Interaction between hesperetin and human serum albumin revealed by spectroscopic methods

In vitro studies of hemoglobin's affinity for the Vitamin B9 and control of its stability character

Vitamin B9, known as folic acid, and hemoglobin play an important biological role in the human body. This study was designed to investigate the nature of the complex through multispectroscopic methods at physiological conditions due to the lack of research on the binding interactions between folic acid and hemoglobin. Structural analysis showed that the interactions between the molecules are mainly hydrophobic with binding constant of 0.73 × 104 L/mol at 37 °C. The secondary structure of the protein was stable after the addition of folic acid with a 20-fold excess of ligand per mol protein. The stability effect of folic acid on hemoglobin was examined as a function of release of iron ions and determination of the level of phenanthroline-Fe2+ complex. The protective function of folic acid was observed at a concentration of 6.12 nmol/L, and the release of iron ions was lower than in the control probe.

Chiral Au(III) chelates exhibit unique NCI-60 cytotoxicity profiles and interactions with human serum albumin

Au(III) bis(pyrrolide-imine) chelates are emerging as a class of versatile, efficacious metallodrug candidates. Here, we synthesised two enantiopure chiral ligands H2L1 and H2L2 (tetradentate cyclohexane-1,2-diamine-bridged bis(pyrrole-imine) derivatives). Metallation of the ligands with Au(III) afforded the chiral cationic complexes AuL1 and AuL2. The in vitro cytotoxicities of AuL1 and AuL2 determined in the NCI-60 single-dose drug screen were 56.5% and 89.1%, respectively. AuL1 was subsequently selected for a five-dose NCI-60 screen, attaining GI50, IC50, and LC50 values of 4.7, 9.3 and 39.8 μM, respectively. Hierarchical cluster analysis of the NCI-60 data indicated that the profile for AuL1 was similar to that of vinblastine sulfate, a microtubule-targeting vinca alkaloid. Reactions of AuL1 with glutathione (GSH) in vitro confirmed its susceptibility to reduction, Au(III) → Au(I), by intracellular thiols. Because human serum albumin (HSA) is responsible for transporting clinically deployed and investigational drugs, we studied the uptake of AuL1 and AuL2 by HSA to delineate how chirality impacts their protein-binding affinity. Steady-state fluorescence quenching data acquired on the native protein and data from site-specific probes showed that the compounds bind at sites close enough to Trp-214 (subdomain IIA) of HSA to quench the fluorophore. The bimolecular quenching rate constants, Kq, were ca. 102 times higher than the maximum diffusion-controlled collision constant of a biomolecule in water (1010 M-1 s-1), confirming that static fluorescence quenching was the dominant mechanism. The Stern-Volmer constants, KSV, were ∼104 M-1 at 37 °C, while the affinity constants, Ka (37 °C), measured ∼2.1 × 104 M-1 (AuL1) and ∼1.2 × 104 M-1 (AuL2) for enthalpy-driven ligand uptake targeting Sudlow's site I. Although far- and near-UV CD spectroscopy indicated that both complexes minimally perturb the secondary and tertiary structure of HSA, substantial shifts in the CD spectra were recorded for both protein-bound ligands. This study highlights the role of chirality in determining the cytotoxicity profiles and protein binding behaviour of enantiomeric Au(III) chelates.

Myricetin: A Potent Anti-Amyloidogenic Polyphenol against Superoxide Dismutase 1 Aggregation

Amyotrophic lateral sclerosis (ALS) is believed to be caused by the aggregation of misfolded or mutated superoxide dismutase 1 (SOD1). As there is currently no treatment, research into aggregation inhibitors continues. Based on docking, molecular dynamics (MD) simulations, and experimental observations, we propose that myricetin, a plant flavonoid, can act as a potent anti-amyloidogenic polyphenol against SOD1 aggregation. Our MD simulation results showed that myricetin stabilizes the protein interface, destabilizes the preformed fibril, and decreases the rate of fibril elongation. Myricetin inhibits the aggregation of SOD1 in a dose-dependent manner as shown by the ThT aggregation kinetics curves. Our transmission electron microscopy, dynamic light scattering, and circular dichroism experiments indicate that fewer shorter fibrils have formed. Fluorescence spectroscopy results predict the involvement of a static quenching mechanism characterized by a strong binding between protein and myricetin. Importantly, size exclusion chromatography revealed the potential of myricetin for fibril destabilization and depolymerization. These experimental observations complement the MD results. Thus, myricetin is a potent SOD1 aggregation inhibitor that can reduce the fibril load. Using the structure of myricetin as a reference, it is possible to design more effective therapeutic inhibitors against ALS that prevent the disease and reverse its effects.

