Bimolecular Quenching Rate Constants Research Articles

Exploring Conformational Changes in β-Lactoglobulin (β-LG) Induced by Silibinin: A comprehensive analysis of polyphenol-protein interaction

This study investigates the comprehensive characterization of the interaction between β-lactoglobulin (β-LG) and Silibinin using various spectroscopic techniques. Fluorescence quenching experiments at different temperatures (298, 303, 308, and 313 K) revealed substantive interactions between β-LG and Silibinin, as indicated by a reduction in fluorescence intensity and a red shift in emission maxima. Further analysis, including Stern-Volmer quenching constants (KSV), bimolecular quenching rate constants (kq), and thermodynamic parameters demonstrated static quenching mechanism and strong binding affinities (Ka range: 0.138–1.483 × 105 M−1) between BLG-Silibinin complex. Thermodynamic study suggested positive enthalpy and entropy changes (ΔHo=43.31 kcal mol−1; ΔSo=164.33 cal mol−1 K−1), suggesting a spontaneous reaction with negative ΔGo values (-5.66 to -7.30 kcal mol−1). Forster resonance energy transfer (FRET) measurements confirmed optimal distances (r and Ro) for FRET occurrence, endorsing the static quenching mechanism. Molecular docking supported these findings, showcasing a 1:1 stoichiometric binding ratio for β-LG: Silibinin. The β-LG and Silibinin complex is primarily stabilized by hydrogen bonds and hydrophobic interactions. Molecular dynamics simulations over 200 ns highlighted stability in the β-LG-Silibinin complex, indicated by RMSD convergence, consistent RMSF values, and compactness illustrated by Rg. Conformational changes in β-LG upon Silibinin binding were further confirmed through UV-Vis absorption spectroscopy, FTIR, far-UV CD, synchronous fluorescence, and 3D fluorescence analyses. Functionally, the antioxidant capacity of β-LG increased after complexation with silibinin as quantified by DPPH assay. These results collectively depict the intricate network of interactions between Silibinin and β-LG, shedding light on the molecular details of their binding and offering insights into potential functional implications in biological contexts.

Anomalous Fluorescence Quenching in Fluorous Solvent-Added Media.

Due to the inherent high electronegativity of fluorine, perfluorocarbons have the potential to exhibit unusual characteristics. Fluorous solvents, in this context, may afford an anomalous solubilizing behavior compared to their hydrocarbon analogues. Addition of perfluorodecalin (PFD) to n-hexane results in unusual fluorescence quenching of polycyclic aromatic hydrocarbons (PAHs) by the quencher nitromethane. As more viscous PFD is added to less viscous n-hexane, the dynamic viscosity (η) of the media increases. The bimolecular quenching rate constants (kq) of the PAHs, instead of decreasing, increase as PFD is added to n-hexane until an equimolar mixture composition is obtained; kq exhibits an expected decrease only in the PFD-rich region of the mixture. The expected decrease in kq in the hydrocarbon analogue decalin added to n-hexane is observed across all compositions. It is proposed that highly electronegative fluorines on PFD stabilize the partial positive charge (δ+) that develops on excited PAHs during electron/charge transfer to the quencher nitromethane, facilitating quenching in the process. In the PFD-rich region, however, increased η starts to dominate the quenching, resulting in the expected decrease in kq. A monotonic decrease in kq is observed in the PFD-added n-hexane system for fluoranthene quenching by triethylamine (TEA) as TEA acts as the electron/charge donor and a partial negative charge (δ-) develops on excited fluoranthene during the quenching process. No such stabilization by PFD is observed when nitrobenzene is employed as the quencher. This is attributed to the significantly higher quenching (KS > 104 M-1) of PAH fluorescence by nitrobenzene due to the presence of aromatic π-π interactions between PAH and nitrobenzene, further facilitated by the higher electron affinity of nitrobenzene as compared to nitromethane. The role of fluorous media in facilitating electron/charge transfer processes is clearly established.

N-Perfluorooctane versus n-Octane: Pyrene Fluorescence to Compare and Contrast Solute Solvation.

