Extractive-spectrophotometric methods for determination of anti-Parkinsonian drug in pharmaceutical formulations and in biological samples using sulphonphthalein acid dyes

A simple, accurate and highly sensitive spectrophotometric methods are proposed for the rapid and accurate determination of amantadine HCl (AMD) using bromocressol green (BCG), bromophenol blue (BPB) and bromothymol blue (BTB). The developed methods involve formation of stable yellow colored chloroform extractable ion-associate complexes of the amino derivative (basic nitrogen) of the AMD with three sulphonphthalein acid dyes, namely; BCG, BPB and BTB, in acidic medium. The ion-associates exhibit absorption maxima at 415, 412 and 414 nm for BCG, BPB and BTB, respectively. AMD can be determined up to 1.5–16.5, 1.4–14.0 and 1.6–17 μg mL^?1, respectively. The effect of optimum conditions via acidity, reagent concentration, time, and solvent was studied. The stoichiometry of the reaction was found to be 1:1 in all cases. The low relative standard deviation values indicate good precision and high recovery values. These methods have been successfully applied for the assay of AMD in pharmaceutical formulations. Statistical comparison of the results with the reference method shows excellent agreement and indicates no significant difference in accuracy and precision.     

Exploring weak interactions to assemble a nitrosyl–ruthenium compound able to release NO under visible light irradiation

This work reports on a novel nitrosyl–ruthenium complex bearing the azanaphthalene ligand quinazoline (qui) in its coordination sphere. The product crystallizes with an additional quinazoline molecule, yielding the compound cis-[Ru(bpy)₂(qui)NO](PF₆)₃ · (qui). This feature leads to an absorption band at λ_max = 430 nm in CH₃CN and λ_max = 420 nm in phosphate buffer, which promotes the photorelease of nitric oxide under visible light irradiation (λ > 400 nm), in 1 ethanol:1 water (v/v) mixture or under physiological pH. Both the intensity and energy of this transition are dependent on solvent and solution pH, suggesting that the transition has a charge transfer nature, and that the association of the second quinazoline molecule with the complex is driven by weak interactions, possibly of the π-stacking type.     

Correlation between molecular structure and optical properties for the bis(2-(2-hydroxyphenyl)benzothiazolate) complexes

A series of methyl-substituted bis(2-(hydroxyphenyl)benzothiazolate)zinc derivatives [Zn(n-MeBTZ)₂, n = 3 (1a), 4 (1b), 5 (1c)] were synthesized to investigate the correlation between molecular structures and optical properties. The results indicate that the blue-emitting (λ_max = 470 nm) complex 1b is monomer with a higher PL quantum efficiency than complexes 1, 1a, 1c. Two green-emitting (λ_max = 507 nm and 499 nm) complexes 1a and 1c have special bi-molecular structures. The molecular structure for Zn(BTZ)₂ (complex 1) is dimer. Bilayer organic light-emitting devices were fabricated by using these complexes as emitting layer. The maximum emission wavelengths of the devices are in the range of 501–553 nm. The devices show turn-on voltages at 9.2, 12.7, 2.3 and 10.7 V for complex 1, 1a, 1b, and 1c, respectively. In particular, the device with complex 1b shows a higher brightness than the other complexes under the same conditions.     

Excited state properties of [Pd0(NHC)(quinone)]2 with NHC = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and quinone = 1,4-naphthoquinone

The carbene complex [Pd⁰(NHC)(quinone)]₂with NHC = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene and quinone = 1,4-naphthoquinone shows two long-wavelength absorptions at 312 and 399 nm which are assigned to (NHC→quinone) LLCT and (Pd⁰ → quinone) MLCT transitions. The MLCT state is not reactive, but emissive (λ_max = 564 nm at 77 K). At r.t., the complex undergoes a photoredox decomposition which is initiated by the LLCT state.     

Photophysical properties of 5-hydroxyflavone

To investigate the role of the excited triplet state in the deactivation process of 5-hydroxyflavone (5HF), the photophysical process of 5HF was studied by transient absorption, phosphorescence spectroscopies, and semiempirical calculations. The triplet–triplet absorption (T–T) spectra of 5HF and 5-methoxyflavone (5MF) were observed upon direct and triplet-sensitized excitation. The T–T spectrum of 5HF (λ_max=350 nm, τ_T=2.8 μs) was different from that of 5MF (λ_max=360 nm, τ_T=6.8 μs). Estimations of the triplet energies of 5HF and 5MF by quenching experiments, phosphorescence, and semiempirical (PM3/CI4) calculation revealed that 5HF underwent an intramolecular hydrogen atom transfer and formed the tautomer in the excited triplet state. The triplet energy of the normal form of 5HF was 260 kJ mol⁻¹, while that of the tautomer form (5HF′) was 197 kJ mol⁻¹. The triplet energy of 5MF, the model compound of the normal form of 5HF, was 261 kJ mol⁻¹. The PM3/CI4 calculation supported the experimental observations and suggested that the most stable conformer in the triplet state of 5HF is the tautomer form.     

