Study of caffeine binding to human serum albumin using optical spectroscopic methods

The binding of caffeine to human serum albumin (HSA) under physiological conditions has been studied by the methods of fluorescence, UV-vis absorbance and circular dichroism (CD) spectroscopy. The mechanism of quenching of HSA fluorescence by caffeine was shown to involve a dynamic quenching procedure. The number of binding sites n and apparent binding constant K _b were measured by the fluorescence quenching method and the thermodynamic parameters ΔH, ΔG, ΔS were calculated. The results indicate that the binding is mainly enthalpy-driven, with van der Waals interactions and hydrogen bonding playing major roles in the reaction. The distance r between donor (HSA) and acceptor (caffeine) was obtained according to the Förster theory of non-radiative energy transfer. Synchronous fluorescence, CD and three-dimensional fluorescence spectroscopy showed that the microenvironment and conformation of HSA were altered during the reaction.     

Studies of the Interaction Between Ronidazole and Human Serum Albumin by Spectroscopic and Molecular Docking Methods

Ronidazole (RNZ) is widely used for the therapeutic treatment of farmed animals and is suspected of being a human carcinogen and mutagen. The interaction between RNZ and human serum albumin (HSA) was investigated systematically by fluorescence spectroscopy, synchronous fluorescence, three-dimensional fluorescence, CD spectroscopy, UV–vis absorption spectroscopy and a molecular docking study. The results indicate that the probable quenching mechanism of HSA by RNZ is dynamic quenching. The corresponding thermodynamic parameters, such as ΔH, ΔS and ΔG, etc., were calculated according to the van’t Hoff equation. The results indicate that the forces acting between RNZ and HSA are mainly hydrogen bonds and van der Waals forces. The conformational changes in the interaction were studied by synchronous fluorescence, CD spectroscopy and three-dimensional fluorescence spectra. The results reveal that the microenvironment and conformation of HSA has been changed. A molecular modeling study further confirmed the binding mode obtained by the experimental studies.     

Interaction Between Plumbagin and Human Serum Albumin by Fluorescence Spectroscopy

The interaction of plumbagin (PLU) with human serum albumin (HSA) in physiological buffer (pH=7.4) was studied by fluorescence spectroscopy. Results obtained from analysis of the fluorescence spectra indicated that PLU has a strong ability to quench the intrinsic fluorescence of HSA through a static quenching procedure. Fluorescence quenching data revealed that the quenching constants (K) are 4.43×10⁴, 3.26×10⁴ and 1.69×10⁴ L?mol^?1 at 293, 303 and 313 K, respectively. The thermodynamic parameters ΔH° and ΔS° were calculated to be ?36.63 kJ?mol^?1, and ?35.702 J?mol^?1?K^?1 respectively, which suggested that van der Waals interactions and hydrogen bonds play a major role in the interaction of PLU with HSA. The distance between donor (HSA) and acceptor (PLU) was calculated to be 3.76 nm based on Förster’s non-radiative energy transfer theory. The results of synchronous fluorescence spectra showed that binding of PLU to HSA can induce conformational changes in HSA.     

In Vitro Binding of Furadan to Bovine Serum Albumin

Under physiological conditions, the interaction between furadan (FRD) and bovine serum albumin (BSA) was investigated by spectroscopy including fluorescence emission, UV-visible absorption, scattering, circular dichroism (CD) spectra, synchronous and three-dimensional fluorescence spectra. The observed binding constant K _b and the number of binding sites n were determined by the fluorescence quenching method. The distance r between donor (BSA) and acceptor (FRD) was obtained according to the Förster theory of non-radiation energy transfer. The enthalpy change (ΔH ^θ ), Gibbs energy change (ΔG ^θ ) and entropy change (ΔS ^θ ) at four different temperatures were calculated. The process of binding was proposed to be a spontaneous process since the ΔG ^θ values were negative. The positive ΔS ^θ and ΔH ^θ values indicated that the interaction of FRD and BSA was driven mainly by hydrophobic interactions. The addition of FRD to BSA solutions leads to enhancement in scattering intensity, exhibiting the formation of an aggregate in solution. CD spectra, synchronous and three-dimensional fluorescence spectra were used to measure the structural change of BSA molecules with FRD present.     