Fluorous solvents may offer distinctly different solvation environments to a solute compared to their hydrocarbon analogues due to the inherently high electronegativity associated with fluorine. Solute solvation within n-perfluorooctane (PFO) is compared with that in n-octane using the well-established polycyclic aromatic hydrocarbon (PAH) fluorescence probe pyrene in the temperature range of 288 to 318 K. Both density (ρ) and dynamic viscosity (η) of PFO are considerably higher than those of n-octane. UV-vis molecular absorbance, fluorescence emission/excitation, and excited-state emission intensity decay reveal the cybotactic region of pyrene to be more nonpolar in PFO than that in n-octane. Bimolecular quenching rate constants (kq) for the pyrene-nitrobenzene fluorophore-quencher pair adhere to the Stokes-Einstein formulation; however, they are considerably higher than the estimated rate constants for the diffusion-controlled process (kdiff). This is due to the high electron affinity of nitrobenzene leading to aromatic π-π interactions between pyrene and nitrobenzene. For a nonaromatic low electron affinity quencher, such as nitromethane, while kq < kdiff in n-octane, kq > kdiff in PFO. This is due to the fact that highly electronegative fluorines on PFO stabilize the partial positive charge (δ+) that develops on excited pyrene during electron/charge transfer to the quencher nitromethane, facilitating quenching in the process. Exciplex formation between pyrene and triethylamine (TEA) is more favored in PFO as opposed to n-octane although ηPFO > ηn-octane. The developing charge on the exciplex is stabilized by the electronegative fluorines of the PFO. The pyrene-TEA exciplex appears to form exclusively in the excited state of pyrene, and the kinetics of exciplex formation is in the subnanosecond regime. On the contrary, the efficiency of exciplex formation between pyrene and N,N-dimethylaniline (DMA) is comparable in PFO and n-octane, and the kinetics is slower in comparison to that of the pyrene-TEA exciplex. Certain ground-state heterogeneity is detected for the pyrene-DMA system in PFO due to the low solubilizing ability of the fluorous solvent. Highly electronegative fluorines on perfluorohydrocarbon solvents are found to offer unusual and unique solvation characteristics.

Exploring the molecular binding mechanism of 6-fluoro, 4-hydroxy, 2- methyl quinoline with TiO2 nanoparticles: A spectroscopic, thermodynamic, and insights into the solvatochromic effect.

This study investigates the interaction between titanium oxide nanoparticles (TiO2 NPs) and the heterocyclic fluorophore 6-fluoro,4-hydroxy,2-methylquinoline (6-FHMQ), aiming to understand fluorescence quenching mechanisms and thermodynamic characteristics. Spectroscopic techniques including spectrofluorometry (FL) and spectrophotometry (UV-Vis) were used, with a lifetime decay (τ) of 0.18ns for 6-FHMQ measured using time correlated single photon counting (TCSPC). The interaction between 6-FHMQ and TiO2 NPs revealed a mix of static and dynamic fluorescence quenching mechanisms, with increasing quenching constants (Ksv) and a higher bimolecular quenching rate constant (Kq). The dynamic nature was highlighted by a temperature-dependent increase in binding sites from 1 to ~ 2. Spontaneous complexation was affirmed by negative change in free energy (ΔG), with negative change in enthalpy (ΔH) and a positive change in entropy (ΔS) values indicating favorable electrostatic and ionic interactions. The impact of varying TiO2 NP concentrations on 6-FHMQ absorption was analyzed using the Benesi-Hildbrand equation, with a quantum yield of 0.61 determined. By forster resonance energy transfer (FRET) theory, the proximity between 6-FHMQ and TiO2 NPs was found to be less than 70Å. Ground and excited state dipole moments of 6-FHMQ in different solvents were calculated to demonstrate solvent sensing ability and charge transfer properties. Ultimately, this study serves as a testament to the power of scientific innovation in the realms of drug delivery and tissue engineering.

