Bimolecular Quenching Rate Constants Research Articles

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.

A stable and sensitive luminescent photoprobe based on tris(3‐acetylindole) terbium(III) complex: Molecular modeling, luminescence quenching, and Ab initio molecular dynamics

We have been successfully used a new photoprobe based on Tb(III)(3‐acetyleindole)3 complex (TbAcI) as a novel, sensitive, accurate, and precise sensor for detecting shallow concentration (pico) of epinephrine (EPI) in different serum samples. Here, we report on and discuss the molecular structure of the interacting species using density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT). The simulation results reveal the strong binding energy of TbAcI (−148.95 kcal/mol) and explain the outcomes of Stern‐Volmer bimolecular quenching analysis due to EPI's presence in the proximity (at 3.22 Å apart from Tb ion) of the emissive TbAcI. The bimolecular quenching rate constant (k2) obtained from the Stern‐Volmer rate constant (Ksv) and the complex luminescence lifetime points to a diffusion‐controlled dynamic quenching. We used the Ab initio molecular dynamic (AIMD) simulations to explain EPI's effect on the TbAcI molecular kinetic energy changes in its S1 state. Quenching begins at 14 fs when the EPI‐TbAcI distance is about 3.22 Å. The excited state AIMD simulations proposes a new opportunity for future research on luminescence quenching.

A study on the interaction of nile blue with Uracils: A spectroscopic and computational approach.

The present work focuses the investigation on fluorescence quenching of nile blue (NB) in presence of various substituted uracil molecules. UV-Visible absorption studies signify the possibility of ground state complex formation between NB and uracil molecules. The increase in concentration of quencher molecules greatly influences the emission spectra of NB. The bimolecular quenching rate constant (kq) were calculated and found to depend on the position and electronic properties of substituent in quencher molecules. Fluorescence quenching experiments were performed at different temperature to calculate the thermodynamic parameters. The fluorescence lifetime measurements show that the quenching process proceeds through static quenching. The mechanism of fluorescence quenching includes the possibility of proton transfer. The bond dissociation enthalpy (BDE) reveals the release of H from the quencher molecules. The quencher molecules possess antioxidant activity and identified using deoxyribose degradation assay. The position of substituent and its electronic property are key features to address the antioxidant activity of uracil molecules.

Spectroscopic characterization of dipicolinic acid and its photoproducts as thymine photosensitizers.

Dipicolinic acid (DPA), present in large amount in bacterial spores, has been proposed to act as an endogenous photosensitizer in spore photoproduct formation. The proposed mechanism involves a triplet-triplet energy transfer from DPA to thymine. However, up to now, no spectroscopic studies have been performed to determine the interaction between the endogenous compound and the nucleobase, probably due to its photolability in aqueous solutions. Here, triplet excited state properties of DPA are reported together with its bimolecular quenching rate constant by thymidine, kq of ca. 5.3×109M-1s-1. To run more reliable studies, a stable methyl ester derivative of DPA, which exhibits the same spectroscopic properties as the parent compound, is also described. Finally, DPA photoproducts are characterized. Studies of their triplet excited state properties have demonstrated that, interestingly, one of them is able to photosensitize thymidine triplet excited state formation.

Study of Bimolecular Quenching Reactions of Coumarin Dye C1 by the Fluorescence Behavior in Toluene and Butanol Binary Mixtures

AbstractInferable from the significance of solvent mixtures in practical chemistry, toluene (TL) and butanol (BL) mixtures are used to study fluorescence behavior of coumarin dye C1 namely 3‐hydroxy‐3‐[2‐oxo‐2‐(2‐oxo‐2H‐chromen‐3‐yl)‐ethyl]‐1,3‐dihydro‐indol‐2‐one. Bimolecular quenching reaction studies of C1 with aniline as quencher are made in mixtures of TL–BL to know the effect of viscosity and dielectric constant variation at room temperature. The quenching process is studied in all solvent mixtures by steady state method. Quenching is characterized by Stern–Volmer (S–V) plots having upward curvature. Analysis of modified S–V equations which accounts both static and dynamic quenching allows calculating bimolecular quenching rate constant. The bimolecular quenching reactions are found to be significantly larger. Further finite sink approximation model is invoked so as to check whether reactions are diffusion limited. The extents of these rate parameters demonstrate that positive deviations in the S–V plot are because of the presence of apparent static and dynamic quenching process.