Kinetics of the diazotization and azo coupling reactions of procaine in the micellar media

The kinetics of the diazotization reaction of procaine in the presence of anionic micelles of sodium dodecyl sulfate (SDS) and cationic micelles of cetyltrimethyl ammonium bromide (CTAB), dodecyltrimethyl ammonium bromide (DDTAB) and tetradecyltrimethyl ammonium bromide (TDTAB) were carried out spectrophotometrically at λ_max = 289 nm. The values of the pseudo first order rate constant were found to be linearly dependent upon the [NaNO₂] in the concentration range of 1.0 × 10⁻³ mol dm⁻³ to 12.0 × 10⁻³ mol dm⁻³ in the presence of 2.0 × 10⁻² mol dm⁻³ acetic acid. The concentration of procaine was kept constant at 6.50 × 10⁻⁵ mol dm⁻³. The addition of the cationic surfactants increased the reaction rate and gave plateau like curve. The addition of SDS micelles to the reactants initially increased the rate of reaction and gave maximum like curve. The maximum value of the rate constant was found to be 9.44 × 10⁻³ s⁻¹ at 2.00 × 10⁻³ mol dm⁻³ SDS concentration. The azo coupling of diazonium ion with β-naphthol (at λ_max = 488) nm was found to linearly dependent upon [ProcN₂⁺] in the presence of both the cationic micelles (CTAB, DDTAB and TDTAB) and anionic micelles (SDS). Both the cationic and anionic micelles inhibited the rate of reactions. The kinetic results in the presence of micelles are explained using the Berezin pseudophase model. This model was also used to determine the kinetic parameters e.g. k_m, K_s from the observed results of the variation of rate constant at different [surfactants].     

Synthesis and characterization of three Cu(II), Zn(II), Cd(II) supramolecular complexes bridged by biphenyl 2,2′-dicarboxylate

Three complexes, M₂(bpy)₂(bpdc)₂·xH₂O [M = Cu, x = 0; M = Zn or Cd, x = 2], have been hydrothermally synthesized by 1,1′-biphenyl-2,2′-dicarboxylic acid (H₂bpdc) with 2,2′-bipyridine (bpy) to form binuclear molecules. In each, the two bpdc groups align the two opposing planar [M(bpy)]²⁺ cations. The molecules are connected by C–H⋯O hydrogen bonds, π–π stacking, and C–H⋯π interactions to form three dimensional supramolecular networks. Furthermore, at room temperature, complex 3 exhibits strong photoluminescence.     

pH effects on the rate of heterogeneous electron transfer across a fluorine doped tin oxide/monolayer interface

Spontaneously adsorbed monolayers of [Ru(bpy)₂PIC](PF₆)₂ have been formed on fluorine doped tin oxide macro- and microelectrodes, bpy is 2,2′-bipyridyl and PIC is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]phenanthroline. These monolayers exhibit well-defined, almost ideal electrochemical responses over a wide range of voltammetric scan rates. The formal potential of the Ru^2+/3+ process shifts by less than 30 mV upon immobilization suggesting that the monolayers are well solvated. Significantly, chronoamperometry, conducted on a microsecond timescale, reveals that protonation of the PIC bridging ligand modulates the rate of interfacial electron transfer. The heterogeneous electron transfer rate constant, measured at an overpotential of +50 mV, decreases from 7.0 ± 1.1 × 10⁵ to 0.7 ± 0.1 × 10⁵ s⁻¹ as the pH of the supporting electrolyte is increased from 1.7 to 9.3. These observations are consistent with the redox mechanism occurring via a heterogeneous electron transfer process, the rate of PIC which depends on the energy difference between the metal dπ-orbitals and the lowest unoccupied molecular orbital (LUMO) of the bridge. Protonation of the bridging ligand decreases this energy gap, resulting in an overall increase in the rate of the redox reaction. Significantly, despite the close proximity of the luminophores to a conducting surface, the monolayers remain luminescent suggesting that the electronically excited state is only weakly coupled to the electrode surface. This is consistent with bipyridyl as the site of the excited state in the metal complex.     