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

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

Food Antioxidants and Their Interaction with Human Proteins

Common to all biological systems and living organisms are molecular interactions, which may lead to specific physiological events. Most often, a cascade of events occurs, establishing an equilibrium between possibly competing and/or synergistic processes. Biochemical pathways that sustain life depend on multiple intrinsic and extrinsic factors contributing to aging and/or diseases. This article deals with food antioxidants and human proteins from the circulation, their interaction, their effect on the structure, properties, and function of antioxidant-bound proteins, and the possible impact of complex formation on antioxidants. An overview of studies examining interactions between individual antioxidant compounds and major blood proteins is presented with findings. Investigating antioxidant/protein interactions at the level of the human organism and determining antioxidant distribution between proteins and involvement in the particular physiological role is a very complex and challenging task. However, by knowing the role of a particular protein in certain pathology or aging, and the effect exerted by a particular antioxidant bound to it, it is possible to recommend specific food intake or resistance to it to improve the condition or slow down the process.

Human Albumin-Based Hydrogels for Their Potential Xeno-Free Microneedle Applications

Nowadays, hydrogels-based microneedles (MNs) have attracted a great interest owing to their outstanding qualities for biomedical applications. For the fabrication of hydrogels-based microneedles as tissue engineering scaffolds and drug delivery carriers, various biomaterials have been tested. They are required to feature tunable physiochemical properties, biodegradability, biocompatibility, nonimmunogenicity, high drug loading capacity, and sustained drug release. Among biomaterials, human proteins are the most ideal biomaterials for fabrication of hydrogels-based MNs; however, they are mechanically weak and poorly processible. To the best of the knowledge, there are no reports of xeno-free human protein-based MNs so far. Here, human albumin-based hydrogels and microneedles for tissue engineering and drug delivery by using relatively new processible human serum albumin methacryloyl (HSAMA) are engineered. The resultant HSAMA hydrogels display tunable mechanical properties, biodegradability, and good biocompatibility. Moreover, the xeno-free HSAMA microneedles display a sustained drug release profile and significant mechanical strength to penetrate the model skin. In vitro, they also show good biocompatibility and anticancer efficacy. Sustainable processible human albumin-based biomaterials may be employed as a xeno-free platform in vivo for tissue engineering and drug delivery in clinical trials in the future.

Impacts of hesperidin on whey protein functionality: Interacting mechanism, antioxidant capacity, and emulsion stabilizing effects

The objective of this work was to explore the possibility of improving the antioxidant capacity and application of whey protein (WP) through non-covalent interactions with hesperidin (HES), a citrus polyphenol with nutraceutical activity. The interaction mechanism was elucidated using several spectroscopic methods and molecular docking analysis. The antioxidant capacity of the WP-HES complexes was analyzed and compared to that of the proteins alone. Moreover, the resistance of oil-in-water emulsions formulated using the WP-HES complexes as antioxidant emulsifiers to changes in environmental conditions (pH, ion strength, and oxidant) was evaluated. Our results showed that HES was incorporated into a single hydrophobic cavity in the WP molecule, where it was mainly held by hydrophobic attractive forces. As a result, the microenvironments of the non-polar tyrosine and tryptophan residues in the protein molecules were altered after complexation. Moreover, the α-helix and β-sheet regions in the protein decreased after complexation, while the β-turn and random regions increased. The antioxidant capacity of the WP-HES complexes was greater than that of the proteins alone. Non-radiative energy transfer from WP to HES was detected during complex formation. Compared to WP alone, the WP-HES complexes produced emulsions with smaller mean droplet diameters, exhibited higher pH and salt stability, and had better oxidative stability. The magnitude of these effects increased as the HES concentration was increased. This research would supply valuable information on the nature of the interactions between WP and HES. Moreover, it may lead to the creation of dual-function antioxidant emulsifiers for application in emulsified food products.