Lanthanide (III) ions as multichannel acceptors for bimolecular photoinduced electron transfer reactions with coumarin dyes

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions is of considerable research interest in photochemical sciences. In this study, interactions between trivalent lanthanide ions (Ln(III)) as electron acceptors and coumarin dyes as electron donors have been investigated in aqueous media using ground state absorption, steady-state (SS) emission, time-resolved (TR) fluorescence and laser flash photolysis (LFP) techniques. Among different Ln(III) ions, only Eu(III), Yb(III) and Sm(III) are found to display substantial interaction with the excited singlet (S1) state of the coumarin dyes, which correspond well with the favorable free energy changes (negative ΔG0) associated with the PET reactions in these coumarin-Ln(III) systems. For other Ln(III) ions, as ΔG0 is not favorable, they do not show any interaction with the excited coumarin dyes. Among the interacting coumarin-Ln(III) systems, bimolecular quenching rate constant (kq) increases consistently with the increasing exergonicity (−ΔG0), when each of the Ln(III) ions are considered independently, except for Eu(III), for which the kq values have already reached the diffusion controlled limit. However, considering all the Ln(III) ions together, the kq values do not show any common correlation with the changing ΔG0 values. The kq values are in general much lower for Yb(III) quencher as compared to Sm(III), even though the exergonicity values for coumarin-Yb(III) systems are more favorable than the coumarin-Sm(III) systems. Correlating the observed results based on the diffusion mediated bimolecular reaction model, we infer that depending on the ΔG0 values, some Ln(III) quenchers possibly also engage their upper electronic energy states as well to assist multichannel ET with the excited coumarin donors, and thereby modulating the kq values very substantially for different coumarin-Ln(III) systems. Apparently, the involvement of multichannel ET causes the total reorganization energy (λ) for ET reactions to differ largely for different Ln(III) ions, which would otherwise be very similar for all the coumarin-Ln(III) pairs. Observed results suggest that the intramolecular reorganization energy (λi) associated with lower electronic states of the hydrated Ln(III) ions is much larger than that associated with higher electronic states of these ions. While the involvement of multichannel ET is itself an intriguing inference, modulation in the ET kinetics and total λ values for different Ln(III) quenchers through the differential participation of the multichannel PET is also expected to find potential uses in the controlled utilization of such PET reactions in many practical areas.

Thermodynamic analysis of albumin interaction with monosodium glutamate food additive: Insights from multi-spectroscopic and molecular docking approaches

Monosodium glutamate (MSG) is an example of food additive, which is used as a flavor enhancer in various foodstuffs. Because of its extensive use in food production, the toxicology and possible negative effects of MSG additive on albumin must be well investigated. Therefore, for the first time, the binding of BSA and MSG have been studied using multispectroscopic and molecular modeling approaches under physiological conditions. BSA fluorescence intensity has been diminished upon the addition of increased MSG concentrations. The Stern–Volmer quenching constant (KSV) value increase with rising temperature and bimolecular quenching rate constant of protein (kq) value was larger than 2.0 × 1010 L mol− 1 s− 1, which shows that the fluorescence quenching was both dynamic and static due to the BSA-MSG complex formation. The positive values for both ΔS0 (888.291 J mol−1K−1) and ΔH0 (243.903 kJ mol−1) proposed that the hydrophobic forces played predominant roles in the binding of MSG to BSA. Also, the negative value of ΔG0 indicated that the interaction between two compounds is spontaneous and enthalpy driven. The UV–VIS spectra results confirmed the binding of MSG to BSA. The circular dichroism analysis exhibited that BSA conformation change occur upon interaction with MSG. Moreover, molecular modeling analysis indicated that glutamate remarkably tends to bind into a cavity above Sudlow II with lowest calculated binding energy of - 8.24 kcal/mol.

