-bis-(5-chloro-2-hydroxy-3-isopropyl-6-methyl-benzyl)-1,2-diaminocyclohexane (H₂L³) have been synthesized by following Mannich condensation reaction. Reaction of these ligands with their corresponding vanadium metal precursors gave one oxidomethoxidovanadium(V) [V^VOL¹(OCH₃)] (1) and two monooxido-bridged divanadium (V, V) complexes [V^VOL^2-3]₂(μ-O) (2-3). The complexes were characterized by IR, UV-vis, NMR and ESI mass spectrometry. Also, the structure of all the complexes (1-3) was confirmed by the Single-Crystal X-ray diffraction analysis, which revealed a distorted octahedral geometry around the metal centres. The solution behavior of the [V^VOL¹(OCH₃)] (1) reveals the formation of two different types of V(V) species in solution, the structurally characterized compound 1 and its corresponding monooxido-bridged divanadium (V, V) complex [V^VOL¹]₂(μ-O), which was further studied by IR, and NMR spectroscopy. The electrochemical behavior of all the complexes was evaluated through cyclic voltammetry. Interaction of the salan-V(V) complexes with human serum albumin (HSA) and bovine serum albumin (BSA) were analysed through fluorescence quenching, UV-vis absorption titration, synchronous fluorescence, circular dichroism studies, and förster resonance energy transfer (FRET). Finally, the in vitro cytotoxicity of the complexes was investigated against MCF-7 and HT-29 and NIH-3T3 cell lines. Cytotoxicity value of complexes in both MCF-7 and HT-29 follows the same trend that is 3 > 1 > 2 which is in line with protein binding affinity of the complexes.

Interaction of Conazole Pesticides Epoxiconazole and Prothioconazole with Human and Bovine Serum Albumin Studied Using Spectroscopic Methods and Molecular Modeling

The interactions of epoxiconazole and prothioconazole with human serum albumin and bovine serum albumin were investigated using spectroscopic methods complemented with molecular modeling. Spectroscopic techniques showed the formation of pesticide/serum albumin complexes with the static type as the dominant mechanism. The association constants ranged from 3.80 × 104-6.45 × 105 L/mol depending on the pesticide molecule (epoxiconazole, prothioconazole) and albumin type (human or bovine serum albumin). The calculated thermodynamic parameters revealed that the binding of pesticides into serum albumin macromolecules mainly depended on hydrogen bonds and van der Waals interactions. Synchronous fluorescence spectroscopy and the competitive experiments method showed that pesticides bind to subdomain IIA, near tryptophan; in the case of bovine serum albumin also on the macromolecule surface. Concerning prothioconazole, we observed the existence of an additional binding site at the junction of domains I and III of serum albumin macromolecules. These observations were corroborated well by molecular modeling predictions. The conformation changes in secondary structure were characterized by circular dichroism, three-dimensional fluorescence, and UV/VIS absorption methods.

Evaluation of parent and nano-encapsulated terbium(III) complex toward its photoluminescence properties, FS-DNA, BSA binding affinity, and biological applications

BACKGROUND

There is a crucial need for finding and developing new compounds as the anticancer and antimicrobial agents with better activity, specific target, and less toxic side effects.

OBJECTIVES

Base on the potential anticancer properties of lanthanide complexes, in the paper, the biological applications of terbium (Tb) complex, containing 2,9-dimethyl- 1,10-phenanthroline (Me2Phen) such as anticancer, antimicrobial, DNA cleavage ability, the interaction with FS-DNA (Fish-Salmon DNA) and BSA (Bovine Serum Albumin) was examined.

METHODS

The interaction of Tb-complex with BSA and DNA was studied by emission spectroscopy, absorption titration, viscosity measurement, CD spectroscopy, competitive experiments, and docking calculation. Also, the ability of this complex to cleave DNA was reported by gel electrophoresis. Tb-complex was concurrently screened for its antibacterial activities by different methods. Besides, the nanocarriers of Tb-complex (lipid nanoencapsulation (LNEP) and the starch nanoencapsulation (SNEP)), as active anticancer candidates, were prepared. MTT technique was applied to measure the antitumor properties of these compounds on human cancer cell lines.