Spectroscopic Characterization and Biological Activity of Hesperetin Schiff Bases and Their Cu(II) Complexes

The three Schiff base ligands, derivatives of hesperetin, HHSB (N-[2,3-dihydro-5,7-dihydroxy-2-(3-hydroxy-4-methoxyphenyl)chromen-4-ylidene]isonicotinohydrazide), HIN (N-[2,3-dihydro-5,7-dihydroxy-2-(3-hydroxy-4-methoxyphenyl)chromen-4-ylidene]benzhydrazide) and HTSC (N-[2,3-dihydro-5,7-dihydroxy-2-(3-hydroxy-4-methoxyphenyl)chromen-4-ylidene]thiosemicarbazide) and their copper complexes, CuHHSB, CuHIN, and CuHTSC were designed, synthesized and analyzed in terms of their spectral characterization and the genotoxic activity. Their structures were established using several methods: elemental analysis, FT-IR, UV-Vis, EPR, and ESI-MS. Spectral data showed that in the acetate complexes the tested Schiff bases act as neutral tridentate ligand coordinating to the copper ion through two oxygen (or oxygen and sulphur) donor atoms and a nitrogen donor atom. EPR measurements indicate that in solution the complexes keep their structures with the ligands remaining bound to copper(II) in a tridentate fashion with (O^-, N, O_ket) or (O^-, N, S) donor set. The genotoxic activity of the compounds was tested against model tumour (HeLa and Caco-2) and normal (LLC-PK1) cell lines. In HeLa cells the genotoxicity for all tested compounds was noticed, for HHSB and CuHHSB was the highest, for HTSC and CuHTSC-the lowest. Generally, Cu complexes displayed lower genotoxicity to HeLa cells than ligands. In the case of Caco-2 cell line HHSB and HTSC induced the strongest breaks to DNA. On the other side, CuHHSB and CuHTSC induced the highest DNA damage against LLC-PK1.

Discovery of MAO-B Inhibitor with Machine Learning, Topomer CoMFA, Molecular Docking and Multi-Spectroscopy Approaches

Alzheimer’s disease (AD) is the most common type of dementia and is a serious disruption to normal life. Monoamine oxidase-B (MAO-B) is an important target for the treatment of AD. In this study, machine learning approaches were applied to investigate the identification model of MAO-B inhibitors. The results showed that the identification model for MAO-B inhibitors with　K-nearest neighbor(KNN) algorithm had a prediction accuracy of 94.1% and 88.0% for the 10-fold cross-validation test and the independent test set, respectively. Secondly, a quantitative activity prediction model for MAO-B was investigated with the Topomer CoMFA model. Two separate cutting mode approaches were used to predict the activity of MAO-B inhibitors. The results showed that the cut model with q² = 0.612 (cross-validated correlation coefficient) and r² = 0.824 (non-cross-validated correlation coefficient) were determined for the training and test sets, respectively. In addition, molecular docking was employed to analyze the interaction between MAO-B and inhibitors. Finally, based on our proposed prediction model, 1-(4-hydroxyphenyl)-3-(2,4,6-trimethoxyphenyl)propan-1-one (LB) was predicted as a potential MAO-B inhibitor and was validated by a multi-spectroscopic approach including fluorescence spectra and ultraviolet spectrophotometry.

Characterization and activity-folding relationship of serine protease from Antarctic krill (Euphausia superba)

Euphausia superba (Antarctic krill) serine protease (ESP) was investigated to gain insights into the activity-structural relationship, folding behavior, and regulation of the catalytic function. We purified ESP from the krill muscle and characterized biochemical distinctions via enzyme kinetics. Studies of inhibition kinetics and unfolding in the presence of a serine residue modifier, such as phenylmethanesulfonyl fluoride, were conducted. Structural characterizations were measured by spectrofluorimetry, including 1-anilinonaphthalene-8-sulfonate dye labeling for hydrophobic residues. The computational simulations such as docking and molecular dynamics were finally conducted to detect key residues and folding behaviors in a nano-second range. The kinetic parameters of ESP were measured as K_m^BANH = 0.97 ± 0.15 mM and k_cat/K_m^BANH = 4.59 s^-1/mM. The time-interval kinetics measurements indicated that ESP inactivation was transformed from a monophase to a biphase process to form a thermodynamically stable state. Spectrofluorimetry measurements showed that serine is directly connected to the regional folding of ESP. Several osmolytes such as proline and glycine only partially protected the inactive form of ESP by serine modification. Computational molecular dynamics and docking simulations showed that three serine residues (Ser183, Ser188, and Ser207) and Cys184, Val206, and Gly209 are key residues of catalytic functions. Our study revealed the functional roles of serine residues as key residues of catalytic function at the active site and of the structural conformation as key folding factors, where ESP displays a flexible property of active site pocket compared to the overall structure.Communicated by Ramaswamy H. Sarma.

