Association Equilibrium of Methylene Blue by Spectral Titration and Chemometrics Analysis: A Thermodynamic Study

The monomer‐dimer equilibrium of methylene blue (MB, Scheme I) has been investigated by means of UV‐Visible spectroscopy in aqueous solutions. The self aggregation of MB in water has been investigated by recording absorption spectra in the wavelength range of 450–750 nm, and in different ionic strengths using concentrated KCl solutions in the temperature range of 20–90°C. Chemometrics analysis of the spectral data gave a dimerization constant, individual spectra of the monomer and dimer forms of the dye molecule. The quantitative analysis of the data of the undefined mixture was carried out by simultaneous resolution of the overlapping spectral bands in the whole set of absorption spectra. The dimerization constants of MB determined by mathematical deconvolution of the thermometric spectral titration data show dependency on temperature variations. The concentration range of MB was 6.00 × 10^?5‐3.00 × 10^?4 M. Utilizing the van't Hoff relation, which describes the dependence of the equilibrium constant on temperature, the thermodynamics parameters ΔH° and ΔS° of the aggregation process were determined. The compensation effect was verified by the thermodynamics results of the dimerization process of the dye.     

亚甲基蓝与鲱鱼精DNA相互作用的光谱法研究

以吖啶橙(AO)作为光谱探针, 采用UV和荧光光谱等方法研究了亚甲基蓝(MB)与鲱鱼精DNA的作用机制. 确定了在低浓度MB时, MB与DNA以嵌插方式作用; 而在高浓度MB时, MB与DNA之间为混合作用方式. 结合比n(MB)∶n(DNA)＝10∶1, 结合常数 ＝2.46×10⁵ L•mol^－1, MB-DNA复合物的表观摩尔吸光系数ε＝5.70×10⁶ L•mol^－1•cm^－1. 同时研究了酸度和温度等对MB与DNA相互作用的影响, 热力学研究推导了MB结合DNA为焓驱动反应.     

Urea-Induced Dissociation of Nerve Growth Factor from Venom of Chinese Cobra by SEC

The urea-induced dissociation of nerve growth factor from venom of Chinese cobra (_cNGF) was studied by intrinsic fluorescence emission spectra, SEC, urea-gradient polyacrylamide gel electrophoresis, assays of biological activity and thermodynamic parameters. The results showed that when urea concentration was lower than or equal to 4.0 mol L⁻¹ or higher than or equal to 8.0 mol L⁻¹, _cNGF existed only in native homodimer form or monomer form, respectively; whereas when urea concentration was higher than 4.0 mol L⁻¹ and lower than 8.0 mol L⁻¹, they existed simultaneously in the native homodimer and monomer forms and the former decreased, while the latter increased with the increase in urea concentration. Based on the association–dissociation equilibrium between _cNGF and urea molecules, an equation, which includes two characteristic dissociation parameters K and ∆m, was presented to describe the urea-induced dissociation process of _cNGF. As the reaction temperature increased from 15 to 35 °C, positive enthalpy and entropy changes were observed, and the parameter K increased from 2.72 × 10⁻¹³ to 5.18 × 10⁻¹² (L mol⁻¹), while the parameters ∆m and ∆G, respectively, decreased from 10.18 to 8.42 and from −10.27 to −18.67 (kJ mol⁻¹), which means that the urea-induced dissociation of _cNGF was spontaneous and entropy-driven and the higher temperature was favorable for the dissociation process. Using the procedures and equations mentioned in the paper, the urea-induced dissociation of _cNGF is first comprehensively described. Furthermore, this work presents a useful method for people to study the dissociation of dimer or multimer proteins induced by denaturants, inducers, pH, etc.

     

Urea-Induced Dissociation of Nerve Growth Factor from Venom of Chinese Cobra by SEC

