Molecular‐Recognition‐Assisted pKa Shifts and Metal‐Ion‐Induced Fluorescence Regeneration in p‐Sulfonatocalix[6]arene‐Encapsulated Acridine

The host–guest interactions of cationic (AcH⁺) and neutral (Ac) forms of the dye acridine with the macrocyclic host p‐sulfonatocalix[6]arene (SCX6) were investigated by using ground‐state absorption, steady‐state and time‐resolved fluorescence, and NMR measurements. The cationic form undergoes significant complexation with SCX6 (K_eq=2.5×10⁴ M ^?1), causing a sharp decrease in the fluorescence intensity and severe quenching in the excited‐state lifetime of the dye. The strong binding of the AcH⁺ form of the dye with SCX6 is attributed to ion–ion interactions involving the sulfonato groups (SO₃^?) of SCX6 and the positively charged AcH⁺ at pH of approximately 4.3. Whereas, the neutral Ac form of the dye undergoes weak complexation with SCX6 (K_eq=0.9×10³ M ^?1) and the binding constant is lowered by one order of magnitude compared with that of the SCX6–AcH⁺ system. The strong affinity of SCX6 to the protonated form leads to a large upward pK_a shift (≈2 units) in the dye. In contrast, strong emission quenching upon SCX6 interaction and the regeneration of fluorescence intensity of the dye in the presence of Gd³⁺ through competitive binding have also been demonstrated.     

Study on the inclusion interactions of β-cyclodextrin and its derivative with dyes by spectrofluorimetry and its analytical application

The inclusion interactions of β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) with dyes were developed by spectrofluorimetry, and the inclusion constants of inclusion complexes were determined by direct fluorescence technique. The main factors (the host molecule, the guest molecule, and the pH) for the inclusion interaction were discussed in detail. At the same time, the inclusion interaction of HP-β-CD and vitamin B₆ (VB₆) was investigated with the competitive fluorescence inclusion method and the inclusion constant of HP-β-CD and vitamin B₆ (VB₆) was obtained by indirect fluorescence technique. On the basis of the linear relationship between the change of fluorescence intensity (ΔF) and the concentration of VB₆, a competitive fluorescence inclusion method was used to the determination of VB₆. The method has been successfully applied to the analysis of VB₆ in synthetic samples, tablets and injections with satisfactory results.     

Analysis of binding interaction between vitamin B2 and trypsin

The interaction between vitamin B₂ (VB₂), a type of necessary nutrient for the body’s metabolism and repair, and trypsin, a serine protease found in the digestive system, has been investigated in vitro under a simulated physiological condition by UV–Vis spectrophotometry and fluorescence spectrometry. The intrinsic fluorescence intensity of trypsin was strongly quenched by VB₂. Spectrophotometric observations are rationalized in terms of a static quenching process at lower concentrations of VB₂ and a combined quenching (both dynamic and static) process at higher concentrations of VB₂. The binding parameters, such as the binding constants and the number of binding sites, can be evaluated by fluorescence quenching experiments. The apparent binding constants K between VB₂ and trypsin at different temperatures were 1.406, 1.264, and 0.543 × 10⁶ L mol^?1 and the numbers of binding sites n were 1.386, 1.391, and 1.319, which were all evaluated by the fluoresence quenching experiments. The negative values of ΔG for the formation of the trypsin–VB₂ complex implied that the binding was a spontaneous process. According to the van’t Hoff equation, the standard enthalpy change (ΔH) and standard entropy change (ΔS) for the reaction were calculated to be ?49.817 kJ mol^?1 and ?56.219 J mol^?1 K^?1, respectively, indicating that the hydrophobic interaction played a significant role in VB₂ binding to trypsin. In addition, the binding distance between VB₂ (acceptor) and trypsin (donor) was estimated to be 1.11 nm according to Förster’s resonance energy transfer theory. The results obtained here will be of biological significance in pharmacology and clinical medicine.     

