Volmer Constant Research Articles

The Interaction of Bovine β-Lactoglobulin with Caffeic Acid: From Binding Mechanisms to Functional Complexes.

In this study, the interaction of native and transglutaminase (Tgase) cross-linked β-lactoglobulin (β-LG) with caffeic acid (CA) was examined, aiming to obtain functional composites. Knowledge on the binding affinity and interaction mechanism was provided by performing fluorescence spectroscopy measurements, after heating the native and cross-linked protein at temperatures ranging from 25 to 95 °C. Regardless of the protein aggregation state, a static quenching mechanism of intrinsic fluorescence of β-LG by CA was established. The decrease of the Stern–Volmer constants with the temperature increase indicating the facile dissociation of the weakly bound complexes. The thermodynamic analysis suggested the existence of multiple contact types, such as Van der Waals’ force and hydrogen bonds, between β-LG and CA. Further molecular docking tests indicated the existence of various CA binding sites on the β-LG surface heat-treated at different temperatures. Anyway, regardless of the simulated temperature, the CA-β-LG assemblies appeared to be unstable. Compared to native protein, the CA-β-LG and CA-β-LGTgase complexes (ratio 1:1) exhibited significantly higher antioxidant activity and inhibitory effects on α-glucosidase, α-amylase, and pancreatic lipase, enzymes associated with metabolic syndrome. These findings might help the knowledge-based development of novel food ingredients with valuable biological properties.

Spectroscopic analysis of the interaction between Co3O4 nanoparticles and acid phosphatase

In this study, the structure and stability of acid phosphatase in its interaction with Co3O4 nanoparticles are evaluated through absorbance, enzyme activity assay, fluorescence, thermal stability, and circular dichroism spectroscopy. It had been found that with the enhancement of Co3O4 nanoparticle concentration, the intensity of the enzyme’s ultraviolet spectrum was increasing. In fact, under these conditions (pH 4.8, T = 310 K), the activity of acid phosphatase was raised too. Also, it was revealed that by enhancing the concentration of the nanoparticles, the enzyme thermal stability increased from 342.0 to 346.0 K. Also, far-UV CD investigations illustrated that the Co3O4 nanoparticles could alter the secondary structure of acid phosphatase through an increase in the value of the α-helix structure (from 10.8 to 13.9%) and a decrease in the β-sheet (from 30.2 to 28.0%). By raising the temperature from 298 to 308 K, the Stern–Volmer constant decreased from 5.72 × 104 to 4.39 × 104 M−1. Also, it was found which Co3O4 nanoparticles quenched the intrinsic fluorescence of acid phosphatase by the static quenching mechanism. Therefore, the thermodynamic parameters showed that the binding process was spontaneous because the value of ∆G was negative. Also, van der Waals forces and hydrogen bonding interactions had the main effects on the interaction of Co3O4 nanoparticles with acid phosphatase because the values of ∆H and ∆H were negative. So, Co3O4 nanoparticles increased the stability and activity of acid phosphatase.

Synthesis, characterization, CT‐DNA binding and docking studies of novel selenated ligands and their palladium complexes

A novel selenated Schiff base (S)‐L1Hhas been synthesized from (2S)‐1‐(benzylselanyl)‐3‐phenylpropan‐2‐amine which upon reduction formed a reduced Schiff base (S)‐L2H. Palladium (II) complexes (S)‐1and (S)‐2of ligands (S)‐L1Hand (S)‐L2Hrespectively were successfully synthesized. The structures of all four compounds were thoroughly identified by analytical and various spectroscopic techniques. The absolute molecular structures of the above two complexes were further confirmed by single crystal X‐ray diffraction. Both (S)‐L1Hand (S)‐L2Hcoordinated as monobasic ((S)‐L1–2), chelating, tridentate (Se,N,O−) ligands resulting in the complexes of composition (S)‐[PdCl(L1/2)] [(S)‐1/2]. In the crystals of complexes (S)‐1and (S)‐2, there were moderate to strong Se⋯O, CH⋯Cl and CH⋯O types of intermolecular secondary interactions. CT‐DNA binding activity of these selenium‐containing ligands and their palladium complexes bearing a Pd–Se bond have been evaluated for the first time by performing electronic absorption titration and fluorescence emission quenching using CT‐DNA‐EB and viscometric experiments. These ligands and complexes exhibited remarkable DNA binding activity as shown by their intrinsic DNA binding constants (Kb) and Stern–Volmer constants (Ksv) in the ranges 5.2–9.9 × 104and 3.6–4.7 × 104, respectively. The viscosity of CT‐DNA decreases with increasing concentration of these compounds. The results of the DNA‐binding studies revealed that all of the compounds interact with DNA at a minor groove which was further confirmed by molecular docking studies.