RESULTS

The experimental and docking results suggest significant binding between DNA as well as BSA with terbium-complex. Besides, groove binding plays the main role in the binding of this compound with DNA and BSA. The competitive experiment with hemin demonstrated that the terbium complex was bound at site III of BSA, which was confirmed by the docking study. Also, Tb-complex was concurrently screened for its DNA cleavage, antimicrobial, and anticancer activities. The anticancer properties of LNEP and SNEP are more than the terbium compound.

CONCLUSIONS

Tb-complex can bond to DNA/BSA with high binding affinity. Base on biological applications of Tb-complex, it can be concluded that this complex and its nanocarriers can suggest as novel anticancer, antimicrobial candidates.

Antioxidant, Antidiabetic and Anticancer Activities of L-Phenylalanine and L-Tyrosine Ester Surfactants: In Vitro and In Silico Studies of their Interactions with Macromolecules as Plausible Mode of Action for their Biological Properties

Background:

Aromatic amino acid-based surfactants have been found to have interesting biological properties such as antibacterial and hemolytic activities. Recently, we have reported the antibacterial activity of a range of ester hydrochloride surfactants derived from L-Phenylalanine and LTyrosine. This study aims at assessing the antioxidant, α-glycosidase inhibitory and cytotoxic activities of a series of L-Phenylalanine and L-Tyrosine ester hydrochlorides. Molecular docking and BSA binding studies were also carried out in order to investigate their potential therapeutic targets.

Methods:

L-Phenylalanine and L-Tyrosine surfactants were tested as potential lipophilic antioxidants using the DPPH and ABTS assays. These surfactants were also tested for their α-glycosidase inhibitory activity using 4-nitrophenyl α -D-glucopyranoside (pNPG) as substrate. Their cytotoxicity effects were screened using HeLa and KB cell lines. Glide version 5.7 as implemented in Schrödinger suite 2013-1, was used for performing docking studies of L-Phenylalanine and L-Tyrosine dodecyl esters. The interaction of the ester hydrochlorides of L-Phenylalanine and L-Tyrosine with bovine serum albumin (BSA) was investigated using fluorometric titration.

Results:

The presence of the phenolic moiety in L-Tyrosine-based surfactants was found to enhance the antioxidant and α-glucosidase inhibitory activities compared to the L-Phenylalanine derivatives. The α- glucosidase and anticancer activities of the phenylalanine surfactants were found to increase with chain length up to C12 above which the activities exhibited a downward trend. In the case of the tyrosine series, an increase in chain length from C8 to C14 was found to decrease the α-glucosidase inhibitory activity and increase the anticancer activity of the surfactants. Binding studies with bovine serum albumin showed that the tyrosine surfactants displayed greater affinity for the serum albumin, owing to the presence of the phenolic group which altered the orientation of the surfactant molecule within the hydrophobic core of BSA.

Conclusion:

L-Tyrosine esters having a phenolic moiety were found to possess enhanced biological activity in terms of both the antioxidant and antidiabetic activities as well as also bind more strongly to Bovine serum albumin. Molecular docking studies of the phenylalanine and tyrosine surfactants of similar chain length with target proteins showed direct correlation with their anticancer and antidiabetic activity. Therefore, the findings show that these aromatic based surfactants derived from L-Tyrosine can act as promising antioxidant, antidiabetic and anticancer agents, and they can also be efficiently transported and eliminated in the body, making them useful candidates for drug designs.

