Structural insights into the binding behavior of isoflavonoid glabridin with human serum albumin

A fluorescent "Turn-ON" probe with rapid and differential response to HSA and BSA: quantitative detection of HSA in urine

The present study provides insight into the differential response of a benzimidazole-malononitrile fluorescent "Turn-ON" probe on interaction with two structurally similar proteins, BSA and HSA. Compound 6 shows more sensitivity towards the two SAs, which is completely lost in the case of compound 7, synthesized by substitution on 6. The aggregates of compound 6 show absorption maxima at 385 nm and weak emission maxima at 565 nm. Compound 6 forms a new emission band at 475 nm on gradual addition of BSA (200 μM) along with a slight increase in the emission band at 565 nm. However, on addition of HSA (50 μM), a new band at 475 nm is formed. In contrast to BSA, in the case of HSA, 50% quenching is observed in the emission band of compound 6 at 565 nm. The new band formed on the interaction of 6 with BSA shows four-fold more enhancement compared to HSA. Furthermore, the mechanism of interaction of 6 with serum albumin has been investigated through lifetime-fluorescence analysis, site-selective drug experiments, dynamic light scattering, FE-SEM, FT-IR, etc. Molecular docking studies and site marker drug displacement experiments reveal differential interactions of 6 towards the two structurally similar proteins. Aggregates of 6 with an average hydrodynamic size of 100-190 nm are disassembled on adding BSA and HSA, and the size of the serum albumin and 6 complex decreases to 10-20 nm, revealing the ligand's encapsulation in the serum albumin cavity. Practical applicability for the quantitative detection of HSA in human urine samples is also demonstrated. The high binding affinity, sensitivity, selectivity and differential response of probe 6 towards two serum albumins (HSA and BSA) and significant quantification of HSA in urine samples shows the potential ability of this probe in medical applications.

Disinfectant efficacy of glabridin against dried and biofilm cells of Listeria monocytogenes and the impact of residual organic matter

Glabridin is an antimicrobial compound which can be extracted from plants, such as liquorice (Glycyrrhiza glabra) roots. Although its activity against foodborne pathogens and spoilage microorganisms has already been reported, the investigation of potential applications as a surface disinfectant is still largely unexplored. Hence, this study evaluated the disinfectant efficacy of glabridin against Listeria monocytogenes. The activity of glabridin was first tested in vitro in a nutrient-rich medium against eight strains of L. monocytogenes, including food isolates and the model strain EGDe. The tested strains showed similar susceptibility with minimal inhibitory and bactericidal concentrations of 12.5 µg/mL and 25 µg/mL, respectively. Subsequently, L. monocytogenes L6, FBR17 and EGDe were selected to assess the efficacy of glabridin against dried cells (according to the European standard EN 13697:2015 + A1:2019) and biofilm cells on stainless steel surfaces. Moreover, the impact of food residual organic matter was investigated using skim milk, cantaloupe and smoked salmon solution as soiling components. Our results showed that applying 200 µg/mL of glabridin resulted in a substantial reduction (>3 log10) of dried and biofilm cells of L. monocytogenes in standard conditions (i.e. low level of residual organic matter). Cantaloupe soiling components slightly reduced the activity of glabridin, while the efficacy of glabridin when tested with salmon and skim milk residuals was substantially affected. Comparative analysis using standardized protein contents provided evidence that the type of food matrices and type of proteins may impact the activity of glabridin as a disinfectant. Overall, this study showed low strain variability for the activity of glabridin against L. monocytogenes and shed light on the possible application of this natural antimicrobial compound as a surface disinfectant.

Investigating the Serum Albumin Binding Behavior of Naphthalimide Based Fluorophore Conjugates: Spectroscopic and Molecular Docking Approach

In the present study, naphthalimide-pyrazole-benzothiazole based fluorescent analogs were synthesized by substituting different primary and secondary amines on the naphthalimide nucleus and were evaluated for their sensitivity and selectivity towards serum albumin. Among various synthesized analogues compound 25 showed the most significant change with serum albumin and was further studied for selective detection and mode of interaction with serum albumin. Here, we compared the binding interaction of fluorescent probe 25 for variation/detection of two 76 % structurally resembling proteins HSA and BSA, by spectroscopic experiments. The compound shows more selectivity for HSA and BSA with a higher binding constant and evident visible change in the emission spectra of two serum albumins among different bioanalytes. The mode of interaction of 25 with human serum albumin and bovine serum albumin was investigated by FT-IR, circular dichroism, and DLS techniques to find out the change in the microenvironment and variation in the structure of serum albumin proteins. Higher binding affinity and specific selectivity of 25 with a limit of detection of 0.69 μM and 1.4 μM towards HSA and BSA compared to other bioanalytes make it a significant fluorescent probe for quantitatively detecting serum albumins at the very early stage of many fatal diseases.