Mangiferin and Hesperidin Transdermal Distribution and Permeability through the Skin from Solutions and Honeybush Extracts (Cyclopia sp.)-A Comparison Ex Vivo Study

Polyphenolic compounds—mangiferin and hesperidin—are, among others, the most important secondary metabolites of African shrub Cyclopia sp. (honeybush). The aim of this study was to compare the percutaneous absorption of mangiferin and hesperidin from solutions (water, ethanol 50%, (v/v)) and extracts obtained from green and fermented honeybush (water, ethanol 50%, (v/v)). Research was performed with the Bronaugh cells, on human dorsal skin. The mangiferin and hesperidin distributions in skin layers (stratum corneum, epidermis, and dermis) and in acceptor fluid (in every 2, 4, 6, and 24 h) were evaluated by HPLC–Photodiode Array Coulometric and Coulometric Electrochemical Array Detection. The transdermal distribution of hesperidin was also demonstrated by fluorescence microscopy. Results indicated that mangiferin and hesperidin were able to cross the stratum corneum and penetrate into the epidermis and dermis. An advantage of hesperidin penetration into the skin from the water over ethanol solution was observed (451.02 ± 14.50 vs. 357.39 ± 4.51 ng/cm²), as well as in the mangiferin study (127.56 ± 9.49 vs. 97.23 ± 2.92 ng/cm²). Furthermore, mangiferin penetration was more evident from nonfermented honeybush ethanol extract (189.85 ± 4.11 ng/cm²) than from solutions. The permeation of mangiferin and hesperidin through the skin to the acceptor fluid was observed regardless of whether the solution or the honeybush extract was applied. The highest ability to permeate the skin was demonstrated for the water solution of hesperidin (250.92 ± 16.01 ng/cm²), while the hesperidin occurring in the extracts permeated in a very low capacity. Mangiferin from nonfermented honeybush ethanol extract had the highest ability to permeate to the acceptor fluid within 24 h (152.36 ± 8.57 ng/cm²).

Hesperetin modulates the Sirt1/Nrf2 signaling pathway in counteracting myocardial ischemia through suppression of oxidative stress, inflammation, and apoptosis

Hesperetin (HSP) is a natural flavonoid that offers useful curative effects for cardiovascular diseases, but its effect on myocardial ischemia and its precise mechanism remains unclear. The aim of this study is to explore the potential cardioprotective mechanism of HSP on myocardial ischemia caused by isoproterenol (ISO). Adult male Kunming mice were randomly divided into five groups: control, ISO, low-dose HSP (L-HSP, 25 mg/kg/d), high-dose HSP (H-HSP, 50 mg/kg/d), and verapamil (VER) group. Treatment groups of mice received HSP or VER for seven days, and the groups other than the control group were injected with ISO (100 mg/kg/d) subcutaneously for two consecutive days to establish a model of myocardial ischemia. Electrocardiogram and heart-histology changes were used to assess changes in myocardial architecture. The activities and the content of oxidative stress markers and inflammatory cytokines were determined and assayed using kits respectively. The expressions of proteins associated with apoptosis and the Sirt1/Nrf2 pathway were evaluated by Western blotting. The results demonstrate that VER, L-HSP and H-HSP significantly reduced the J-point displacement, heart rate, cardiac pathomorphological changes, and the levels of creatine kinase, lactated dehydrogenase, malonaldehyde, interleukin-6, and tumor necrosis factor-α in serum while promoting the activation of superoxide dismutase, catalase, glutathione in serum in the ISO-treated animals. Furthermore, L-HSP and H-HSP also reversed the ISO-induced apoptosis and the changes in the Sirt1/Nrf2 signaling pathway, as evident from the levels of proteins Bax, Bcl-2, caspase-3, Sirt1, Nrf2, NQO-1, and HO-1. In conclusion, HSP plays a protective role in ISO-induced myocardial ischemia by modulating oxidative stress, inflammation, and apoptosis via Sirt1/Nrf2 pathway activation.