In vitro anticancer activity of parent and nano-encapsulated samarium(iii) complex towards antimicrobial activity studies and FS-DNA/BSA binding affinity

Based on the potential anticancer properties of lanthanide complexes, the anticancer activity of the Sm(iii) complex containing a 2,2′-bipyridine ligand (bpy) and its interaction with FS-DNA (Fish-Salmon DNA) and BSA (Bovine Serum Albumin) were examined experimentally and by molecular docking in this paper. Absorption and fluorescence spectroscopic methods were used to define the thermodynamic parameters, binding constant (K_b), and the probable binding mechanism. It was concluded that the Sm complex interacts with FS-DNA through a minor groove with a K_b of 10⁵ M⁻¹. Also, the K_b for the BSA binding at 298 K was found to be 5.89 × 10⁵ M⁻¹, showing relatively a high tendency of the Sm complex to DNA and BSA. Besides, the Sm complex was docked to BSA and DNA by the autodock program. The results of the docking calculations were in good agreement with the experimental examinations. Additionally, the antifungal and antibacterial properties of this complex were investigated. The anticancer tests on the effect of the Sm complex, starch nano-encapsulation, and lipid nano-encapsulation in MCF-7 and A-549 cell lines were performed by the MTT method. It can be observed that the Sm complex and its nanocarriers presented a selective inhibitory effect on various cancer cell growths.

The biological properties of the Sm-complex, such as its interaction with FS-DNA and BSA, anticancer, and antimicrobial activities were studied.

Experimental and theoretical investigations of Dy(III) complex with 2,2'-bipyridine ligand: DNA and BSA interactions and antimicrobial activity study

In this study, the interactions of a novel metal complex [Dy(bpy)₂Cl₃.OH₂] (bpy is 2,2'-bipyridine) with fish salmon DNA (FS-DNA) and bovine serum albumin (BSA) were investigated by experimental and theoretical methods. All results suggested significant binding between the Dy(III) complex with FS-DNA and BSA. The binding constants (K_b), Stern-Volmer quenching constants (K_SV) of Dy(III)-complex with FS-DNA and BSA at various temperatures as well as thermodynamic parameters using Van't Hoff equation were obtained. The experimental results from absorption, ionic strength, iodide ion quenching, ethidium bromide (EtBr) quenching studies and positive ΔH˚ and ΔS˚ suggested that hydrophobic groove-binding mode played a predominant role in the binding of Dy(III)-complex with FS-DNA. Indeed, the molecular docking results for DNA-binding were in agreement with experimental data. Besides, the results found from experimental and molecular modeling indicated that the Dy(III)-complex bound to BSA via Van der Waals interactions. Moreover, the results of competitive tests by phenylbutazone, ibuprofen, and hemin (as a site-I, site-II and site-III markers, respectively) considered that the site-III of BSA is the most possible binding site for Dy(III)-complex. In addition, Dy(III) complex was concurrently screened for its antimicrobial activities. The presented data provide a promising platform for the development of novel metal complexes that target nucleic acids and proteins with antimicrobial activity.Communicated by Ramaswamy H. Sarma.

Spectroscopic and cytotoxicity studies on the combined interaction of (-)-epigallocatechin-3-gallate and anthracycline drugs with human serum albumin

The interactions of (-)-epigallocatechin-3-Gallate (EGCG) and anthracycline drugs (doxorubicin, DOX and epirubicin, EPI) alone or in combination with human serum albumin (HSA) under physiological condition were studied by fluorescence spectroscopy, UV-vis absorption spectroscopy, circular dichroism (CD) spectroscopy, and dynamic light scattering (DLS). The cytotoxic activity of the single drug, combined drugs, and their complexes with HSA against human cervical cancer HeLa cell line was determined by MTT assay. Fluorescence quenching result and difference spectra of UV absorption revealed the formation of static complex between EGCG, DOX, or EPI and HSA. The binding of EGCG with HSA was driven by both enthalpy and entropy while the binding of DOX or EPI was mainly entropy driven. The nature of binding was expounded based on the effect of sodium chloride, tetrabutylammonium bromide, and sucrose which interfere in electrostatic, hydrophobic, and hydrogen bonding interactions, respectively. Site marker competitive experiments combined with synchronous fluorescence spectra showed that these three ligands mainly bound to subdomain IIA of HSA and were closer to tryptophan residues. In EGCG + DOX/EPI + HSA ternary system, the effect of one drug on the binding ability of another drug was discussed. The influences of the individual and combined binding of EGCG and DOX/EPI on the secondary structure and particle size of HSA were investigated by CD spectroscopy and DLS, respectively. Moreover, the synergistic cytotoxicity of EGCG and DOX/EPI as well as their complexes with HSA were discussed. Obtained results would provide beneficial information on the combination of EGCG and anthracyclines in clinic.