Interaction studies of cannabidiol with human serum albumin by surface plasmon resonance, spectroscopy, and molecular docking

The binding interaction of cannabidiol (CBD) and human serum albumin (HSA) under physiological blood pH conditions (pH 7.4) was conducted by surface plasmon resonance (SPR), fluorescence spectroscopy, UV-Visible spectrophotometry, and molecular docking. The responses from SPR measurement increased with the increase in CBD concentration until equilibrium was reached at the equilibrium dissociation constant (KD) of 9.8 × 10-4 M. The results from fluorescence and UV-Visible spectroscopy showed that CBD bound to HSA at one site in a spontaneous manner to form protein-CBD complexes. The quenching process involved both static and dynamic mechanisms while the static mechanism contributed predominantly to the binding between CBD and albumin. The binding constants estimated from the fluorescence studies were from 0.16 × 103 to 8.10 × 103 M-1, which were calculated at different temperature conditions using Stern-Volmer plots. The thermodynamic parameters demonstrated that the binding interaction was a spontaneous reaction as Gibbs free energy had negative values (ΔG = -12.57 to -23.20 kJ.mol-1). Positive ΔH and ΔS values (ΔH = 2.46 × 105 J.mol-1 and ΔS = 869.81 J.mol-1K-1) indicated that the hydrophobic force was the major binding interaction. Finally, confirmation of the type and extent of interaction was provided using UV-spectroscopy and molecular docking studies. The outcomes of this study are expected to serve as a platform to conduct future studies on binding interactions and toxicological research of CBD.Communicated by Ramaswamy H. Sarma.

Hydroxyl group-induced enhancement of antioxidant activity of resveratrol over pterostilbene by binding to lactoferrin

The reduced antioxidant capacity of trans-resveratrol (Res) than the second generation of Res, namely pterostilbene (Pte), severely prohibits its in-depth intriguing radical-scavenging applications in food formulations. Herein, a unique chemical structure-dependent strategy was proposed to specifically enhance the radical scavenging activity of Res over Pte, relying on the two more hydroxyl groups on the A-benzene ring of Res, thus facilitating its binding with lactoferrin (LF) to form stable complexes through more hydrogen bonds. We prepared LF-Res and LF-Pte complexes, revealed their binding mechanisms by multispectral analysis and molecular docking/dynamics simulations, further evaluated their antioxidant properties via ABTS and DPPH assays and a model of inhibiting apple browning, eventually elucidated their structure-binding-property relationships. This contribution offers a new approach to restore the antioxidant capability of Res, also paves the way to precisely regulate the fascinating bioactivities of hydrophobic compounds by protein-binding in a chemical structure-, especially hydroxyl group-dependent manner.

Co-assembled whey protein and proanthocyanidins as a promising biocarrier for hydrophobic pterostilbene: Fabrication, characterization, and cellular antioxidant potential

The usage of food-derived polyphenols with different polarities has been limited by their instability and incompatibility. Therefore, a biocarrier was developed by co-assembly of whey protein isolate (WPI) and hydrophilic proanthocyanidin (PC) for loading hydrophobic pterostilbene (PTE). Such biocarrier has superior affinity for PTE than WPI alone, as determined by encapsulation efficiency and loading capacity assay, fluorescence quenching analysis, and molecular docking, whereas the assembly process was characterized by particle size and zeta potential, 3-dimensional fluorescence, and scanning electron microscopy. Circular dichroism and Fourier transform infrared spectroscopy spectra confirmed the α-helix to β-sheet and random coil transition of proteins during the formation of nanocomplexes. Whey protein isolate acted as a mediator through altering the binding mode of PC and PTE, allowing them to perform significant synergistic effects in enhancing 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) and 2,2-diphenyl-1-picrylhydrazyl radical scavenging and reducing H2O2-induced cell damage. This research may serve to develop new protein/polyphenol co-loading systems and offer a reliable nutritional fortification.

Enhancing Stability and Antioxidant Activity of Resveratrol-Loaded Emulsions by Ovalbumin-Dextran Conjugates

A reliable strategy for improving the stability and shelf life of protein-stabilized systems is by covalently attaching the protein onto a polysaccharide. In this study, ovalbumin (OVA) was modified with dextran (DEX) of different molecular weights by the Maillard reaction, and was used to enhance the stability of emulsions loaded with resveratrol. The surface hydrophobicity, thermal stability, and FT-IR spectroscopy of the OVA-DEX conjugates were evaluated. The results showed that the surface hydrophobicity of OVA decreased, while the thermal stability of OVA was significantly improved after DEX covalent modification. The OVA-DEX1k-stabilized emulsion exhibited high encapsulation efficiency of resveratrol, with the value of 89.0%. In addition, OVA-DEX was considerably more effective in droplet stabilization against different environmental stresses (heat, pH, and ionic strength). After 28 days of storage at 25 °C, the OVA-stabilized emulsion showed faster decomposition of resveratrol, whereas the OVA-DEX-conjugate-stabilized emulsion had approximately 73% retention of resveratrol. Moreover, the antioxidant activity of resveratrol-loaded emulsions stabilized by OVA-DEX was higher during storage under different temperatures. These results proved that the OVA-DEX conjugates had the potential to form stable, food-grade emulsion-based delivery systems against environmental stresses, which strongly supports their potential in the field of food and biomedical applications.

Insights into the effect of glucose on the binding between human serum albumin and the nonsteroidal anti-inflammatory drug nimesulide

Glucose interacts with human serum albumin (HSA, the main protein responsible for the biodistribution of drugs in the bloodstream) and consequently affects the binding capacity of exogenous compounds. Thus, in this work, the interactive profile between HSA and the anti-inflammatory drug nimesulide (NMD, used mainly by patients with diabetic neuropathy to relieve acute or chronic pains) was characterized in nonglycemic, normoglycemic (80 mg/dL), and hyperglycemic (320 mg/dL) conditions by biophysics techniques. There is a spontaneous and ground-state association HSA:NMD under physiological conditions. Therefore, the Stern-Volmer constant (Ksv) can also be used to estimate the binding affinity. The Ksv values for nonglycemic, normoglycemic, and hyperglycemic conditions are around 104 M-1, indicating a moderate affinity of NMD to albumin that was slightly improved by glucose levels. Additionally, the binding is enthalpically and entropically driven mainly into subdomains IIA or IIIA. The binding perturbs weakly the α-helix content of albumin, however, glucose potentially stabilizes the tertiary structure, decreasing the structural perturbation upon NMD binding and improves the complex HSA:NMD stability. Overall, the biophysical characterization indicated that glucose levels might slightly positively impact the pharmacokinetic profile of NMD, allowing NMD to achieve its therapeutical potential without affecting drastically its effective dosages.