A combined laser flash photolysis,density functional theory and atoms in molecules study of the photochemical hydrogen abstraction by pyrene-4,5-dione

The photochemical hydrogen abstraction reaction of pyrene-4,5-dione (3) has been investigated by laser flash photolysis. Excitation (λ = 355 nm) of a degassed solution of 3 in acetonitrile resulted in the formation of a detectable transient with absorption maxima at 380 and 470 nm. This transient decays with a lifetime of around 4.8 μs and is quenched by oxygen. This transient is most probably a triplet state of 3. Addition of hydrogen donors, such as 2-propanol; 1,4-cyclohexadiene or 4-methoxyphenol led to the formation of a new transient with λ_max at 380, 500 nm and a broad absorption at 640 nm. This new transient slowly decays with second order kinetics and was assigned to the semiquinone radical obtained from the hydrogen abstraction reaction. Using DFT and AIM calculations the reactivity of 3 and 9,10-phenanthrenequinone (1) is best interpreted as a proton coupled electron transfer like mechanism for the hydrogen abstraction from 2-propanol.     

A selective hydration of nitriles catalysed by a Pd(OAc)2-based system in water

In situ formation of a [Pd(OAc)₂bipy] (bipy = 2,2′-bipyridyl) complex in water selectively catalyses the hydration of a wide range of organonitriles at 70 °C. Catalyst loadings of 5 mol% afford primary amide products in excellent yields in the absence of hydration-promoting additives such as oximes and hydroxylamines.     

Thermodynamic properties of multicomponent electrolyte solutions in mixed solvents {HCl + NaCl-tert-C4H9OH + H2O} at (278.15 to 318.15) K

The thermodynamic properties of (HCl (m_A) + NaCl (m_B)-tert-C₄H₉OH + H₂O) system were studied by means of e.m.f. measurements in the cell without liquid junction at constant total ionic strength I=1.00 mol · kg⁻¹ from 278.15 K to 318.15 K. The standard electrode potential of Ag–AgCl and activity coefficient of HCl in the mixed solvents have been determined. The results show that the activity coefficient of HCl in HCl–NaCl solution still obeys Harned’s Rule and the logarithm of HCl activity coefficient, lgγ_A, is a linear function reciprocal of temperature at constant composition of the mixture. The standard transfer Gibbs free energy of HCl was calculated.     

Diffusion of levodopa in aqueous solutions of hydrochloric acid at 25 °C

Ternary mutual diffusion coefficients (D₁₁, D₂₂, D₁₂ and D₂₁) measured by the Taylor dispersion method are reported for aqueous solutions of {levodopa (l-dopa) + HCl} solutions at 25 °C and HCl concentrations up to 0.100 mol · dm⁻³. The coupled diffusion of l-dopa (1) and HCl (2) is significant, as indicated by large negative cross-diffusion coefficients. D₂₁, for example, reaches values that are larger than D₁₁, the main coefficient of l-dopa. Combined Fick and Nernst–Planck equations are used to analyze the proton coupled diffusion of l-dopa and HCl in terms of the binding of H⁺ ions to l-dopa and ion migration in the electric field generated by l-dopa and HCl concentration gradients.     

Preparation and thermal properties of lanthanide complexes with 2,3-dichlorobenzoic acid and 1,10-phenanthroline

Three lanthanide complexes with a general formula [Ln(2,3-DClBA)₃phen]₂ (Ln(III) = Eu(1), Tb(2), Ho(3); 2,3-DClBA = 2,3-dichlorobenzoate; phen = 1,10-phenanthroline) were synthesized and characterized by elemental analysis, molar conductance, infrared and ultraviolet spectra and powder X-ray diffraction (XRD). The luminescent properties of the complexes 1 and 2 were studied. The thermal behaviors of the complexes were also discussed by thermogravimetric (TG), differential thermogravimetric (DTG) and infrared spectra (IR) techniques. The heat capacities of the complexes were measured from 259.15 to 493.02 K by means of Differential scanning calorimeter (DSC). The dependence of heat capacity on the reduce temperature x (x = [T ? (T_max + T_min)/2]/[(T_max ? T_min)/2]) was fitted to a polynomial equation with the least squares method for each complex. Furthermore, based on the fitted polynomial, the smoothed heat capacities and the derived thermodynamic functions (H_T ? H_298.15 K), (S_T ? S_298.15 K) and (G_T ? G_298.15 K) in the measured temperature range were obtained with an interval of 10 K.     

