Characterization of the Binding of Metoprolol Tartrate and Guaifenesin Drugs to Human Serum Albumin and Human Hemoglobin Proteins by Fluorescence and Circular Dichroism Spectroscopy

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.

Investigation of the interaction between S-isoalkyl derivatives of the thiosalicylic acid and human serum albumin

S-isoalkyl derivatives of thiosalicylic acid (isopropyl-(L1), isobutyl-(L2) and isoamyl-(L3)) were selected in order to investigate the binding interaction with the human serum albumin (HSA) using different spectroscopic methods and molecular docking simulation. Association constants and number of binding sites were used to analyze the quenching mechanism. The experimental results showed that the fluorescence quenching of HSA by L1, L2 and L3 occurs because of static quenching and that binding processes were spontaneous, with the leading forces in bonding by hydrogen bonding, hydrophobic interactions, and electrostatic interactions. Fluorescence spectroscopy, UV-Vis spectroscopy and synchronous fluorescence spectroscopy showed that ligands (L1, L2 and L3) can bind to HSA and that the binding of ligands induced some microenvironmental and conformational changes in HSA. The calculated distance between the donor and the acceptor according to fiFörster's theory confirms the energy transfer efficiency between the acceptor and HSA. Results of site marker competitive experiments showed that the tested compounds bind to HSA in domain IIA (Site I). Molecular dynamics and docking calculations demonstrated that L3 binds to the Sudlow site I of HSA with lower values of binding energies compared to L1 and L2, indicating the formation of the most stable ligand-HSA complex. Understanding the binding mechanisms of S-isoalkyl derivatives of the thiosalicylic acid to HSA may provide valuable data for the future studies of their biological activity and application as potential antitumor drugs.Communicated by Ramaswamy H. Sarma.

Spectroscopic Analysis of an Antimalarial Drug’s (Quinine) Influence on Human Serum Albumin Reduction and Antioxidant Potential

Quinine (Qi) is a well-known drug used in malaria therapy; it is also a potential anti-arrhythmic drug used in the treatment of calf cramps, rheumatoid arthritis, colds, and photodermatitis. Moreover, it is used in the food industry for the production of tonics. This study aimed to analyze the interaction between quinine and a transporting protein—human serum albumin (HSA)—as well as the influence of Qi on both protein reduction and antioxidant potential. It was found that Qi (via spectrofluorometric measurements and circular dichroism spectroscopy) binds to HSA with a low affinity and slightly affects the secondary structure of albumin. As demonstrated by the use of ABTS and FRAP assays, HSA has a higher antioxidant and reduction potential than Qi, while their mutual interaction results in a synergistic effect in antioxidant activity and reduction potential.

The Effect of Antihypertensive Drugs on NADH in Newly Diagnosed Primary Hypertension

Background

Some antihypertensive medications alter cellular energy production, presumably by modification of the mitochondrial function. In vivo studies of such effects are challenging in humans. We applied a noninvasive forearm skin measurement of the 460-nm fluorescence specific for the reduced form of nicotinamide adenine dinucleotide (NADH) to study the 6-week effects of four different antihypertensive medications on mitochondrial function using the Flow-Mediated Skin Fluorescence (FMSF).

Methods

In a prospective open-label study, we compared the long-term effects of a 6-week treatment with either amlodipine (5 mg), perindopril (5 mg), nebivolol (5 mg), or metoprolol (50 mg) on the dynamic flow-mediated changes in the skin NADH content in 76 patients (29 women) with untreated primary arterial hypertension (HA). Patients underwent 24-hour ambulatory blood pressure monitoring. To study mitochondrial function, the FMSF was measured at rest, during 100-second ischemia and postischemic reperfusion. The control group consisted of 18 healthy people (7 women).

Results

There were no significant differences in the FMSF parameters between the control and the study group before medication. After the 6-week treatment, all drugs similarly reduced blood pressure. Neither amlodipine, perindopril, nor nebivolol changed the flow-mediated 460-nm skin fluorescence significantly. However, metoprolol raised this fluorescence at rest, during ischemia and reperfusion (P at most <0.05), indicating an increase in the total NADH skin content.

Conclusion

Amlodipine, perindopril, and nebivolol appear neutral for the skin NADH content during the 6-week antihypertensive treatment. Similar treatment with metoprolol increased skin NADH at rest, during ischemia and reperfusion, probably due to an effect on microcirculation and altered mitochondrial function. Explanation of the potential mechanisms behind metoprolol influence on the skin NADH metabolism requires further investigation.