Characterization, Anti-glycation, Anti-inflammation, and Lipase Inhibitory Properties of Rauvolfia vomitoria Leaf Extract: In Vitro and In Silico Evaluations for Obesity Treatment

Pancreatic lipase (PLP) is an enzyme responsible for the catalytic hydrolysis of fats and its inhibition is relevant for obesity management. Side effects linked with orthodox inhibitors have, however, paved the way for an increased search for safe natural sources. The present study investigated the anti-glycation, anti-inflammatory, and anti-lipase properties of Rauvolfia vomitoria aqueous (ARV), ethanolic (ERV), and methanolic (MRV) leaf extracts coupled with the molecular interactions of selected bioactive compounds with PLP using in vitro and in silico techniques. Phytochemical constituents were characterized using spectroscopic techniques. Drug-likeness and chemical reactivity profile of selected bioactive compounds were analyzed using SwissADME and quantum chemical calculations. FT-IR and GC-MS affirmed the presence of phenolic compounds including 3-phenyl-2-ethoxypropylphthalimide and 5-methyl-2-phenyl-1H-indole. All extracts showed moderate anti-glycation, anti-inflammatory, and lipase inhibitory capacities relative to standard controls. However, MRV exhibited the highest lipase inhibition (IC50, 0.17 ± 0.01 mg/mL), using a mixed-inhibition pattern. MRV interaction with PLP resulted in decreased secondary structure components of PLP (α-sheet, β-turn). MRV compounds (MCP20, MCP28, etc.) exhibited low chemical hardness, EHOMO-ELUMO energy gap, and high chemical reactivity. Foremost MRV compounds obeyed Lipinski's rule of five for drug-likeness and interacted with PHE-78 amongst others at PLP catalytic domain with high binding affinity (≥ - 9.3 kcal/mol). Pi-alkyl hydrophobic interaction and hydrogen bonding were predominantly involved. Our findings provide scientific insights into the ethnotherapeutic uses of R. vomitoria extracts for the management of obesity and related complications, plus useful information for optimizable drug-like candidates against obesity.

Unleashing the binding interaction of chrysin-Cu(II) complex with the biomacromolecular targets: further studies of cell cytotoxicity and radical scavenging properties

Flavonoids are significant dietary components and have ability to coordinate with metal ions to produce novel drug discovery leads that are superior to those of the parent flavonoids. Here, in this report, we have synthesized chrysin-Cu(II) complex (as per reported article) and characterized it further with different analytical techniques. The synthesized complex was evaluated for radical scavenging and cell cytotoxicity studies where it exhibited enhanced activity as compared to bare chrysin. The interaction studies of the complex with ct-DNA (Kb ⁓ 105 M-1), human serum albumin (HSA) and ovalbumin (Kb ⁓ 104 M-1) were evaluated using multi-spectroscopic and molecular docking studies. Groove binding mode with ct-DNA was observed as confirmed from competitive displacement studies, viscosity measurement, melting temperature estimation and docking analyses. The complex exhibited comparatively higher affinity towards ct-DNA which indicated it efficient transportation by the carrier proteins and controlled release in the target DNA.Communicated by Ramaswamy H. Sarma.

Complexation of turmeric and curcumin mediated silver nanoparticles with human serum albumin: Further investigation into the protein-corona formation, anti-bacterial effects and cell cytotoxicity studies

Biosynthesized noble metal nanoparticles have been of recent interest due to their broad implications in the future biomedicinal field. We have synthesized silver nanoparticle using turmeric-extract and its major component curcumin as reducing and stabilizing agents. Further, we have investigated the protein-NPs interaction focusing the inspection of the role of biosynthesized AgNPs on any conformational changes of the protein, binding and thermodynamic parameters using spectroscopic techniques. Fluorescence quenching studies revealed that both CUR-AgNPs and TUR-AgNPs have moderate binding affinities (∼10⁴ M^-1) towards human serum albumin (HSA) and static quenching mechanism was involved in the binding. Estimated thermodynamic parameters indicate the involvement of hydrophobic forces in the binding processes. The surface charge potential of the biosynthesized AgNPs became more negative upon complexation with HSA as observed from Zeta potential measurements. Antibacterial efficacies of the biosynthesized AgNPs were evaluated against Escherichia coli (gram-negative) and Enterococcus faecalis (gram-positive) bacterial strains. The AgNPs were found to destroy the cancer (HeLa) cell lines in vitro. The overall findings of our study successfully outline the detailed insight of the protein corona formation by biocompatible AgNPs and their biological applications concerning the future scope in the biomedicinal field.

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.

