Investigating the binding of curcumin derivatives to bovine serum albumin

Assessing Partial Inhibition of Ribonuclease A Activity by Curcumin through Fluorescence Spectroscopy and Theoretical Studies

Molecular interactions and controlled expression of enzymatic activities are fundamental to all cellular functions in an organism. The active polyphenol in turmeric known as curcumin (CCM) is known to exhibit diverse pharmacological activities. Ribonucleases (RNases) are the hydrolytic enzymes that plays important role in ribonucleic acid (RNA) metabolism. Uncontrolled and unwanted cleavage of RNA by RNases may be the cause of cell death leading to disease states. The protein ribonuclease A (RNase A) in the superfamily of RNases cleaves the RNA besides its role in different diseases like autoimmune diseases, and pancreatic disorders. Interaction of CCM with RNase A have been reported along with the possible role of CCM to inhibit the RNase A enzymatic activity. The interaction strength was found to be 104 M-1 order from spectroscopic results. Quenching of RNase A fluorescence by CCM was 104 M-1 order. Non-radiative energy transfer from RNase A (donor) to CCM (acceptor) suggested a distance of 2.42 nm between the donor-acceptor pair. Circular dichroism studies revealed no structural changes in RNase A after binding. Binding-induced conformational variation in protein was observed from synchronous fluorescence studies. Agarose gel electrophoresis revealed a partial inhibition of the RNase A activity by CCM though not significant. Molecular docking and molecular dynamics studies suggested the residues of RNase A involved in the interaction with supporting the experimental finding for the partial inhibition of the enzyme activity. This study may help in designing new CCM analogues or related structures to understand their differential inhibition of the RNase A activity.

Biocompatible curcumin coupled nanofibrous membrane for pathogens sterilization and isolation

Airborne transmission of pathogens is the most probable cause for the spread of respiratory diseases, which can be intercepted by personal protective equipment such as masks. In this study, an efficient antiviral personal protective filter was fabricated by coupling the biocompatible curcumin (CCM) with nanofibrous polytetrafluoroethylene (PTFE) membrane. The CCM extracted from plants was first dissolved in acidified ethanol at a certain pH and temperature to optimize its loading concentration, antiviral activation, and binding forces on the polyethylene terephthalate (PET) support to form a pre-filtration layer at the front section of the filter. Ultrathin PTFE membrane was then fabricated on the antibacterial-antiviral PET support (A-A PET) by controllable heating lamination. This functional layer of the filter exhibits good gas permeance (3423.6 m³/(m²·h·kPa)) and ultrafine particles rejection rate (>98.79%). Moreover, the obtained A-A filter exhibit a high antibacterial rate against a variety of bacteria (E. coli, B. subtilis, A. niger, and Penicillium were 99.84%, 99.02%, 93.60%, 95.23%, respectively). Forthwith virucidal (SARS-CoV-2) efficiency of the A-A filter can reach 99.90% for 5 min. The filter shows good stability after 10 heating cycles, demonstrating its reusability.

Strategies for Improving Bioavailability, Bioactivity, and Physical-Chemical Behavior of Curcumin

Curcumin (CCM) is one of the most frequently explored plant compounds with various biological actions such as antibacterial, antiviral, antifungal, antineoplastic, and antioxidant/anti-inflammatory properties. The laboratory data and clinical trials have demonstrated that the bioavailability and bioactivity of curcumin are influenced by the feature of the curcumin molecular complex types. Curcumin has a high capacity to form molecular complexes with proteins (such as whey proteins, bovine serum albumin, β-lactoglobulin), carbohydrates, lipids, and natural compounds (e.g., resveratrol, piperine, quercetin). These complexes increase the bioactivity and bioavailability of curcumin. The current review provides these derivatization strategies for curcumin in terms of biological and physico-chemical aspects with a strong focus on different type of proteins, characterization methods, and thermodynamic features of protein–curcumin complexes, and with the aim of evaluating the best performances. The current literature review offers, taking into consideration various biological effects of the CCM, a whole approach for CCM-biomolecules interactions such as CCM-proteins, CCM-nanomaterials, and CCM-natural compounds regarding molecular strategies to improve the bioactivity as well as the bioavailability of curcumin in biological systems.