Insights into the Binding of Dietary Phenolic Compounds to Human Serum Albumin and Food-Drug Interactions

The distribution of drugs and dietary phenolic compounds in the systemic circulation de-pends on, among other factors, unspecific/specific reversible binding to plasma proteins such as human serum albumin (HSA). Phenolic substances, present in plant-derived feeds, foods, beverages, herbal medicines, and dietary supplements, are of great interest due to their biological activity. Recently, considerable research has been directed at the formation of phenol-HSA complexes, focusing above all on structure-affinity relationships. The nucleophilicity and planarity of molecules can be altered by the number and position of hydroxyl groups on the aromatic ring and by hydrogenation. Binding affinities towards HSA may also differ between phenolic compounds in their native form and conjugates derived from phase II reactions. On the other hand, food-drug interactions may increase the concentration of free drugs in the blood, affecting their transport and/or disposition and in some cases provoking adverse or toxic effects. This is caused mainly by a decrease in drug binding affinities for HSA in the presence of flavonoids. Accordingly, to avoid the side effects arising from changes in plasma protein binding, the intake of flavonoid-rich food and beverages should be taken into consideration when treating certain pathologies.

Study on the antibacterial activities of emodin derivatives against clinical drug-resistant bacterial strains and their interaction with proteins

Background

Novel haloemodin (HEI2) synthesized by modifying emodin, a traditional Chinese medicine component, possesses remarkable antibacterial activity, being much more effective than its parent nucleus, emodin.

Methods

Firstly, we discovered that HEI2 increases bacterial cell membrane permeability to potassium ions more drastically than emodin. We thus further investigated the interaction of haloemodin and protein using a fluorescence quenching and circular dichroism (CD) study based on bovine serum albumin (BSA).

Results

HEI2 spontaneously bound to BSA at Trp 212 residue (subdomain IIA) by hydrogen bonds and van der Waals interactions to forms HEI2-BSA complexes, and this binding decreased the α-helical content of BSA. We also confirmed that emodin bound to BSA by hydrophobic interaction alone.

Conclusions

These results suggest that the main responses for the substantial antibacterial activities of HEI2 are a disruption of the bacterial plasma membrane function and the interaction with biological functional proteins. Furthermore, the study of the interaction of drugs with BSA, which has a fluorescent group tryptophan residue similar to many bio-functional proteins, will be a simple and inexpensive scope-reducing method in screening new drugs.

Inhibitory effect of α-ketoglutaric acid on α-glucosidase: integrating molecular dynamics simulation and inhibition kinetics

The inhibition of α-glucosidase is used as a key clinical approach to treat type 2 diabetes mellitus and thus, we assessed the inhibitory effect of α-ketoglutaric acid (AKG) on α-glucosidase with both an enzyme kinetic assay and computational simulations. AKG bound to the active site and interacted with several key residues, including ASP68, PHE157, PHE177, PHE311, ARG312, TYR313, ASN412, ILE434 and ARG439, as detected by protein-ligand docking and molecular dynamics simulations. Subsequently, we confirmed the action of AKG on α-glucosidase as mixed-type inhibition with reversible and rapid binding. The relevant kinetic parameter IC₅₀ was measured (IC₅₀ = 1.738 ± 0.041 mM), and the dissociation constant was determined (K_{i Slope} = 0.46 ± 0.04 mM). Regarding the relationship between structure and activity, a high AKG concentration induced the slight modulation of the shape of the active site, as monitored by hydrophobic exposure. This tertiary conformational change was linked to AKG inhibition and mostly involved regional changes in the active site. Our study provides insight into the functional role of AKG due to its structural property of a hydroxyphenyl ring that interacts with the active site. We suggest that similar hydroxyphenyl ring-containing compounds targeting key residues in the active site might be potential α-glucosidase inhibitors. AbbreviationsAKGalpha-ketoglutaric acidpNPG4-nitrophenyl-α-d-glucopyranosideANS1-anilinonaphthalene-8-sulfonateMDmolecular dynamics.Communicated by Ramaswamy H. Sarma.