Profiling the interaction of a novel toxic pyruvate dehydrogenase kinase inhibitor with human serum albumin

To discover novel pyruvate dehydrogenase kinase (PDK) inhibitors, a new compound 2,2-dichloro-1-(4-((4-isopropylphenyl)amino)-3-nitrophenyl)ethan-1-one, namely XB-1 was identified, which inhibited PDK activity with a half maximal inhibitory concentration (IC₅₀) value of 337.0 nM, and reduced A549 cell proliferation with a half maximal effective concentration (EC₅₀) value of 330.0 nM. However, the compound appears to exhibit a negligible selectivity between cancer cell and normal one, indicating a potential toxicity existed for the compound. Herein, the interaction of the toxic XB-1 to human serum albumin (HSA) was firstly explored by spectroscopic approaches with the aim to reduce/avoid the toxicity of PDK inhibitors in the next hit-to-lead campaign. In detail, it was found that the XB-1 could effectively bind to HSA mainly via hydrogen bond interaction in PBS buffer (pH = 7.4, 10.0 mM), resulting in the formation of HSA-XB-1 complex. The negative value of ΔG showed that the binding of XB-1 to HSA is a spontaneous process. The result from site-selective binding assay suggested that the XB-1 bound to the site I of HSA by competing with warfarin, which was perfect in agreement with the molecular docking method. The results of this paper may offer a valuable theoretical basis to study the toxicity of biofunctional molecules and may offer thoughts about how to avoid/reduce toxicity for a small molecule.

Ligand binding constants for human serum albumin evaluated by ratiometric analysis of DSC thermograms

This paper describes an expanded application of our recently reported method (Eskew et al., Analytical Biochemistry 621,1 2021) utilizing thermogram signals for thermal denaturation measured by differential scanning calorimetry. Characteristic signals were used to quantitatively evaluate ligand binding constants for human serum albumin. In our approach the ensemble of temperature dependent calorimetric responses for various protein-ligand mixtures and native HSA were compared, in a ratiometric manner, to extract binding constants and stoichiometries. Protein/ligand mixtures were prepared at various ligand concentrations and subjected to thermal denaturation analysis by calorimetry. Measurements provided the melting temperature, T_m, and free-energy ΔG_cal(37°C) for melting ligand-bound Albumin as a function of ligand concentration. Concentration dependent behaviors of these parameters derived from protein/ligand mixtures were used to construct dose-response curves. Fitting of dose-response curves yielded quantitative evaluation of the ligand binding constant and semi-quantitative estimates of the binding stoichiometry. Many of the ligands had known binding affinity for Albumin with binding constants reported in the literature. Evaluated binding parameters for the ligands impressively agreed with reported literature values determined using other standard experimental methods. Results are reported for 29 drug ligands binding to Albumin. These validate our calorimetry-based process for applications in pre-clinical drug screening.

Spectroscopic studies of simultaneous binding of cyclophosphamide and imatinib mesylate to human holo-transferrin

The interactions of proteins with drugs are very important from a pharmacological point of view. Holo-transferrin is a blood-plasma glycoprotein whose main function is iron-binding and the transport of other ligands. Additionally, the protein is only transferrin-form recognized by TfR1 and TfR2 receptors at the surface of rapidly proliferating malignant cells. Imatinib mesylate is a tyrosine-kinase inhibitor mainly used in the treatment of blood cancers, frequently in multidrug therapy with cyclophosphamide. In this study the effect of cyclophosphamide on the interaction of imatinib mesylate with human holo-transferrin has been investigated. Using spectroscopic techniques such as fluorescence, circular dichroism, ultraviolet-visible and electrophoretic light scattering additive parameters, system stability and the effect of the ligands on the protein conformation at varying pH values have been defined. Calculated quenching constants are in the order of 2 × 10⁴ M^-1 and the type of interaction depends on the reaction medium. Under physiological conditions binding constant is 1.329 × 10⁶ M^-1 whereas in an environment similar to that of cancer cells the constant is significantly lower, K_a = 6.060 × 10⁴ M^-1. N values are approximate to 1 in all cases. Moreover, some changes are observed in the α-helical structure of the protein after interaction with the drugs and the presence of cyclophosphamide slightly stabilizes the protein secondary structure. All collected data proves the effect of cyclophosphamide on the interaction between imatinib mesylate and human holo-transferrin. It is of great clinical interest due to anticancer, multidrug therapies including both imatinib mesylate and cyclophosphamide.