Skin-targeted delivery of extracellular vesicle-encapsulated curcumin using dissolvable microneedle arrays

Therapeutic benefits of curcumin for inflammatory diseases have been demonstrated. However, curcumin's potential as a clinical therapeutic has been hindered due to its low solubility and stability in vivo. We hypothesized that a hybrid curcumin carrier that incorporates albumin-binding and extracellular vesicle (EV) encapsulation could effectively address the current challenges of curcumin delivery. We further postulated that using dissolvable microneedle arrays (dMNAs) for local delivery of curcumin-albumin-EVs (CA-EVs) could effectively control skin inflammation in vivo. Mild sonication was used to encapsulate curcumin and albumin into EVs, and the resulting CA-EVs were integrated into tip-loaded dMNAs. In vitro and in vivo studies were performed to assess the stability, cellular uptake, and anti-inflammatory bioactivity of dMNA-delivered CA-EVs. Curcumin in CA-EVs exhibited at least five-fold higher stability in vitro than naïve curcumin or curcumin-EVs without albumin. Incorporating CA-EVs into dMNAs did not alter their cellular uptake or anti-inflammatory bioactivity. The dMNA embedded CA-EVs retained their bioactivity when stored at room temperature for at least 12 months. In rat and mice models, dMNA delivered CA-EVs suppressed and significantly reduced lipopolysaccharide and Imiquimod-triggered inflammation. We conclude that dMNA delivery of CA-EVs has the potential to become an effective local-delivery strategy for inflammatory skin diseases. STATEMENT OF SIGNIFICANCE: We introduce and evaluate a skin-targeted delivery system for curcumin that synergistically combines albumin association, extracellular-vesicle encapsulation, and dissolvable microneedle arrays (dMNAs) . In vitro, curcumin-albumin encapsulated extracellular vesicles (CA-EVs) inhibit and reverse the LPS-triggered expression of inflammatory transcription factor NF-κB. The integration of CA-EVs into dMNAs does not affect them physically or functionally. Importantly, dMNAs extend EV storage stability for at least 12 months at room temperature with minimal loss in their bioactivity. We demonstrate that dMNA delivered CA-EVs effectively block and reverse skin inflammation in vivo in mouse and rat models.

The potency of heterocyclic curcumin analogues: An evidence-based review

Curcumin, a potent phytochemical, has been a significant lead compound and has been extensively investigated for its multiple bioactivities. Owing to its natural origin, non-toxic, safe, and pleiotropic behavior, it has been extensively explored. However, several limitations such as its poor stability, bioavailability, and fast metabolism prove to be a constraint to achieve its full therapeutic potential. Many approaches have been adopted to improve its profile, amongst which, structural modifications have indicated promising results. Its symmetric structure and simple chemistry have prompted organic and medicinal chemists to manipulate its arrangement and study its implications on the corresponding activity. One such recurring and favorable modification is at the diketo moiety with the aim to achieve isoxazole and pyrazole analogues of curcumin. A modification at this site is not only simple to achieve, but also has indicated a superior activity consistently. This review is a comprehensive and wide-ranged report of the different methods adopted to achieve several cyclized curcumin analogues along with the improvement in the efficacy of the corresponding activities observed.

Characterization of the binding of cyanidin-3-glucoside to bovine serum albumin and its stability in a beverage model system: A multispectroscopic and chemometrics study

Anthocyanins as one of the main natural groups of food colorants undergo quick color fading, which can be diminished through protein association. The stabilization of cyanidin-3-glucoside (CYG) through binding to bovine serum albumin (BSA) was investigated at pH 3.0 using atomic force microscopy and differential scanning calorimetry along with UV-Vis absorption, steady-state fluorescence, circular dichroism, and three-dimensional emission spectral analyses merged with the multivariate curve resolution-alternative least square method. The stabilized CYG molecules were found at the site II of BSA with combined static and dynamic quenching mechanisms. Approximately 93% of the BSA binding sites were occupied in the BSA-CYG complex through hydrogen bonds and van der Waals forces with the binding constant and stoichiometry ratio of 1.88 × 10⁵ M^-1 and 1:13, respectively. The results also revealed that CYG molecules caused partial unfolding of the BSA structure, while it was not enough for significant alteration of denaturation temperature. The binding results also indicated that the reduction of H₂O₂-induced-CYG oxidation rate (34.78%) at pH 3.0 was mainly driven via the BSA-hemiketal association, although the colored species of CYG had a greater affinity towards BSA in the equilibrated system at pHs 1.0 and 5.0.