The urea-induced dissociation of nerve growth factor from venom of Chinese cobra (_cNGF) was studied by intrinsic fluorescence emission spectra, SEC, urea-gradient polyacrylamide gel electrophoresis, assays of biological activity and thermodynamic parameters. The results showed that when urea concentration was lower than or equal to 4.0 mol L^?1 or higher than or equal to 8.0 mol L^?1, _cNGF existed only in native homodimer form or monomer form, respectively; whereas when urea concentration was higher than 4.0 mol L^?1 and lower than 8.0 mol L^?1, they existed simultaneously in the native homodimer and monomer forms and the former decreased, while the latter increased with the increase in urea concentration. Based on the association–dissociation equilibrium between _cNGF and urea molecules, an equation, which includes two characteristic dissociation parameters K and ?m, was presented to describe the urea-induced dissociation process of _cNGF. As the reaction temperature increased from 15 to 35 °C, positive enthalpy and entropy changes were observed, and the parameter K increased from 2.72 × 10^?13 to 5.18 × 10^?12 (L mol^?1), while the parameters ?m and ?G, respectively, decreased from 10.18 to 8.42 and from ?10.27 to ?18.67 (kJ mol^?1), which means that the urea-induced dissociation of _cNGF was spontaneous and entropy-driven and the higher temperature was favorable for the dissociation process. Using the procedures and equations mentioned in the paper, the urea-induced dissociation of _cNGF is first comprehensively described. Furthermore, this work presents a useful method for people to study the dissociation of dimer or multimer proteins induced by denaturants, inducers, pH, etc.     

Electric conductivity of polybutadienyl- and polyisoprenyl-lithium in tetrahydrofurane and dimethoxyethane

The electric conductivity of polybutadienyl-lithium and polyisoprenyl-lithium has been investigated in tetrahydrofuran and dimethoxyethane solutions over wide temperature and concentration ranges (0 to ?70°; 7·10^?2 to 1·10^?7 and 5·10^?3 to 1·10^?7 mol/l). Dissociation constants, enthalpy and entropy of dissociation of ion pairs, relative constants for formation of ion triplets, and interionic distances were calculated for each system investigated. The nature of “living” chains under these conditions is considered.     

The dimerization study of some cationic monomethine cyanine dyes by chemometrics method

The thermodynamics of the monomer-dimer equilibrium of five cationic monomethine cyanine dyes has been studied by means of UV-vis spectroscopy and advanced chemometrics methods. The dimerization constants were determined by studying the dependence of their absorption spectra on temperature in the range 20?C75°C and were found to be: 1.91 × 10³, 1.53 × 10³, 6.81 × 10³, 5.18 × 10³, and 8.28 × 10³ dm³ mol^?1 for 1, 2, 3, 4, and 5 respectively. The absorption spectra of the monomer and dimer forms of the dyes were also determined, as well as the enthalpy and entropy of the dimerization.     

Preparation and ESR-Spectroscopical Investigation of Remarkably Persistent Oxoisobenzofuranyl Radicals

The synthesis of the diastereoisomeric 1,1′-diaryl-1,1′-bi(isobenzofuran)-3,3′(1H,1′H)-diones 3a–d starting from the readily available 2-aroylbenzoic acids 1a–d is described (Scheme 1). Of the colourless dimers 3a–d , only the sterically congested 3a and 3b dissociate at ambient temperature in solution to the deep red free 3-oxoisobenzofuran-1-yl radicals 4a and 4b , respectively. The radicals 4a, b are extremely persistent in the absence of O₂. The structures of these radicals are confirmed and the coupling constants assigned by ESR and ENDOR spectroscopy and computer simulation of their ESR spectra. The dissociation equilibrium constant at 20° in toluene for 3a is determined to be 1.18 · 10^?5 M . By studying the steady-state radical concentration as a function of temperature, the enthalpy and entropy changes for the homolytic dissociation of 3a are determined.     

A numerical study of time-dependent schr?dinger equation for multiphoton vibrational interaction of NO molecule, modelled as Morse oscillator, with intense far-infrared femtosecond lasers

For the NO molecule, modelled as a Morse oscillator, time-dependent (TD) nuclear Schr?dinger equation has been numerically solved for the multiphoton vibrational dynamics of the molecule under a far-infrared laser of wavelength 10503 nm, and four different intensities,I = 1 × 10⁸, 1 × 10¹³, 5 × 10¹⁶, and 5 × 10¹⁸ W cm⁻² respectively. Starting from the vibrational ground state at zero time, various TD quantities such as the norm, dissociation probability, potential energy curve and dipole moment are examined. Rich high-harmonics generation (HHG) spectra and above-threshold dissociation (ATD) spectra, due to the multiphoton interaction of vibrational motions with the laser field, and consequent elevation to the vibrational continuum, have been obtained and analysed. Dedicated to Professor C N R Rao on his 70th birthday An erratum to this article is available at .     