Investigation on the Inclusion Behavior of ApoCopC with Vitamin B6

In neutral phosphate buffer solution of pH 7.0, the interaction between apoCopC and Vitamin B₆ has been investigated in detail by means of fluorescence spectroscopy. According to the change of Vitamin B₆ fluorescence spectra and fluorescence polarization, we can conclude that a novel supramolecular system is generated. ApoCopC can form a 1:5 host-guest inclusion supramolecular complex with Vitamin B₆, and the formation constant has been calculated to be (2.24 ± 0.08) × 10⁴ M^? 1. It suggests the strong inclusion ability of apoCopC to the guest molecules. In addition, the mechanism of the apoCopC protein fluorescence quenching by Vitamin B₆ was also discussed. And based on the Stern-Volmer equation, the apparent quenching constant was estimated to be (2.75 ± 0.05) × 10⁴ M^? 1.     

Complexes of Pt(II) and Pt(IV) with disubstituted purines

Mixed-ligand complexes of Pt(II) and Pt(IV) with 2,6-diaminopurine and 6-thioguanine were synthesized and characterised. The complexes were prepared in acidic and basic media. The binding of the ligands to the metal ion varies according to the pH of the medium. Thus, in the complexes of 6-thioguanine, the ligand acts as a monodentate ligand coordinating through the neutral C₆-SH group in the acidic medium and in the basic medium as a bidentate ligand binding to the metal ion through C₆S^? and N₇, forming a five-membered chelate ring. In an acidic medium 2,6-diaminopurine forms mononuclear complexes with Pt(II) and Pt(IV) binding through N₇. In a basic medium binuclear hydroxobridged complexes are formed with Pt(IV) and the ligand is monodentate, coordinating through N₇.     

Electrocatalytic Oxidation of Vitamin B6 by 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl

Introduction Nitroxides such as 2,2,6,6-tetramethylpiperidine-N- oxyl (TEMPO) are well-known stable free radicals which have been extensively used in spin labeling,1 spin trapping2 and as antioxidants.3 Nitroxides are also easy to undergo reversible one-electron oxidation to form the corresponding oxoammonium ions.4 Oxoammonium ions are mild one-electron oxidants which have been used in organic synthesis5-8 and to generate radical cations.9 We10 found recently that electrochemically generat…     

Determination of thiabendazole in aqueous solutions using a cucurbituril-enhanced fluorescence method

The complexation of thiabendazole (TBZ) with the cucurbit[6]uril (Q[6]), cucurbit[7]uril (Q[7]) and symmetric tetramethyl-cucurbit[6]uril (TMeQ[6]) in aqueous solution has been investigated using UV–vis and fluorespectrometry. The experimental results show 1:1 host–guest inclusion complexes at pH 6.5 for all three macrocyclic hosts, and the corresponding formation constants by UV and fluorescence methods are (5.37?±?1.05)?×?10⁴?L?mol^?1 and (1.47?±?0.41)?×?10⁴?L?mol^?1 for the Q[6]-TBZ system (7.76?±?0.51)?×?10⁴?L?mol^?1 and (9.36?±?0.22)?×?10⁴?L?mol^?1 for the Q[7]-TBZ system (1.28?±?0.78)?×?10⁴?L?mol^?1 and (2.69?±?0.55)?×?10⁴?L?mol^?1 for the TMeQ[6]-TBZ system, respectively. Based on the enhancement of the fluorescence intensity of TBZ with the addition of Q[n]s in neutral media, a fluorespectrometry method for the determination of TBZ in aqueous solution in the presence of Q[n] was established. In the range of 6.0?×?10^?8?mol?L^?1–8.0?×?10^?6?mol?L^?1 a linear relationship was obtained between fluorescence intensity and TBZ concentration. The detection limit was found to be between 5.51 and 8.85?×?10^?9?mol?L^?1. The interference of coexisting ions was found to be slight. The proposed method has been successfully applied to the determination of TBZ in different aqueous solutions with satisfactory recoveries of 92–103%. The method seems to be suitable for environmental water analysis.     