Synthesis of new morpholine containing 3-amido-9-ethylcarbazole derivative and studies on its biophysical interactions with calf thymus DNA/HSA

In this work, we presented the synthesis and investigation of binding properties of the new morpholine containing 3-amido-9-ethylcarbazole derivative (CMR) to calf thymus DNA (ctDNA) and human serum albumin (HSA) by fluorescence spectroscopy, UV absorption spectroscopy and molecular docking method. A decrease in Stern–Volmer constants was obtained with increase in temperature; it shows that static quenching mechanism leads to formation of new CMR-DNA/HSA complexes, which have hydrophobic interaction as the predominant role in the binding modes. Also, binding properties of DNA were investigated with competition assays on two probes (EB and H33258) by absorption, ionic strength and iodide ion quenching methods. The results suggested that CMR entered into the minor groove binding on the A-T region of DNA. The spectral data further confirmed by molecular docking which elicited that CMR complexes have similar interaction and conformation trends to each target, DNA and HSA. The experimental and computational results show that CMR has been classified as a promising molecule in drug designing of other carbazole derivatives. Communicated by Ramaswamy H. Sarma

Interaction of the Coffee Diterpenes Cafestol and 16-O-Methyl-Cafestol Palmitates with Serum Albumins.

The main coffee diterpenes cafestol, kahweol, and 16-O-methylcafestol, present in the bean lipid fraction, are mostly esterified with fatty acids. They are believed to induce dyslipidaemia and hypercholesterolemia when taken with certain types of coffee brews. The study of their binding to serum albumins could help explain their interactions with biologically active xenobiotics. We investigated the interactions occurring between cafestol and 16-O-methylcafestol palmitates with Bovine Serum Albumin (BSA), Human Serum Albumin (HSA), and Fatty Free Human Serum Albumin (ffHSA) by means of circular dichroism and fluorimetry. Circular Dichroism (CD) revealed a slight change (up to 3%) in the secondary structure of fatty-free human albumin in the presence of the diterpene esters, suggesting that the aliphatic chain of the palmitate partly occupies one of the fatty acid sites of the protein. A warfarin displacement experiment was performed to identify the binding site, which is probably close but not coincident with Sudlow site I, as the affinity for warfarin is enhanced. Fluorescence quenching titrations revealed a complex behaviour, with Stern–Volmer constants in the order of 103–104 Lmol−1. A model of the HSA-warfarin-cafestol palmitate complex was obtained by docking, and the most favourable solution was found with the terpene palmitate chain inside the FA4 fatty acid site and the cafestol moiety fronting warfarin at the interface with site I.

In-Situ Gold-Ceria Nanoparticles: Superior Optical Fluorescence Quenching Sensor for Dissolved Oxygen.

Cerium oxide (ceria) nanoparticles (NPs) have been proved to be an efficient optical fluorescent material through generating visible emission (~530 nm) under violet excitation. This feature allowed ceria NPs to be used as an optical sensor via the fluorescence quenching Technique. In this paper, the impact of in-situ embedded gold nanoparticles (Au NPs) inside ceria nanoparticles was studied. Then, gold–ceria NPs were used for sensing dissolved oxygen (DO) in aqueous media. It was observed that both fluorescence intensity and lifetime were changed due to increased concentration of DO. Added gold was found to enhance the sensitivity of ceria to DO quencher detection. This enhancement was due to optical coupling between the fluorescence emission spectrum of ceria with the surface plasmonic resonance of gold nanoparticles. In addition, gold caused the decrease of ceria nanoparticles’ bandgap, which indicates the formation of more oxygen vacancies inside the non-stoichiometric crystalline structure of ceria. The Stern–Volmer constant, which indicates the sensitivity of optical sensing material, of ceria–gold NPs with added DO was found to be 893.7 M−1, compared to 184.6 M−1 to in case of ceria nanoparticles only, which indicates a superior optical sensitivity to DO compared to other optical sensing materials used in the literature to detect DO. Moreover, the fluorescence lifetime was found to be changed according to the variation of added DO concentration. The optically-sensitivity-enhanced ceria nanoparticles due to embedded gold nanoparticles can be a promising sensing host for dissolved oxygen in a wide variety of applications including biomedicine and water quality monitoring.