Microbubble-Mediated Enhanced Delivery of Curcumin to Cervical Cancer Cells

The major bottleneck in the current chemotherapy treatment of cancer is the low bioavailability and high cytotoxicity. Targeted delivery of drug to the cancer cells can reduce the cytotoxicity and increase the bioavailability. In this context, microbubbles are currently being explored as drug-delivery vehicles to effectively deliver drug to the tumors or cancerous cells. Microbubbles when used along with ultrasound can enhance drug uptake and inhibit the growth of tumor cells. Several potential anticancer molecules exhibit poor water solubility, which limits their use in therapeutic applications. Such poorly water soluble molecules can be coadministered with microbubbles or encapsulated within or loaded on the microbubbles surface, to enhance the effectiveness of these molecules against cancer cells. Curcumin is one of such potential anticancer molecules obtained from the rhizome of herbal spice, turmeric. In this work, curcumin-loaded protein microbubbles were synthesized and examined for effective in vitro delivery of curcumin to HeLa cells. Microbubbles in the size range of 1-10 μm were produced using perfluorobutane as core gas and bovine serum albumin (BSA) as shell material and were loaded with curcumin. The amount of curcumin loaded on the microbubble surface was estimated using UV-vis spectroscopy, and the average curcumin loading was found to be ∼54 μM/10⁸ microbubbles. Kinetics of in vitro curcumin release from microbubble surface was also estimated, where a 4-fold increase in the rate of curcumin release was obtained in the presence of ultrasound. Sonication and incubation of HeLa cells with curcumin-loaded BSA microbubbles enhanced the uptake of curcumin by ∼250 times. Further, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay confirmed ∼71% decrease in cell viability when HeLa cells were sonicated with curcumin-loaded microbubbles and incubated for 48 h.

Domain-Specific Association of a Phenanthrene-Pyrene-Based Synthetic Fluorescent Probe with Bovine Serum Albumin: Spectroscopic and Molecular Docking Analysis

In this report, the interaction between a phenanthrene-pyrene-based fluorescent probe (PPI) and bovine serum albumin (BSA), a transport protein, has been explored by steady-state emission spectroscopy, fluorescence anisotropy, far-ultraviolet circular dichroism (CD), time-resolved spectral measurements, and molecular docking simulation study. The blue shift along with emission enhancement indicates the interaction between PPI and BSA. The binding of the probe causes quenching of BSA fluorescence through both static and dynamic quenching mechanisms, revealing a 1:1 interaction, as delineated from Benesi-Hildebrand plot, with a binding constant of ∼10⁵ M^-1, which is in excellent agreement with the binding constant extracted from fluorescence anisotropy measurements. The thermodynamic parameters, ΔH°, ΔS°, and ΔG°, as determined from van't Hoff relationship indicate the predominance of van der Waals/extensive hydrogen-bonding interactions for the binding phenomenon. The molecular docking and site-selective binding studies reveal the predominant binding of PPI in subdomain IIA of BSA. From the fluorescence resonance energy transfer study, the average distance between tryptophan 213 of the BSA donor and the PPI acceptor is found to be 3.04 nm. CD study demonstrates the reduction of α-helical content of BSA protein on binding with PPI, clearly indicating the change of conformation of BSA.

Impact of multiple quaternary ammonium salts on dynamic properties of BSA adsorption layer at different pH values

The interaction mechanism of multiple quaternary ammonium salts (MQAS) with bovine serum albumin (BSA) was examined by the fluorescence quenching method and circular dichroism (CD) spectra. Moreover, the effects of MQAS on the dynamic properties of BSA adsorption layers at different pH values were investigated using dilational interfacial rheology. Results show that the quenching constants increase with an increase in pH values and decrease with an increase in the experiment temperature at pH 5.3. The quenching mechanism is static quenching, and the electrostatic force dominates the interaction between MQAS and BSA at pH 5.3. Due to three positive head groups, MQAS can significantly affect the dynamic interfacial activity of BSA molecules at a relatively low concentration. At pH 4.3, the electrostatic repulsion is unfavorable for the formation of MQAS/BSA complexes. Consequently, MQAS molecules will replace BSA molecules from the interface by competitive adsorption. At the pH value above the isoelectric point of BSA, the electrostatic attraction is better for the formation of MQAS/BSA complexes, which exhibit a rapid adsorption rate and an enhanced interfacial activity. Moreover, the kinetic dependencies of interfacial dilational elasticity for the MQAS/BSA mixtures become nonmonotonous. The appearance of the maximum interfacial elasticity values can be attributed to the formation of tails and loops, which suggests that the addition of MQAS destroys the secondary and tertiary structure of protein in the bulk phase. In addition, the effects of MQAS on the secondary structure of protein were demonstrated by CD spectra.