Mechanism of rhodamine 6G molecular aggregation in montmorillonite colloid

Stopped-flow mixing device and visible absorption spectroscopy were used for the analysis of dye rhodamine 6G (R6G) molecular aggregation in the colloids based on Na-saturated montmorillonite. Two stages of the reaction were identified: The first stage was very short and taking only several seconds, involving the adsorption of R6G cations and their initial aggregation on the surface of colloid particles. The initially formed J-aggregates exhibited similar spectral properties as monomeric form of R6G. In the second stage, initially formed aggregates converted to sandwich-type H-aggregates absorbing light at significantly lower wavelengths and adsorbed monomers. The aggregate rearrangement took several hours. Monomers, with the spectral properties identical to R6G solution, were also identified as a component in complex spectra using principal component analysis (PCA) and multivariate curve resolution (MCR). Partial bleaching of the dye was also proven. Reaction kinetics of the rearrangement of the aggregates followed the model considering a complex mechanism of the molecular aggregation. Data fits using stretched-exponential function led to the determination of rate constants, which had been in the range 10^?3?4×10^?3s^?1.      

Investigation on the self-association of an inorganic coordination compound with biological activity (Casiopeína III-ia) in aqueous solution

From studies using different experimental techniques employed to determine the presence of aggregates e.g. isothermal titration calorimetry, surface tension, electrical conductivity, UV–Vis spectrophotometry, dynamic and static light scattering, it is clearly demonstrated that the compound [Cu(4, 4′-dimethyl-2, 2′-bipyridine)(acetylacetonato)H₂O]NO₃ (Casiopeína III-ia), promising member of a family of new generation compounds for cancer treatment, is able to auto associate in aqueous media. Physicochemical properties associated with the formation of the aggregates were determined in pure water and in phosphate buffer media in order to simulate physiological conditions. From isothermal titration calorimetry and electrical conductivity measurements we calculated the dissociation constant of the aggregates, K _D . For pure water the values obtained in both techniques are 2.73 × 10^?4 and 5.93 × 10^?4 M respectively while for the buffer media we obtained 4.61 × 10^?4 and 1.57 × 10^?3 M. The enthalpy of dissociation, ?H _D , calculated from the calorimetric data shows that the presence of the phosphate ions has an energetic effect on the aggregate stability since in pure water a value of 18.79 kJ mol^?1 was obtained in comparison with the buffer media where a value 4 times bigger was found (70.48 kJ mol^?1). With the data collected from these techniques the number of monomers calculated which participate in the formation of the aggregates is around two. From our surface tension, electrical conductivity and UV–Vis spectrophotometry measurements the critical aggregate concentration, cac, was determined. For each technique specific concentration ranges were obtained but we can summarize that the cac in pure water is between 3 and 3.5 mM and for the buffer media is between 3.5 and 4 mM. Dynamic light scattering measurements provide us with the hydrodynamic diameter of the aggregates and from static light scattering measurements we determined the molecular weight of the Casiopeína III-ia aggregates to be of 1000.015 g mol^?1 which is two times the molecular weight of the Casiopeína III-ia molecule. This value is in agreement with the number of monomers which participate in the formation of the aggregates obtained from isothermal titration calorimetry and electrical conductivity data analysis.     

Steric Exclusion Liquid Chromatography Studies in Urea on the Denaturation of the Bovine Eye Lens Protein α-Crystallin

Abstract

Steric exclusion liquid chromatography in the presence of intermediate urea concentrations with lowangle laser light scattering detection was used to investigate the stepwise dissociation of the multimeric bovine eye lens protein α-crystallin. The change in the quaternary structure of α-crystallin as a function of increasing urea concentration clearly resembled dissociation by increasing alkaline pH, urea or guanidine-HC1 concentrations when studied by sedimentation velocity analysis. Next to native and native-like threelayer aggregates (M_r 6.5 ? 7.5 × 10⁵), the first dissociation products (two-layer molecules M_r 4 ? 5.5 × 10⁵), the second dissociation products (core molecules M_r 2.5 ? 3 × 10⁵), and monomeric subunits (M_r 20 000) could be characterized. In the range from 2.6 to 4.4 M urea, we found a gradual decrease in the proportion of the remaining three-layer aggregates and an increase in that of monomeric subunits. The fluorescence emission maxima showed increasing solvent exposure of the tryptophan residues going from three-layer aggregates to monomeric subunits. The subunit compositions for most dissosiation products did not significantly differ from that of native α-crystallin. The interpretation of earlier results on Sephacryl-S200 steric exclusion chromatography in 3.8 M urea appeared to be an oversimplification.     