Study of the Binding of Herbacetin to Bovine Serum Albumin by Fluorescence Spectroscopy

In this paper, the interaction between herbacetin and BSA was investigated by fluorescence and three-dimensional fluorescence spectroscopy under simulated physiological conditions. It was proved that the fluorescence quenching of BSA by herbacetin was mainly the result of the formation of a herbacetin–BSA complex. The modified Stern–Volmer quenching constant and the corresponding thermodynamic parameters ΔH ⁰, ΔG ⁰ and ΔS ⁰ were calculated at different temperatures. The results indicated that electrostatic interactions were the predominant intermolecular forces in stabilizing the complex. The distance r=3.23 nm between the donor (BSA) and acceptor (herbacetin) was obtained according to Förster’s nonradioactive energy transfer theory. The synchronous fluorescence and three-dimensional fluorescence spectra results showed that the hydrophobity of amino acid residues increased in the presence of herbacetin. These results revealed that the microenvironment and conformation of BSA changed during the binding reaction.     

Investigating the interaction of juglone (5-hydroxy-1, 4-naphthoquinone) with serum albumins using spectroscopic and in silico methods

The interaction between juglone at the concentration range of 10–110 µM and bovine serum albumin (BSA) or human serum albumin (HSA) at the constant concentration of 11 µM was investigated by fluorescence and UV absorption spectroscopy under physiological-like condition. Performing the experiments at different temperatures showed that the fluorescence intensity of BSA/HSA was decreased in the presence of juglone by a static quenching mechanism due to the formation of the juglone–protein complex. The binding constant for the interaction was in the order of 10³ M^?1, and the number of binding sites for juglone on serum albumins was determined to be equal to one. The thermodynamic parameters including enthalpy (ΔH), entropy (ΔS) and Gibb’s free energy (ΔG) changes were obtained by using the van’t Hoff equation. These results indicated that van der Waals force and hydrogen bonding were the main intermolecular forces stabilizing the complex in a spontaneous association reaction. Moreover, the interaction of BSA/HSA with juglone was verified by UV absorption spectra and molecular docking. The results of synchronous fluorescence, UV–visible and CD spectra demonstrated that the binding of juglone with BSA/HSA induces minimum conformational changes in the structure of albumins. The increased binding affinity of juglone to albumin observed in the presence of site markers (digoxin and ibuprofen) excludes IIA and IIIA sites as the binding site of juglone. This is partially in agreement with the results of molecular docking studies which suggests sub-domain IA of albumin as the binding site.     

Insight into the binding evaluation of two antitumor Pd(II) complexes with human serum albumin

The interaction between two novel water-soluble palladium(II) complexes (Pd(bpy)(pyr-dtc)]NO₃, complex I and ([Pd(phen)(pyr-dtc)]NO₃, complex II, where bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline and pyr-dtc = pyrrolidinedithiocarbame) and human serum albumin (HSA) was investigated by fluorescence quenching spectroscopy, synchronous, fluorescence resonance energy transfer (FRET) and three-dimensional fluorescence combined with UV–Vis absorption spectroscopy and circular dichroism technique under simulative physiological conditions. Fluorescence analysis demonstrated that the quenching mechanism of HSA by Pd(II) complexes was static fluorescence quenching and hydrogen bonds and van der Waals interactions were the main intermolecular force based on thermodynamic data. The HSA–Pd(II) complex interaction had a high affinity of 10⁵ M^?1, and the number of binding sites n is almost 1. The results of synchronous fluorescence, three-dimensional fluorescence spectra, UV–Vis absorption and CD spectroscopy indicated that these two complexes may induce the microenvironment around the tryptophan residues and the conformation of human serum albumin. The binding distance (r) in the interaction between Pd(II) complex and HSA was estimated by the efficiency of fluorescence resonance energy transfer (FRET). Furthermore, results from multiple spectroscopic studies are consistent and indicate that the antitumor Pd(II) complexes can efficiently bind with human serum albumin molecules, providing a reasonable model that can help in understanding the design, transportation and toxic effects of anticancer agents.     

Characterization of the interaction between a platinum(II) complex and human serum albumin: spectroscopic analysis and molecular docking

In this study, the interaction between (2,2?-bipyridine)(pyrrolidinedithiocarbamato) platinum(II) nitrate, [Pt(bpy)(pyr-dtc]NO₃, and human serum albumin (HSA) was investigated by various spectroscopic methods (UV–vis, fluorescence, CD and FT-IR) and molecular docking technique at three temperatures. UV–vis absorption spectroscopy showed that Pt(II) complex can denature the protein at moderate concentrations. The results of emission quenching at two temperatures has revealed that the quenching mechanism of Pt(II) complex with HSA was static quenching mechanism. Binding constants (K), binding site number (n) and corresponding thermodynamic parameters ?G?, ?H? and ?S? were calculated and revealed that hydrophobic forces played a major role when Pt(II) complex interacted with HSA. The binding distance (r) between above complex and HSA based on Förster?s theory of non-radiation energy transfer was calculated as 3.22 nm. Alterations of HSA secondary structure induced by complex were confirmed by FT-IR and CD measurements. Also, a molecular docking study was performed for identification of key structural features of binding of the Pt complex into the receptor and predicting bioactive conformers. Our results may provide valuable information to understand the mechanistic pathway of drug delivery and to pharmacological behavior of drug.     