Exploration of binding mechanism of apigenin to pepsin: Spectroscopic analysis, molecular docking, enzyme activity and antioxidant assays

Pepsin plays an important role in nutrient metabolism. Apigenin (AP) is a beneficial polyphenol to human health. To enhance the bioavailability of AP and elucidate the inhibitory effect of AP on pepsin, the interaction mechanism of AP with pepsin was investigated using spectroscopic analysis and molecular docking, and the activity of pepsin and antioxidant activity of AP was also evaluated. Specifically, AP performed static quenching of pepsin and had only one binding site on pepsin. More interestingly, the interaction between AP and pepsin was spontaneous, while hydrogen bonds and van der Waals forces were the main binding forces. Generally, synchronous and three-dimensional fluorescence confirmed that AP induced the conformational changes of pepsin, and molecular docking proved the above results and illustrated the specific binding patterns. Specifically, AP inhibited the activity of pepsin, while pepsin decreased the antioxidant activity of AP. These results provided useful information for elucidating the interactions between AP and pepsin.

Improved Analytical Performance of an Amphiphilic Probe upon Protein Encapsulation: Spectroscopic Investigation along with Computational Rationalization

An easily synthesizable pyrene-based amphiphilic probe (Pybpa) has been developed, which exhibited no responses with metal ions in the pure aqueous medium despite possessing a metal ion-chelating bispicolyl unit. We believe that spontaneous aggregation of Pybpa in aqueous medium makes the ion binding unit not accessible to the metal ions. However, the sensitivity and selectivity of Pybpa toward Zn²⁺ ions drastically improve in the presence of serum albumin protein, HSA. The differences in the microenvironment inside the protein cavity, in terms of local polarity, and conformational rigidity might be attributing factors for that. The mechanistic investigations also suggest that there might be the involvement of polar amino acid residues that take part in coordination with Zn²⁺ ions. Pybpa shows no detectable spectroscopic changes with Zn²⁺ ions in aqueous medium in the absence of HSA. However, it can effectively recognize Zn²⁺ ions in the protein-bound form. Moreover, the photophysical behavior of Pybpa and its zinc complex have been investigated with DFT and docking studies. Noteworthy, such an unusual sensing aspect of Zn²⁺ exclusively in the protein-bound state and particularly in aqueous medium is truly rare and innovative.

Physicochemical and functional properties of capsaicin loaded cricket protein isolate and alginate complexes

As people become more aware of the health benefits of foods and their nutritional benefits for preventing diseases and promoting health, the demand for functional foods rich in proteins, fiber, and bioactives like capsaicin (CAP) is constantly rising. This study hypothesized that the electrostatic complexes developed by cricket protein isolate (CPI) and alginate (AL) could be utilized to encapsulate CAP, making it more water-soluble and protecting it at acidic pHs. Quantitative analysis revealed that CAP was efficiently encapsulated into the CPI-AL complexes with a maximum encapsulation efficiency of 91%, improving its aqueous solubility 45-fold. In vitro release tests showed that CAP was retained at acidic pHs (3.0 and 5.0) in CPI-AL complexes but released steadily at neutral pH (7.4), which will protect CAP in the stomach while enabling its release in the small intestine. Moreover, the antioxidant activity of CAP-CPI-AL complexes was superior to that of their individual bare equivalents. The complexes also demonstrated enhanced emulsifying capabilities and stability at acidic pHs (2.0-5.0) as the CPI fraction in the complexes increased. Our findings thus contribute to the growing body of knowledge that validates protein-polysaccharide complexation as a promising strategy for developing edible delivery systems.

Unveiling interaction mechanisms between myricitrin and human serum albumin: Insights from multi-spectroscopic, molecular docking and molecular dynamic simulation analyses

Myricitrin is a natural polyhydroxy flavonoid and is mainly derived from the bark and leaves of the Chinese Bayberry tree (Myrica rubra). It has different pharmacological activities, including antioxidative, anti-inflammatory, hypoglycemic, antiviral, liver protection and cholagogue properties, and may be added to foods, pharmaceuticals, and cosmetic products for antioxidant purposes. In this study, the interaction mechanism between myricitrin and human serum albumin (HSA) was investigated using spectroscopic methods, molecular docking techniques, and molecular dynamic simulations. We showed that the HSA/myricitrin interaction exhibited a static fluorescence quenching mechanism, and that binding processes were spontaneous in nature, with the main forces exemplified by hydrogen bonding, hydrophobic interactions, and electrostatic interactions. Fluorescence spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, synchronous fluorescence spectroscopy, three-dimensional (3D) fluorescence spectroscopy, micro-Fourier transform infrared spectroscopy (micro-FTIR), and circular dichroism (CD) spectroscopy showed that myricitrin binding altered the HSA conformation to some extent. Competitive binding and molecular docking studies showed that the preferred binding of myricitrin on HSA was in the sub-structural domain IIA (Site I); molecular dynamic simulations revealed that myricitrin interacted with HSA to produce a well stabilized complex, and it also generated a conformational change in HSA. The antioxidant capacity of the HSA-myricitrin complex was reduced when compared with free myricitrin. The identification of HSA-myricitrin binding mechanisms provides valuable insights for the application of myricitrin to the food and pharmaceutical industries.

Study of Competitive Displacement of Curcumin on α-zearalenol Binding to Human Serum Albumin Complex Using Fluorescence Spectroscopy

α-zearalenol (α-ZOL) is a mycotoxin with a strong estrogen effect that affects the synthesis and secretion of sex hormones and is transported to target organs through human serum albumin (HSA). Additionally, it has been reported that curcumin can also bind to HSA with high affinity at the same binding site as α-ZOL. Additionally, several studies reported that reducing the bound fraction of α-ZOL contributes to speeding up the elimination rate of α-ZOL to reduce its hazard to organs. Therefore, to explore the influence of a nutrition intervention with curcumin on α-ZOL effects, the competitive displacement of α-ZOL from HSA by curcumin was investigated using spectroscopic techniques, ultrafiltration techniques and HPLC methods. Results show that curcumin and α-ZOL share the same binding site (subdomain IIA) on HSA, and curcumin binds to HSA with a binding constant of 1.12 × 10⁵ M⁻¹, which is higher than that of α-ZOL (3.98 × 10⁴ M⁻¹). Ultrafiltration studies demonstrated that curcumin could displace α-ZOL from HSA to reduce α-ZOL’s binding fraction. Synchronous fluorescence spectroscopy revealed that curcumin could reduce the hydrophobicity of the microenvironment of an HSA–α-ZOL complex. This study is of great significance for applying curcumin and other highly active foodborne components to interfere with the toxicokinetics of α-ZOL and reduce its risk of its exposure.

