Aqueous Phosphate Buffer Research Articles

Carbon nanotubes/pentacyaneferrate-modified chitosan nanocomposites platforms for reagentless glucose biosensing

The design, characterization and applicability of a nanostructured biosensor platform are described. The biosensor is developed through the immobilization of three components: a polymeric chitosan network previously modified with a redox mediator (denoted as PCF-Pyr-Ch), an enzyme (glucose oxidase, chosen as a model) and carbon nanotubes onto a solid glassy carbon electrode (C). In order to assess the influence of the nanomaterial in the performance of the resulting analytical device, a second biosensor, free of carbon nanotubes, is developed. The characterization of both biosensing platforms was performed in aqueous phosphate buffer solutions using atomic force microscopy technique. In the presence of glucose, both systems exhibit a clear electrocatalytic activity, and glucose could be amperometrically determined at +0.35 V versus Ag/AgCl. The performance of both biosensors was evaluated in terms of sensitivity, detection limit and linear response range. Finally, the enhancement of the analytical response induced by the presence of carbon nanotubes was evaluated.

Biotransformation of cyclohexene to value-added oxygenated derivative catalyzed by chloroperoxidase in the presence of small quantities of quaternary ammonium salts

Biotransformation of cyclohexene catalyzed by chloroperoxidase (CPO) from Caldariomyces fumago was employed to prepare value-added oxygenated derivative: trans-1,2-cyclohexanediol (CHD) using H 2O 2 as oxidants. The conversion of substrate was enhanced to 89.2% in the presence of small quantities of quaternary ammonium salts (QAS) compared to that of only 48.1% in aqueous phosphate buffer after 150 min. The enhancement was dependent on the concentration and alkyl group length of QAS. QAS were found playing multiple functions in reaction media: phase transfer catalysis and tuning the composition of product. UV–vis, fluorescence and circular dichroism (CD) spectral assay were employed to investigate the effect of QAS on micro-environment around active center and structure of protein. The strengthening of α-helix structure of CPO and more exposure heme for easily access of substrate was found responsible for the improvement of catalytic performance of CPO. Moreover, the determined kinetic parameters indicated the affinity and selectivity of CPO to substrate was improved according to the observation of a decrease of Michaelis constant K m while an increase of the second order rate constants k cat/ K m. The strategy reported in this work is environment-friendly, straightforward and applicable to large scale preparation.

The stability of trifluorothymidine: hydrolysis in buffered aqueous solutions.

The decomposition of trifluorothymidine in aqueous solution, and borate and phosphate buffers has been studied using an h.p.l.c. method. In aqueous solution accelerated studies demonstrate that a storage life, to 10% loss, of more than 30 years at 4 degrees C is predicted, but steam sterilization causes extensive breakdown. Considerable differences occur in the kinetics and routes of decomposition in borate and phosphate buffers in the pH range 4-8. Hydroxide ion attack producing 5-carboxy-2'-deoxyuridine is suppressed in borate buffers. Complex kinetics of decomposition in phosphate buffer are found and possible explanations suggested. Steam sterilization in buffered solution also leads to unacceptable breakdown.

Effect of anti-inflammatory drugs on partitioning characteristics of propranolol and oxprenolol.

The influence of a range of nine anti-inflammatory drugs on the octan-1-ol aqueous phosphate buffer pH 7.4 apparent partition coefficients of propranolol and oxprenolol has been examined. All produced a change in the apparent partition coefficient which can be explained in terms of partitioning of hydrophobic ion-pairs formed between the ionic anti-inflammatory compound and the protonated cationic drug.