Transient spectra study on photo-dynamics of curcumin

A novel mechanism of DNA damage induced by photosensitized curcumin (Cur) was explored using laser flash photolysis, pulse radiolysis and gel electrophoresis. Cur neutral radical (Cur) was confirmed as an identical product in photo-sensitization of Cur by laser flash photolysis and pulse radiolysis. A series of reaction rate constants between Cur and nucleic acid bases/nucleotides were determined by pulse radiolysis. Gel electrophoresis was carried out to investigate damage induced by photosensitized Cur to biologically active DNA. The results indicate that the damage to DNA may be caused by Cur produced from the photosensitization of Cur.

Effects of urea, metal ions and surfactants on the binding of baicalein with bovine serum albumin

The interaction of baicalein with bovine serum albumin (BSA) was investigated with the help of spectroscopic and molecular docking studies. The binding affinity of baicalein towards BSA was estimated to be in order of 10⁵ M⁻¹ from fluorescence quenching studies. Negative ΔH° (−5.66±0.14 kJ/mol) and positive (ΔS°) (+79.96±0.65 J/mol K) indicate the presence of electrostatic interactions along with the hydrophobic forces that result in a positive ΔS°. The hydrophobic association of baicalein with BSA diminishes in the presence of sodium dodecyl sulfate (SDS) due to probable hydrophobic association of baicalein with SDS, resulting in a negative ΔS° (−40.65±0.87 J/mol K). Matrix-assisted laser desorption ionization/time of flight (MALDI--TOF) experiments indicate a 1:1 complexation between baicalein and BSA. The unfolding and refolding phenomena of BSA were investigated in the absence and presence of baicalein using steady-state and fluorescence lifetime measurements. It was observed that the presence of urea ruptured the non-covalent interaction between baicalein and BSA. The presence of metal ions (Ag⁺, Mg²⁺, Ni²⁺, Mn²⁺, Co²⁺and Zn²⁺) increased the binding affinity of ligand towards BSA. The changes in conformational aspects of BSA after ligand binding were also investigated using circular dichroism (CD) and Fourier transform infrared (FT-IR) spectroscopic techniques. Site selectivity studies following molecular docking analyses indicated the binding of baicalein to site 1 (subdomain IIA) of BSA.

Multispectroscopic analysis and molecular modeling to investigate the binding of beta lactoglobulin with curcumin derivatives

Bovine beta lactoglobulin (β-lg), the major whey protein, has a great affinity for a wide range of organic compounds like fatty acids, retinoletc.

Inhibition of amyloid fibrillation of hen egg-white lysozyme by the natural and synthetic curcuminoids

As one of the strategies for synthesis of novel amyloid inhibitors, chemical modification of the natural curcuminoids framework can be introduced.

Nanocomplexation between curcumin and soy protein isolate: influence on curcumin stability/bioaccessibility and in vitro protein digestibility

The complexation of nanoparticles in unheated and heated (at 75-95°) soy protein isolate (SPI) with curcumin and the effects on curcumin stability/bioaccessibility and in vitro protein digestibility were investigated. The nanoparticles did not display noticeable changes in size and morphology upon nanocomplexation with curcumin, except their surface hydrophobicity. The encapsulation efficiency of curcumin progressively decreased with increasing initial curcumin concentration in the dispersion, while the load amount linearly increased. The solubility of curcumin in water was enhanced by the complexation above 98000-fold (vs free curcumin in water). The formation of the nanocomplexes considerably improved the storage stability of curcumin. In vitro simulated digestion experiments indicated that the complexation also improved the bioaccessibility of curcumin; the bioaccessibility was greatly impaired by hydrolysis-induced protein aggregation. Addtionally, the nanocomplexation significantly improved the in vitro protein digestibility of both unheated and heated SPI.