The dipeptidyl peptidase-4 inhibitory effect of flavonoids is hindered in protein rich environments

Dipeptidyl peptidase-4 (DPP-4) inhibitors present a unique approach for the management of type 2 diabetes (T2D). In the present study, the inhibition of DPP-4 was evaluated for a large panel of flavonoids, important components of the human diet, using in vitro and ex vivo models. The activity of the isolated enzyme was assayed in vitro. Subsequently, the most active flavonoids were tested ex vivo in human whole blood and plasma. In this study, contrary to the in vitro fluorometric tests, flavonoids did not show inhibitory activity against DPP-4. Due to the discrepancy in the results between the in vitro and ex vivo approaches, plasma protein binding values were determined, presenting values from 43.9 to 100.0%. This work provides a new insight into the inhibitory activity for DPP-4, based on the flavonoid scaffold. Additionally, the obtained results showed that the inhibitory effect of flavonoids against DPP-4 was hindered in protein rich environments, like that occurring in blood, and indicated the need for experimental refinement in drug discovery for blood targets.

Locating the binding domains of lysozyme with ionic liquids in aqueous solution via spectroscopic studies

The binding domains of lysozyme with ionic liquids (ILs, [C₄mim]BF₄, [C₄mim]Cl, [C₄mim]Br and [dmim]I) in aqueous solution was investigated by studying molecular interactions using spectroscopic techniques. Ultraviolet spectroscopy (UV) showed that the addition of ILs increased the absorption peak intensity of lysozyme at 210 nm by enhancing peptide bond valence electron transition. It is also found that a weak interaction between ILs and lysozyme chromophore groups was generated by analyzing the changes of absorption peak intensity near 280 nm. Fluorescence and Synchronous Fluorescence spectra results showed that four ILs had quenching effect on the fluorescent substances of lysozyme, and the quenching effect rose with increasing ILs concentration. Meanwhile, the interaction between lysozyme and ILs molecules is mainly based on Van der Waals force and two Tryptophan (Trp) residues (Trp62 or Trp108) at the active site of lysozyme molecules play a critical role in binding ILs to their own molecules.

Studies on the Interaction between Three Small Flavonoid Molecules and Bovine Lactoferrin

The interaction between three flavonoids, i.e., Luteolin (LTL), Quercetin (QCT), and Naringenin (NGN) and bovine lactoferrin (BLF) at pH 7.4 was investigated by fluorescence quenching spectra, synchronous fluorescence spectra, and UV-visible absorption spectra. The results indicate the fluorescence of BLF quenched by Luteolin (LTL), Quercetin (QCT), and Naringenin (NGN) via static quenching. The main force between QCT and LTL with BLF was van der Waals interactions and hydrogen bonds. Electrostatic interactions played a major role in the binding process of interaction between NGN and BLF. Synchronous fluorescence was used to study the conformational changes of BLF. The values of binding constant (Ka) and number of binding sites (n) at different temperatures (300K, 305K, 310K) were also calculated, respectively. The results of corresponding thermodynamic parameters as well as binding distance between BLF and LTL, QCT, or GNG were obtained. These results implied that Luteolin (LTL), Quercetin (QCT), and Naringenin (NGN) could provide important guides for compound quantity (e.g., medicine dosage) and the design of new compounds (or drugs).

Dynamics of Ionic Liquid-Assisted Refolding of Denatured Cytochrome c: A Study of Preferential Interactions toward Renaturation