Photodegradation of Folate Sensitized by Riboflavin

Folate is shown to react with singlet-excited state of riboflavin in a diffusion controlled reaction and with triplet-excited state of riboflavin in a somewhat slower reaction with (3)k(q) = 4.8 × 10(8) L mol(-1) s(-1) in aqueous phosphate buffer at pH 7.4, ionic strength of 0.2 mol L(-1), and 25°C. Singlet quenching is assigned as photo-induced reductive electron transfer from ground state folate to singlet-excited riboflavin, while triplet quenching is assigned as one-electron transfer rather than hydrogen atom transfer from folate to triplet-excited riboflavin, as the reaction quantum yield, φ = 0.32, is hardly influenced by solvent change from water to deuterium oxide, φ = 0.37. Cyclic voltammetry showed an irreversible two-electron anodic process for folate, E = 1.14 V versus NHE at a scan-rate of 50 mV s(-1), which appears to be kinetically controlled by the heterogeneous electron transfer from the substrates to the electrode. Main products of folate photooxidation sensitized by riboflavin were pterin-6-carboxylic acid and p-aminobenzoyl-L-glutamic acid as shown by liquid chromatographic ion-trap mass spectrometry (LC-IT-MS).

Electrochemical behavior of antioxidants: I. Mechanistic study on electrochemical oxidation of gallic acid in aqueous solutions at glassy-carbon electrode

Mechanism of electrochemical oxidation of gallic acid in aqueous phosphate buffer solutions of different pH’s was studied at glassy-carbon electrode. The study was performed using cyclic, convolution–deconvolution sweep voltammetry, chronoamperometry and chronocoulometry. It gives two irreversible diffusion-controlled cyclic voltammetric waves at the entire range of pH. The electrochemical oxidation mechanism was proposed to be an ECEC-first order mechanism in which the two electron transfer steps and the two chemical follow-up deprotonation reactions are irreversible. The proposed mechanism was confirmed by digital simulation and the electrode kinetic parameters are estimated on comparing the simulated response with the experimental ones.

Synthesis, characterization and in vitro hydrolysis of a gemfibrozil-nicotinic acid codrug for improvement of lipid profile

Combination therapy of fibrates and nicotinic acid has been reported to be synergistic. Herein, we describe a covalent codrug of gemfibrozil (GEM) and nicotinic acid (NA) that was synthesized and characterized by 1H NMR, 13C NMR, FT-IR, MS analysis and elemental analysis. A validated HPLC method was developed that allows for the accurate quantitative determination of the codrug and its hydrolytic products that are formed during the in vitro chemical and enzymatic hydrolysis. The physico-chemical properties of codrug were improved compared to its parent drugs in term of water solubility and partition coefficient. The kinetics of hydrolysis of the codrug was studied using accelerated hydrolysis experiments at high temperatures in aqueous phosphate buffer solution in pH 1.2, 6.8 and 7.4. Using the Arrhenius equation, the extrapolated half-life at 37 °C were 289 days at pH 1.2 for the codrug and 130 and 20,315 days at pH 6.8 for the codrug and gemfibrozil 2-hydroxyethyl ester (GHEE), respectively. The shortest half-lives were at pH 7.4; 42 days for the codrug and 5837 days for GHEE, respectively. The hydrolysis of the latter was studied, alone, at 80 °C and pH 1.2 and compared to its hydrolysis when it is produced from the codrug using similar conditions. The k obs was found in both cases to be 1.60 × 10 −3 h −1. The half-lives in plasma were 35.24 min and 26.75 h for the codrug and GHEE, respectively. With regard to liver homogenate, the hydrolysis half-lives were 1.96 min and 48.13 min for the codrug and GHEE, respectively. It can be expected that in vivo, the codrug will liberate NA immediately in plasma then GEM will be liberated from its 2-hydroxyethyl ester in the liver.

A promising codrug of nicotinic acid and ibuprofen for managing dyslipidemia. I: Synthesis and in vitro evaluation