Effect of processing on physicochemical characteristics and bioefficacy of β-lactoglobulin-epigallocatechin-3-gallate complexes

Varying amounts of epigallocatechin-3-gallate (EGCG) were encapsulated in β-lactoglobulin (β-Lg) nanoparticles, either native or processed, denoted as heated or desolvated protein. The stability, physical properties, and bioactivity of the β-Lg-EGCG complexes were tested. Native β-Lg-EGCG complexes showed comparable stability and binding efficacy (EGCG/β-Lg molar ratio of 1:1) to heated β-Lg nanoparticles (1% and 5% protein w/w). The sizes of heated and desolvated β-Lg nanoparticles were comparable, but the latter showed the highest binding affinity for EGCG. The presence of EGCG complexed with β-Lg did not affect the interfacial tension of the protein when tested at the soy oil-water interface but caused a decrease in dilational elasticity. All β-Lg complexes (native, heated, or desolvated) showed a decrease in cellular proliferation similar to that of free ECGC. In summary, protein-EGCG complexes did not alter the bioefficacy of EGCG and contributed to increased stability with storage, demonstrating the potential benefits of nanoencapsulation.

Photo-ionization and photo-excitation of curcumin investigated by laser flash photolysis

Curcumin (Cur) has putative antitumor properties. In the current study, we examined photophysical and photochemical properties of Cur using laser flash photolysis. The results demonstrated that Cur could be photo-ionized at 355 nm laser pulse to produce radical cation (Cur(+)) and solvated electron e(sol)(-) in 7:3 ethanol-water mixtures. The quantum yield of Cur photo-ionization and the ratio of photo-ionization to photo-excitation were also determined. Cur(+) could be transferred into neutral radical of Cur (Cur) via deprotonation with the pKa 4.13. The excited singlet of Cur ((1)Cur* could be transferred into excited triplet ((3)Cur*, which could be quenched by oxygen to produce singlet oxygen (1)O2*. Reaction of (3)Cur* with tryptophan was confirmed. The results encourage developing curcumin as a photosensitive antitumor agent.

Binding of curcumin with bovine serum albumin in the presence of ι-carrageenan and implications on the stability and antioxidant activity of curcumin

This work studied the influences of formation of BSA/ι-carrageenan complexes on the binding, stability, and antioxidant activity of curcumin. In the presence of BSA and ι-carrageenan, curcumin gives higher intensities of absorption and fluorescence than free curcumin and curcumin only combined with BSA. The added ι-carrageenan is observed to promote curcumin for quenching the instrinsic fluorescence of BSA. These results are explained in terms of the formation of BSA/ι-carrageenan complexes, which help to stabilize the folded structure of BSA for providing curcumin with a more hydrophobic microenvironment. The small difference in anisotropy values of curcumin with BSA alone and of BSA/ι-carrageenan complexes suggests that ι-carrageenan acts as outer stretch conformation in BSA/ι-carrageenan complexes but does not directly disturb the hydrophobic pockets inside BSA, where curcumin is hydrophobically located. The determined values of the binding constant are higher for curcumin with BSA/ι-carrageenan complexes than with BSA alone. Moreover, BSA/ι-carrageenan complexes are found to be superior to single BSA for enhancing the stability and DPPH radical-scavenging ability of curcumin.

Study of fluorescence quenching of Barley α-amylase

The fluorescence quenching of Barley α-amylase by acrylamide and succinimide has been studied in water using steady-state and time-resolved fluorescence techniques. The steady-state fluorescence quenching technique has been performed in three different pHs (i.e., 6, 7 and 8) of water. Ground state and excited state binding constants (K(g) &K(e)) have been calculated. From the calculated binding constants (K(g) &K(e)) the free energy changes for the ground (ΔG(g)) and excited (ΔG(e)) states have been calculated and are presented in tables. UV and FTIR spectra have also been recorded to prove the binding of Barley α-amylase with acrylamide and succinimide.

A spectroscopic investigations of anticancer drugs binding to bovine serum albumin

The binding of anticancer drugs (i) Uracil (U), (ii) 5-Fluorouracil (5FU) and (iii) 5-Chlorouracil (5ClU), to bovine serum albumin (BSA) at two levels of temperature was studied by the fluorescence of quenching method. UV/Vis, time-resolved fluorescence, Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance ((1)H NMR) and scanning electron microscope (SEM) analyses were also made. Binding constants (K(a)) and binding sites (n) at various levels of temperature were calculated. The obtained binding sites were found to be equal to one for all the three quenchers (U, 5FU and 5ClU) at two different temperature levels. Thermodynamic parameters ΔH, ΔG and ΔS have been calculated and were presented in tables. Change in FTIR absorption intensity shows strong binding of anticancer drugs to BSA. Changes in chemical shifts of NMR and fluorescence lifetimes of the drugs indicate the presence of interaction and binding of BSA to anticancer drugs. (1)H NMR spectra and SEM photographs also conform this binding.