In vitro refolding of denatured protein and the influence of the alkyl chain on the refolding of a protein were tested using long chain imidazolium chloride salts, 1-methyl-3-octylimidazolium chloride [C₈mim][Cl], and 1-decyl-3-methylimidazolium chloride [C₁₀mim][Cl]. The horse heart cytochrome c (h-cyt c) was denatured by urea and guanidinium hydrochloride (GdnHCl), as well as by base-induced denaturation at pH 13, to provide a broad overview of the overall refolding behavior. The variation in the alkyl chain of the ionic liquids (ILs) showed a profound effect on the refolding of denatured h-cyt c. The ligand-induced refolding was correlated to understand the mechanism of the conformational stability of proteins in aqueous solutions of ILs. The results showed that the long chain ILs having the [C₈mim]⁺ and [C₁₀mim]⁺ cations promote the refolding of alkali-denatured h-cyt c. The IL having the [C₁₀mim]⁺ cation efficiently refolded the alkali-denatured h-cyt c with the formation of the MG state, whereas the IL having the [C₈mim]⁺ cation, which is known to be compatible for protein stability, shows slight refolding and forms a different transition state. The lifetime results show successful refolding of alkaline-denatured h-cyt c by both of the ILs, however, more refolding was observed in the case of [C₁₀mim][Cl], and this was correlated with the fast and medium lifetimes (τ₁ and τ₂) obtained, which show an increase accompanied by an increase in secondary structure. The hydrophobic interactions plays an important role in the refolding of chemically and alkali-denatured h-cyt c by long chain imidazolium ILs. The formation of the MG state by [C₁₀mim][Cl] was also confirmed, as some regular structure exists far below the CMC of IL. The overall results suggested that the [C₁₀mim]⁺ cation bound to the unfolded h-cyt c triggers its refolding by electrostatic and hydrophobic interactions that stabilize the MG state.

Inhibitory effect of pyrogallol on α-glucosidase: Integrating docking simulations with inhibition kinetics

In this study we conducted serial kinetic studies integrated with computational simulations to judge the inhibitory effect of pyrogallol on α-glucosidase, due to the association between this enzyme and the treatment of type 2 diabetes. As a result, we found that pyrogallol bound to the active site of α-glucosidase, interacting with several key residues, such as ASP68, MET69, TYR71, PHE157, PHE158, PHE177, GLN181, HIS348, ASP349, ASP406, VAL407, ASP408, ARG439, and ARG443, which was predicted by performing a protein-ligand docking simulation. Subsequently, we evaluated the inhibitory effect of pyrogallol on α-glucosidase, and found that it induced a mixed type of inhibition in a reversible and quick-binding manner. The relevant kinetic parameters were evaluated to be: IC₅₀=0.72±0.051mM; K_i=0.37±0.018mM. A tertiary conformational change was synchronized with pyrogallol inhibition and modulation of the shape of the active site was correspondingly observed. Our study provides insight into the functional inhibitory role of pyrogallol, which results from its triple-hydroxyl groups interacting with the active site of α-glucosidase. We suggest that compounds similar to pyrogallol (phenolic hydroxyl compounds) which target the key residues of the active site of α-glucosidase could be potential agents for α-glucosidase inhibition.

Aggregation behaviour of biocompatible choline carboxylate ionic liquids and their interactions with biomolecules through experimental and theoretical investigations

The self-assembly of long-chain choline carboxylates accompanied by their interaction with BSA protein were investigated with focus on environmental sustainability.

Molecular interaction study of flavonoids with human serum albumin using native mass spectrometry and molecular modeling

Noncovalent interactions between proteins and small-molecule ligands widely exist in biological bodies and play significant roles in many physiological and pathological processes. Native mass spectrometry (MS) has emerged as a new powerful tool to study noncovalent interactions by directly analyzing the ligand-protein complexes. In this work, an ultrahigh-resolution native MS method based on a 15-T SolariX XR Fourier transform ion cyclotron resonance mass spectrometer was firstly used to investigate the interaction between human serum albumin (HSA) and flavonoids. Various flavonoids with similar structure were selected to unravel the relationship between the structure of flavonoids and their binding affinity for HSA. It was found that the position of the hydroxyl groups and double bond of flavonoids could influence the noncovalent interaction. Through a competitive experiment between HSA binding site markers and apigenin, the subdomain IIA (site 1) of HSA was determined as the binding site for flavonoids. Moreover, a cooperative allosteric interaction between apigenin and ibuprofen was found from their different HSA binding sites, which was further verified by circular dichroism spectroscopy and molecular docking studies. These results show that native MS is a useful tool to investigate the molecular interaction between a protein and its ligands. Graphical abstract Unravel the relationship between the structure of flavonoids and their binding affinity to HSA by native MS.