Nicotinic acid is therapeutically the optimum antihyperlipidemic agent, yet its intolerable cutaneous flushing hinders its wide clinical implication. The codrug of nicotinic acid and ibuprofen (IBP) was synthesized in the aim of overcoming the troublesome side effect of nicotinic acid by blockade of prostaglandin synthesis through released IBP, thus enhance patient’s compliance. The physico-chemical properties of codrug namely solubility, partition coefficient, and pKa were determined. Its solubility in aqueous and organic solvents was highest in 0.1 M HCl and isopropanol, respectively. The kinetics of hydrolysis of the codrug and IBP 2-hydroxyethyl ester was studied in aqueous phosphate buffer solution in pH 1.2, 6.8, and 7.4 at 70°C, 80°C, and 90°C. The hydrolysis was found to be pH dependent and followed Arrhenius equation. The half-life of codrug and IBP 2-hydroxyethyl ester at 25°C in pH 7.4 was 218 days and 3 years, respectively. In vitro enzymatic hydrolysis of codrug and IBP 2-hydroxyethyl ester was studied in human plasma and rat liver homogenate. Codrug and IBP 2-hydroxyethyl ester exhibited faster in vitro enzymatic hydrolysis than in vitro chemical hydrolysis. The pseudo-first-order rate constants were 0.0113, 0.177 min−1 for codrug and 0.0006, 0.0569 min−1 for IBP 2-hydroxyethyl ester in human plasma and rat liver homogenate, respectively. Thus, nicotinic acid will be rapidly released from codrug to manage dyslipidemia, followed by the later release of IBP from IBP 2-hydroxyethyl ester to alleviate nicotinic acid cutaneous flushing.

Rapid assays of clozapine and its metabolites in dried blood spots by liquid chromatography and microextraction by packed sorbent procedure

A novel analytical approach has been developed for the determination of clozapine and its metabolites in dried blood spots on filter paper, using a chromatographic method coupled with a microextraction by packed sorbent procedure. The analytes were separated on a RP-C18 column using a mobile phase composed of 20% methanol, 16% acetonitrile and 64% aqueous phosphate buffer. Coulometric detection was used, setting the guard cell at +0.050 V, the first analytical cell at −0.200 V and the second analytical cell at +0.500 V. Clozapine and its metabolites were extracted from dried blood spots with phosphate buffer and, then, a microextraction by packed sorbent procedure for the sample clean-up was implemented obtaining good extraction yields. The calibration curve was linear over the 2.5–1000 ng mL −1 blood concentration ranges for all the analytes. The method validation gave satisfactory results in terms of sensitivity, precision, selectivity and accuracy. The analytical method was successfully applied to dried blood spots from several psychiatric patients for therapeutic drug monitoring purpose.

Thermodynamic characterization of phthalocyanine–human serum albumin interaction

The thermodynamic of the binding of nickel (II) tetrasulfonated phthalocyanine anion (Ni(tspc) 4− ), to human serum albumin (HSA) was investigated in 5 mM aqueous phosphate buffer of pH 7.40 at 25 ◦ C using optical absorption spectroscopy. The results show that (Ni(tspc) 4− ) does not have any affinity for aggregation due to increasing of salt concentration and exists as monomers even in homogeneous aqueous solutions of high ionic strengths (more than 2 M NaCl). The binding constant (K) was obtained by analysis of optical absorption spectra of mentioned complex at various HSA concentrations using SQUAD software. The value of K was estimated to be 4.89 × 10 5 ± 0.03 (M −1 )a t 25 ◦ C. The thermodynamic parameters were calcu- lated by van't Hoff equation. The enthalpy and entropy changes were 28.08 kJ/mol and 203.09 J/(mol · K) at 25 ◦ C, respectively. The results indicate that the binding is mainly entropy driven and the enthalpy is unfavorable for it, the hydrophobic forces thus playing a major role in the binding process.

Thermodynamic characterization of phthalocyanine–human serum albumin interaction

The thermodynamic of the binding of nickel (II) tetrasulfonated phthalocyanine anion [Ni(tspc)4–], to human serum albumin (HSA) was investigated in 5 mM aqueous phosphate buffer of pH 7.40 at 25°C using optical absorption spectroscopy. The results show that [Ni(tspc)4–] does not have any affinity for aggregation due to increasing of salt concentration and exists as monomers even in homogeneous aqueous solutions of high ionic strengths (more than 2 M NaCl). The binding constant (K) was obtained by analysis of optical absorption spectra of mentioned complex at various HSA concentrations using SQUAD software. The value ofKwas estimated to be 4.89×105±0.03 (M–1) at 25°C. The thermodynamic parameters were calculated by van’t Hoff equation. The enthalpy and entropy changes were 28.08 kJ/mol and 203.09 J/(mol?·?K) at 25°C, respectively. The results indicate that the binding is mainly entropy driven and the enthalpy is unfavorable for it, the hydrophobic forces thus playing a major role in the binding process.