Naphthalene-fused (α-alkoxycarbonyl)methylene-γ-butyrolactones: antiproliferative activity and binding to bovine serum albumin and DNA

A naphthalene-fused (α-alkoxycarbonyl)methylene-γ-butyrolactone (methyl 2-[7-hydroxy-2-oxonaphtho[1,2-b]furan-3(2H)-yliden]acetate) has been prepared as a representative compound of a potential class of cytotoxic agents. In vitro cytotoxicity has been evaluated against HCT-15 colon and MCF-7 breast cancer cells and IC(50) was 64-66 μM, causing morphological changes in cells, such as loss of adhesion, rounding, cell shrinkage, and detachment from the substratum. The binding constant K of the complex between the naphthyl lactone with bovine serum albumin (8 × 10(3) M(-1)) suggests a minor change in protein folding. The K of the binding with DNA (1.06 × 10(4) M(-1)) suggests nonspecific electrostatic interactions with DNA and this was confirmed by melting point data (Tm<0.6 °C). Therefore, naphthalene-fused (α-alkoxycarbonyl)methylene-γ-butyrolactone should not be able to intercalate with DNA but its interaction should occur at the level of DNA surface.

Stereo-selectivity of human serum albumin to enantiomeric and isoelectronic pollutants dissected by spectroscopy, calorimetry and bioinformatics

1-naphthol (1N), 2-naphthol (2N) and 8-quinolinol (8H) are general water pollutants. 1N and 2N are the configurational enantiomers and 8H is isoelectronic to 1N and 2N. These pollutants when ingested are transported in the blood by proteins like human serum albumin (HSA). Binding of these pollutants to HSA has been explored to elucidate the specific selectivity of molecular recognition by this multiligand binding protein. The association constants (K(b)) of these pollutants to HSA were moderate (10(4)-10(5) M(-1)). The proximity of the ligands to HSA is also revealed by their average binding distance, r, which is estimated to be in the range of 4.39-5.37 nm. The binding free energy (ΔG) in each case remains effectively the same for each site because of enthalpy-entropy compensation (EEC). The difference observed between ΔC(p) (exp) and ΔC(p) (calc) are suggested to be caused by binding-induced flexibility changes in the HSA. Efforts are also made to elaborate the differences observed in binding isotherms obtained through multiple approaches of calorimetry, spectroscopy and bioinformatics. We suggest that difference in dissociation constants of pollutants by calorimetry, spectroscopic and computational approaches could correspond to occurrence of different set of populations of pollutants having different molecular characteristics in ground state and excited state. Furthermore, our observation of enhanced binding of pollutants (2N and 8H) in the presence of hemin signifies that ligands like hemin may enhance the storage period of these pollutants in blood that may even facilitate the ill effects of these pollutants.

Multitargeting by curcumin as revealed by molecular interaction studies

Curcumin (diferuloylmethane), the active ingredient in turmeric (Curcuma longa), is a highly pleiotropic molecule with anti-inflammatory, anti-oxidant, chemopreventive, chemosensitization, and radiosensitization activities. The pleiotropic activities attributed to curcumin come from its complex molecular structure and chemistry, as well as its ability to influence multiple signaling molecules. Curcumin has been shown to bind by multiple forces directly to numerous signaling molecules, such as inflammatory molecules, cell survival proteins, protein kinases, protein reductases, histone acetyltransferase, histone deacetylase, glyoxalase I, xanthine oxidase, proteasome, HIV1 integrase, HIV1 protease, sarco (endo) plasmic reticulum Ca(2+) ATPase, DNA methyltransferases 1, FtsZ protofilaments, carrier proteins, and metal ions. Curcumin can also bind directly to DNA and RNA. Owing to its β-diketone moiety, curcumin undergoes keto-enol tautomerism that has been reported as a favorable state for direct binding. The functional groups on curcumin found suitable for interaction with other macromolecules include the α, β-unsaturated β-diketone moiety, carbonyl and enolic groups of the β-diketone moiety, methoxy and phenolic hydroxyl groups, and the phenyl rings. Various biophysical tools have been used to monitor direct interaction of curcumin with other proteins, including absorption, fluorescence, Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopy, surface plasmon resonance, competitive ligand binding, Forster type fluorescence resonance energy transfer (FRET), radiolabeling, site-directed mutagenesis, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), immunoprecipitation, phage display biopanning, electron microscopy, 1-anilino-8-naphthalene-sulfonate (ANS) displacement, and co-localization. Molecular docking, the most commonly employed computational tool for calculating binding affinities and predicting binding sites, has also been used to further characterize curcumin's binding sites. Furthermore, the ability of curcumin to bind directly to carrier proteins improves its solubility and bioavailability. In this review, we focus on how curcumin directly targets signaling molecules, as well as the different forces that bind the curcumin-protein complex and how this interaction affects the biological properties of proteins. We will also discuss various analogues of curcumin designed to bind selective targets with increased affinity.