Determination of the Solubility, Dissolution Enthalpy and Entropy of Donepezil Hydrochloride Polymorphic Form III in Different Solvents

The solubilities of donepezil hydrochloride polymorphic form III in methanol, ethanol, 1-propanol and dimethyl formamide were measured at temperatures ranging from 278.15 to 333.15 K at atmosphere pressure using a gravimetrical method. The modified Apelblat model fitted the experimental data well with the root-mean-square deviations less than 6.287 × 10^?4. The dissolution enthalpy and entropy of solute were predicted from the solubility data in different solvents using the van’t Hoff equation. The relationships among solubility, temperature, the intermolecular force and hydrogen bonding between solute and solvent, and the viscosity of solvents were investigated. The viscosity and surface tension of solvents affect the dissolution enthalpy and entropy of donepezil hydrochloride polymorphic form III.     

Ionic conduction in LiSCN/dimethylformamide/poly(propylene oxide) system I. Dissociation equilibria of LiSCN

Dissociation equilibria of lithiumthiocyanate (LiSCN) in N,N-dimethylformamide (DMF) solutions of poly (propylene oxide) (PPO) were investigated by using infrared spectroscopy. The stretching bands due to the thiocyanate ions SCN^?1 and the LiSCN ion pair were found at 2058 and 2072 cm^?1, respectively. At high LiSCN concentration C of ca. 20 wt %, another weak band due to the dimer (LiSCN)₂ was observed. From the ratio of the areas of the absorption bands, the dissociation constant K₁ for the equilibrium LiSCN ? Li⁺ + SCN^?1 and that K₂ for (LiSCN)₂ ? 2LiSCN have been determined. With increasing DMF content, K₁ increases from 1 × 10^?4 for bulk PPO to 4.8 × 10^?1 for pure DMF at 299 K. Log K¹ is not linear against inverse of the dielectric constant ? of the medium and decreases with increasing temperature. The enthalpy (ΔH) and entropy (ΔS) changes for the dissociation of LiSCN are both negative. ©1995 John Wiley & Sons, Inc.     

Microcalorimetry and spectroscopic studies on the binding of dye janus green blue to deoxyribonucleic acid

The interaction of the phenazinium dye janus green blue (JGB) with deoxyribonucleic acid was investigated using isothermal titration calorimetry and thermal melting experiments. The calorimetric data were supplemented by spectroscopic studies. Calorimetry results suggested the binding affinity of the dye to DNA to be of the order of 10⁵ M^?1. The binding was predominantly entropy driven with a small negative favorable enthalpy contribution to the standard molar Gibbs energy change. The binding became weaker as the temperature and salt concentration was raised. The temperature dependence of the standard molar enthalpy changes yielded negative values of standard molar heat capacity change for the complexation revealing substantial hydrophobic contribution in the DNA binding. An enthalpy–entropy compensation behavior was also observed in the system. The salt dependence of the binding yielded the release of 0.69 number of cations on binding of each dye molecule. The non-polyelectrolytic contribution was found to be the predominant force in the binding interaction. Thermal melting studies revealed that the DNA helix was stabilized against denaturation by the dye. The binding was also characterized by absorbance, resonance light scattering and circular dichroism spectral measurements. The binding constants from the spectral results were close to those obtained from the calorimetric data. The energetic aspects of the interaction of the dye JGB to double-stranded DNA are supported by strong binding revealed from the spectral data.     

A predictive-kinetic method for the quantitation of amino acids with ninhydrin

The development and evaluation of a predictive-kinetic method for quantifying amino acids based on reactions with ninhydrin are described. Conditions are developed for which reactions are pseudo-first-order in the amino acid. Absorbance vs. time data from the kinetic region of the reaction (1–3 half-lives) are fitted to a first-order model to predict the total absorbance change that would occur if the reaction were monitored to completion. Computed absorbance changes vary linearly with amino acid concentration between 1 × 10^?5 and 5 × 10^?5 mol l^?1. Results are virtually independent of changes in temperature (± 1° C) and ninhydrin concentration (± 3 × 10^?3 mol l^?1).     

Use of multivariate curve resolution analysis in the spectroelectrochemistry of 9,10-anthraquinone reduction in dimethylformamide solution

A multivariate curve resolution analysis has been conducted to deconvolute the spectroelectrochemical data of 9,10-anthraquinone (AQ) reduction. The experiments were carried out in dimethylformamide solution containing small amounts of benzoic acid as a proton source at an optically transparent thin layer electrode (OTTLE). Two electrochemical techniques, namely a potential step method and chronoabsorptiometry were employed. In the former, the absorbance spectra of anthraquinone solutions at different applied potentials were recorded, and in the latter technique, a −1.90 V potential was applied to the OTTLE and the absorbance spectra of the solution were recorded at different time intervals. For both techniques, factor analysis revealed that three chemical components coexist in the system, which can be attributed to AQ⁻, AQH⁻ and AQ²⁻. By using a multivariate curve resolution analysis, the concentration profiles and pure spectra of the components were determined. The results of the potential step method revealed that the maximum amount of AQ⁻ is produced at −1.25 V, and application of more negative potentials causes an decreased amount of AQ⁻ at the expense of the evolution of AQH⁻. Finally, the AQ²⁻ species is produced at −1.6 V. The chronoabsorptiometric results showed that AQ⁻ is not stable at 1.9 V and rapidly converts to AQ²⁻.     