Complex Formation of Hematoporphyrin with Cyclodextrins and 1,1′-Diheptyl-4,4′-bipyridinium Dibromide in Aqueous Solutions

At around 5×10^-6?mol?dm^-3 of hematoporphyrin (HP), an HP dimer exists as well as an HP monomer. The equilibrium constant for the dimerization of HP in pH 10.0 buffer has been evaluated to be 1.70×10⁵?mol^-1?dm³ from the HP concentration dependence of the absorption spectrum. In aqueous solution, HP forms 1:1 inclusion complexes with β-cyclodextrin (β-CD), γ-cyclodextrin (γ-CD), and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TM-β-CD). The fluorescence of HP is significantly enhanced by the addition of CDs. From simulations of the fluorescence intensity changes, the equilibrium constants for the formation of the CD–HP inclusion complexes have been estimated to be 200, 95.7, and 938?mol^-1?dm³ for β-CD, γ-CD, and TM-β-CD, respectively. HP forms a 1:1 complex with 1,1′-diheptyl-4,4′-bipyridinium dibromide (DHB) in aqueous solution. In contrast to the addition of CDs, the HP fluorescence is significantly quenched by the addition of DHB. The equilibrium constant for the formation of the HP–DHB complex has been evaluated to be 1.98×10⁵?mol^-1?dm³ from the fluorescence intensity change of HP. The addition of DHB to an HP solution containing β-CD induces a circular dichroism signal of negative sign, indicating the formation of a ternary inclusion complex involving β-CD, HP, and DHB. In contrast, there is no evidence for the formation of a ternary inclusion complex of HP with DHB and TM-β-CD.     

Spectrofluorometry study of β-cyclodextrin and N-phenyl-1-naphthylamine inclusion complex and its analytical application via artificial neural network

The interaction between β-cyclodextrin (βCD) and N-phenyl-1-naphthylamine has been studied spectrofluorimetrically and found to form a 1:1 (βCD:NPN) inclusion complex at optimum conditions of pH 8 with the formation time of 120?s. The association constant of the complex was established to be 6.0?×?10²?M, while a remarkable enhancement in fluorescence intensity was also observed at 445?nm with excitation of 334?nm. A spectroflurometric method for the detection of N-phenyl-1-naphthylamine has been developed having a dynamic range linear up to 4.67?×?10^?7?M with a limit of detection of 0.58?nM. The repeatability study at two different βCD concentrations of 1.0?×?10^?4 and 4.0?×?10^?4?M was found to give RSD values of 2.40 and 1.42%, respectively. Artificial neural network (ANN) has been utilised to model the analytical system and successfully extended the analytical dynamic range up to 8.0?×?10^?7?M from the original 4.67?×?10^?7?M, brief network training and the optimum parameters of are described in this work.     

Fluorescence resonance energy transfer between acridine orange and rhodamine 6G and its analytical application for vitamin B12 with flow-injection laser-induced fluorescence detection

By coupling flow-injection with laser-induced fluorescence detection, a setup was developed and a novel method combining fluorescence resonance energy transfer (FRET) and flow-injection analysis (FIA) was proposed for the determination of vitamin B₁₂ (VB₁₂) based on its fluorescence quenching on the system of acridine orange (AO)/rhodamine 6G (R6G). The effective energy transfer could occur between AO and R6G in the dodecyl benzene sodium sulfonate (DBS) while 454 nm argon laser was used as the excitation source, and as a result, the fluorescence emission of R6G has been increased significantly. It was found that the fluorescence of the above system could be sharply diminished by VB₁₂. By using the mixed solution AO-R6G-DBS and the same solution containing VB₁₂ as the carrier and sample, respectively, a series of negative peaks which could be applied for the quantification of VB₁₂ were obtained. The detection limit for VB₁₂ was 1.65 × 10⁻⁶ mol/L. The linear range for determining VB₁₂ was 4 × 10⁻⁴ to 2 × 10⁻⁶ mol/L (correlation coefficient, r = 0.9923). The method was applied to measure VB₁₂ injections with satisfactory results.     