The interaction between 4-aminoantipyrine and bovine serum albumin: multiple spectroscopic and molecular docking investigations

4-Aminoantipyrine (AAP) is widely used in the pharmaceutical industry, in biochemical experiments and in environmental monitoring. AAP as an aromatic pollutant in the environment poses a great threat to human health. To evaluate the toxicity of AAP at the protein level, the effects of AAP on bovine serum albumin (BSA) were investigated by multiple spectroscopic techniques and molecular modeling. After the inner filter effect was eliminated, the experimental results showed that AAP effectively quenched the intrinsic fluorescence of BSA via static quenching. The number of binding sites, the binding constant, the thermodynamic parameters and binding subdomain were measured, and indicated that AAP could spontaneously bind with BSA on subdomain IIIA through electrostatic forces. Molecular docking results revealed that AAP interacted with the Glu 488 and Glu 502 residues of BSA. Furthermore, the conformation of BSA was demonstrably changed in the presence of AAP. The skeletal structure of BSA loosened, exposing internal hydrophobic aromatic ring amino acids and peptide strands to the solution.

Interaction of the anticancer plant alkaloid sanguinarine with bovine serum albumin

Background

Interaction of the iminium and alkanolamine forms of sanguinarine with bovine serum albumin (BSA) was characterized by spectroscopic and calorimetric techniques.

Methodology/Principal Findings

Formation of strong complexes of BSA with both iminium and alkanolamine forms was revealed from fluorescence quenching of sanguinarine. Binding parameters calculated from Stern-Volmer quenching method revealed that the neutral alkanolamine had higher affinity to BSA compared to the charged iminium form. Specific binding distances of 3.37 and 2.38 nm between Trp 212 (donor) and iminium and alkanolamine forms (acceptor), respectively, were obtained from Forster resonance energy transfer studies. Competitive binding using the site markers warfarin and ibuprofen, having definite binding sites, demonstrated that both forms of sanguinarine bind to site I (subdomain IIA) on BSA. Sanguinarine binding alters protein conformation by reducing the α-helical organization and increasing the coiled structure, indicating a small but definitive partial unfolding of the protein. Thermodynamic parameters evaluated from isothermal titration calorimetry suggested that the binding was enthalpy driven for the iminium form but favoured by negative enthalpy and strong favourable entropy contributions for the alkanolamine form, revealing the involvement of different molecular forces in the complexation.

Conclusions/Significance

The results suggest that the neutral alkanolamine form binds to the protein more favourably compared to the charged iminium, in stark contrast to the reported DNA binding preference of sanguinarine.

Circular dichroism and fluorescence spectroscopic study on the interaction of bisdemethoxycurcumin and diacetylbisdemethoxycurcumin with human serum albumin

The interactions of bisdemethoxycurcumin (BDMC) as a bioactive constituent of turmeric and diacetylbisdemethoxycurcumin (DABC) as a novel synthetic derivative of curcumin with human serum albumin (HSA) have been investigated by fluorescence and circular dichroism (CD) spectroscopy. The binding parameters, including the number of substantive binding sites and the binding constants, have been estimated from the analysis of fluorescence measurements. The estimated binding parameters indicated that BDMC has higher affinity than DABC to bind HSA, suggesting the essential role of the phenolic OH groups of BDMC, which are acetylated in DABC. It was found that the binding site for BDMC and DABC is located in the vicinity of Trp-214 in subdomain IIA, which is the same as binding site for curcumin (CUR). The minor changes on the far-UV circular dichroism spectra resulted in partial changes in the calculated secondary structure contents of HSA. The negligible alteration in the secondary structure of HSA indicated that ligand-induced conformational changes are localized in the binding site and do not involve considerable changes in the protein folding. The visible CD spectra indicated that the optical activity observed during the ligand binding is due to induced-protein chirality.