DNA/BSA binding of a new oxovanadium (IV) complex of glycylglycine derivative Schiff base ligand

In this study, a new oxovanadium (IV) Schiff base complex was synthesized by the reaction of {[5-(triethylammoniummethyl)salicylaldehyde]chloride}triethylammonium and vanadyl sulfate. The complex was characterized by elemental analysis, FT-IR and UV–Vis spectroscopy. The interaction between the complex and bovine serum albumin (BSA) was studied by absorption and emission titration methods. The quality of the spectral changes and the large calculated value of both the binding constant (Kb = 7.53 × 104 M−1) and the Stern-Volmer quenching constant (KSV = 7.65 × 104 M−1) proved the strong affinity of the complex to BSA. The static quenching mechanism was confirmed by comparing the calculated Kb values at four different temperatures. The negative changes in enthalpy and Gibbs free energy, proved that the BSA-complex binding is exothermic and spontaneous. The fluorescence data also indicated 1:1 binding stoichiometry of the complex on one binding site of BSA. Competitive experiments with site markers (phenylbutazone and ibuprofen) and synchronous measurements identified site I of BSA as the main binding site. Molecular docking results confirmed the binding pocket of site I (subdomain IIA) as the binding site. In the second part of the work, the binding of the complex to fish sperm DNA (FS-DNA) was investigated by UV–Vis absorption spectroscopy, fluorescence spectroscopy, viscosity measurement and cyclic voltammetry. The Stern-Volmer quenching and bimolecular quenching rate constants (KSV = 6.13 × 102 M−1, kq = 6.13 × 1010 M−1S−1) were calculated. Absorption and emission studies could present types of interactions for example groove binding and electrostatic. A minor changes in viscosity showed a groove binding mode. In cyclic voltammogram study, decrease in current and also shift of peak potentials positively after adding DNA supported a partial intercalation mode. Theoretical docking study proposed the minor groove as the main part for DNA-complex binding.

Preferential solvation and bimolecular quenching reactions of boronic acid dye at very low quencher concentrations studied by fluorescence spectrum in toluene and butanol binary mixtures

Inferable from the significance of solvent mixtures in practical chemistry, toluene (TL) and butanol (BL) mixtures are used to studyfluorescence behavior of boronic acid dye 2-methoxy-5-fluoro-phenyl boronic acid (2MEFPBA). At the inception, preferential solvation is examined in TL-BL, to understand specific and nonspecific interactions. Suppan’s dielectric enrichment model is further used to understand the nonideality and dielectric enrichment in TL-BL mixtures for preferential solvation. Bimolecular quenching reaction studies of 2MEFPBA with aniline as quencher are made in mixtures of TL–BLto know the effect of viscosity and dielectric constant variation at room temperature. The quenching process is studied in all solvent mixtures by steady state and transient state method. Quenching is characterized by S-V plots having upward curvature. Analysis of modified S-V equations which accounts both static and dynamic quenching allows calculating bimolecular quenching rate constant. The bimolecular quenching reactions are found to be significantly larger. Further finite sink approximation model is invoked so as to check whether reactions are diffusion limited. The extents of these rate parameters demonstrate that positive deviations in the Stern-Volmer (S-V) plot are because of the presence of apparent static and dynamic quenching process.

Fluorescence Quenching of Dipyrenylalkanes by an Electron/Charge Acceptor.

Compounds possessing two fluorophoric moieties may exhibit dual fluorescence, one characterizing the monomeric fluorophore unit and the other characterizing the intramolecular aggregate. Fluorescence quenching of two dipyrenylalkanes, 1,3-bis(1-pyrenyl)propane [1Py(3)1Py] and 1,10-bis(1-pyrenyl)decane [1Py(10)1Py] having different alkyl chains separating the two termini pyrenyl groups that are capable of forming an intramolecular excimer, by an electron/charge-accepting quencher, nitromethane, is investigated in four different solvents-nonpolar (cyclohexane (CH)), polar-aprotic (acetonitrile (ACN)), polar-protic (ethanol (EtOH)), and chlorinated (dichloromethane (DCM))-under ambient conditions. For a given probe in a solvent, fluorescence from the monomer and the intramolecular excimer are quenched with similar efficiencies; the efficiency of quenching is higher for the probe with the longer alkyl chain separating the two fluorophores. Quenching efficiency is significantly higher in chlorinated solvent DCM. The bimolecular quenching rate constants for intramolecular excimer, however, are either comparable or lower for the longer alkyl chain compound. It is suggested that, while the donor electronic excited-state energetics is more favorable for the long alkyl chain compound, the approach of the quencher to the intramolecular excimer appears to be hindered by the presence of a longer alkyl chain.