UV/Vis and fluorescence study on the interaction of Ni(II) complex of Schiff base of glycine and chiral auxiliary (S)-2-[N-(N′-benzylprolyl)amino]benzophenone with bovine serum albumin

Binding interaction of Ni(II) complex of Schiff base of glycine and chiral auxiliary (S)-2-[N-(N′-benzylprolyl)amino]benzophenone (Ni(II)-(S)-BPB-Gly) with bovine serum albumin (BSA) at different temperatures (293, 298, and 308 K) has been studied using UV/Vis and fluorescence spectroscopy methods. A gradual decrease in the Stern–Volmer constant with the increase in temperature showed the static mechanism of BSA fluorescence quenching in the presence of Ni(II)-(S)-BPB-Gly initiated by the formation of ground-state complex. Analysis of the binding constants, number of binding sites, and thermodynamic parameters (∆H0, ∆S0, ∆G0) suggested that Ni(II)-(S)-BPB-Gly spontaneously and exothermically binds to BSA mainly driven by hydrogen bonding and van der Waals forces. The binding distances between Ni(II)-(S)-BPB-Gly and BSA in the temperature range 293–308 K determined using Forster’s resonance energy transfer theory (found to be 2.413, 2.445, and 2.459 nm) showed that the binding properties decrease with the temperature rising.

Effect of Acetonitrile on the Colloidal Behavior of Conventional Cationic Surfactants: A Combined Conductivity, Surface Tension, Fluorescence and FTIR Study

A comprehensive study the effect of acetonitrile (ACN) with four cationic surfactants, viz. tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC) and cetylpyridinium bromide (CPB) was made by using the conductivity, surface tension, fluorescence and FTIR techniques. Significant micellar, interfacial and thermodynamic properties were studied by the tensiometeric and conductivity methods. The critical micelle concentration (CMC), aggregation number (Nagg), and Stern–Volmer constants (Ksv) have also been studied by the steady state fluorescence method using pyrene as probe. The fluorescence study also supports the CMC results obtained from conductivity and surface tension. FTIR was used to ascertain that the strength of intermolecular interactions such as hydrogen bonding, ion–ion pair interactions and induced dipole interactions between the surfactants and ACN depend upon the head-group of the surfactants. The interaction of surfactants with ACN is energetically favorable and occurs via direct interactions between the surfactants and ACN. The results further revealed that the strength of interactions between the surfactants and ACN follows the order: TTAB > CTAB > CPC > CPB.

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.

Biophysical and computational approaches to unravel the molecular interaction mechanism of bromodeoxyuridine, a proliferative marker with human serum albumin

The interaction of a nucleoside analogue bromodeoxyuridine (BrdU) with human serum albumin (HSA) was studied to investigate the binding phenomenon and analyse the protein conformation upon BrdU binding. Multiple spectroscopic techniques, viz. intrinsic and 3-D fluorescence, UV–Vis absorption, and circular dichroism (CD) spectroscopy along with molecular docking were used. Decrease in the Stern–Volmer constant (Ksv) as well as the association constant (Ka) with increase in temperature suggested BrdU–HSA complex formation. Intermediate binding affinity between BrdU and HSA was evident from the Ka values (2.49–3.97 × 104 mol–1 dm3), while BrdU–HSA complex formation was driven by hydrophobic and van der Waals interactions along with hydrogen bonds, as revealed by thermodynamic data (ΔS = + 28.48 J mol−1 K−1; ΔH = − 17.16 kJ mol−1). Minor changes occur in both secondary and tertiary structures as well as in the fluorophores’ microenvironment of HSA, as recognized from the CD spectral results in the far-UV and the near-UV regions and 3-D fluorescence spectra, respectively. Use of site markers (warfarin and indomethacin for site I; diazepam for site II) as well as docking results suggested BrdU binding to both site I (more preferred) and site II, located in subdomains IIA and IIIA, respectively, of HSA.