Lysozyme-dalargin self-organization at the aqueous-air and liquid-liquid interfaces

An experimental study of protein-peptide binding was performed by means of radiochemical and spectroscopic methods. Lysozyme and dalargin were chosen due to their biological and physiological importance. By means of tensiometry and radiochemical assays, it was found that dalargin possesses rather high surface activity at the aqueous-air and aqueous-p-xylene interfaces to be substituted by protein. Dalargin forms a hydrophobic complex with lysozyme in which the secondary structure of lysozyme is preserved. When lysozyme forms a mixed adsorption layer with dalargin at the aqueous-air surface, the peptide prevents protein from concentrating in the subsurface monolayer. In the presence of p-xylene protein in the interface, reorganization occurs quickly, so there is no lag in the interfacial tension time dependence. The interfacial tension in this case is controlled by protein and/or protein-peptide complexes. An increase in the enzymatic activity of lysozyme in the presence of dalargin was confirmed by a docking model that suggests the formation of hydrogen bonds between dalargin and amino acid residues in the active site.

Spectroscopic studies on the interaction of aspartame with human serum albumin

In this work the binding of artificial sweetener aspartame with human serum albumin (HSA) was studied at physiological pH. Binding studies of aspartame (APM) with HSA are useful to understand APM -HSA interaction, mechanism and providing guidance for the application and design of new and more efficient artificial sweeteners. The interaction was investigated by spectrophotometric, spectrofluorometric competition experiment and circular dichroism (CD) techniques. The results indicated that the binding of APM to HSA caused fluorescence quenching of HSA through static quenching mechanism with binding constant 1.42 × 10⁺⁴ M^-1 at 298 K and the number of binding sites is approximately one. Thermodynamic parameters, enthalpy changes (ΔH) and entropy changes (ΔS) were calculated to be -41.20 kJ mol^-1 and -58.19 J mol^-1 K^-1, respectively, according to van't Hoff equation, which indicated that reaction is enthalpically driven. Quenching of the fluorescence of HSA was found to be a static quenching process. The binding constants and number of binding sites were obtained at three different temperatures (298, 308 and 318 K). Combining above results and those of spectrofluorometric competition experiment and circular dichroism (CD), indicated that APM binds to HSA via Sudlow's site I. Furthermore, the study of molecular docking on HSA binding also indicated that APM can strongly bind to the site I (subdomain IIA) of HSA mainly by hydrophobic interaction and hydrogen bond interactions exist between APM and HSA.

Ex vivo binding studies of the anti-cancer drug noscapine with human hemoglobin: a spectroscopic and molecular docking study

Noscapine binds human hemoglobin spontaneously forming a stable complex that affects noscapine's ADMET profile, bioavailability and toxicity.

Acrylamide-hemoglobin adduct: A spectroscopic study

Acrylamide is a neurotoxic and carcinogenic organic compound that is able to bind to several biomolecules and form adducts, through nucleophilic addition and in vivo by the Maillard Reaction, interfering with the biological functions of these molecules. Hemoglobin is one of the most abundant intracellular blood proteins, and thus it is of high interest to understand whether the binding of acrylamide can alter its properties. The interaction of acrylamide with hemoglobin was assessed in a 20:1 ratio, and after a 72 h-incubation period, a decrease of ca. 50% in the absorbance of the hemoglobin's Soret band was observed at 37 °C. This together with the analysis of circular dichroism spectra indicate that acrylamide binds in close proximity to the heme group. These perturbations were confirmed to not correspond to the loss of the heme group and were mostly reverted after passing the protein through a size-exclusion chromatographic matrix, suggesting a dominant non-covalent interaction for the observed effect. The thermodynamic parameters of unfolding in the absence and presence of acrylamide, suggest an interaction based on H-bonds and van der Waals forces that slightly stabilizes hemoglobin. The oxygen binding capacity of hemoglobin does not seem to be hindered, as no differences in the Q bands were observed in the adduct.