Synthesis of a novel p-hydroxycinnamic amide and its interaction with bovine serum albumin

A novel p-hydroxycinnamic amides ((E)-4-(3-((4-(N-(5-methoxy-pyrimidin-2-yl) sulfamoyl) phenyl)amino)-3-oxoprop-1-en-1-yl)phenyl acetate (MSAP) was synthesized and confirmed by 1H NMR, 13CNMR ESI-MS, elemental analysis and single-crystal X-ray diffraction analysis. The crystal structure of the complex is monoclinic system and space group is P21/c. The interactions between MSAP and bovine serum albumin (BSA) was investigated with fluorescence and UV–Vis spectral analysis and molecular simulation. According to the fluorescence quenching calculation, the corresponding thermodynamic parameters (ΔH, ΔG and ΔS etc.), bimolecular quenching rate constant (kq), apparent quenching constants (Ksv) and binding sites number (n) were calculated. The distances (r) between BSA and its derivatives were obtained according to Foster’s non-radioactive energy transfer theory. Conformational changes in BSA were observed from synchronous fluorescence spectra. Furthermore, the site marker competitive experiments indicated that the binding of compounds to BSA primarily took place in site I (sub-domain IIA).

Formation mechanism and photo physical behaviors of Pyrene-Methyl-beta-cyclodextrin complex at excited state

Structural characterization of the Pyrene-Methyl-beta-cyclodextrin (Py-Me-β-CD) complex, obtained in aqueous medium, was carried out through Fourier Transform Infrared spectroscopy (FTIR), X-ray diffractometry (XRD), 1H NMR and two-dimensional correlated spectroscopy (2D COSY). Thermal characterization of the Py-Me-β-CD complex was performed with differential scanning calorimetry (DSC) and thermogravimetry (TG). Gibbs free energy and excited singlet state energy values of the complex were calculated. The stoichiometry of the complex was determined to be 1:2 by the Benesi-Hildebrand method. The complex binding constant (Kb), which was 1.5 × 1011 M−1, indicated a strong inclusion complex. Both thermal and XRD analyzes showed that the crystallinity of the pyrene decreased and its thermal stability increased as a result of complex formation. The most obvious quenching effect in the transition metals (cobalt, iron, zinc, copper, manganese and nickel) was obtained with iron. Calculated Stern-Volmer constant (Ksv) and the quenching activation energy (Ea) values showed that iron interacted with the complex differently. The lifetime (τ) and bimolecular quenching rate constant (kq) calculations pointed to diffusion controlled dynamic quenching for the metals, other than iron. In addition, with the energy band calculations made, it was shown by means of Tauc plots, that electron transfer was made to iron and manganese metals from the complex fluorophore group, as part of the quenching mechanism. The value of the activation energy for iron indicated a weakly bound encounter complex formation.

Direct Observation of the Reduction of Aryl Halides by a Photoexcited Perylene Diimide Radical Anion.

The ability of the doublet excited state of perylene diimide anion radical 2(PDI-•)* to reduce aromatic electron acceptors was probed by picosecond time-resolved transient absorption (TA) spectroscopy. Excitation of PDI-• produces visible TA due to 2(PDI-•)* that decays with τ = 160 ps. Aromatic electron acceptors with varying reduction potential quench 2(PDI-•)* and, in some cases, give a new visible region absorption that is attributed to the products of bimolecular photoinduced electron transfer, 2(PDI-•)* + Ar-X → PDI + Ar-X-•. Stern-Volmer quenching of 2(PDI-•)* accomplished with a series of acceptors provides bimolecular quenching rate constants as a function of acceptor reduction potential. Rehm-Weller analysis of the electron transfer quenching data affords the potential for the (*PDI-•/PDI) electrochemical half-reaction as -1.87 V vs SCE.

Interaction of KMP-11 and its mutants with ionic liquid choline dihydrogen phosphate: Multispectroscopic studies aided by docking and molecular dynamics simulations