Molecular Insight into the Binding of Astilbin with Human Serum Albumin and Its Effect on Antioxidant Characteristics of Astilbin

Astilbin is a dihydroflavonol glycoside identified in many natural plants and functional food with promising biological activities which is used as an antioxidant in the pharmaceutical and food fields. This work investigated the interaction between astilbin and human serum albumin (HSA) and their effects on the antioxidant activity of astilbin by multi-spectroscopic and molecular modeling studies. The experimental results show that astilbin quenches the fluorescence emission of HSA through a static quenching mechanism. Astilbin and HSA prefer to bind at the Site Ⅰ position, which is mainly maintained by electrostatic force, hydrophobic and hydrogen bonding interactions. Multi-spectroscopic and MD results indicate that the secondary structure of HSA could be changed because of the interaction of astilbin with HSA. DPPH radical scavenging assay shows that the presence of HSA reduces the antioxidant capacity of astilbin. The explication of astilbin–HSA binding mechanism will provide insights into clinical use and resource development of astilbin in food and pharmaceutical industries.

Unraveling the phase behavior of cricket protein isolate and alginate in aqueous solution

The associative phase behavior of cricket protein isolate (CPI) and sodium alginate (AL) in aqueous solutions was explored using turbidimetry, methylene blue spectroscopy, zeta potentiometry, dynamic light scattering, and confocal microscopy as a function of pH, biopolymer ratio, total biopolymer concentration (C_T), and ionic strength. When both biopolymers had net-negative charges, soluble complexes formed between pH 6.0 and 8.0, however when both biopolymers had opposing net charges, insoluble complexes formed as complex coacervates below pH 5.5, defined as pH_φ1, followed by precipitates below another critical pH 3.0 (pH_p). Increasing the CPI:AL weight ratio or C_T facilitated complex formation, and the addition of salts (NaCl/KCl) had a salt-enhancement and salt-reduction impact at low and high salt concentrations, respectively. Ionic interactions between oppositely charged CPI and AL were mainly responsible for the formation of their insoluble complexes, while hydrogen bonding and hydrophobic interactions also played significant roles.

Milk β-casein as delivery systems for luteolin: Multi-spectroscopic, computer simulations, and biological studies

β-Casein, a highly amphiphilic calcium-sensitive phosphoprotein, has specific features that promote its application as a nanocarrier for hydrophobic bioactives. Luteolin is a flavonoid with rich biological activities existing in vegetables and fruits. It is important to understand the interaction of β-casein with luteolin for the development of β-casein-based delivery systems. Here, the interaction mode between luteolin and β-casein was investigated with multispectral techniques, computer simulation, and biological methods. The results demonstrated that luteolin could bind to β-casein spontaneously which is driven by hydrophobic interactions and statically quench the intrinsic fluorescence of β-casein. Molecular docking and molecular dynamics simulation showed that β-casein formed a stable complex with luteolin. It could be concluded that luteolin was encapsulated in β-casein micelles and exhibited higher antioxidant activity than luteolin alone. These results would be helpful to understand the interaction mechanism of luteolin with β-casein and indicated that β-casein micelles were very promising as delivery vehicles for luteolin. PRACTICAL APPLICATIONS: Adding bioactive compounds to food is an efficient method of functional food processing, and protein is an excellent natural carrier for these substances. β-Casein is a milk protein with a unique amphiphilic structure that makes it a natural nanocarrier for active ingredients. This study created β-casein nanocarriers and encapsulated luteolin based on the interaction mechanism between β-casein with luteolin. Luteolin encapsulated in β-casein micelles demonstrated higher antioxidant activity when compared to free luteolin. This research will provide useful data for the development of functional foods based on β-casein and luteolin in the food industry.

Interaction of ovalbumin with lutein dipalmitate and their effects on the color stability of marigold lutein esters extracts

In this study, the interaction of ovalbumin with lutein dipalmitate and the effect of ovalbumin on marigold lutein esters extracts were investigated. Lutein dipalmitate quenched the fluorescence of ovalbumin by static quenching. Binding and thermodynamic parameters proved that lutein dipalmitate bound to ovalbumin spontaneously by van der Waals force and hydrogen bond, and the complex stoichiometry was 1:1. Through three-dimensional fluorescence spectroscopy, Fourier transform infrared spectroscopy and circular dichroism experiments, the conformation of ovalbumin was unfolded, and alteration in the ovalbumin secondary structure induced by lutein dipalmitate was observed. The results of transmission electron microscopy and particle size revealed that there were spherical and nano-sized aggregates in the ovalbumin-lutein dipalmitate system, indicating the lutein dipalmitate not only could bind to ovalbumin at molecular level, but also promote the aggregation of ovalbumin. Additionally, the addition of ovalbumin had a positive effect on the stability of marigold lutein esters extracts.