Removal of 1,2-benzanthracene via the intercalation of 1,2-benzanthracene with DNA and magnetic bead-based separation

In this study, DNA-functionalize-magnetic beads were investigated as sorbent materials for effective removing 1,2-benzanthracene (BaA) from water. In order to reveal the removal mechanism, the interaction mode between BaA and DNA was evaluated by using various characterization tools such as UV-visible and circular dichroism spectroscopy, fluorescence and resonance scattering spectroscopy, and agarose gel electrophoresis. In the presence of BaA, the melting temperature of DNA increased from 76.2 °C to 82.3 °C, which closely related to the intercalating of BaA. It was found that a part of the ethidium bromide (EB) binding sites to DNA were occupied by BaA in EB competing study. The results indicated that a new complex appeared between hsDNA and BaA, and the number of the binding sites (n) and the binding constants (K_A) at different temperatures were obtained. DNA binding saturation value (≈0.80) was obtained by resonance scattering spectra study. BaA could be enriched and removed by DNA-functionalize-magnetic beads via the intercalation, and the removal efficiency was 97.73% when the initial concentration was 2.45 x10^-6mol·L^-1 (559.31 μg/L).

Studies on binding of single-stranded DNA with reduced graphene oxide-silver nanocomposites

The binding reaction of reduced graphene oxide-silver nanocomposites (rGO-AgNCs) with calf thymus single-stranded DNA (ssDNA) was studied by ultraviolet-visible absorption, fluorescence spectroscopy and circular dichroism (CD), using berberine hemisulphate (BR) dye as a fluorescence probe. The absorbance of ssDNA increases, but the fluorescence intensity is quenched with the addition of rGO-AgNCs. The binding of rGO-AgNCs with ssDNA was able to increase the quenching effects of BR and ssDNA, and induce the changes in CD spectra. All of the evidence indicated that there was a relatively strong interaction between ssDNA and rGO-AgNCs. The data obtained from fluorescence experiments revealed that the quenching process of ssDNA caused by rGO-AgNCs is primarily due to complex formation, i.e. static quenching. The increasing trend of the binding equilibrium constant (Ka) with rising temperature indicated that the binding process was an endothermic reaction. The calculated thermodynamic parameters showed that the binding process was thermodynamically spontaneous, and hydrophobic association played predominant roles in the binding of ssDNA to the surface of rGO-AgNCs.

Structural peculiarities of lysozyme - PLURONIC complexes at the aqueous-air and liquid-liquid interfaces and in the bulk of aqueous solution

Interaction between proteins and synthetic polymers that represent a perspective potential in drug delivery or/and already used in medicine plays a key role in biological functioning of both molecules along with a system as a whole. In present study association between hen egg white lysozyme and Pluronic triblock-copolymers (L121, P123 and F127) in the bulk of the solution as well as at the aqueous-air and liquid-liquid interfaces was analyzed by means of spectroscopic and radiochemical assay. In protein-Pluronic complexes lysozyme keeps the secondary structure (CD and SAXS data results), while fluorescence and UV-analysis indicates changes in the local surrounding of fluorophoric amino acid residues. Radiochemical assay in combination with molecular docking reveals the formation of the complexes, in which proline residues turned to the interface between water and hydrophobic medium.

Caffeine and catechin towards prevention of drug induced oxidation of hemoglobin: A spectroscopic study

Administration of low doses of nitrates over prolonged periods in patients suffering from coronary heart disease may lead to chronic methemoglobinemia, a disease caused by oxidation of hemoglobin. Previous reports have shown that natural products like curcumin, vitamin E, vitamin C, resveratrol, etc., are capable of inhibition of nitrite induced oxidation of hemoglobin. Hence in this study we aimed to investigate the preventive role of antioxidants present in our diet, like caffeine and catechin hydrate which are commonly found in coffee and tea towards methemoglobin (met-Hb) formation. Our study revealed that when the hemolysate was pre-incubated with equimolar concentration of caffeine and its metabolite with respect to nitrite, the rate of the nitrite induced oxidation of HbA decreased from (7.33 ± 0.54) × 10^-5 min^-1 to (7.09 ± 1.05) × 10^-5 min^-1 and (2.98 ± 0.06) × 10^-5 min^-1 respectively. Hence it was evident that the metabolite of caffeine, 1-methyluric acid, exhibited better efficiency at physiological concentration than its precursor. On the contrary, only 4 mM catechin hydrate could enhance the rate of methemoglobin formation even in absence of nitrite and the rate of the reaction was (6.088 ± 0.31) × 10^-5 min^-1 which is comparable with that of 400 μM nitrite.