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.

A Naphthalimide-Benzothiazole Conjugate as Colorimetric and Fluorescent Sensor for Selective Trinitrophenol Detection

Although chemical structural modification of naphthalimides is widely employed for the purpose of sensing explosives, the effects of such modification have been little explored. Herein, we report the design and synthesis of a new naphthalimide-benzothiazole conjugate (1) and its ability to sense various nitrophenols by means of its colorimetric and fluorescent characteristics. Under long-range UV light (365 nm), 1 displayed a color change of its solution from bluish to colorless only upon addition of 2,4,6-trinitrophenol (TNP). Photoluminescence spectroscopy showed quantitative fluorescence quenching by TNP of the emission peaks of 1 at 398 nm and 418 nm due to donor–acceptor electron transfer. The interaction of 1 with TNP was via a cooperative, non-covalent hydrogen-bonding interaction. Receptor 1 exhibited high sensitivity and selectivity towards TNP over various aromatic nitro analytes. The binding constant (K) and Stern–Volmer constant (Ksv) between 1 and TNP were found to be 5.332 × 10−5 M and 2.271 × 106 M−1, respectively. Furthermore, the limit of detection was calculated and found to be as low as 1.613 × 10−10 M.

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

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

Aligned Tubular Conjugated Microporous Polymer Films for the Aggregation‐Induced Emission‐Based Sensing of Explosives

AbstractThis work shows the sensing performance of conjugated microporous polymer (CMP) tubes. Aligned tubular CMP films (CMP‐AT) are synthesized by a template method. The Sonogashira coupling of tetra(4‐ethynyl)phenylethylene with 1,4‐diiodobenzene in the cylindrical pores of anodic aluminum oxide (AAO) plates and the etching of templates result in the CMP‐AT films. Due to the tetraphenylethylene moieties in the materials, the CMP‐AT films show aggregation‐induced emission (AIE). Based on emission‐quenching behavior, the sensing performance of CMP‐AT films toward model explosives, nitrotoluenes, is studied. The CMP‐AT films having longer CMP tubes with thinner wall thickness show better sensing performance with the Stern–Volmer constant (Ksv) values up to 92 400 M−1 toward 2,4‐dinitrotoluene. The reduced diffusion pathway of substrates by the thin wall of the CMP tubes is critical for the AIE quenching‐based sensing of nitrotoluenes. These observations indicate that the functionality of CMP materials can be further enhanced by their morphological engineering. Due to the chemical stability of CMP materials, the CMP‐AT‐5 film can be recycled at least five times, maintaining the original sensing performance and tubular morphology.

Luminescent Lanthanide-Based Probes for the Detection of Nitroaromatic Compounds in Water.

A new mixed pyridyl–carboxylate ligand with two picolinate chromophores and a flexible linear spacer, potassium 2,2′-(butane-1,4-diylbis((pyridin-2-ylmethyl)azanediyl))diacetate (K2bpbd), which is obtained in high yield and spectroscopically characterized, has been utilized to make new lanthanide complexes, namely, [Ln(bpbd) (H2O)2(NO3)]·xH2O, where Ln = Tb (1) and x = 6, Ln = Sm (2) and x = 7, and Ln = Dy (3) and x = 7. These complexes have been extensively characterized by various spectroscopic techniques (UV–vis and Fourier transform infrared spectroscopy), elemental analyses, thermogravimetric analysis, field emission scanning electron microscopy, and powder X-ray diffraction. These show very intense characteristic luminescence features that confirm the antenna effect of the ligand on the metal center. These complexes have been utilized for the detection of various nitroaromatic compounds. Among these three complexes, 1 is found to be the best for the selective sensing of 2,4,6-trinitrophenol in water with a detection limit of (0.35 ± 0.05) ppm. Its Stern–Volmer constant, KSV [(5.48 ± 0.1) × 104 M–1], is one of the highest among similar sensors reported so far.