Mesophase and magnetic behavior in cobalt(II) and iron(II) compounds

The metal complexes with long alkyl chains [Co(C16-terpy)₃](BF₄)₂ (1) and [Fe(C16-terpy)₂](BF₄)₂ (2) were synthesized and the physical properties of the complex were characterized by magnetic susceptibility, Mössbauer spectroscopy, polarizing optical microscopy, differential scanning calorimetry, and X-ray scattering, where C16-terpy is 4′-hexadecyloxy-2,2′:6′,2′′-terpyridine. Variable-temperature magnetic susceptibility measurements and/or Mössbauer studies revealed that the complex 1 exhibited unique spin transition (T_1/2↓ = 217 K and T_1/2↑ = 260 K) induced by structural phase transition, and the complex 2 was in the low-spin state in the temperature region of 5–400 K before the first mesophase transition. The cobalt(II) and iron(II) complexes exhibited liquid-crystal properties in the temperature range of 371–528 K and 466–556 K, respectively. After mesophase transition, the complex 1 exhibited only slight spin transition (T_1/2↓ = 266 K and T_1/2↑ = 279 K), and the complex 2 was in the low-spin state. The compounds with multifunction, i.e., magnetic property and liquid-crystal properties, are important in the development of molecular materials.     

Multi-technique approach on the effect of surfactant concentrations on the thermal unfolding of rabbit serum albumin: Formation and solubilization of the protein aggregates

Effect of cationic surfactant, cetyltrimethylammonium bromide (CTAB) addition on the thermal denaturation of rabbit serum albumin (RSA) has been studied by employing small-angle neutron scattering (SANS), circular dichroism (CD), intrinsic fluorescence and ultra violet (UV) spectroscopy. The studies were performed at three different temperatures viz., 30, 50 and 70 °C and at two different concentrations of CTAB: the low concentration of CTAB used was 1 mM and the higher concentration was 80 mM (for SANS) and 20 mM (for CD, fluorescence and UV). A collective effect of high temperature and low concentration of CTAB led to the protein aggregation followed by solubilization of these aggregates at higher concentration of surfactant. At 1 mM CTAB and 30 °C, the protein–surfactant complex has a prolate ellipsoidal shape with semi-major axis of 88.9 Å and semi-minor axis of 19.6 Å which are slightly greater than the values of the native RSA. At 50 °C, the size of the semi-major axis increases while at 70 °C an increase in the size of both axes was found. The thermal outcome at higher concentration of CTAB (80 mM) was rather different. Higher concentration of CTAB unfolds the protein by the formation of micelle-like aggregates along the polypeptide chains of the protein and the complex was stabilized at higher temperatures, which was not found with lower concentration of CTAB. The CD results were found to be consistent with the SANS results, i.e., decrease in α-helicity of RSA was more when less amount of surfactant was present as compared to the system with higher surfactant concentration. In a similar fashion, results of relative fluorescence intensity (RFI) reveal that increase in temperature causes decrease in λ_max of native RSA as well as RSA + 1 mM CTAB, whereas the λ_max remains unchanged for RSA + 20 mM CTAB systems. That means the structure remains compact in presence of 20 mM CTAB while the structure becomes loose when low or zero amount of surfactant was present. The UV results indicate that the protein aggregation takes place in presence of low amount of CTAB and these aggregates become soluble at high concentration of CTAB.     

Use of hydrophobins in formulation of water insoluble drugs for oral administration

The poor water solubility of many drugs requires a specific formulation to achieve a sufficient bioavailability after oral administration. Suspensions of small drug particles can be used to improve the bioavailability. We here show that the fungal hydrophobin SC3 can be used to make suspensions of water insoluble drugs. Bioavailability of two of these drugs, nifedipine and cyclosporine A (CyA), was tested when administered as a SC3-based suspension. SC3 (in a 1:2 (w/w) drug:SC3 ratio) or 100% PEG400 increased the bioavailability of nifedipine to a similar degree (6 ± 2- and 4 ± 3-fold, respectively) compared to nifedipine powder without additives. Moreover, SC3 (in a 7:1 (w/w) drug:hydrophobin ratio) was as effective as a 20-fold diluted Neoral^® formulation by increasing bioavailability of CyA 2.3 ± 0.3-fold compared to CyA in water. Interestingly, using SC3 in the CyA formulation resulted in a slower uptake (p < 0.001 in T_max) of the drug, with a lower peak concentration (C_max 1.8 mg ml^?1) at a later time point (T_max 9 ± 2 h) compared to Neoral^® (C_max 2.2 mg ml^?1; T_max 3.2 ± 0.2). Consequently, SC3 will result in a more constant, longer lasting drug level in the body. Taken together, hydrophobins are attractive candidates to formulate hydrophobic drugs.     