The interaction between bovine serum albumin and [6]-,[8]- and [10]-gingerol: An effective strategy to improve the solubility and stability of gingerol

In this study, the binding mechanism between bovine serum albumin (BSA) and three gingerols ([6]-, [8]- and [10]-gingerol) was evaluated to explore an effective strategy for improving solubility and stability of gingerols. The fluorescence analysis suggested gingerols could bind with BSA to form a stable BSA/gingerols complex and [10]-gingerol had the strongest binding affinity (K_a = 4.016 × 10⁴ L/mol) at 298 K. Thermodynamic parameters and molecular modeling validated that hydrophobic interaction and hydrogen bonds were the main driving force for the interaction of BSA/gingerols. Gingerols bound to BSA at site I (subdomain IIA) resulted in a conformational change of BSA with a structure shrinkage, which was responsible for the decrease of surface hydrophobicity. The formation of BSA/gingerols complexes promoted the solubility of [6]-, [8]- and [10]-gingerol increasing by 1.50, 6.04 and 23.50 times, respectively. In addition, the stability and antioxidant capacity of gingerols was significantly improved after binding with BSA.

Curcumin-Loaded Human Serum Albumin Nanoparticles Prevent Parkinson’s Disease-like Symptoms in C. elegans

Parkinson’s disease is one of the most common degenerative disorders and is characterized by observable motor dysfunction and the loss of dopaminergic neurons. In this study, we fabricated curcumin nanoparticles using human serum albumin as a nanocarrier. Encapsulating curcumin is beneficial to improving its aqueous solubility and bioavailability. The curcumin-loaded HSA nanoparticles were acquired in the particle size and at the zeta potential of 200 nm and −10 mV, respectively. The curcumin-loaded human serum albumin nanoparticles ameliorated Parkinson’s disease features in the C. elegans model, including body movement, basal slowing response, and the degeneration of dopaminergic neurons. These results suggest that curcumin nanoparticles have potential as a medicinal nanomaterial for preventing the progression of Parkinson’s disease.

Potent cyclometallated Pd(II) antitumor complexes bearing α-amino acids: synthesis, structural characterization, DNA/BSA binding, cytotoxicity and molecular dynamics simulation

A rational approach was adopted to design high-potential metal-based antitumor agents. A series of organometallic Pd(ii) complexes with a general formula of [Pd{κ2(C,C)-[(C6H4-2)PPh2]CH(CO)C6H4Ph-4}{κ2(N,O)}] (N,O = alanine (Pd-A), valine (Pd-V), leucine (Pd-L), l-isoleucine (Pd-I) and phenylalanine (Pd-F)) were prepared by cyclopalladation of the phosphorus ylide, bridge cleavage reaction and subsequent chelation of natural α-amino acids. The complexes were fully identified using IR and multinuclear 1H, 13C, 31P NMR spectroscopic methods. X-ray crystallography exhibited that the Pd(ii) atom is located in a slightly distorted square-planar environment surrounded by C,C-orthometallated phosphorus ylide as well as NO-pendant amino acid functionality. In vitro cytotoxicity evaluation of new cyclometallated Pd(ii) complexes toward a human leukemia (K562) cancer cell line indicated promising results. The highest cytotoxic activity was discovered in the case of phenylalanine (CH2C6H5). IC50 values of this complex on a panel of human tumor cell lines representative of liver (HepG2), breast (SKBR-3), and ovarian (A2780-Resistance/Sensitive) cancers also indicated promising antitumor effects in comparison with standard cisplatin. The binding interaction ability of the phenylalanine-containing orthopalladated complex, as the most efficient compound, with calf-thymus deoxyribonucleic acid (CT-DNA) and bovine serum albumin (BSA) was investigated. UV-Vis spectroscopy, competitive emission titration, and circular dichroism (CD) techniques demonstrated the intercalative binding of the Pd(ii) complex with DNA. Molecular docking studies also fully agreed with the experimental data. Examination of the reactivity towards the protein BSA revealed that the static quenching mechanism of BSA intrinsic fluorescence by the Pd(ii) complex with a binding constant (Kb) of ∼105 is indicative of the high affinity of the complex. The competitive binding experiment using site markers with definite binding sites demonstrated that the hydrophobic cavities of site I (subdomain IIA) are responsible for the bimolecular interaction between protein BSA and the complex. Molecular docking studies effectively confirmed the significance of hydrophobic interactions in Pd(ii)-BSA binding. The results of this study could greatly contribute to exploring new potent metal-based anticancer drugs.

Monosaccharides interact weakly with human serum albumin. Insights for the functional perturbations on the binding capacity of albumin

Monosaccharides, e.g. fructose, glucose, and arabinose are present in most foods consumed daily, whether, in natural or industrialized forms, and their concentration in the human bloodstream can impact the formation of advanced glycation end-products (AGEs, prevalent in people with diabetes) impacting the profile of Human Serum Albumin (HSA) in biodistribution of endogenous and exogenous compounds. Multiple spectroscopic techniques (UV-vis, circular dichroism, steady-state, and time-resolved fluorescence) combined with molecular docking showed that carbohydrates interact weakly and spontaneously via a ground-state association with HSA. The binding is enthalpically and entropically driven in the subdomain IIA (site I) and perturb weakly the secondary structure of the albumin. Hydrogen bonding and van der Waals forces are the main intermolecular interactions involved in the ligand binding, as well as hydrophobic effects related to the release of hydration shell upon ligand binding. Overall, the results indicated that an increase in glucose, fructose or arabinose level in the human bloodstream may cause functional perturbation on the binding capacity of albumin. Therefore, there is the necessity of carbohydrate level control in the bloodstream to not compromise the interaction and distribution of exogenous and endogenous compounds by HSA.