A novel cadmium(II) complex of bipyridine derivative: synthesis, X-ray crystal structure, DNA-binding and antibacterial activities

A mononuclear cadmium(II) complex of formula [Cd(5,5′-dmbipy)2(OAc)2]·2H2O (5,5′-dmbipy = 5,5′-dimethyl-2,2′-bipyridine and OAc = acetato ligand) has been synthesized and characterized by FT-IR, UV–Vis, 1H-NMR, elemental analysis and single-crystal X-ray structure analysis. The molecular structure of the complex shows a distorted tetragonal antiprism CdN4O4 coordination geometry around the cadmium atom, resulting in coordination by four nitrogen atoms from two 5,5′-dmbipy ligands and four oxygen atoms from two acetate anions. The interaction of this complex to FS-DNA (fish sperm DNA) has also been studied by electronic absorption, fluorescence and gel electrophoresis techniques. Binding constant (Kb), Stern–Volmer constant (Ksv), number of binding sites (n) and bimolecular quenching rate constant (kq) have been calculated from these spectroscopic data. These results have revealed that the metal complex can bind effectively to FS-DNA via groove binding. The calculated thermodynamic parameters (ΔH°, ΔS° and ΔG°) show that hydrogen bonding and van der Waals forces have an important function in the Cd(II) complex–DNA interaction. The antibacterial effects of the synthesized cadmium complex have also been examined in vitro against standard bacterial strains: one Gram-positive (Staphylococcus aureus, ATCC 25923) and one Gram-negative (Escherichia coli, ATCC 25922) bacteria, using disk diffusion and macro-dilution broth methods. The obtained results show that the Cd(II) complex exhibits a marked antibacterial activity which is significantly better than those observed for its free ligand and metal salt for both Gram-positive and Gram-negative bacteria. However, this metal complex is a more potent antibacterial agent against the Gram-positive than that of the Gram-negative bacteria.Communicated by Ramaswamy H. Sarma

Influence of solvent, electrolytes, β-CD, OTAB on the krafft temperature and aggregation of sodium tetradecyl sulfate

The krafft temperature (Tk) of sodium tetradecyl sulfate (STS) has been determined conductometrically in aqueous, different aquo-organic solvent media (dioxane, formamide, and ethylene glycol), different concentrations of additives viz. NaCl, β-CD, and octadecyltrimethylammonium bromide (OTAB). The change in Tk in different media has been notified and summarized in tabular form. The micellar properties of STS also have been determined in binary aqueous mixtures of dioxane (DO) and formamide (FA) by tensiometry, conductometry and isothermal titration calorimetry. Different interfacial parameters, Gibbs surface excess (Γmax), area minima per surfactant (Amin), surface pressure (ΠCMC), and packing parameter (P) have been calculated from related well-known equations. Thermodynamic parameters, such as free energy for micellization (ΔGm0), free energy of adsorption (ΔGads0), change in enthalpy (ΔHm0), and entropy change associated with micellization (ΔSm0) have been also deduced. Aggregation number and Stern Volmer constant of STS in DO and FA have been determined from steady state fluorescence quenching study using fluorimetric technique.

Ordering selected Zn(II), Cu(II), Pd(II) and Co(III) complex compounds: their separately and combinedly antibacterial therapy and DNA-binding studies