Evaluating the biomolecular interaction between delamanid/formulations and human serum albumin by fluorescence, CD spectroscopy and SPR: Effects on protein conformation, kinetic and thermodynamic parameters

Delamanid is an anti-tuberculosis drug used for the treatment of drug-resistant tuberculosis. Since delamanid has a high protein bound potential, even patients with low albumin levels should experience high and rapid delamanid clearance. However, the interaction between delamanid and albumin should be better controlled to optimize drug efficacy. This study was designed to evaluate the binding characteristics of delamanid to human serum albumin (HSA) using various methods: fluorescence spectroscopy, circular dichroism (CD), surface plasmon resonance (SPR), and molecular docking simulation. The fluorescence emission band without any shift indicated the interaction was not affected by the polarity of the fluorophore microenvironment. The reduction of fluorescence intensity at 344 nm was proportional to the increment of delamanid concentration as a fluorescence quencher. UV-absorbance measurement at the maximum wavelength (λmax, 280 nm) was evaluated using inner filter effect correction. The HSA conformation change was explained by the intermolecular energy transfer between delamanid and HSA during complex formation. The study, which was conducted at temperatures of 298 K, 303 K, and 310 K, revealed a static quenching mechanism that correlated with a decreased of bimolecular quenching rate constant (kq) and binding constant (Ka) at increased temperatures. The Ka was 1.75–3.16 × 104 M−1 with a specific binding site with stoichiometry 1:1. The negative enthalpy change, negative entropy change, and negative Gibbs free energy change demonstrated an exothermic-spontaneous reaction while van der Waals forces and hydrogen bonds played a vital role in the binding. The molecular displacement approach and molecular docking confirmed that the binding occurred mainly in subdomain IIA, which is a hydrophobic pocket of HSA, with a theoretical binding free energy of −9.33 kcal/mol. SPR exhibited a real time negative sensorgram that resulted from deviation of the reflex angle due to ligand delamanid-HSA complex forming. The binding occurred spontaneously after delamanid was presented to the HSA surface. The SPR mathematical fitting model revealed that the association rate constant (kon) was 2.62 × 108 s−1M−1 and the dissociation rate constant (koff) was 5.65 × 10−3 s−1. The complexes were performed with an association constant (KA) of 4.64 × 1010 M−1 and the dissociation constant (KD) of 2.15 × 10−11 M. The binding constant indicated high binding affinity and high stability of the complex in an equilibrium. Modified CD spectra revealed that conformation of the HSA structure was altered by the presence of delamanid during preparation of the proliposomes that led to the reduction of secondary structure stabilization. This was indicated by the percentage decrease of α-helix. These findings are beneficial to understanding delamanid-HSA binding characteristics as well as the drug administration regimen.

Spectroscopic and in silico evaluation on the interactive behavior between substituted β-2,3-dihydrofuran naphthoquinones and human serum albumin

The interactive profile between human serum albumin (HSA, the main carrier of endogenous and exogenous compounds in the human bloodstream) and five potential anticandidal agents, namely 2,3-dihydro-2-phenyl-naphtho[1,2-b]furan-4,5‑dione (1) and its derivatives 2–5 was studied by biophysical techniques, i.e., UV–vis, circular dichroism, steady-state and time-resolved fluorescence, combined with in silico calculations. The Stern-Volmer quenching constant (KSV) trend in three different temperatures and the bimolecular quenching rate constant (kq ≈ 1012 M−1s−1) values indicated a ground-state association HSA:1–5 which was confirmed by time-resolved fluorescence decays. Both KSV and modified Stern-Volmer binding constant (Ka) values are in the order of 104 M−1, indicating a moderate binding affinity, following the same reported trend for 1,4-naphthoquinones (the isomeric form of the naphthoquinones under study). There is only one main binding site for HSA:1–5, probably the subdomain IIA where the Trp-214 residue can be found. The association is driven both enthalpically and entropically, perturbing both the microenvironment around Trp-214 residue and weakly the α-helix content of albumin. Overall, the β-2,3-dihydrofuran naphthoquinones 1–5 showed favorable binding affinity to HSA to achieve their therapeutical potential and control of effective dosages.

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.