Thermodynamics’ study of the adsorption process of methylene blue on activated carbon at different ionic strengths

The adsorption constant of aqueous solution of methylene blue (MB), (800 · 10⁻⁶ mol · dm³) on activated carbon (AC) has been investigated by means of UV–Visible spectroscopy. The adsorption process is very fast and is physical in nature; however, it is complicated by the factors inherent in the structures of both the MB solution and AC, such as tendency of MB to form molecular aggregates in solution. In this work, the effects of various experimental parameters such as temperature and ionic strength have been investigated, using a batch adsorption technique to obtain information on treating effluents from the dye industry. Utilizing the van’t Hoff relation, which describes the dependence of equilibrium constant on temperature, as a constraint, we determine the spectral responses of the free and adsorbed amounts, as well as the enthalpy of the adsorption equilibrium.     

Characterization of a Layered Methylene Blue/Vanadium Oxide Nanocomposite and its Application in a Reagentless H2O2 Biosensor

Layered nanocomposite of methylene blue (MB)-intercalated vanadium oxide was obtained through a simple hydrothermal synthesis method using MB, V₂O₅, and NaI as starting materials. The intercalation reaction was proven to be successful using X-ray diffraction pattern. The MB-V₂O₅ nanocomposite was characterized using a scanning electron micrograph, infrared spectra, thermogravimetric analysis, UV spectra, and electrochemical measurements. The intercalated MB cations showed a fine diffusion-controlled electrochemical redox process and facilitated the immobilized horseradish peroxidase’s (HRP) good catalytic reduction upon H₂O₂. The as-prepared MB-V₂O₅/HRP biosensor showed a linear response to H₂O₂ over a range from 2.0?×?10^?6 to 9.5?×?10^?5 M with a detection limit of 9.7?×?10^?7 M (S/N ratio?=?3).     

Concentration controlled multilevel self‐assembly of 3‐armed poly(ethylene glycol)‐b‐poly(ε‐caprolactone) block copolymers investigated by AFM

Concentration dependent morphology of 3‐armed poly(ethylene glycol)‐b‐poly(ε‐caprolactone) copolymer aggregates in aqueous system was investigated by atomic force microscopy (AFM). The AFM results show that, at a low concentration, 4 × 10^?5 g/mL, spherical micelles occur, and unmicellized molecules are not distributed homogeneously in the copolymer aqueous solution. Unequal outspread clusters composed of wormlike aggregates are formed at a moderate copolymer concentration, 4 × 10^?4 g/mL, those wormlike aggregates are orderly packed in the clusters. At a high concentration of 0.05 g/mL, the copolymer aqueous system is indeed a gel at room temperature, outspread clusters of wormlike aggregates join together to forma network structure. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1412–1418, 2008     

Effects of the thermal history and concentration on the aggregation of Erwinia gum in an aqueous solution

The aggregation of Erwinia (E) gum in a 0.2 M NaCl aqueous solution was investigated by multi‐angle laser light scattering and gel permeation chromatography (GPC) combined with light scattering. The GPC chromatograms of five fractions contained two peaks; the fractions had the same elution volume but different peak areas, suggesting that aggregates and single chains coexisted in the solution at 25 °C. The apparent weight‐average molecular weights (M_w) of the aggregates and single chains for each fraction were all about 2.1 × 10⁶ and 7.8 × 10⁴, respectively. This indicates that the aggregates were composed of about 27 molecules of E gum in the concentration range used (1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ g/mL). The weight fraction of the aggregates (w_ag) increased with increasing concentration, but the aggregates still existed even in an extremely dilute solution. The fractionation process and polymer concentration hardly affected the apparent aggregation number but significantly changed w_ag. The E‐gum M_w decreased sharply with an increase in temperature. When the E‐gum solution was kept at 100 °C, w_ag decreased sharply for 20 h and leveled off after 100 h. Once the aggregates were decomposed at a higher temperature, no aggregation was observed in the solution at 25 °C, indicating that the aggregation was irreversible. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1352–1358, 2000