Small molecular floribundiquinone B derived from medicinal plants inhibits acetylcholinesterase activity

Being a neurodegenerative disorder, Alzheimer's disease (AD) is the one of the most terrible diseases. And acetylcholinesterase (AChE) is considered as an important target for treating AD. Acetylcholinesterase inhibitors (AChEI) are considered to be one of the effective drugs for the treatment of AD. The aim of this study is to find a novel potential AChEI as a drug for the treatment of AD. In this study, instead of using the synthetic compounds, we used those extracted from plants to investigate the interaction between floribundiquinone B (FB) and AChE by means of both the experimental approach such as fluorescence spectra, ultraviolet-visible (UV-vis) absorption spectrometry, circular dichroism (CD) and the theoretical approaches such as molecular docking. The findings reported here have provided many useful clues and hints for designing more effective and less toxic drugs against Alzheimer's disease.

Biophysical exploration of protein-flavonol recognition: effects of molecular properties and conformational flexibility

The current work explores the biomolecular recognition of a series of flavonols by a protein and then uncovers the influences of the structural features of flavonols and the protein's own characteristics, e.g. the dynamics and flexibility, on the bioavailability of flavonols by using the pivotal biomacromolecule hemoglobin as a model. The experimental results revealed that flavonol may lead to a notable decrease in the steady-state fluorescence intensity of the β-37 Trp residue, and in the meantime the R-T transition of the protein transpired. Such noncovalent recognition forms the ground-state adduct, with an association intensity of 3.991 × 10(4) M(-1) in the reaction process, which has already been authenticated by the detailed analysis of time-resolved fluorescence and UV/vis absorption spectra. Furthermore, flavonol can form hydrogen bonds and π-conjugation effects with several amino acid residues on the polypeptide chain, for example, Trp-37, Arg-40, Asp-99 and Asn-102, and this event would induce self-regulation of the compact, regular conformation of the protein to a certain extent, which explicitly corroborates the results of circular dichroism. According to the study of molecular docking and structure-activity relationships, we could see that the recognition capacities of the protein-flavonols are inversely interrelated with the C log P values of the flavonol molecules. Moreover, the properties of the substituents in the structural B-ring unit of flavonols, i.e. polarity, position and number, will also prominently affect the degree of affinity and bioavailability of the protein-flavonol complexes. The analytical results of molecular dynamics (MD) simulation testified that the discussions of the structure-activity relationships are entirely logical, and the conformations of the amino acid residues forming noncovalent interactions tend to be stable in the MD simulation, as further elucidated from the dynamics data. Plainly, molecular recognition of the protein-flavonols might noticeably cause relatively large changes in protein flexibility, and then manifest different recognition strengths and corresponding biological activities. This issue will be carefully validated by the interpretation of root-mean-square fluctuation.

Physiologically based kinetic modeling of hesperidin metabolism and its use to predict in vivo effective doses in humans

SCOPE

To develop a physiologically based kinetic (PBK) model that describes the absorption, distribution, metabolism, and excretion of hesperidin in humans, enabling the translation of in vitro concentration-response curves to in vivo dose-response curves.

METHODS AND RESULTS

The PBK model for hesperidin in humans was developed based on in vitro metabolic parameters. Hesperidin was predicted to mainly occur in the systemic circulation as different monoglucuronides. The plasma concentrations of hesperidin aglycone (hesperetin) was predicted to be <0.02 mg/L at an oral dose of 50 mg/kg bw. The developed PBK model allowed conversion of in vitro concentration-response curves for different effects to in vivo dose-response curves. The BMD₀₅ (benchmark dose for 5% response) values for protein kinase A inhibition ranged between 135 and 529 mg/kg bw hesperidin, and for inhibition of endothelial cell migration and prostaglandin E₂ and nitric oxide production ranged between 2.19 and 44 mg/kg bw hesperidin. These values are in line with reported human data showing in vivo effects by hesperidin and show that these effects may occur at Western dietary and supplementary intake of hesperidin.

CONCLUSIONS

The developed PBK model adequately predicts absorption, distribution, metabolism, and excretion of hesperidin in humans and allows to evaluate the human in vivo situation without the need for human intervention studies.