Interactive Profile between 1,4-Naphthoquinone Derivatives and Human Serum Albumin

The interactive profile between four 1,4-naphthoquinone derivatives (1-4) and human serum albumin (HSA) was studied by spectroscopic techniques and in silico calculations. The bimolecular quenching rate constant (kq ca. 1012 L mol-1 s-1) and the time-resolved fluorescence decays indicated a static fluorescence quenching mechanism. Thus, there is a spontaneous ground-state association, and based on both Stern-Volmer, modified Stern-Volmer, and van’t Hoff approaches, the association is moderate mainly driven by hydrophobic forces. The circular dichroism (CD) analysis indicated that until the proportion albumin:compound of 1:8 there is a weak perturbation on the structural content of albumin, while molecular docking results suggested subdomain IIA (site I), a positive electrostatic pocket, as the main binding site. Overall, even though the hydrogen atom replacement by methyl, fluorine, or chlorine atoms in the para position of the aromatic ring in the benzo[g]chromene-5,10-dione moiety changes the lipophilicity, it does not change the binding profile to HSA.

Synthetic dimethoxyxanthones bind similarly to human serum albumin compared with highly oxygenated xanthones

Spectroscopic studies were carried out for the interaction of the bioactive compounds 2,3-dimethoxyxanthone (1) and 3,4-dimethoxyxanthone (2) with human serum albumin (HSA). The UV absorption, steady-state, and time-resolved fluorescence spectroscopy studies showed that 1 and 2 interact with HSA through a ground-state association. The decrease in the Stern-Volmer constant (KSV) values with increasing temperature, and the bimolecular quenching rate constant (kq) values in the order of 1012 M−1s−1 indicated a static fluorescence quenching mechanism, corroborating with time-resolved fluorescence decays. The association constant (Ka) values in the order of 104 M−1 indicated a moderate interaction between these xanthones and HSA. Circular dichroism experiments showed weak perturbation on the secondary structure of albumin after the interaction with 1 and 2. Thermodynamic parameters suggested that the binding is entropically (for HSA:1, ΔS° 0.012 ± 0.003 and for HSA:2 0.008 ± 0.002 J/molK) and enthalpically (for HSA:1, ΔH° -24.0 ± 0.99 and for HSA:2 -25.6 ± 0.67 kJ/mol) controlled, with a Gibbs free energy change (ΔG°) close to -28.0 kJ/mol in both cases. Competitive drug-displacement and molecular docking results showed that 1 and 2 could interact not only with subdomain IIA, where Trp-214 residue can be found, but also with subdomains IIIA and IB. Hydrogen bonding, electrostatic, and hydrophobic interactions were detected as the main binding forces to stabilize the association HSA:1 and HSA:2. Overall, the dimethoxyxanthones have similar binding affinity compared with highly oxygenated xanthones, e.g., mangiferin, gentiacaulein, and norswertianin.

Use of computational and wet lab techniques to examine the molecular association between a potent hepatitis C virus inhibitor, PSI-6206 and human serum albumin.

This study explores the plausible molecular interaction between a potent hepatitis C virus inhibitor, PSI-6206 (PSI), and human serum albumin (HSA), a primary transporter in blood plasma. Results obtained from both computational viz. molecular docking and molecular dynamics (MD) simulation and wet lab techniques such as UV absorption, fluorescence, circular dichroism (CD), and atomic force microscopy (AFM) complemented each other. While docking results identified PSI binding to subdomain IIA (Site I) of HSA by forming six hydrogen bonds, MD simulations signified the complex stability throughout the 50,000ps. A consistent cutback in the Stern-Volmer quenching constant (Ksv) along with rising temperatures supported the static mode of fluorescence quenching in response to PSI addition and implied the development of the PSI-HSA complex. This discovery was backed by the alteration of the HSA UV absorption spectrum, a larger value (>1010 M-1.s-1) of the bimolecular quenching rate constant (kq) and the AFM-guided swelling of the HSA molecule, in the presence of PSI. Moreover, the fluorescence titration results revealed a modest binding affinity (4.27-6.25×103 M-1) in the PSI-HSA system, involving hydrogen bonds, van der Waals and hydrophobic interactions, as inferred from ΔS = + 22.77Jmol-1 K-1 and ΔH = - 11.02 KJ mol-1values. CD and 3D fluorescence spectra reminded significant adjustment in the 2° and 3° structures and modification in the Tyr/Trp microenvironment of the protein in the PSI-bound state. The results obtained from drug competing experiments also advocated the binding location of PSI in HSA as Site I.