Evaluation the binding of chelerythrine, a potentially harmful toxin, with bovine serum albumin

Chelerythrine (CHE), a benzophenanthridine alkaloid, is usually used as a nutritional and functional additive in variety of health foods. However, it should be paid enough attention because of its potential toxicity to human health. In this work, the binding mechanism of CHE with bovine serum albumin (BSA) was systematically investigated with spectroscopic approaches. The results showed that the mixture of BSA with CHE could spontaneously cause the formation of BSA-CHE complex through electrostatic interaction under simulative physiological conditions (0.01 mol L^-1 Tris-HCl, 0.015 mol L^-1 NaCl, pH = 7.4). Site marker competitive displacement experiments exhibited that CHE was primarily bound to the hydrophobic pocket of the site II (subdomain IIIA) of BSA. It has been reported that the binding of small functional molecules to serum albumins remarkably impacts their absorption, distribution, metabolism, conformation, and excretion features. Therefore, this study might be helpful for human to have an in-depth understanding of the biological effect of CHE in vivo and guide human to take it safely and reasonably.

Metal-Biosurfactant Complexes Characterization: Binding, Self-Assembly and Interaction with Bovine Serum Albumin

Studies on the specific and nonspecific interactions of biosurfactants with proteins are broadly relevant given the potential applications of biosurfactant/protein systems in pharmaceutics and cosmetics. The aim of this study was to evaluate the interactions of divalent counterions with the biomolecular anionic biosurfactant surfactin-C₁₅ through molecular modeling, surface tension and dynamic light scattering (DLS), with a specific focus on its effects on biotherapeutic formulations. The conformational analysis based on a semi-empirical approach revealed that Cu²⁺ ions can be coordinated by three amide nitrogens belonging to the surfactin-C₁₅ cycle and one oxygen atom of the aspartic acid from the side chain of the lipopeptide. Backbone oxygen atoms mainly involve Zn²⁺, Ca²⁺ and Mg²⁺. Subsequently, the interactions between metal-coordinated lipopeptide complexes and bovine serum albumin (BSA) were extensively investigated by fluorescence spectroscopy and molecular docking analysis. Fluorescence results showed that metal-lipopeptide complexes interact with BSA through a static quenching mechanism. Molecular docking results indicate that the metal-lipopeptide complexes are stabilized by hydrogen bonding and van der Waals forces. The biosurfactant-protein interaction properties herein described are of significance for metal-based drug discovery hypothesizing that the association of divalent metal ions with surfactin allows its interaction with bacteria, fungi and cancer cell membranes with effects that are similar to those of the cationic peptide antibiotics.

Calorimetric and spectroscopic detection of the interaction between a diazo dye and human serum albumin

Dye effluents are one of the main causes of water pollution. Azo dyes, the most widely applied colorants, are particularly difficult to degrade. Exposure of such dyes to the aquatic environment is hazardous to human health and biota due to their intrinsic harmful mutagenic and carcinogenic properties. Congo Red (CR) is an anionic and synthetic diazo dye, which is recalcitrant to the biodegradative process and metabolizes to produce a potential carcinogen. Research on the interaction of this toxic dye with serum albumin, as a transport protein, is of paramount significance because the physiological and toxicological behaviours of the dye in vivo are associated with its interactive characteristics with the proteins. In this regard, a detailed binding profile of CR with human serum albumin (HSA) was studied using isothermal titration calorimetry (ITC) along with various spectroscopic and microscopic methods. The thermodynamic results from ITC indicated that the CR-HSA non-covalent interaction occured primarily due to favorable entropy and unfavorable enthalpy with a Ka of 106 M-1 at lower concentrations and 105 M-1 at higher concentrations. Steady-state fluorescence data revealed that the intrinsic fluorescence of HSA was quenched in the presence of CR via the static quenching mechanism. Using Förster's non-radioactive energy transfer theory (FRET), the specific binding distance r (2.73 nm) between the donor (Trp-214 from HSA) and the acceptor (CR) was calculated. Our preliminary results indicated that CR had a high affinity to HSA, which can have significant implications in the distribution and elimination of this toxic dye upon exposure.