Kinetic spectrophotometric method for trace determination of thiocyanate based on its inhibitory effect

A kinetic spectrophotometric method for the determination of thiocyanate, based on its inhibitory effect on silver(I) catalyzed substitution of cyanide ion, by phenylhydrazine in hexacyanoferrate(II) is described. Thiocyanate ions form strong complexes with silver(I) catalyst which is used as the basis for its determination at trace level. The progress of reaction was monitored, spectrophotometrically, at 488 nm (λ_max of [Fe(CN)₅PhNHNH₂]^3?, complex) under the optimum reaction conditions at: 2.5 × 10^?3 M [Fe(CN)₆]^4?, 1.0 × 10^?3 M [PhNHNH₂], 8.0 × 10^?7 M [Ag⁺], pH 2.8 ± 0.02, ionic strength (μ) 0.02 M (KNO₃) and temperature 30 ± 0.1 °C. A linear relationship obtained between absorbance (measured at 488 nm at different times) and inhibitor concentration, under specified conditions, has been used for the determination of [thiocyanate] in the range of 0.8–8.0 × 10^?8 M with a detection limit of 2 × 10^?9 M. The standard deviation and percentage error have been calculated and reported with each datum. A most plausible mechanistic scheme has been proposed for the reaction. The values of equilibrium constants for complex formation between catalyst–inhibitor (K_CI), catalyst–substrate (K_s) and Michaelis–Menten constant (K_m) have been computed from the kinetic data. The influence of possible interference by major cations and anions on the determination of thiocyanate and their limits has been investigated.     

Electrochemical kinetics of the hydrogen reaction on platinum in concentrated HCl(aq)

The hydrogen reaction in concentrated HCl(aq) solutions is a key reaction for the CuCl(aq)/HCl(aq) electrolytic cell. Here, electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) were used to obtain new data for the hydrogen reaction on platinum submerged in highly concentrated acidic solutions at 25 °C and 0.1 MPa. LSV and EIS data were collected for Pt in 0.5 mol/L H₂SO₄(aq), 1 mol/L HCl(aq) and 7.71 mol/L HCl(aq) solutions. It was found that exchange current density (j₀) values varied between 1 and 2 mA/cm². An equivalent circuit model was used to obtain comparable j₀ and limiting current density values from EIS data relative to values obtained with LSV data. It was found that as the concentration of acid increased, a noticeable decrease in the performance was observed.     

Crystal structures,luminescence, and thermal properties of lanthanide complexes with 2,3,4-trimethoxybenzoic acid and 1,10-phenanthroline

The complexes of [Ln(2,3,4-tmoba)₃phen]₂ (Ln = Dy (1), Eu (2), Tb (3); 2,3,4-tmoba = 2,3,4-trimethoxybenzoate; phen = 1,10-phenanthroline) were synthesized and characterized by a series of techniques including the elemental analysis, IR and fluorescent spectra and TG/DSC-FTIR technology. The crystal structures were determined by X-ray crystallography. Each complex include two Ln³⁺ ions, six 2,3,4-tmoBA and two phen molecules forming a binuclear structure, giving the coordination number of nine. The three-dimensional IR accumulation spectra of gaseous products for the complexes 1 to 3 are analyzed and the thermal decomposition processes are further authenticated. Through means of differential scanning calorimeter (DSC), two solid-solid phase transition endothermic peaks were found in the complex 2, which was different from the complexes 1 and 3. The heat capacities of these complexes were measured and fitted to a polynomial equation with the least squares method for each complex on the basis of the reduce temperature x (x = [T − (T_max + T_min)/2]/[(T_max − T_min)/2]) over the range from (256.15 to 476.15) K. Subsequently, the smoothed molar heat capacities and thermodynamic functions (H_T−H_298.15 K), (S_T−S_298.15 K), and (G_T−G_298.15 K) of the complexes 1 to 3 were calculated based on the fitted polynomial of the heat capacities. The fluorescent intensity of the complexes 2 and 3 are markedly improved as well.