Multispectral and molecular modeling investigations on the binding behaviors of two anticoccidials with serum albumins

The interaction properties of monensin/clopidol with bovine/human serum albumin (BSA/HSA) were determined via multispectral together with molecular modeling techniques in the report. Fluorescence quenching spectra at different temperatures and fluorescence lifetime determination demonstrated that the fluorescence quenching belonged to a static quenching type. In the case of monensin-BSA, clopidol-BSA, monensin-HSA and clopidol-HSA, the binding constants K_a (291 K) were 5.42 × 10⁴, 4.96 × 10⁴, 3.22 × 10⁴ and 2.99 × 10⁴ M^-1, respectively; the binding distances r₀ were 1.88, 2.53, 2.19 and 2.02 nm, respectively. Monensin and clopidol bound strongly with BSA/HSA with binding free energies equal to -26.37/-25.11 and -26.11/-24.93 kJ mol^-1, respectively. The spontaneous binding process was dominated by hydrogen bonds and van der Waals forces as reflected in thermodynamic parameters analyses. Synchronous, CD, FTIR and UV-vis spectra assays confirmed that serum albumins conformations were altered. Using competitive experiment, monensin/clopidol was observed to bind at site I of serum albumins, which were reconfirmed by the results of molecular modeling.Communicated by Ramaswamy H. Sarma.

Assessment on the binding characteristics of residual marbofloxacin in animal-derived food to bovine/human serum albumin by spectroscopy and molecular modelling

To assess the toxicity of residual marbofloxacin from animal-derived food, the interaction characteristics of marbofloxacin to bovine/human serum albumins (BSA/HSA) were explored using spectroscopic methods combined with molecular modelling. According to fluorescence spectra and time-resolved fluorescence spectra measurements, quenching of BSA/HSA fluorescence induced by marbofloxacin was characterized as static quenching. A 1:1 ground-state complex of marbofloxacin to BSA/HSA was formed with binding constant (K_a ) 1.66 × 10⁴ /9.74 × 10³ M^-1 at 291 K. The location of marbofloxacin binding at site I within BSA/HSA was clarified by site marker competitive experiments. Molecular modelling demonstrated that the binding region for marbofloxacin to BSA and HSA were at site I with the lowest binding free energies of -22.86 and -21.60 kJ mol^-1 , respectively. Hydrogen bonds and van der Waals forces were dominantly involved in the spontaneous binding. Nonradiation energy transferred from BSA and HSA to marbofloxacin, due to the close distance (r₀ ) between marbofloxacin and Trp residues of BSA (4.28 nm) and HSA (3.34 nm). As explained by circular dichroism (CD) spectra, an increased BSA/HSA α-helix structure was observed after binding to marbofloxacin. Ultraviolet-visible (UV-vis) and Fourier transform infrared (FT-IR) spectra suggested that conformation of the two proteins was altered by marbofloxacin.

Protective effects of three structurally similar polyphenolic compounds against oxidative damage and their binding properties to human serum albumin

Brazilin (Bra), hematoxylin (Hto) and hematein (Hte) are structurally similar polyphenols having rich biological activities, but their antioxidant ability has not been well studied. Here, their protective ability against human serum albumin (HSA) oxidative degradation were investigated using 2,2'-Azobis (2-methylpropionamidine) dihydrochloride (AAPH), NaClO and Fenton like reactions methods. The results indicated that polyphenols inhibited the oxidative injuries of HSA in the order: Hto > Bra > Hte. Additionally, the biological effects of polyphenols were mostly influenced by their binding to protein. Therefore, the structure-affinity relationships of polyphenols binding to HSA were also explored. Fluorescence experiments indicated that polyphenols bound to HSA through static quenching mechanism. Furthermore, some conformational changes of HSA could be observed in the presence of polyphenols. Altogether, molecular structure of polyphenols played a significant role in their protective effect against HSA oxidative damage and binding ability, which provided fundamental insights into their application as health care foods.

Delineating the interaction mechanism of glabridin and ovalbumin by spectroscopic and molecular docking techniques

The interaction between ovalbumin (OVA) and isoflavonoid glabridin (GB) was investigated using spectroscopic and molecular docking techniques. Fluorescence spectroscopy revealed that GB was bound to OVA mainly due to hydrogen bonding and hydrophobic forces. FT-IR spectroscopy showed that the combination of GB and OVA resulted in a decrease in the β-sheet content of OVA and an increase in the α-helix and extended-chain content. All these experimental results were supported and clarified by molecular docking simulations. GB binding was able to inhibit chemical denaturant-induced structural changes in OVA as observed by intrinsic tryptophan and ANS fluorescence. Moreover, GB-OVA complex increased the aqueous solubility of GB by about 4.45 times at pH 7.0. These results provided insights into the interaction between GB and OVA that contributes to the utilization of GB in the food and pharmaceutical industries.

Recreational drugs 25I-NBOH and 25I-NBOMe bind to both Sudlow's sites I and II of human serum albumin (HSA): biophysical and molecular modeling studies

25I-NBOH and 25I-NBOMe simultaneously bind to sites I and II of HSA, which may affect their distribution and effects.