Interaction of repaglinide with bovine serum albumin: Spectroscopic and molecular docking approaches

Repaglinide (RPG) regulates the amount of glucose by stimulating the pancreas to release insulin in the blood. In view of its biological importance, we have examined the interaction between RPG and a model protein, bovine serum albumin (BSA) employing various spectroscopic, electrochemical and molecular docking methods. Fluorescence spectra of BSA were recorded in the presence and absence of RPG in phosphate buffer of pH 7.4. Fluorescence intensity of BSA was decreased upon the addition of increased concentrations of RPG, indicating the interaction between RPG and BSA. Stern-Volmer quenching analysis results revealed that RPG quenched the intensity of BSA through dynamic quenching mechanism. This was further confirmed from the time-resolved fluorescence measurements. The binding constant as calculated from the spectroscopic and voltammetric results was observed to be in the order of 10⁴ M⁻¹ at 298 K, suggesting the moderate binding affinity between RPG and BSA. Competitive experimental results revealed that the primary binding site for RPG on BSA was site II. Absorption and circular dichroism studies indicated the changes in the secondary structure of BSA upon its interaction with RPG. Molecular simulation studies pointed out that RPG was bound to BSA in the hydrophobic pocket of site II.

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Dynamic mode of quenching mechanism was noticed in RPG-BSA interaction.

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RPG was bound to BSA at the Sudlow’s site II and the resultant RPG-BSA complex was mainly stabilized by hydrophobic forces.

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The binding constant of RPG-BSA of the order of 10⁴ M⁻¹ at 298 K indicated the non-covalent interactions.

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Secondary structural changes in BSA upon binding to RPG were evident from absorption and circular dichroism studies.

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The influence of β-cyclodextrin and metal ions on RPG-BSA binding affinity was examined.

Interaction between quinoline yellow and human serum albumin: spectroscopic, chemometric and molecular docking studies

BACKGROUND

Quinoline yellow (QY), a synthetic colourant widely used in the food industry, has caused extensive concerns because of its potentially harmful effects on human health. In the present work, the interactions between QY and human serum albumin (HSA) were characterized by multiple spectroscopic methods, a chemometric algorithm, and molecular modelling studies.

RESULTS

The concentration profiles and pure spectra obtained for the components (QY, HSA and QY-HSA complex) from analyses of the expanded UV-visible absorption data matrices by multivariate curve resolution alternating least squares confirmed the QY-HSA interaction process. QY quenched the fluorescence of HSA through formation of a QY-HSA complex that was stabilized by moderate affinity. Hydrophobic forces and hydrogen bonding play major roles in the binding of QY to HSA. Site-specific marker-induced displacement results suggest that QY binds to subdomain IIA of HSA. This was corroborated by the molecular docking results. Decreases in HSA surface hydrophobicity and free sulfhydryl group content indicate that QY causes a contraction of the peptide strand in HSA, hiding the hydrophobic patches of the protein. Analyses by UV-visible absorption, circular dichroism, and three-dimensional fluorescence spectroscopy found that QY causes microenvironmental perturbations around the fluorophores and secondary structure changes in HSA.

CONCLUSION

This work shows that QY binds to HSA, affecting its structural and functional properties, and provides new insights into the binding mechanism and a comprehensive understanding of the toxicity of QY to biological processes. © 2018 Society of Chemical Industry.