Interactions of the cis and trans states of an azobenzene photoswitch with lysozyme induced by red and blue light

Exploring the interaction between an azobenzene-based photoswitch and natural protein can help elucidate how the photo-control of an optical molecule participates in the transmission and delivery of proteins, as well as the effects of azo-switch trans and cis states on protein configurations. In this study, fluorescence analysis, circular dichroism spectroscopy, molecular docking, and molecular dynamics simulations were used to study the interaction among different configurations of tetra-ortho-methoxy substituted azobenzene di-maleimide (toM-ABDM), a red light-induced optical azo-switch, and lysozyme (LYZ). Results showed that toM-ABDM caused the static quenching of LYZ. The cis toM-ABDM had stronger binding affinity than trans toM-ABDM. The noncovalent interaction, hydrogen bonds and van der Waals forces, could not regulate the conformation of LYZ in photo-control. A binding model of toM-ABDM and LYZ in different forms induced by red and blue light was further established by computer simulation.

Sphingomyelin-induced structural modification of native human hemoglobin and its chemically and thermally disrupted secondary structure: A photophysical exploration

Sphingomyelin-induced structural modification of Human Hemoglobin (Hb) has been investigated in its native and unfolded conformers that are partially denatured in presence of ∼ 4 M urea, completely denatured in ∼ 8 M urea and thermally disrupted (at ∼ 65 °C) state. The absorption studies unveil ground state complexation between Hb and SM. From steady-state fluorescence and quenching studies alteration of the micro-environments around Trp residues of Hb in above mentioned different cases has been determined. Moreover, lesser exposure of Trp residues to SM in thermally disrupted Hb can be accounted for the exceptionally interesting outcomes in other experiments. The alterations in the time-resolved decay profiles of native Hb, partially and totally chemically denatured as well as thermally disrupted Hb with gradual addition of SM also affirm the amendment of the proteinous micro-environment surrounding Trp residues in a view of FRET between Trp residues and heme group. Wavelength-sensitive emission spectral studies reveal that the protein shows red edge effect in its different conformations in presence and absence of SM. Interestingly, the wavelength-responsive time-resolved study at a constant excitation wavelength demonstrates that with addition of lipid the increment of the average fluorescence lifetime signifies a considerable modulation of solvation dynamics of the fluorescent Trp residues in their excited state being greatest in case of thermally disrupted Hb. Nevertheless, the loss of α-helicity of Hb at its various conformers with addition of SM has been portrayed thoroughly by means of far-UV CD spectral studies in a view of disruption of secondary structure of the protein.

Effects of piceatannol on the structure and activities of bovine serum albumin: A multi-spectral and molecular modeling studies

Piceatannol (PIC) displays a wide spectrum of biological activities, such as antioxidation, antibacterial activity and anti-inflammation, but the biochemical and molecular mechanism is not fully understood. In this study, the interaction of PIC with bovine serum albumin (BSA) was studied by fluorescence spectroscopy, ultraviolet-visible absorption spectroscopy, circular dichroism spectroscopy and molecular simulation. The effects of PIC on BSA non-enzymatic glycosylation, fibrillation, thermal stability, and structure information were also studied. The results showed that the formation of PIC-BSA complex by mainly hydrogen-bonding forces resulted in the conformational changes of protein. PIC inhibited the formation of β-sheets structures of BSA. BSA still maintained the esterase-like good activity in the presence of PIC. In addition, PIC significantly reduced the degree of BSA glycosylation. These results provided a basis for the molecular interaction between PIC and protein, and suggested the potential effect of PIC in preventing the progression of diabetes mellitus.

In-depth investigation of the binding of flavonoid taxifolin with bovine hemoglobin at physiological pH: Spectroscopic and molecular docking studies

The use of bioactive flavonoids as drugs has long mesmerized the scientific world. Their small size and planar structure enables them to interact with limitless substrates especially biomolecules. Taxifolin is a flavonoid well known for its anti-oxidizing and metal chelating properties. Its interaction with a few biomolecules has been studied so far to exploit its pharmacological activities. Hemoglobin, an iron containing macromolecule acts as a major carrier protein and is also associated with the occurrence of many diseases. Our present study lays emphasis on the interaction of flavanonol taxifolin with bovine hemoglobin at physiological pH. This was achieved by monitoring the changes in the absorbance, fluorescence, anisotropic, lifetime and circular dichroic spectra. Benesi-Hildebrand plot determined a binding constant value of 20.0 × 10³ M^-1 at 25 °C. Stern-Volmer quenching studies reveal that the binding is associated with a static mode of quenching. The complexation is thermodynamically favored as indicated by the negative value of enthalpy and positive value of entropy changes seen from the van't Hoff plot. Theoretical DFT calculations were used to find out an optimized geometry and HOMO-LUMO energy gap for taxifolin. Molecular docking studies revealed the location of taxifolin inside the hemoglobin moiety.