High performance liquid chromatographic determination of some guaiphenesin-containing cough-cold preparations

This paper presents different HPLC methods for the simultaneous determination of some guaiphenesin-containing cough-cold preparations. Three pharmaceutically available combinations were analyzed: salbutamol sulfate (SAL) and guaiphenesin (GUA), combination I; ascorbic acid (ASC), paracetamol (PAR) and guaiphenesin (GUA), combination II; and theophylline anhydrous (THE), guaiphenesin (GUA) and ambroxol hydrochloride (AMB), combination III. A 250 × 4.6 mm C-18 column was used for all combinations. The mobile phase for the three combinations consisted of a mixture of methanol and 0.01 M aqueous phosphate buffer solution. The pH of the mobile phase was adjusted to 3.2, 6.2 and 3.8 for combinations I, II and III, respectively. The proposed HPLC methods were successfully applied to the determination of the investigated drugs, both in synthetic mixtures and in pharmaceutical preparations, without any matrix interference and with high precision and accuracy. Different aspects of analytical validation are presented in the text.

Simultaneous determination of disulfiram and bupropion in human plasma of alcohol and nicotine abusers

An isocratic high-performance liquid-chromatographic method has been developed for the simultaneous determination of disulfiram and bupropion in human plasma samples. Analyses were carried out on a C(8) reversed-phase column using a mobile phase composed of 50% acetonitrile and 50% aqueous phosphate buffer, containing triethylamine. Diode-array detection was used, operating at a wavelength of 250 nm. For the clean-up of plasma samples, a solid phase extraction procedure, based on C(2) cartridges, was implemented. Extraction yields of the analytes were satisfactory, being always higher than 84%. The calibration curve was linear over the 5-500 ng mL(-1) plasma concentration range for both disulfiram and bupropion. The method showed a high sensitivity (limit of detection of 1.5 ng mL(-1)) and satisfactory precision, selectivity and accuracy. The application to human plasma samples obtained from some alcohol and nicotine abusers also gave good results.

What is modulating solubility in simulated intestinal fluids?

The aim of this study was to understand which parameters are responsible for the selective modulation of compounds solubility in simulated intestinal fluids. The solubility of 25 chemically diverse reference compounds was measured in simulated intestinal fluid (FaSSIF-V2) and in aqueous phosphate and maleate buffers. Electrostatic interactions between compounds and the bio-relevant medium components seem to explain the different solubility behavior observed for acids and bases. The solubility of ionized acids is not increased in FaSSIF-V2 probably due to electrostatic repulsions with the media components. Lipophilicity plays an important role but mainly for charged bases with a log P > 4 (or log D 6.5 > 1.9). When the aqueous solubility is mainly driven by lipophilicity, the FaSSIF-V2 components seem to improve the solubility of basic compounds to a greater extent than for compounds whose solubility is limited by crystal packing. These results suggest that ionization, lipophilicity and crystal packing play important but peculiar roles in controlling solubility in FaSSIF-V2 compared to that in aqueous buffer and this information could be useful to guide medicinal chemists and formulation scientists.

Evaluation of extraction methods for use with NMR-based metabolomics in the marine polychaete ragworm, Hediste diversicolor