A Chalcone-Based Potential Therapeutic Small Molecule That Binds to Subdomain IIA in HSA Precisely Controls the Rotamerization of Trp-214

The principal intent of this work is to explore whether the site-specific binding of a newly synthesized quinoline-appended anthracenyl chalcone, (E)-3-(anthracen-10-yl)-1-(6,8-dibromo-2-methylquinolin-3-yl)prop-2-en-1-one (ADMQ), with an extracellular protein of the human circulatory system, human serum albumin (HSA), can control the rotamerization of its sole tryptophan residue, Trp-214. With this aim, we have systematically studied the binding affinity, interactions, and localization pattern of the title compound inside the specific binding domain of the transport protein and any conformation alteration caused therein. Multiple spectroscopic experiments substantiated by an in silico molecular modeling exercise provide evidence for the binding of the guest ADMQ in the hydrophobic domain of HSA, which is primarily constituted by residues Trp-214, Arg-218, Arg-222, Asp-451, and Tyr-452. Rotationally restricted ADMQ prefers to reside in Sudlow site I (subdomain IIA) of HSA in close proximity (2.45 nm) to the intrinsic fluorophore Trp-214 and is interestingly found to control its vital rotamerization process. The driving force for this rotational interconversion is predominantly found to be governed by the direct interaction of ADMQ with Trp-214. However, the role of induced conformational perturbation in the biomacromolecule itself upon ADMQ adoption cannot be ruled out completely, as indicated by circular dichroism, 3D fluorescence, root-mean-square deviation, root-mean-square fluctuation, and secondary structure element observations. The comprehensive spectroscopic study outlined herein provides important information on the biophysical interaction of a chalcone-based potential therapeutic candidate with a carrier protein, exemplifying its utility in having a regulatory effect on the microconformations of Trp-214.

Difference in Binding of Long- and Medium-Chain Fatty Acids with Serum Albumin: The Role of Macromolecular Crowding Effect

Fatty acids (FAs) are transported by serum albumin in plasma. Studies have been undertaken to address the binding of MCFAs or LCFAs to human plasma albumin (HPA) and bovine serum albumin (BSA) by characterizing the binding affinities. Previous research on FA binding to serum albumin was usually performed in dilute solutions that are not sufficiently concentrated for the interpretation of the significance of the results under normal physiological conditions. How macromolecular crowded media affect fatty acids and bovine serum albumin (BSA) binding remains unknown. In this article, we investigated the mechanism of FA-BSA binding in a polyethylene glycol crowding environment by using thermodynamic and spectroscopic methods. Molecular crowding increased the binding constant for saturated medium-chain fatty acids (MCFAs) but significantly decreased the binding constant for unsaturated long-chain FAs. The binding sites tended to increase in all the cases. Further investigation revealed that crowding media might loosen the structure of BSA, facilitating MCFA-BSA binding. This research is useful for understanding the transportation of FAs by BSA under physiological conditions and may also help to control digestion by the eventual incorporation of macromolecular crowding agents into food formulations.

Simultaneous Binding of Folic Acid and Methotrexate to Human Serum Albumin: Insights into the Structural Changes of Protein and the Location and Competitive Displacement of Drugs

Protein structure can be flexible to adopt multiple conformations to house small molecules. In this paper, we have attempted to experimentally figure out how the structure of a transport protein steer the drug-drug competition (DDC) by maintaining the equilibrium distribution of the bound and unbound fractions of drugs. This understanding is an important facet in biophysical and medicinal chemistry to ascertain the effectiveness of drugs. It is important to note that majority of studies involving small-molecule-transport protein interaction aimed at characterizing the binding process, and because these proteins can interact with thousands of molecules, there are hundreds of reports on such interactions. This ultimately led to an impression among the readers that any studies involving serum albumin may not lead to any new findings except for traditional binding explorations. However, in the present paper, we would like to draw the attention of the readers that our findings are very surprising, new, and important, involving the phenomenon of ligand-protein interaction. Here, we have studied two structurally similar drugs methotrexate (MTX) and folic acid (FA), which attempt to bind the primary binding site (subdomain IIA), one at a time, of human serum albumin. Details of binding analyses reveal that when both of the drugs are present, the single-site binding mode of FA prefers to occupy the primary binding site and hence pushes the primary-site-bound MTX to another location (subdomain IIIA), which is the second binding site of MTX. The structural analysis indicates that DDC has occurred in a cooperative fashion so that the loss of the protein secondary structure is minimum when both drugs are bound to the protein, which means that in the case of duo-drug binding, the conventional interaction between the drug and the protein is altered to undergo restoration of the protein structure. This can indeed regulate the DDC by modifying the bound and unbound fractions of MTX in plasma. The present study emphasizes that in vitro structural characterizations of the transport protein provide important information to improve the molecular-level understanding of DDC for further therapeutic applications with combination drug.