Spectroscopic, voltammetric and computational approaches shed light on the combination characteristics of an anticancer agent, bexarotene with human serum albumin

The combination process of bexarotene (BEX) with the primary transporter protein in blood plasma, human serum albumin (HSA) was explored to comprehend the anticancer agent’s transport in the circulatory system. The HSA fluorescence spectrum was discovered to be successively quenched by BEX. Involvement of static quenching process as well as the BEX–HSA complexation were suggested by the bimolecular quenching rate constant (kq) values. This finding was further corroborated by ultraviolet absorption spectral analysis. The binding affinity of BEX–HSA interaction was found as moderate on the basis of Ka values (Ka = 3.19–8.72 × 104 M−1) for the system. Thermodynamic particulars (ΔS = 175.93 J mol−1 K−1; ΔH = 25.85 kJ mol−1) revealed that the stabilizing intermolecular forces associated in the reaction were hydrophobic interactions and hydrogen bonds. BEX binding altered the microenvironment near Trp and Tyr residues of HSA, but it improved the protein’s resistance to heat stress. Both site-specific ligand displacement and computational docking outcomes indicated that Sudlow’s Site II was detected as the position for BEX binding to HSA with higher priority. The BEX–HSA complex constancy was demonstrated by the complex maintaining equilibrium during the course of the simulations, according to analysis of the molecular dynamics simulation results.

Correlation of solute diffusion with dynamic viscosity in lithium salt-added (choline chloride + glycerol) deep eutectic solvents.

Due to their favorable physicochemical properties, deep eutectic solvents (DESs) are finding increased use in chemistry. Metal salt-added DESs are currently being investigated for their potential applications in electrochemistry as a replacement for organic electrolytes. Insights into solute diffusion in salt-added DESs, in this context, are of the utmost importance. Solute diffusion in a LiCl-added DES composed of the H-bond acceptor choline chloride and the H-bond donor glycerol in a 1 : 2 mole ratio, named glyceline, is assessed as a function of temperature and LiCl concentration. For relative translational diffusion, the fluorophore-quencher pair of pyrene-nitromethane is used, whereas for rotational diffusion a fluorescent anisotropic rotor, perylene, is selected. The fluorescence quenching of pyrene by nitromethane was found to be purely dynamic in nature. The estimated bimolecular quenching rate constant (kq) exhibits excellent adherence to the Stokes-Einstein relation, suggesting relative translational diffusion of the solute to be controlled by the dynamic viscosity of the LiCl-added glyceline solution. The rotational reorientation time (θ) of the rotor perylene is also found to scale with dynamic viscosity and obey the Stokes-Einstein relation satisfactorily. Linear correlation between θ and dynamic viscosity (η) improves for glyceline solutions with fixed LiCl concentrations hinting at the possible change in the hydrodynamic volume with LiCl concentration within the DES. Control of rotational diffusion of the solute by the dynamic viscosity is established nonetheless. The effect of earlier reported micro- and/or nano-heterogeneities within salt-added DES systems on solute diffusion dynamics is found to be minimal. The work highlights DESs in offering a solubilizing medium for solutes where the diffusion dynamics are simply controlled by the dynamic viscosity.