Host–guest Complex of a Water-soluble Cucurbit[6]uril Derivative with the Hydrochloride Salt of 3-amino-5-phenylpyrazole

Interaction between tetramethylcucurbit[6]uril and 3-amino-5-phenylpyrazole hydrochloride in aqueous solution has been investigated by using ¹H NMR spectroscopy, electronic absorption spectroscopy and fluorescence spectroscopy, as well as by a single crystal X-ray diffraction determination. The ¹H NMR spectra analysis established a basic interaction model in which an inclusion complex with a host:guest ratio of 1:1 forms, in which the host selectively binds the phenyl moiety of the guest. Absorption spectrophotometric and fluorescence spectroscopic analysis in aqueous solution defined the stability of the host–guest inclusion complexes quantitatively as 6.8 × 10⁵ mol^? 1 L at pH 2.6; the interaction is pH dependent, decreasing as pH rises. The single crystal X-ray structure of the isolated inclusion complex shows the phenyl moiety of the guest inserted into the host cavity, which supports particularly the ¹H NMR spectroscopic study in solution. In the crystal structure of the inclusion complex, the host–guest interaction involves both inter- and intra-complex hydrogen bonding, forming 2:2 dimers that stack in one dimension as supramolecular tubes.     

Fluorescence of auramine O and its binding to poly(vinylbenzo-18-crown-6) and to polyvinylbenzoglyme in water

Binding of the cationic dye auramine O (AuO) to the polysoap-type polymers poly(vinylbenzo-18-crown-6) (P18C6) and polyvinylbenzoglyme (PVBG) in water were studied by fluorimetry and dialysis. The quantum yield of P18C6-bound AuO was found to be 0.028, the value being 0.018 for AuO bound to PVBG. The intrinsic binding constants were found to be 2.2 × 10⁴M^?1 (P18C6) and 1.2 × 10⁴M^?1 (PVBG), the respective first binding constants being 317 and 63M^?1. Addition of crown-ether-complexable cations such as K⁺, Tl⁺, or Cs⁺ converts the neutral poly(crown ether) into a polycation, causing repulsion of the cationic dye and a strong decrease in the AuO fluorescence. AuO fluorescence was also studied in the absence of polymer in ether solvents, giving θ values of 0.011 and 0.018 in THF and dioxane. Traces of water rapidly form a nonfluorescent species. Solutions of AuO in water without polymer present exhibit very strong fluorescence on addition of BPh₄ anions owing to formation of AuO⁺, BPh₄^? ion pairs and higher aggregates.     

Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N‐Oxide Complexes

A study of the strong N?X???^?O?N⁺ (X=I, Br) halogen bonding interactions reports 2×27 donor×acceptor complexes of N‐halosaccharins and pyridine N‐oxides (PyNO). DFT calculations were used to investigate the X???O halogen bond (XB) interaction energies in 54 complexes. A simplified computationally fast electrostatic model was developed for predicting the X???O XBs. The XB interaction energies vary from ?47.5 to ?120.3 kJ mol^?1; the strongest N?I???^?O?N⁺ XBs approaching those of 3‐center‐4‐electron [N?I?N]⁺ halogen‐bonded systems (ca. 160 kJ mol^?1). ¹H NMR association constants (K_XB) determined in CDCl₃ and [D₆]acetone vary from 2.0×10⁰ to >10⁸ m ^?1 and correlate well with the calculated donor×acceptor complexation enthalpies found between ?38.4 and ?77.5 kJ mol^?1. In X‐ray crystal structures, the N‐iodosaccharin‐PyNO complexes manifest short interaction ratios (R_XB) between 0.65–0.67 for the N?I???^?O?N⁺ halogen bond.     

Kinetics of the photoaquation of Hexacyanoferrate(II) ion

Kinetics of the photoaquation of hexacyanoferrate(II) ion in aqueous solution were studied potentiometrically and spectrophotometrically. Supposing the simplest mechanism (see Fig. 3. in text), the photoaquation in alkaline medium can be well described. The value of the constants at pH = ll.0 are: ø = 0.8-1.0, k₆ = (3.0 ± 0.5) × 10^?8 s^?1 and k_?6 = 1.5 ± 0.2 mol^?1 dm₃ s^?1. To describe the photoaquation in neutral medium t was extended (k′ = 3.33 x 10² mol^?1 dm³s^?1). The quantum yield in acidic medium can be calculated by combination of ø values of different protonated complexes. The reversibility of photoaquation in alkaline medium is also explained by the scheme.     