Exploration of interaction of canthaxanthin with human serum albumin by spectroscopic and molecular simulation methods

The interaction between the food colorant canthaxanthin (CA) and human serum albumin (HSA) in aqueous solution was explored by using fluorescence spectroscopy, three-dimensional fluorescence spectra, synchronous fluorescence spectra, UV-vis absorbance spectroscopy, circular dichroism (CD) spectra and molecular docking methods. The thermodynamic parameters calculated from fluorescence spectra data showed that CA could result in the HSA fluorescence quenching. From the K_SV change with the temperature dependence, it was concluded that HSA fluorescence quenching triggered by CA is the static quenching and the number of binding sites is one. Furthermore, the secondary structure of HSA was changed with the addition of CA based on the results of synchronous fluorescence, three-dimensional fluorescence and CD spectra. Hydrogen bonds and van der Waals forces played key roles in the binding process of CA with HSA, which can be obtained from negative standard enthalpy (ΔH) and negative standard entropy (ΔS). Furthermore, the conclusions were certified by molecular docking studies and the binding mode was further analyzed with Discovery Studio. These conclusions can highlight the potential of the interaction mechanism of food additives and HSA.

Probing the binding properties of dicyandiamide with pepsin by spectroscopy and docking methods

Dicyandiamide (DCD), considered to be a nitrification inhibitor, poses threat to human's health with exposure from milk, infant formula and other food products. In this work, DCD was investigated for its binding reaction with pepsin using spectroscopy and docking methods. Fluorescence experiments indicated DCD quenched the fluorescence of pepsin through a static process. Thermodynamic analysis of the binding data (ΔH⁰ = -21.72 kJ mol^-1 and ΔS⁰ = 17.61 J mol^-1 K^-1) suggested the involvement of hydrophobic and hydrogen bonding in the complex formation. The pepsin interacted with DCD at a hydrophobic cavity, leading to a conformational changes in the pepsin, as revealed from UV-vis absorption, Fourier transform infrared, the time-resolved fluorescence, three-dimensional fluorescence and circular dichroism spectral results.

Fluorescence spectroscopic and molecular docking studies of the binding interaction between the new anaplastic lymphoma kinase inhibitor crizotinib and bovine serum albumin

Binding of the recently introduced anti-cancer drug, crizotinib (CRB) with the bovine serum albumin (BSA) was comprehensively studied with the aid of fluorescence and UV-Vis spectroscopic as well as molecular docking techniques. The collective results of the study under the simulated physiological conditions proposed a static type of binding occurring between the CRB and BSA with binding constants of 10⁴Lmol^-1. BSA conformational changes were investigated using three dimensional (3D) and synchronous fluorescence measurements. Moreover, the results of site marker competitive experiments and molecular docking, it could be deduced that CRB was inserted into the subdomain IIA (site I) of BSA yielding a more stabilized system. This was further confirmed with the molecular docking results which revealed that CRB is located in the active site residues Try149, Glu152, Ser191, Arg194, Arg198, Trp213, Arg217, Arg256, His287, Ala290, Glu291, Ser343, Asp450 within a radius of 6Å. Combining the molecular docking studies and the computed thermodynamic parameters, it can be inferred that hydrophobic and electrostatic interactions are the major binding forces involved in formation of the CRB-BSA complex.