In this study, four Co(III)-, Cu(II)-, Zn(II)- and Pd(II)-based potent antibacterial complexes of formula K3[Co(ox)3]·3H2O (I), [Cu(phen)2Cl]Cl·6.5H2O (II), [Zn(phen)3]Cl2 (III) and [Pd(phen)2](NO3)2 (IV) (where ox is oxalato and phen is 1,10-phenanthroline) were synthesized. They were characterized by elemental analysis, molar conductivity measurements, UV–vis, Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H-NMR) techniques. These metal complexes were ordered in three combination series of I+II, I+II+III and I+II+III+IV. Antibacterial screening for each metal complex and their combinations against Gram-positive and Gram-negative bacteria revealed that all compounds were more potent antibacterial agents against the Gram-negative than those of the Gram-positive bacteria. The four metal complexes showed antibacterial activity in the order I > II > III > IV, and the activity of their combinations followed the order of I+II+III+IV > I+II+III > I+II. The DNA-binding properties of complex (I) and its three combinations were studied using electronic absorption and fluorescence (ethidium bromide displacement assay) spectroscopy. The results obtained indicated that all series interact effectively with calf thymus DNA (CT-DNA). The binding constant (Kb), the number of binding sites (n) and the Stern–Volmer constant (Ksv) were obtained based on the results of fluorescence measurements. The calculated thermodynamic parameters supported that hydrogen bonding and van der Waals forces play a major role in the association of each series of metal complexes with CT-DNA and follow the above-binding affinity order for the series.Communicated by Ramaswamy H. Sarma

Inclusion complex of Tramadol in β-cyclodextrin enhances fluorescence by preventing self-quenching

Fluorescence self-quenching occurs at high concentration. Inhibition of self-quenching by inclusion of fluorescence emitters inside the hydrophobic cavity of β-cyclodextrin (β-CD) has been addressed taking the example of the fluorescence behavior of Tramadol hydrochloride. Indeed complexation by β-CD enhanced fluorescence emission of Tramadol under conditions where self-quenching was operative. A quantitative account of self-quenching and its inhibition by β-CD was done through determination of complexation equilibrium by 1H NMR experiments and a detailed study of absorption and fluorescence properties. Tramadol and β-CD associate as a complex of 1:1 stoichiometry with a formation constant K11 = 260. Complexation of Tramadol by β-CD does not cause modification of its absorbance and fluorescence spectra. Fluorescence self-quenching of Tramadol above ∼ 1 mmol·L−1 was characterized by a Stern–Volmer constant K = 810 L·mol−1. Inhibition of self-quenching by formation of an inclusion complex was manifested by lower Stern–Volmer constants in the presence of β-CD. Such study required a correct account of Inner Filter Effects on fluorescence, which is mandatory in all physicochemical studies using fluorescence where concentrations are rather high.

A new multifunctional two-dimensional cobalt(II) metal–organic framework for electrochemical detection of hydrogen peroxide, luminescent sensing of metal ions, and photocatalysis

A new multifunctional cobalt(II)-based metal–organic framework, [Co(DCTP)(L)(H2O)2]n (NCST-1, NCST = North China University of Science and Technology), was synthesized by self-assembly of Co(OAc)2·4H2O with 1,4-bis(benzimidazol-1-ylmethyl)-benzene (L) and 2,5-dichloroterephthalic acid (H2DCTP). Single-crystal X-ray diffraction analyses reveal that NCST-1 is a 2D (4,4) sheet, which is further expanded into a 3D supramolecular network by OH⋯O hydrogen bond interactions. A non-enzyme electro-catalytic sensor was developed for sensing hydrogen peroxide (H2O2) based on the NCST-1, which was cast-coated on glassy carbon electrode (GCE). The modified sensor exhibited a remarkable performance for H2O2 reduction with an extended liner range (5 μM–7 mM, 7 mM–18 mM), a fast response time (3 s), a high sensitivity (52.55 μA mM−1 cm−2), and a low detection limit (6.86 μM) in alkaline solution. Furthermore, NCST-1, as a luminescent probe, displayed prominent multifunctional sensing property for Fe3+, Hg2+ as well Ag+ through fluorescence quenching effect. The emission intensity of NCST-1 increases linearly with the increasing concentration of Fe3+ with a Stern–Volmer constant (Ksv) of 1.12 × 104 M−1 and a detection limit (LOD) of 0.13 μM in aqueous solution. For mercury ion (II), Ksv = 7.58 × 103 M−1 and LOD = 1.10 μM. For silver ion (I), Ksv = 6.22 × 103 M−1 and LOD = 1.33 μM. Finally, the photocatalytic activities of NCST-1 were evaluated in the degradation of methylene blue (MB), Rhodamine B (RhB), and methyl orange (MO). The degradation efficiency for MB reached up to 91.3%, although NCST-1 hardly showed degradation ability towards RhB and MO.