Insight into anti-oxidative carbohydrate polymers from medicinal plants: Structure-activity relationships, mechanism of actions and interactions with bovine serum albumin

Recently, research associated with natural anti-oxidants leads to the chemical characterization of many compounds possessing strong anti-oxidant activity. Among these anti-oxidants, naturally occurring carbohydrate polymers containing pectic arabinogalactans esterified with phenolic acids in monomeric and dimeric forms are noteworthy. The presence of highly branched arabinogalactan type II side chains and sugar linked phenolic acid residues have been resolved as important parameters. The anti-oxidant activity of these compounds depend on their ability to convert free radicals into stable by-products and themselves oxidized to more stable and less reactive resonance stabilized radicals. Moreover, these carbohydrate polymers form water soluble stable complexes with protein. Such findings support their applications in a diversity of fields including food industry and pharmacy. This review highlights experimental evidences supporting that the carbohydrate polymers containing phenolic polysaccharides may become promising drug candidate for the prevention of aging and age related diseases.

Comprehensive investigations about the binding interaction of acesulfame with human serum albumin

In this work, the binding interaction of an artificial sweetener, acesulfame (ACS) with human serum albumin (HSA) are investigated at the molecular level by using spectral methods and molecular modeling. ACS has the ability to induce static quenching of the intrinsic fluorescence of HSA by a complex formed between HSA and ACS through weak multi-noncovalent forces including hydrophobic, hydrogen bond and van der Waals forces. ACS enters subdomain IIA of HSA to induce the tertiary structure changes of HSA and decreased the hydrophobicity of protein. In addition, ACS binding does not obviously alter the secondary structure of HSA. This study is hoped to provide some crucial information for further investigations of the biosafety of sweetener.

Probing the Interaction between Human Serum Albumin and 9-Hydroxyphenanthrene: A Spectroscopic and Molecular Docking Study

9-Hydroxyphenanthrene (9-OHPhe), the representative hydroxyl metabolite of phenanthrene, has generated increasing concern as it is potentially hazardous to organisms. Herein, multispectroscopic and molecular docking techniques were applied to investigate the molecular interaction of human serum albumin (HSA) with 9-hydroxyphenanthrene (9-OHPhe) under simulated physiological conditions. Steady-state fluorescence and time-resolved fluorescence spectral analysis showed that 9-OHPhe quenched HSA fluorescence through a mixed static and dynamic process. HSA can bind with 9-OHPhe to form a 1:1 complex, with binding constants of 1.28 × 10⁵, 1.36 × 10⁵, and 1.26 × 10⁵ L·mol^-1 at 298.15, 303.15, and 308.15 K, respectively. The strong binding between HSA and 9-OHPhe is spontaneous and entropy-driven. Molecular docking indicated that the optimal binding site of 9-OHPhe with HSA was located in the IA subdomain of HSA. Thermodynamic analysis and molecular docking results suggested that hydrophobic interactions and hydrogen bond force dominated the binding process of HSA with 9-OHPhe. Specifically, 9-OHPhe formed hydrophobic interactions with LEU134, LEU139, ILE142, LEU154, PHE157, ALA158, and TYR161 and formed a 1.86 Å hydrogen bond with LEU135. Circular dichroism spectral analysis showed that the α-helical content of HSA decreased from 52.3 to 50.9% after adding 9-OHPhe with a ratio of 1:1. The obtained results are hoped to provide basic data for understanding the potential effects of the hydroxyl metabolites of PAHs on functional biomacromolecules.

Biophysical insight into the interaction of levocabastine with human serum albumin: spectroscopy and molecular docking approach

Interaction of levocabastine with human serum albumin (HSA) is investigated by applying fluorescence spectroscopy, circular dichroism spectroscopy and molecular docking methods. Levocabastine is an important drug in treatment of allergy and currently a target drug for drug repurposing to treat other diseases like vernal keratoconjuctivitis. Fluorescence quenching data revealed that levocabastine bind weakly to protein with binding constant in the order of 10³ M^-1. Förster resonance energy transfer results indicated the binding distance of 2.28 nm for levocabastine. Synchronous fluorescence result suggest slight blue shift for tryptophan upon levocabastine binding, binding of levocabastine impelled rise in α-helical structure in protein, while there are minimal changes in tertiary structure in protein. Moreover, docking results indicate levocabastine binds to pocket near to the drug site-I in HSA via hydrogen bonding and hydrophobic interactions. Understanding the interaction of levocabastine with HSA is significant for the advancement of therapeutic and diagnostic strategies for optimal treatment results.Communicated by Ramaswamy H. Sarma.

Systematic FTIR Spectroscopy Study of the Secondary Structure Changes in Human Serum Albumin under Various Denaturation Conditions

Human serum albumin (HSA) is the most abundant protein in blood plasma. HSA is involved in the transport of hormones, fatty acids, and some other compounds, maintenance of blood pH, osmotic pressure, and many other functions. Although this protein is well studied, data about its conformational changes upon different denaturation factors are fragmentary and sometimes contradictory. This is especially true for FTIR spectroscopy data interpretation. Here, the effect of various denaturing agents on the structural state of HSA by using FTIR spectroscopy in the aqueous solutions was systematically studied. Our data suggest that the second derivative deconvolution method provides the most consistent interpretation of the obtained IR spectra. The secondary structure changes of HSA were studied depending on the concentration of the denaturing agent during acid, alkaline, and thermal denaturation. In general, the denaturation of HSA in different conditions is accompanied by a decrease in α-helical conformation and an increase in random coil conformation and the intermolecular β-strands. Meantime, some variation in the conformational changes depending on the type of the denaturation agent were also observed. The increase of β-structural conformation suggests that HSA may form amyloid-like aggregates upon the denaturation.