Interactions of two food colourants with BSA: Analysis by Debye-Hückel theory

We have focused on exploring pH- and ionic strength-modulated binding of acid red 1 (AR1) and acid green 50 (AG50) with bovine serum albumin (BSA) by fluorescence, UV-vis absorption and FTIR spectra. The results implied that the quenching mechanism of BSA-AR1/AG50 system was a static quenching, electrostatic force dominated the formation of BSA-AR1/AG50 complex, and the binding affinity of AR1 was greater than that of AG50 on the subdomain IIA of BSA. Moreover, their true thermodynamic binding constant (Keq), true free energy change (ΔG(0)I→0), and effective charge (ZP) in the anion receptor pocket of BSA were calculated using Debye-Hückel limiting law. The local charge bound by AR1/AG50 rather than the overall or surface charge of BSA played a key role in determining their interaction strength. Besides, the thermal and structural stabilization of BSA was discussed by analyzing the changes of Tm and Hurea without/with the addition of AR1/AG50, respectively.

Solvent assisted formation of ruthenium(III) and ruthenium(II) hydrazone complexes in one-pot with potential in vitro cytotoxicity and enhanced LDH, NO and ROS release

A set each of new bivalent and trivalent ruthenium complexes, [Ru(III)(HL)Cl2(EPh3)2] and [Ru(II)(L)(CO)(EPh3)2] (E = P (complexes and ) or As (complexes and )) were synthesised from the reactions of [Ru(III)Cl3(EPh3)3] with 2-hydroxynaphthaldehyde benzoic acid hydrazone (H2L) in methanol-chloroform and characterized by elemental analysis, spectral data and XRD study. A suitable mechanism to account for the formation of bivalent ruthenium carbonyl complexes from the corresponding trivalent precursors is provided by considering the role of added base in the reaction. Interaction of complexes with CT-DNA/bovine serum albumin was analysed with absorption and emission spectral titration studies. In vitro cytotoxic potential of the above ruthenium hydrazone complexes assayed against the A549 cell line revealed a significant growth inhibition. The test complexes added in IC50 concentration into the cell culture medium enhanced the release of lactate dehydrogenase, NO and reactive oxygen species in comparison with the control. Cell death induced by the complexes was studied using a propidium iodide staining assay and showed noticeable changes in the cell morphology which resembled apoptosis.

Synthesis of imidazole derivatives and the spectral characterization of the binding properties towards human serum albumin

Small molecular drugs that can combine with target proteins specifically, and then block relative signal pathway, finally obtain the purpose of treatment. For this reason, the synthesis of novel imidazole derivatives was described and this study explored the details of imidazole derivatives binding to human serum albumin (HSA). The data of steady-state and time-resolved fluorescence showed that the conjugation of imidazole derivatives with HSA yielded quenching by a static mechanism. Meanwhile, the number of binding sites, the binding constants, and the thermodynamic parameters were also measured; the raw data indicated that imidazole derivatives could spontaneously bind with HSA through hydrophobic interactions and hydrogen bonds which agreed well with the results from the molecular modeling study. Competitive binding experiments confirmed the location of binding. Furthermore, alteration of the secondary structure of HSA in the presence of the imidazole derivatives was tested.

A Spectroscopic Approach to Investigate the Molecular Interactions between the Newly Approved Irreversible ErbB blocker "Afatinib" and Bovine Serum Albumin

The interaction of afatinib (AFB) with bovine serum albumin (BSA) was examined via fluorescence and UV-Vis spectroscopy. Spectrofluorimetric measurements revealed that AFB can strongly quench the BSA intrinsic fluorescence through producing a non-fluorescent complex. This quenching mechanism was thoroughly investigated with regard to the type of quenching, binding constant, number of binding locations and the fundamental thermodynamic parameters. Subsequently, the association constant of AFB with BSA was computed at three different temperatures and was found to range from 7.34 to 13.19 x10⁵ L mol^-1. Thermodynamic parameters calculations demonstrated a positive ΔS^Ɵvalue with both negative ΔH^ϴand ΔG^ϴvalues for AFB–BSA complex, which in turn infers thata spontaneous binding is taking place with both electrostatic bonding and hydrophobic interactions participating in the binding of AFB and BSA. Similarly, the UV absorption spectra of AFB-BSA system were studied and confirmed the interaction. Conformational alteration of the protein upon binding to AFB was elaborated with the aid of three dimensional fluorescence measurements as well as synchronous fluorescence spectra.