Spectroscopic Studies on the Interaction of Vitamin C with Bovine Serum Albumin

The mechanism of binding of vitamin C (VC) with bovine serum albumin (BSA) was investigated by spectroscopic methods under simulated physiological conditions. VC effectively quenched the intrinsic fluorescence of BSA. The binding constants K _A, and the number of binding sites, n, and corresponding thermodynamic parameters ΔG ^Θ , ΔH ^Θ and ΔS ^Θ between VC and BSA were calculated at different temperatures. The primary binding pattern between VC and BSA was interpreted as being a hydrophobic interaction. The interaction between VC and BSA occurs through static quenching and the effect of VC on the conformation of BSA was also analyzed using synchronous fluorescence spectroscopy. The average binding distance, r, between the donor (BSA) and acceptor (VC) was determined based on Förster’s theory and was found to be 3.65 nm. The effects of common ions on the binding constant of VC-BSA were also examined.     

Computational and experimental study on the interaction of three novel rare earth complexes containing 2,9-dimethyl-1,10-phenanthroline with human serum albumin

In this paper, several rare earth [terbium(III), ytterbium(III) and yttrium(III)] complexes containing 2,9-dimethyl-1,10-phenanthroline (Me₂Phen) were successfully synthesized and characterized by means of elemental analysis (CHN), infrared spectroscopy (FT-IR), UV–vis absorption spectroscopy and ¹HNMR. To explore the potential medicinal value of these complexes (MMe₂Phen), their binding interactions with human serum albumin (HSA) were investigated through UV–vis and fluorescence spectroscopies and also molecular docking examinations. The thermodynamic parameters, binding forces and Förster resonance distance between these complexes and Trp-214 of HSA were estimated from the analysis of fluorescence measurements. The values of estimated binding constants (K_b) ranging for the formation of MMe₂Phen:HSA complex were in the order of 10⁵ M^?1. The thermodynamic parameters determined by van’t Hoff analysis of K_b (ΔH°?<?0 and ΔS°?<?0) clearly indicate the major rules of hydrogen bonds and van der Waals interactions in the formation process of MMe₂Phen:HSA. The values of Stern–Volmer constant and the evaluation of dynamic quenching constant at various temperatures provided good evidences for static quenching mechanism. Furthermore, the results of molecular docking calculation and competitive binding experiments represent the binding of these complexes to site 3 of HSA located in subdomain IB, containing both polar and apolar residues. The consistency of computational and experimental results, according to the binding sites and the order of binding affinities (TbMe₂Phen?>?YbMe₂Phen?>?YMe₂Phen), supports the accuracy of docking calculation.     

Structural and Thermodynamic Investigation into the Protein-Binding Properties of a Natural Product Crytotanshinone

Crytotanshinone (CTSO) is a Chinese herbal medicine active ingredient isolated from Salvia miltiorrhiza. In this work, the interaction of CTSO and human serum albumin (HSA) was studied by fluorescence spectra, ultraviolet spectra, circular dichroism (CD) spectra, molecular probe and molecular modeling methods. The results showed that the endogenous fluorescence of HSA was quenched by CTSO through a static mechanism. The number of binding sites, equilibrium constants, and thermodynamic parameters of the reaction were calculated at three different temperatures. The positive enthalpy change (ΔH^θ) and entropy change (ΔS^θ) revealed that the interaction was an endothermic as well as an entropy-driven process, where hydrophobic power played the major role in stabilizing the structure of the new complex. Site-selective binding experiments were carried out using warfarin and ibuprofen as probes, which proved that CTSO binds to Sudlow’s site II in subdomain IIIA of the HSA molecule. Circular dichroism (CD) spectra was employed to detect the α-helix and β-strand contents in HSA before and after the binding of CTSO. Based on the experimental results, the structure of the CTSO–HSA complex was calculated by docking CTSO to the proven site using molecular modeling. The study obtained comprehensive information on structure and thermodynamics, which is essential for understanding the bioaffinity, delivery process and pharmacological mechanism.     