Twisted intramolecular charge transfer (TICT) based fluorescent probe for lighting up serum albumin with high sensitivity in physiological conditions

Accurate detection of human serum albumin (HSA) in biological samples is quite meaningful for early disease diagnosis and treatment. Herein, a novel fluorescent probe 1-ethyl-4-[2-[4-(diethylamino)-2-hydroxyphenyl]ethenyl]]-pyridinium salt (DEHP) was developed for HSA determination. The inherent fluorescence of DEHP is essentially negligible at physiological conditions assigned to the well-developed twisted intramolecular charge transfer (TICT) protocol. An intriguing fluorescence light up is triggered as the addition of HSA, on account of the inhibited TICT procedure when DEHP enters the hydrophobic cavity of protein HSA. This combination leads to a turn on fluorescent response for HSA with a detection limit of 4.8 nM. After an overall investigation, it has been proved that the strong binding between DEHP and HSA is specific-site-related. In additional, the probe implies a great potential to assist clinical diagnosis due to the usage in actual serum detection. Cell imaging also shows that the probe is expected to monitor HSA production process at cell level.

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.

Biomolecular interaction of a platelet aggregation inhibitor, 3,4-methylenedioxy-β-nitrostyrene with human serum albumin: multi-spectral and computational characterization

Molecular interaction of the 3,4-methylenedioxy-β-nitrostyrene (MNS), an inhibitor of platelet aggregation with the main transport protein, albumin from human serum (HSA) was explored using absorption, fluorescence and circular dichroism (CD) spectroscopy in combination with in silico analyses. The MNS-HSA complexation was corroborated from the fluorescence and absorption spectral results. Implication of static quenching mechanism for MNS-HSA system was predicted from the Stern-Volmer constant, K_SV-temperature relationship as well as the bimolecular quenching rate constant, k_q values. Stabilization of the complex was affirmed by the value of the binding constant (K_a = 0.56-1.48 × 10⁴ M^-1). Thermodynamic data revealed that the MNS-HSA association was spontaneously driven mainly through hydrophobic interactions along with van der Waal's interaction and H-bonds. These results were well supported by in silico interpretations. Far-UV and near-UV CD spectral results manifested small variations in the protein's secondary and tertiary structures, respectively, while three-dimensional fluorescence spectra displayed microenvironmental fluctuations around protein's fluorophores, upon MNS binding. Significant improvement in the protein's thermostability was evident from the temperature-stability results of MNS-bound HSA. Binding locus of MNS, as identified by competitive drug displacement findings as well as in silico analysis, was found to be located in subdomain IIA (Sudlow's site I) of the protein.Communicated by Ramaswamy H. Sarma.

Food additive dye (quinoline yellow) promotes unfolding and aggregation of myoglobin: A spectroscopic and molecular docking analysis

Protein aggregation leads to vast conformational changes and plays a key role in the pathogenesis of various neurodegenerative diseases including Alzheimer's and Parkinson's. In the current piece of work, we have explored the interaction of quinoline yellow (QY) with myoglobin (Mb) at two different pH (3.5 and 7.4). Various spectroscopic techniques such as turbidity, Rayleigh light scattering (RLS), UV-Vis absorbance, fluorescence resonance energy transfer (FRET), far UV-CD along with transmission electron microscopy (TEM) and molecular docking have been utilized to characterize dye-induced aggregation in Mb. Binding results showed that interaction between QY and myoglobin is spontaneous and static in nature with high K_SV value of 2.14 × 10⁴ M^-1. On the other hand, thermodynamics studies (∆H & ∆S) revealed that complex formation was driven by hydrogen and Van der Walls forces. Molecular docking analysis showed strong binding affinity (K_d = 4.95 × 10⁴ M^-1) between QY and Mb at Pro100, Ile101, Lys102, Glu105, Glu136, Arg139, Lys140, and Ala143 residues. The intrinsic fluorescence and circular dichroism studies indicated that QY induced conformational changes in Mb at pH 3.5. Turbidity and RLS studies showed aggregation of Mb in the presence of QY (0.2-5 mM). Moreover, kinetics data revealed nucleation independent aggregation of myoglobin in the presence of QY. TEM analysis further established amorphous nature of Mb aggregate induced by QY. At pH (7.4), QY was unable to induce aggregation in myoglobin; it might be due to repulsive nature of negatively charged dye and myoglobin or partially altered states of protein could be pre-requisite for binding and aggregation.