Further understanding of the multiple equilibria interaction pattern between ionic liquid and β-cyclodextrin

The interactions of the ionic liquids 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C _n mimTf₂N, n = 2, 4, 6, 8, 10, 12) with β-cyclodextrin (β-CD) in aqueous solutions are investigated in this work. The stoichiometry and apparent association constants are obtained by competitive fluorescence method and isothermal titration calorimetry (ITC). The results show that C₂mimTf₂N, C₄mimTf₂N, and C₆mimTf₂N mainly form 1:1 (guest:host) inclusion complexes with β-CD, whereas C₈mimTf₂N, C₁₀mimTf₂N, and C₁₂mimTf₂N form both 1:1 and 1:2 inclusion complexes, the latter of which are mainly attributed to the formation of the C _n mim⁺–2β-CD complexes. Besides, Tf₂N^? only forms the 1:1 complex with β-CD owing to a charge resonance structure that breaks the symmetry of the structure of Tf₂N^?, which is proved by Fourier transform infrared spectra. The thermodynamic parameters obtained by ITC reveal that the formation of the inclusion complexes are enthalpy-controlled for C₂mimTf₂N, C₄mimTf₂N, and C₆mimTf₂N, while for the C₈mimTf₂N/β-CD system, the process becomes entropy and enthalpy driven. Based on high-resolution mass spectrometry used with electrospray ionization results, the interaction between C _n mimTf₂N and β-CD is found to follow the multiple equilibria interaction pattern which was suggested in our previous work.     

A thermodynamic study of α-, β-, and γ-cyclodextrin-complexed m-methyl red in alkaline solutions

The UV/Visible spectra of m-methyl red (m-MR) ({3-[4-(dimethyl-amino) phenylazo] benzoic acid}) were examined in basic, acidic and strongly acidic aqueous solutions. The observed spectra of m-MR were analyzed and compared with the tautomeric and resonance structures that suggested theoretically. Three isosbestic points in the spectra were observed around 508, 464 and 443 nm representing three different equilibriums between four different species of m-MR. The inclusion constant (K_f) for the inclusion of basic form of m-MR with alpha-, Beta-, and gamma-Cyclodextrin (α-, β- and γ-CD) was evaluated at different temperatures using Benesi-Hildebrand method. The values of K_f at 25 °C were found to be 8.70 × 10³, 4.93 × 10³ mol^?1 dm³ and 2.95 × 10⁷ mol^?2 dm⁶ basis on the inclusion complex ratios (m-MR:CD) of 1:1, 1:1, and 2:1 respectively. The values of the thermodynamic quantities ΔH°, ΔS°, ΔG° for the different inclusion processes were calculated by using Van’t Hoff plot. For all cases of the studied inclusion processes, these inclusions were favored through entropy and enthalpy changes.     

Kinetic studies on the thermal dissociation of β-cyclodextrin-cinnamyl alcohol inclusion complex

The stability of β-cyclodextrin-cinnamyl alcohol inclusion complex (β-CD·C₉H₁₀·8H₂O) was investigated using TG and DSC. The mass loss took place in three stages: the dehydration occurred between 50–120°C; the dissociation of β-CD·C₉H₁₀O occurred in the range of 210–260°C; and the decomposition of β-CD began at 280°C. The dissociation of β-CD·C₉H₁₀O was studied by means of thermogravimetry, and the results showed: the dissociation of β-CD·C₉H₁₀O was dominated by a two-dimensional diffusion process (D₂). The activation energyE was 161.2 kJ mol^?1, the pre-exponential factorA was 4.5×10¹³ min^?1. Cyclodextrin is able to form inclusion complexes with a great variety of guest molecules, and the interesting of studies focussed on the energy binding cyclodextrin and the guest molecule. In this paper, β-cyclodextrin-cinnamyl alcohol inclusion complex was studied by fluorescence spectrophotometry and infrared absorption spectroscopy, and the results show: the stable energy of inclusion complexes of β-CD with weakly polar guest molecules consists mainly of Van der Waals interaction.     