Binding interactions of KMP-11 with the IL choline dihydrogen phosphate (CDHP) solvent have been explored from multispectroscopic studies in conjunction with docking and molecular dynamics simulations. The protein KMP-11, a vaccine candidate against ‘Leishmaniasis’, does not contain any tryptophan (Trp) in its WT sequence. For this study, two single site Trp mutant proteins Y5W and Y89W of KMP -11 have been generated as Trp used as biomarker by the aid of intrinsic fluorescence. In the former mutant (Y5W) Trp is inserted in the vicinity of the N-terminal, while in the later (Y89W), the same residue is disposed near the C-terminal of the protein. The binding interactions of CDHP with both the mutant proteins have been explored from UV–Vis, steady state and time resolved fluorescence spectroscopic studies. The bimolecular quenching rate constant (kq) at various temperatures, as obtained from fluorescence spectroscopic studies suggest definite but weak binding interactions between the mutant proteins and CDHP IL solvent. This result is further corroborated from the near and far - UV Circular Dichroism (CD) spectra. The possible binding sites of CDHP in the vicinity of WT and mutant proteins and the corresponding binding forces have been predicted from the molecular docking studies. The results of MD simulation further portend minimal or no unfolding in the secondary structures of the WT and the mutant proteins upon addition of CDHP molecule. We believe that this in-depth study featuring the interaction of KMP-11 with IL CDHP will enlighten the fundamental understanding towards the development of improved protein based vaccines in general and KMP-11 guided vaccines in particular for the prevention of VL.

Interaction of Nitroglycerin with Bovine Serum Albumin and the Influence of Metal Ions on the Binding

The effect of Cu2+, Ca2+, Mg2+and Zn2+ on the interaction between nitroglycerin and bovine serum albumin was investigated. The bimolecular quenching rate constant, the Stern-Volmer quenching constant, the binding constants and the number of binding sites were calculated in the absence and presence of Cu2+, Ca2+, Mg2+and Zn2+. The quenching constants of nitroglycerin to bovine serum albumin were increased in the presence of metal ions. Static quenching mechanism was also confirmed. The binding constants of nitroglycerin to bovine serum albumin were influenced by different metal ions. The enthalpy change, free energy chang, entropy change and the distance between the donor and the acceptor at different temperatures were calculated. The results indicated that energy transfer from bovine serum albumin to nitroglycerin occurs with high probability.

Interaction of Nitroglycerin with Bovine Serum Albumin and the Influence of Metal Ions on the Binding

The effect of Cu2+, Ca2+, Mg2+and Zn2+ on the interaction between nitroglycerin and bovine serum albumin was investigated. The bimolecular quenching rate constant, the Stern-Volmer quenching constant, the binding constants and the number of binding sites were calculated in the absence and presence of Cu2+, Ca2+, Mg2+and Zn2+. The quenching constants of nitroglycerin to bovine serum albumin were increased in the presence of metal ions. Static quenching mechanism was also confirmed. The binding constants of nitroglycerin to bovine serum albumin were influenced by different metal ions. The enthalpy change, free energy chang, entropy change and the distance between the donor and the acceptor at different temperatures were calculated. The results indicated that energy transfer from bovine serum albumin to nitroglycerin occurs with high probability.

Visible-Light Initiated Free Radical Polymerization by Homomolecular Triplet-Triplet Annihilation

Polymerization reactions initiated by ultraviolet light are ubiquitous in scores of industrial applications but would markedly benefit from visible light activation to overcome stability, energy consumption, and sample geometry limitations. The current work leverages visible-light driven homomolecular triplet-triplet annihilation (TTA) in zinc(II) meso-tetraphenylporphyrin (ZnTPP) to initiate facile free-radical polymerization in trimethylolpropane triacrylate (TMPTA) monomers through ultrafast electron transfer quenching. Selective Q-band (S1) excitation of ZnTPP in the green or yellow (𝜆ex = 514.5 or 552 nm) sensitizes TTA occurring between two 3ZnTPP* energized chromophores, ultimately generating the highly reducing S2 excited state on one ZnTPP molecule (Ered = -2.13 V vs SCE). Subsequently, this S2 state engages in electron transfer with TMPTA, thereby initiating the radical polymerization process. Estimation of the free energy of the reaction combined with electrochemistry strongly suggest that electron transfer is only plausible from the S2 excited state of ZnTPP. Consistent with this hypothesis, the S1 and T1 excited states of ZnTPP showed no evidence of static or dynamic quenching by TMPTA. The bimolecular electron transfer was then verified by concentration-dependent dynamic fluorescence quenching of the ZnTPP S2 excited state using optically gated fluorescence upconversion measurements. In acetonitrile, this electron transfer was concentration-dependent, with time constants ranging between 2.57 ps to 1.04 ps, with TMPTA concentrations from 0 to 3.47 M, respectively. The dynamic bimolecular quenching rate constant (9.5 × 1011 M-1s-1, in toluene) was in the same order of magnitude with that obtained from static fluorescence measurements (3.2 × 1011 M-1s-1, in toluene). FT-IR spectroscopy confirmed visible light-initiated polymerization through monitoring consumption of the olefins in TMPTA as a function of photolysis time. EPR experiments, in conjunction with trapping radical species with 2-methyl-2-nitrosopropane (t-BuNO), identified that acrylate radicals were indeed formed through green light activation. Furthermore, we demonstrate that acrylate polymers of various macroscopic shapes as well as micron-scale objects can be produced using numerous low power visible light sources in the presence of only ZnTPP and TMPTA. These well-defined images represent the general applicability of homomolecular TTA mediated photoinitiated polymerization in both micro- and macrofabrication applications.