Streptozocin; a GLUT2 binding drug, interacts with human serum albumin at loci h6DOM3-h7DOM3

Streptozocin (STZ) is a broad range antibiotic, highly genotoxic, antineoplastic and hyperglycemic. HSA is the most abundant protein in physiology and it binds to almost all exogenic and endogenic ligands, including drugs. STZ-induced fluorescence quenching of HSA has been done at pH 7.4, pH 3.5 and at pH 7.4 with 4.5 M urea at temperatures 286 K, 291 K, and 306 K. K_sv found to be 10³ M^-1, binding constant 1.5X10³M^-1 and binding sites ~1. But, K_sv for HSA and glucopyranose interaction was found lesser than that of HSA-STZ binding. Binding of STZ/glucopyranose on HSA seems to result in complex formation as calculated K_q > 10¹⁰ M^-1 s^-1. The number of binding sites, binding constants, and binding energies were increased with temperature. The ΔG⁰, ΔH⁰, and ΔS⁰ for HSA-STZ interaction were found to be -17.7 × 10³ J·mol^-1; 2.34 × 10⁵ J·mol^-1 and 841 JK^-1 mol^-1 respectively at pH 7.4 and 291 K. The comparative bindings of N, F and I states of HSA with STZ and their molecular docking analyses indicate that IIIA-B junction (i.e., inter-helix h6_DOM3-h7_DOM3) is the probable binding site, a locus close to fatty acid binding site-5. These results could be useful for therapeutic and analytical exploitation of STZ, as albumin used as the vehicle for drug delivery.

Mutual influence of piceatannol and bisphenol F on their interaction with pepsin: Insights from spectroscopic, isothermal titration calorimetry and molecular modeling studies

The individual and combined interactions of bisphenol F and piceatannol with pepsin were investigated using spectroscopic methods (fluorescence, UV-vis absorption, and circular dichroism spectroscopy), combined with isothermal titration calorimetry and molecular docking. Thermodynamic data showed that hydrogen bonds and van der Waals forces might play a major role for the binding process. Site marking experiments and molecular docking confirmed the binding sites of these two ligands on pepsin. The discrepancy in the binding constant between the binary and ternary systems indicated the competitive binding of piceatannol and bisphenol F to pepsin. Circular dichroism spectra studies suggested that the binding of the two ligands led to a loosening of pepsin backbone. Enzyme activity assays indicated that the inhibition of pepsin activity by piceatannol and bisphenol F was competitive. These results will be helpful to understand the mechanism of piceatannol and bisphenol F affecting the activity of digestive proteases in the sight of the food security.

New insights into the binding behavior of lomefloxacin and human hemoglobin using biophysical techniques: binary and ternary approaches

Demonstrates the overlap that had been induced between the fluorescence emission spectrum of Hb and the absorption spectrum of drugs, which has proved that there is a high probability to the occurrence of energy transfer from Hb and LMF in the absence and presence of NRF.

Differences between the binding modes of enantiomers S/R-nicotine to acetylcholinesterase