Electroconductivity and transport numbers of solid electrolytes La10−x CaxA6O27−δ and La9.33+δA6−x AlxO26 (A = Si, Ge) with apatite-like structure

The ion-oxygen conductivity of apatite-like compounds based on lanthanum silicates and germanates La₁₀A₆O₂₇ (A = Si, Ge), La_10?x Ca_xSi₆O_27?δ (x = 0.25, 0.5, 1.0), La_9.75Ca_0.25Ge₆O_27?δ and La_9.33+δSi_6?x Al_xO₂₆(x=0.4, 0.8, 1.5) is studied in the interval of partial oxygen pressures pO₂ extending from 10^?16 to 10⁵ Pa, at temperatures of 500–1000°C. The electroconductivity of undoped compounds La₁₀A₆O₂₇ (A = Si, Ge) exceeds that of yttria-stabilized zirconia. The electroconductivity of lanthanum germanate (1.7 × 10^?2 and 8.5 × 10^?2S cm^?1 at 700 and 900°C, respectively) is substantially higher than that of lanthanum silicate (9.8 × 10^?3 and 3.5 × 10^?2 S cm^?1 at 700 and 900°C). Doping lanthanum germanate with calcium raises its electroconductivity (2.7 × 10^?2 and 1.3 × 10^?1 S cm^?1 for La_9.75Ca_0.25Ge₆O_27?δ at 700 and 900°C). Conversely, doping lanthanum silicate with ions of calcium or aluminum reduces the conductivity. In the pO₂ interval studied, the above compounds are ionic conductors and represent a class of solid electrolytes of promise for various electrochemical devices.     

Investigation on the inclusion behavior of Norfloxacin with 2-methyl-β-cyclodextrin

The formation of the complexes of Norfloxacin with 2-methyl-β-cyclodextrin (Me-β-CD) was studied by UV-Vis absorption spectroscopy, fluorescence and nuclear magnetic resonance spectroscopy (NMR). Experimental conditions including Me-β-CD concentration and media acidity were investigated in detail at room temperature. The results suggest that Norfloxacin exists in three molecular forms in aqueous solution at different pH values, namely, the acidic form, the neutral form and the alkaline form). Me-β-CD was more suitable for inclusion of Norfloxacin in the acidic medium. The binding constant (K) of the inclusion complex was determined by fluorescence measurement, and the complexation ratio was determined as 1:1 in the concentration range used in this study. A mechanism was proposed to explain the inclusion process based on the experimental NMR data.     

Synthesis,structure, spectroscopic,and DNA binding properties of Co(III) and Ni(II) complexes with ((E)-2-(amino((pyridin-2-ylmethylene)amino)methylene)maleonitrile)

Two new complexes, [Co(L)₂]Cl·(MeOH)₂ (1) and [Ni(L)₂]₄·EtOH (2) (L?=?(E)-2-(amino((pyridin-2-ylmethylene)amino)methylene)maleonitrile), were synthesized and characterized by X-ray crystallography, IR, UV, and fluorescence spectroscopy. According to X-ray crystallographic studies, each metal was six-coordinate with six nitrogens from two ligands. Both complexes form two-dimensional supramolecular networks via hydrogen bonding and π–π interactions. Ultraviolet and visible spectra showed that absorptions arise from π–π ^?, MLCT, and d–d electron transitions. Fluorescence spectroscopy revealed moderate intercalative binding of these two complexes with EB–DNA, with apparent binding constant (K _app) values of 9.14?×?10⁵ and 3.20?×?10^5?M^?1 for Co(III) and Ni(II) complexes, respectively. UV–visible absorption spectra showed that the absorption of DNA at 260?nm was quenched for 2 but quenched then improved for 1 with addition of complexes, tentatively attributed to the effect of the combined intercalative binding and electrostatic interaction for 1.