Shedding light on the molecular interaction between the Hepatitis B virus inhibitor, clevudine, and human serum albumin: Thermodynamic, spectroscopic, microscopic, and in silico analyses

The molecular interaction between clevudine (CLV), a hepatitis B virus inhibitor, and human serum albumin (HSA), the major transporter in plasma, was investigated via fluorescence, UV–vis absorption, circular dichroism (CD), atomic force microscopy (AFM), and molecular modelling approaches. A steady reduction in the Stern-Volmer quenching constant (Ksv) with increasing temperature endorsed the static mode of the protein fluorescence quenching upon CLV addition, thus confirming CLV-HSA complex formation. This finding was supported by alteration in the UV–vis absorption spectrum of HSA upon CLV addition, higher value (>10 10 M−1 s−1) of the bimolecular quenching rate constant (kq), along with swelling of the HSA molecule in the presence of CLV, as revealed by AFM. Fluorescence quenching titration established a weak binding affinity from the values of the binding constant (Ka=8.65-11.20×103 M−1) at various temperatures (290, 300, 310 and 315 K) for the CLV-HSA system. According to the thermodynamic data (ΔS = +51.89 J mol−1 K−1 and ΔH = −7.69 KJmol−1) of the binding reaction, hydrophobic interactions, hydrogen bonds, and van der Waals forces stabilized the CLV-HSA complex. Lesser changes in both secondary and tertiary structures of HSA upon CLV binding were ratified from the far-UV and the near-UV CD spectral results. In addition, the microenvironment around HSA’s Trp and Tyr residues was also altered, as deduced from three-dimensional fluorescence spectra. The protein showed some resistance against thermal stress in the presence of CLV. The binding site of CLV in subdomain IIA (Site I) of HSA was identified by competitive displacement and molecular docking findings.

Photodynamic Effect of 5,10,15,20-Tetrakis[4-(3-N,N-dimethylaminopropoxy)phenyl]chlorin towards the Human Pathogen Candida albicans under Different Culture Conditions

Photocytotoxic activity sensitized by 5,10,15,20-tetrakis[4-(3-N,N-dimethylaminopropoxy)phenyl]chlorin (TAPC) was investigated in Candida albicans under different culture conditions. Planktonic cells incubated with 2.5 μM TAPC were eradicated after 5 min irradiation with white light. Studies in the presence of reactive oxygen species scavengers indicated the involvement of mainly a type II mechanism. Furthermore, cell growth of C. albicans was suppressed in the presence of 5 μM TAPC. A decrease in pseudohyphae survival of 5 log was found after 30 min irradiation. However, the photokilling of this virulence factor reached a 1.5 log reduction in human serum. The uptake of TAPC by pseudohyphae decreased in serum due to the interaction of TAPC with albumin. The binding constant of the TAPC-albumin complex was ~104 M−1, while the bimolecular quenching rate constant was ~1012 s−1 M−1, indicating that this process occurred through a static process. Thus, the photoinactivation of C. albicans was considerably decreased in the presence of albumin. A reduction of 2 log in cell survival was observed using 4.5% albumin and 30 min irradiation. The results allow optimizing the best conditions to inactivate C. albicans under different culture conditions.

Interaction of CuO and ZnO nanoparticles with 3-N-(N′-methylacetamidino) benzanthrone: A temperature dependent fluorescence quenching study

The interaction of fluorescent probe 3-N-(N'-methylacetamidino) benzanthrone (3-MAB) with copper oxide and zinc oxide nanoparticles is studied, wherein the copper oxide nanoparticles (CuO NPs) and zinc oxide nanoparticles (ZnO NPs) were prepared by combustion method. The prepared CuO and ZnO nanoparticles were characterized by X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM) and High resolution transmission electron microscopy (HRTEM) techniques. The interaction between fluorescent molecule 3-MAB and nanoparticles (CuO and ZnO) was investigated by absorption and fluorescence techniques. Further, the steady state fluorescence quenching of 3-MAB by CuO and ZnO nanoparticles in different solvents was studied at room temperature (298 K) and higher temperatures at 308 K and 318 K to determine the bimolecular quenching rate constant, binding constant and binding sites.