Exploring the interaction mechanism of a dicarboxamide fungicide, iprodione with bovine serum albumin

Binding of iprodione (IPR), a dicarboxamide fungicide, to the carrier protein in bovine circulation, bovine serum albumin (BSA) was characterized with the help of fluorescence, absorption, circular dichroism and Fourier transform infrared (FTIR) spectral measurements in combination with computational analysis. The increase in the KSV (Stern–Volmer constant) value with temperature and absorption spectral results characterized the IPR-induced quenching of BSA fluorescence as dynamic quenching. On the other hand, higher value (> 1011 M−1 s−1) of the bimolecular quenching rate constant (kq) suggested complex formation between IPR and BSA. In view of it, increase in KSV value with temperature can be considered as an indication of the involvement of endothermic apolar (hydrophobic) interactions in stabilizing the IPR–BSA complex, as these forces are maximized at higher temperature. A weak binding affinity was anticipated from the values of the binding constant (Ka = 0.83–2.69 × 103 M−1) for IPR–BSA association. Thermodynamic analysis of the binding data further supported contribution of hydrophobic interactions in the IPR–BSA association process. This result was validated by the computational docking analysis. Spectral results from three-dimensional fluorescence and circular dichroism demonstrated microenvironmental changes around BSA fluorophores and protein’s structural (secondary and tertiary) alterations, respectively, upon IPR binding to the protein. Slight variation in the secondary structures of BSA in the presence of IPR was also verified from FTIR spectral results. IPR was found to bind to both site I (subdomain IIA) and site II (subdomain IIIA) of BSA, showing more preference toward site II, as identified by the competitive drug displacement results and supported by the computational analysis.

An Experimental Study to Synthesize and Characterize Host–Guest Encapsulation of Anthracene, and the Quenching Effects of Co and Ni

A complex of anthracene with methyl-beta-cyclodextrin (A-Me-β-CD) having fluorophoric characteristics was obtained in aqueous medium. Spectroscopic and thermal analyses of the complex were performed by Fourier transform infrared spectroscopy and differential scanning calorimetry-thermogravimetry. The interaction of Me-β-CD and anthracene was also analyzed by means of spectrometry by a UV–Vis spectrophotometer. The stoichiometry of the complex was determined by the Benesi–Hildebrand method. The complex formation constant was found to be (42 ± 3) × 103 L·mol−1. The Gibbs energy and excited singlet state energy were calculated. The mechanism of the quenching effect of Co(II) and Ni(II) metals in the inclusion complex was studied. The Stern–Volmer constant, bimolecular quenching rate constant, lifetime and approximate activation energy values were determined, and the quenching was found to be diffusion controlled dynamic quenching. In addition, with the energy band calculations made, it was shown by means of Taug curves that electron transfer was made to Ni and Co metals from the complex fluorophore group in the quenching mechanism.

Hydrogen Abstraction of Ketoprofen in the Excited Triplet State with Indole and Methylindoles.

Relaxation of excited states and reactivity of ketoprofen (KP), one of the most popular nonsteroidal anti-inflammatory drugs, with indole and methylindoles have been studied with transient absorption and quantum chemical calculations. KP in the excited triplet state, 3KP*, abstracted a hydrogen atom from indole and methylindoles to afford a ketyl radical and a counter radical. The bimolecular quenching rate constants of 3KP* by indole and methylindoles, kq, and the hydrogen atom abstraction rate constants, kr, were obtained. The kr values for methylindoles were larger than that for indole; in addition, transient spectra at around 350 nm, assigned to the corresponding C-centered radical, was observed. These results indicate that 3KP* abstracts a hydrogen atom of the methyl group as well as that of N-H in the indole frame. These findings give us information on the reactivity of excited KP in the vicinity of tryptophan in a KP-protein complex, which will ultimately cause photosensitization on human skin.