Nicotine causes neurotoxic effects because it quickly penetrates the blood–brain barrier after entering the human body. Acetylcholinesterase (AChE) is a key enzyme in the central and peripheral nervous system associated with neurotoxicity. In this study, a spectroscopic method and computer simulation were applied to explore the mode of interaction between AChE and enantiomers of nicotine (S/R-nicotine). Fluorescence spectroscopy showed that the quenching mechanism of endogenous fluorescence of AChE by S/R-nicotine was static, as confirmed by the time-resolved steady-state fluorescence. The binding strength of both nicotine to AChE was weak (S-AChE: K_a = 80.06 L mol⁻¹, R-AChE: K_a = 173.75 L mol⁻¹). The main driving forces of S-AChE system interaction process were van der Waals force and hydrogen bonding, whereas that of R-AChE system was electrostatic force. Computer simulations showed that there were other important forces involved. S/R-Nicotine had a major binding site on AChE, and molecular docking showed that they bound mainly to the cavities enclosed by the active sites (ES, PAS, OH, AACS, and AP) in the protein. UV-vis spectroscopy and 3D spectroscopy indicated that nicotine significantly affected the microenvironment of Trp amino acids in AChE. The CD spectra indicated that S-nicotine increased the α-helical structure of AChE, but the overall conformation did not change significantly. By contrast, R-nicotine significantly changed the secondary structure of AChE. 5,5′-Dithiobis-2-nitrobenzoic acid (DTNB) method indicated that S and R nicotine produced different degrees of inhibition on the catalytic activity of AChE. Both experimental methods and computer simulations showed that R-nicotine had a significantly higher effect on AChE than S-nicotine. This research comprehensively and systematically analyzed the mode of interaction between nicotine and AChE for neurotoxicity assessment.

Study on the binding modes of AChE to S/R-nicotine.

Spectroscopic and molecular modelling studies on glycation modified bovine serum albumin with cyanidin-3-O-glucoside

In this study, we report the glycation mediated effect of bovine serum albumin (BSA) on the molecular interaction mechanism of cyanidin-3-O-glucoside (C3G) by molecular modelling, Uv-visible spectroscopy, transmission electron microscopy (TEM), fluorescence spectroscopy, and circular dichroism spectroscopy studies. The structures of advanced glycation end-products (AGEs) modified BSA were modelled, energy minimized and analyzed for binding affinity by molecular docking studies using Autodock Vina. Glycation experiments are carried out using glucose and methylglyoxal to validate the molecular modelling results on the interaction of modified BSA with C3G. The modified structures were characterized by reduction in the binding pocket volume, surface, depth, hydrophobicity, and hydrogen bond donors/acceptors. Arg-194, Arg-196, Arg-198, Arg-217, Arg-409, Lys-114, Lys-116, Lys-204, Lys 221, and Lys-439 were found to be crucial in the context of glycation of BSA. TEM images represented the formation of unique globular aggregates in the event of glycation. Uv-visible spectroscopic studies showed the formation of new chromophores between 300 and 400 nm in the event of glycation. Fluorescence quenching was observed in a differential manner in the presence of C3G on glycation modified BSA. Circular dichroism studies suggested the loss of helical structure and formation of β-sheeted structure upon glycation, but subsequent C3G binding has resulted in the increase towards helical structure. Our findings suggested that drug binding affinity has been certainly impaired due to glycation and subsequent AGE modification. Arg-p modification has more austere impact on the structure and would affect the binding properties. We conclude that C3G had differential modulation of binding properties on glycated BSA which can help to protect the stability and bioavailability that has been impaired due to glycation mediated structural changes.

Study on the interactions between toxic nitroanilines and lysozyme by spectroscopic approaches and molecular modeling

Being exogenous environmental pollutants, nitroanilines (NAs) are highly toxic and have mutagenic and carcinogenic activity. Being lack of studies on interactions between NAs and lysozyme at molecular level, the binding interactions of lysozyme with o-nitroaniline (oNA), m-nitroaniline (mNA) and p-nitroaniline (pNA) were investigated by means of steady-state fluorescence, synchronous fluorescence, UV-vis absorption spectroscopy, as well as molecular modeling. The experimental results revealed that the fluorescence of lysozyme is quenched by oNA and mNA through a static quenching, while the fluorescence quenching triggered by pNA is a combined dynamic and static quenching. The number of binding sites (n) and the binding constant (K_b) corresponding thermodynamic parameters ΔH^⊖, ΔS^⊖, ΔG^⊖ at different temperatures were calculated. The reactions between NAs and lysozyme were spontaneous and entropy driven and the binding of NAs to lysozyme induced conformation changes of lysozyme. The difference of the position of -NO₂ group affected the binding and the binding constants K_b decreased in the following pattern: K_b (pNA) >K_b (mNA) >K_b (oNA). Molecular docking studies were performed to reveal the most favorable binding sites of NAs on lysozyme. Our recently results could offer mechanistic insights into the nature of the binding interactions between NAs and lysozyme and provide information about the toxicity risk of NAs to human health.