Luminescence quenching of Ru(II)-diimine complexes with Cr(VI) ions: Steady-state and time-resolved studies

Luminescence quenching of [Ru(bpy)(CN)4]2−, [Ru(phen)(CN)4]2−, [Ru(bpy)3]2+ and [Ru(phen)3]2+ by hexavalent chromium ion in aqueous media were investigated by steady-state and time-resolved measurements. Effect of pH on the luminescence quenching process is also investigated. It has been found that luminescence changes due to the interaction of all Ru(II) complexes with Cr(VI) ions were high at pH values less than or equal to 5.0 and negligible at pH ≥ 8.0. The experimental data of the quenching process were analyzed using Stern–Volmer (S–V) equations. The steady state luminescence emission data shows an upward curvature, indicating that the quenching is preceded by both dynamic and static quenching. For all studied complexes, the lifetime of the excited 3MLCT state decreases with increasing Cr(VI) concentration implying that the diffusion is mainly responsible for the quenching process. The Stern–Volmer dynamic quenching constants (KD) and the bimolecular quenching rate constant for the quenching process (kq) were calculated by applying the Stern–Volmer equation to the luminescence decay data. The bimolecular quenching rate constants (Kq) were found to increase in the order [Ru(bpy)(CN)4]2− < [Ru(phen)(CN)4]2− < [Ru(phen)3]2+ < [Ru(bpy)3]2+ which correlates will with the oxidation potentials of Ru(II) complexes. The association constants for Ru(II)-Cr(VI) system were calculated using the Benesi-Hildebrand equation and found to be 3.0 × 103, 2.5 × 103, 1.2 × 103, 7.1 × 103 M−1 for [Ru(bpy)(CN)4]2−, [Ru(phen)(CN)4]2−, [Ru(bpy)3]2+ and [Ru(phen)3]2+ respectively, which is interpreted in terms of the dynamic (diffusion) and the sphere of action (static) quenching models.

Donor-acceptor complex formation in tetra-n-butylammonium chloride: n-decanoic acid deep eutectic solvent.

Complex formation between pyrene (Py) and N,N-dimethylaniline (DMA) is presented in a deep eutectic solvent constituting of tetra-n-butylammonium chloride (TBAC) and n-decanoic acid (DA) in a 1:2 mol ratio, respectively, named TBAC:DA. The addition of DMA to a Py solution of TBAC:DA results in the formation of a fluorescent Py-DMA charge-transfer complex, which is manifested via a broad structureless bathochromically shifted band centered at 550(±2) nm. The solvatochromic nature of the Py-DMA fluorescent complex indicates the solvent polarity of TBAC:DA to be higher than that of methanol. The absence of a negative pre-exponential factor in the intensity decay at 550 nm combined with the excitation scans implies the presence of weak interaction between Py and DMA in the ground-state, leading to the rapid formation of a Py-DMA complex possibly at a sub-nanosecond time scale. The Stern-Volmer quenching constant (KSV) varies from 53(±2) to 96(±1) M-1, and the bimolecular quenching rate constant (kq) varies from 3.0(±0.4) × 108 to 8.8(±1.3) × 108 M-1 s-1 by increasing the temperature (T) from 283.15 to 313.15 K, implying efficient deactivation of electron-acceptor Py in the excited-state induced effectively by the electron-donor DMA within TBAC:DA. ln kq varies linearly with 1/T with an activation energy (Ea) of 26.4(±0.4) kJ mol-1. The linear behavior between kq and 1/η suggests conformity to the Stokes-Einstein relationship within TBAC:DA. The Py-DMA complex formation efficiency increases with an increase in T and reaches maxima at 298.15 K before decreasing with a further increase in T. The initial reduction in η favors Py-DMA complex formation; this effect is overcome by preferential thermal deactivation of the Py-DMA fluorescent complex as compared to that of pyrene.