Spectroscopic and Isothermal Titration Calorimetry Studies of Binding Interactions Between Carbon Nanodots and Serum Albumins

Carbon nanodots (C-dots) have attracted great attention as a new class of luminescent nanomaterials. In order to better understand the basic behavior of C-dots in biological systems, the binding characteristics of C-dots with bovine serum albumin (BSA) and human serum albumin (HSA) were investigated using spectroscopic approaches and isothermal titration calorimetry at pH 7.4. We found that the intrinsic fluorescence of BSA and HSA was quenched by the C-dots with a dynamic quenching mode. It was proved that the C-dots had little influence on the conformation of BSA and HSA by their UV–vis and circular dichroism spectra. Some important thermodynamic parameters were calculated, and the positive values of ΔH° and ΔS° indicate that the binding process was endothermic, and that the interaction was driven by favorable entropy and unfavorable enthalpy. It also showed that the hydrophobic force played a major role in the binding process.     

Fluorometric and molecular modeling deciphering the non-covalent interaction between cyromazine and human serum albumin

Cyromazine (CMZ) had been believed to be one of the safest pesticides and widely used for many years until its carcinogenesis was revealed recently. In this work, the interaction between cyromazine and human serum albumin (HSA) was systematically investigated by multiple spectroscopic methods and molecular docking techniques using warfarin and flufenamic acid as probes. The results demonstrated the fluorescence of HSA had been quenched by CMZ through static mechanism, with new non-covalent complexes formed at ground state. Fluorescence probe experiments indicated CMZ bound to Sudlow’s site I in subdomain IIA of HSA, having no competition with site marker in site II. The number of binding sites, equilibrium constants and thermodynamic parameters were calculated by monitoring the binding equilibriums at different temperatures. The positive enthalpy change (ΔH ^θ) and entropy change (ΔS ^θ) implied the binding was mainly conducted by hydrophobic interactions. The binding was an endothermic, spontaneous (ΔG ^θ?<?0) and entropy-driven process which made the energy distribution of the system more evenly. The force of interaction and the conformation of binding pocket were displayed by molecular simulation and discussed at molecular level. Circular dichroism (CD) spectra indicated distorted α-helix content of HSA decreased while other fine secondary structure increased when CMZ was added.     

Thermodynamics of the complexation of arabinogalactan with salicylic and <Emphasis Type="Italic">p</Emphasis>-aminobenzoic acids in aqueous solutions

The thermodynamics of complexation of arabinogalactan with salicylic and p-aminobenzoic acids in aqueous solutions is studied by means spectroscopy. The standard thermodynamic characteristics (ΔH°; ΔG°; ΔS°) of complexation are calculated.     

Solubility and Solution Thermodynamic Properties of 4-(4-Hydroxyphenyl)-2-butanone (Raspberry Ketone) in Different Pure Solvents

The solubility of 4-(4-hydroxyphenyl)-2-butanone (raspberry ketone) in six pure solvents was experimentally determined at temperatures ranging from 283.15 to 313.15 K under the pressure 0.10 MPa by employing a gravimetrical method. The experimental results indicate that the solubility of raspberry ketone in all studied solvents is temperature dependent, a rise in temperature brings about an increase in solubility. The experimental solubility data of raspberry ketone in six pure solvents (acetone, ethanol, ethyl acetate, n-propyl alcohol, n-butyl alcohol, and distilled water) was correlated by using several commonly used thermodynamic models, including the Apelblat, van’t Hoff and λh equations. The results of the error analysis indicate that the van’t Hoff equation was able to give more accurate and reliable predictions of solubility with root-mean-square deviation less than 0.56%. Furthermore, the changes of dissolution enthalpies (Δ_diss H°), dissolution entropies (Δ_diss S°) and dissolution Gibbs energies (Δ_diss G°) of raspberry ketone in the solvents studied were estimated by the van’t Hoff equation. The positive value of Δ_diss H°, Δ_diss S°, and Δ_diss G° indicated that these dissolution processes of raspberry ketone in the solvents studied were all endothermic and enthalpy-driven.     