Investigation of interaction of vaccinia virus complement control protein and curcumin with complement components c3 and c3b using quartz crystal microbalance with dissipation monitoring technology

C3 and C3b, the components central to the complement activation, also play a damaging role in several inflammatory disorders. Vaccinia virus complement control protein (VCP) and curcumin (Cur) are natural compounds with different biological origins reported to regulate complement activation. However, both VCP and Cur have not been investigated for their interaction with the third component (C3) prior to it being converted to its activated form (C3b). These two compounds have also not been compared to each other with respect to their interactions with C3 and C3b. Quartz crystal microbalance with dissipation monitoring (QCM-D) is a novel technology used to study the interaction of biomolecules. This technology was applied to characterize the interactions of VCP, Cur and appropriate controls with the key complement components. Cur as well as VCP showed binding to both C3 and to C3b, Cur however bound to C3b to a lesser extent.

Interaction of curcumin and diacetylcurcumin with the lipocalin member beta-lactoglobulin

The binding of curcumin (CUR) and diacetylcurcumin (DAC) to bovine beta-lactoglobulin (BLG) genetic variant B was investigated by fluorescence and circular dichroism techniques. The binding parameters including number of substantive binding sites and the binding constants have been evaluated by fluorescence quenching method. The distance (r) between donor (BLG) and acceptor (CUR and DAC) was obtained according to the Förster's theory of non-radiative energy transfer. The far-UV circular dichroism spectra were used to investigate the possible changes in the secondary structure of BLG in the presence of CUR and DAC and showed that these two ligands change the alpha-helix and random coil contents of this protein to some extent. The visible circular dichroism spectra indicated that the optical activity during the ligand binding was observed due to the induced-protein chirality. All of the achieved results suggested the important role of the phenolic OH group of CUR in the binding process resulted in more affinity of CUR than DAC for binding to BLG.

Effective stabilization of curcumin by association to plasma proteins: human serum albumin and fibrinogen

The use of curcumin as an effective wound healing agent is of significant interest currently. It is well established that curcumin undergoes rapid degradation in physiological buffer by hydrolysis. The means by which curcumin is stabilized at the wound site to enable healing is poorly understood because blood plasma is composed of approximately 92% water. Plasma proteins, which constitute the remaining 6-8%, has been shown to stabilize curcumin. It is, however, still unclear which proteins are responsible for this phenomenon. In this study, the effects of major plasma proteins, which include human serum albumin (HSA), fibrinogen, immunoglobulin G (IgG), and transferrin, on stabilizing curcumin are investigated. In particular, we investigate their effects on the hydrolysis of curcumin at pH 7.4. In the presence of both transferrin and IgG, curcumin continues to undergo rapid hydrolysis but this reaction is suppressed by the presence of either HSA or fibrinogen with an impressive yield of approximately 95%. Furthermore, the binding constants of curcumin to HSA and fibrinogen are on the order of 10(4) and 10(5) M(-1), respectively. The binding constants of transferrin and IgG, however, are at least 1 order of magnitude less than those of HSA and fibrinogen. The results support that strong binding occurs at the hydrophobic moieties of HSA and fibrinogen, excluding water access. Therefore, strong interactions with HSA and fibrinogen inhibit hydrolysis of curcumin and in turn lead to effective suppression of degradation.

Molecular interactions of isoxazolcurcumin with human serum albumin: spectroscopic and molecular modeling studies

Curcumin is a nontoxic natural product with diverse pharmacological potencies. We report the interaction of a potent synthetic derivative of curcumin, isoxazolcurcumin (IOC) with human serum albumin (HSA) using various biophysical methods. The observed fluorescence quenching of HSA by IOC is due to a complex formation by a static quenching process with a quenching constant of the order of 10(5) M(-1). The binding affinity and the number of binding sites were obtained from a Scatchard analysis. Thermodynamics reveals that the interaction is entropy driven with predominantly hydrophobic forces. From the observed Förster-type fluorescence resonance energy transfer (FRET), the donor (Trp 214 in HSA) to acceptor (IOC) distance is calculated to be 3.2 nm. The conformational changes of HSA due to the interaction were investigated qualitatively from synchronous fluorescence spectra along with a quantitative estimation of the secondary structure from Fourier Transform Infrared (FTIR) and circular dichroism (CD) spectroscopies. Molecular docking studies were performed to obtain information on the possible residues involved in the interaction process, and changes in accessible surface area of the interacting residues were calculated. The preferred binding site of IOC was analyzed by ligand displacement experiments with 1-anilino-8-naphthalenesulfonate (ANS) and warfarin-bound HSA.