Lysine modification of human serum albumin and its effect on protein conformation and nalidixic acid binding

In order to investigate the involvement of lysine residues of human serum albumin (HSA) in nalidixic acid (NA) binding, various modified preparations of HSA such as 44% carbamylated (C44), 83% carbamylated (C83) and 85% acetylated (A85) were made by treating the HSA solution with a different molar excess of potassium cyanate and acetic anhydride. The extent of modification, charge homogeneity and conformational changes of these derivatives were checked by TNBSA reaction method, polyacrylamide gel electrophoresis (PAGE) and gel filtration using Sephacryl S-200 HR column, respectively. Binding of NA to HSA and its derivatives was examined using fluorescence quenching titration method to determine the binding constant. The emergence of a single band in PAGE and single symmetrical peak in gel filtration results confirmed the charge and size homogeneity of these derivatives. Hydrodynamic properties such as Stokes radius and frictional ratio, as obtained from the analytical gel filtration results suggested molecular expansion in C83 and A85 HSAs while C44 HSA retained the native conformation. Addition of NA to both native and modified HSA derivatives quenched the fluorescence intensity of the protein at 344 ​nm to a different extent. Whereas the values of the Stern-Volmer constant (KSV) and bimolecular quenching rate constant (kq) suggested, NA-HSA complex formation, binding constant (Ka) value suggested an intermediate binding affinity between NA and HSA. Furthermore, the decrease in the Ka value with the extent of modification was indicative of the involvement of lysine residues in NA-HSA interaction.

Fluorescence Quenching Studies on the Interactions between Chosen Fluoroquinolones and Selected Stable TEMPO and PROXYL Nitroxides.

The influence of the stable 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) nitroxide and its six C4-substituted derivatives, as well as two C3-substituted analogues of 2,2,5,5-tetramethylpyrrolidynyl-N-oxyl (PROXYL) nitroxide on the chosen fluoroquinolone antibiotics (marbofloxacin, ciprofloxacin, danofloxacin, norfloxacin, enrofloxacin, levofloxacin and ofloxacin), has been examined in aqueous solutions by UV absorption as well as steady-state and time-resolved fluorescence spectroscopies. The mechanism of fluorescence quenching has been specified and proved to be purely dynamic (collisional) for all the studied systems, which was additionally confirmed by temperature dependence experiments. Moreover, the selected quenching parameters—that is, Stern–Volmer quenching constants and bimolecular quenching rate constants—have been determined and explained. The possibility of electron transfer was ruled out, and the quenching was found to be diffusion-limited, being a result of the increase in non-radiative processes. Furthermore, as the chosen nitroxides affected the fluorescence of fluoroquinolone antibiotics in different ways, an influence of the structure and the type of substituents in the molecules of both fluoroquinolones and stable radicals on the quenching efficiency has been determined and discussed. Finally, the impact of the solvent’s polarity on the values of bimolecular quenching rate constants has been explained. The significance of the project comes from many applications of nitroxides in chemistry, biology and industry.

Binding of metal ions to the curcumin mediated methoxy polyethylene glycol thiol conjugated greenly synthesized gold nanoparticles: A fluorescence spectroscopic study

Curcumin, a nontoxic molecule obtained from plant extract, was used to reduce gold salt to produce gold nanoparticles (AuNPs). First time the particles were functionalized with methoxy polyethylene glycol thiol (mPEGT), which acts as a stabilizing agent in such a manner that the methoxy group is pointed toward the aqueous solution to serve as anchor point to many metal ions. The greenly synthesized gold nanoparticles have a size between ∼40 and ∼83 nm. The prepared material was characterized in detail by SEM, XRD, TGA and FT-IR spectroscopy. These AuNPs were found to be fluorescence active similar to free curcumin suggesting that curcumin is conjugated at the surface of AuNPs. Exposure of the mPEGT functionalized AuNPs to different concentrations of metal cations in the range between 0 and 100 μM resulted in the decrease in the fluorescence intensity of the AuNPs unlike free curcumin in aqueous solution. Excited state lifetime measurements further validated quenching mechanism to be static in nature. The binding constant and number of binding sites for such interaction were evaluated. Based on observed bimolecular quenching rate constants, kQ values, the order of association of the metals studied is Pb (II) > Hg (II) >Hg (I) > Cd (II)>Al (III) >Ni (II) >Cu (II) >Na2HAsO4. These interactions between the mPEGT conjugated AuNPs and the metal ions in solution contribute a proof-of-concept that mPEGT functionalized curcumin mediated AuNPs can be used as a simple, cheap and straightforward, on-site detection system for toxic and even essential metal ions in solution.