Binding interaction and conformational changes of human serum albumin with ranitidine studied by spectroscopic and time-resolved fluorescence methods

This study was designed to examine the interaction of histamine H₂-receptor antagonist drug ranitidine (RTN) with human serum albumin by multi-spectroscopic methods. The experimental results showed the involvement of dynamic quenching mechanism which was further confirmed by lifetime spectral studies. The binding constants (K _a) at three temperatures (288, 298, and 308 K) were 2.058 ± 0.020, 4.160 ± 0.010 and 6.801 ± 0.011 × 10⁴ dm³ mol^?1, respectively, and the number of binding sites (m) were 1.169, respectively; thermodynamic parameters ΔH ⁰ (44.152 ± 0.047 kJ mol^?1), ΔG ⁰ (?26.214 ± 0.040 kJ mol^?1), and ΔS ⁰ (236.130 ± 0.025 J K^?1 mol^?1) were calculated. The distance r between donor and acceptor was obtained (r = 3.40 nm) according to the Förster theory of non-radiative energy transfer. Synchronous fluorescence, CD, AFM and 3D fluorescence spectral results revealed the changes in secondary structure of the protein upon interaction with RTN. A molecular modeling study further confirmed the binding mode obtained by the experimental studies.     

Crystal structure and spectral characteristics of Bis(1-adamantyl acetoacetato)copper(II)

The structure of a new complex bis(1-adamantyl acetoacetato)copper(II) was studied by X-ray crystallography, electronic absorption spectroscopy in the visible, and vibrational spectroscopy. Copper atoms have a square coordination in both solutions and crystals. The crystals are triclinic, space group P \(\bar 1\), a = 6.9389(6) Å, b = 9.0697(8) Å, c = 11.4280(10) Å, α = 109.279(4) °, β = 100.179(4)°, γ = 98.171(4)°, V = 652.45(10) ų, Z = 2. Molecules form the crystals only due to van der Waals forces, because Cu-C intermolecular interactions, which are characteristic of other β-dicarbonyl complexes of copper(II), are absent.     

Mixed-Ligand Complexation of Zinc and Cobalt(II) Complexonates with Amino Acids in an Aqueous Solution

The formation of mixed-ligand complexes in the M(II)–Nta and Ida–L systems (M = Co, Zn; L = His, Orn, Lys, Gly, Im, en), where Ida and Nta are the residues of iminodiacetic and nitrilotriacetic acids, was studied by pH-metry, calorimetry, and NMR spectroscopy. The thermodynamic parameters (logK, Δ _r G⁰, Δ _r H, Δ _r S) of formation for these complexes were determined at 298.15 K and an ionic strength I = 0.5 (KNO3). The most probable pattern of coordination between a complexone and an amino acid in mixed-ligand complexes was revealed.     

Hydrophobic Interaction Between Domain I of Albumin and B Chain of Detemir May Support Myristate-Dependent Detemir-Albumin Binding

The bindings of detemir [LysB29(Nε-tetradecanoyl)des(B30)-insulin] with two highly homologous albumins, HSA (human serum albumin) and BSA (bovine serum albumin), were investigated through CD, spectrofluorophotometry, and molecular docking analysis. The absence of any tryptophanyl residue in detemir makes albumin binding study possible by exclusive tryptophanyl spectral quenching at 340 nm (λem = 296 nm). The interactions found to be static (Kq > 10¹⁰ M^?1 s^?1) with Stern–Volmer constants ≈10³ M^?1. The observed ΔG ⁰ that was negative in all cases concludes the reactions were spontaneous. Domains I and III of an albumin unfold with 5.0 M urea at pH 7.4, although domain II remains intact. Significant decreases in ΔH ⁰ and ΔS ⁰ were due to unfolding explicit that detemir binding may involve domains I and III of albumins. Temperature-dependent changes in binding were higher in HSA than BSA but after unfolding such changes were very less, further indicating the role of domains I and III in detemir binding. Pro28 and Tyr26 of insulin were found to be interacting with Arg114 and Val116 of HSA domain I, while myristate segment of detemir binds to Lys519 of domain III. Interactions seem to be predominantly hydrophobic and entropy driven. Although detemir binds to albumin through myristate, the peptide part shows involvement in binding.     

Characterization of Phenosafranine–Hemoglobin Interactions in Aqueous Solution

Binding of the drug phenosafranine to hemoglobin (Hb) in aqueous solutions was investigated by fluorescence, UV/vis and circular dichroism (CD) spectral methods at pH=7.4. The fluorescence data showed that fluorescence quenching of Hb by phenosafranine is the result of formation of a phenosafranine–Hb complex with a 1:1 molar ratio. Thermodynamic analysis implied that hydrophobic, electrostatic and hydrogen bond interactions are all involved in stabilizing the complex. The molecular distance (r=4.29 nm) between the donor (Hb) and acceptor (phenosafranine) was calculated according to Förster’s theory. The features of phenosafranine-induced secondary structure changes of Hb have been studied by synchronous fluorescence, CD and three-dimensional fluorescence spectroscopy. This study improves our knowledge of the interaction dynamics of phenazinium drugs to the physiologically important protein Hb.