The sediment-dwelling polychaete, Hediste diversicolor, is commonly found in Northern temperate estuaries. Its limited mobility and tolerance to polluted conditions makes it a good candidate for biological monitoring. Moreover, its importance in the functioning of the sediment ecosystem has caused it to be described as a keystone species. Here we present the development of analytical methodology that will enable the use of H. diversicolor in environmental metabolomics studies for the biomonitoring of estuarine ecosystems. Polar and non-polar extraction solvents have been used to solubilise a wide range of metabolites. Extraction solvents assessed include: aqueous phosphate buffer solution, methanol:chloroform:water (1:1:0.9), methanol:water (1:1 and 2:1) and chloroform. The metabolites were analysed using 1-dimensional (1D) 1H nuclear magnetic resonance (NMR) spectroscopy. Using the methanol:water (1:1) method, previous freezing to aid cell rupture did not result in an enhanced extraction. Removal of methanol with a speed vacuum resulted in reduction in yield. Methanol:water (1:1) and chloroform extractions proved to be the most appropriate techniques based on the sample yield and repeatability. NMR-based metabolomics in the ragworm can now be used to understand the ecophysiology of this important estuarine organism and has applications in biomonitoring, biomarker development and ecotoxicological studies.

One-step growth of 3–5 nm diameter palladium electrocatalyst in a carbon nanoparticle–chitosan host and characterization for formic acid oxidation

The electrochemical formation of a palladium nanoparticle catalyst composite material has been investigated. A carbon nanoparticle–chitosan host film deposited onto a carbon substrate electrode has been employed to immobilize PdCl 2 as catalyst precursor. A one-step electrochemical reduction process gave Pd nanoparticles within the chitosan matrix with different levels of loading, on different carbon substrates, and with a reproducible catalyst particle diameter of ca. 3–5 nm. High activity for formic acid oxidation has been observed in aqueous phosphate buffer medium. The oxidation of formic acid has been investigated as a function of pH and maximum catalyst activity was observed at pH 6. When varying the formic acid concentration, limiting behaviour consistent with a “resistance effect” has been observed. A flow cell system based on a screen-printed carbon electrode has been employed to establish the effect of hydrodynamic conditions on the formic acid oxidation. Both increasing the convective-diffusion mass transport rate and increasing the concentration of formic acid caused the oxidation peak current to converge towards the same “resistance limit”. A mechanistic model to explain the resistance effect based on CO 2 flux and localized CO 2 gas bubble formation at the Pd nanoparticle modified carbon nanoparticle–chitosan host film has been proposed.

Synthesis, characterization and in vitro degradation of poly(ester-anhydride)s based on succinic acid and 1,6-bis- p-carboxyphenoxyhexane

This paper describes synthesis and characteristics of functional poly(ester-anhydride)s bearing allyl pendant groups. The polymers were obtained by polycondensation of 1,6-bis- p-carboxyphenoxyhexane (CPH) and oligo(3-allyloxy-1,2-propylene succinate) terminated with carboxyl groups (OSAGE). The carboxyl groups in OSAGE and in CPH were converted to mixed anhydride groups by acetylation with acetic anhydride. After that, prepolymers thus obtained were condensed in vacuum to yield poly(ester-anhydride)s. The structure of copolymers was confirmed by NMR spectroscopy. Influence of molecular weight of OSAGE as well as of the CPH and OSAGE content on selected properties of poly(ester-anhydride)s was examined. Poly(ester-anhydride)s were subjected to hydrolytic degradation at 37 °C, in aqueous phosphate buffer solution of pH 7.41 (PBS). The course of degradation was monitored by determination of weight loss of samples and 1H NMR. Fracture surfaces of samples during degradation were examined by scanning electron microscopy.

Aggregation behavior of tetrakis(N, N′, N″, N‴-tetramethyl tetra-2,3-pyridino)porphyrazine copper(II) and its interaction with ct-DNA: a thermodynamic approach

The association behavior of tetrakis( N , N ′, N ″, N ‴-tetramethyl tetra-2,3-pyridino)porphyrazine copper(II) ([ Cu(II) 2,3- tmtppa ]4+) was investigated in aqueous solutions at 25 °C and various ionic strengths using optical absorption and resonance light scattering (RLS) spectroscopies. The results show that ([ Cu(II) 2,3- tmtppa ]4+) does not have any affinity for aggregation due to increasing of salt concentration and exists as monomers even in homogeneous aqueous solutions of high ionic strengths (more than 1 M NaCl ). Interaction of ([ Cu(II) 2,3- tmtppa ]4+) with calf thymus DNA has also been studied in 1 mM aqueous phosphate buffer of pH 7.0, by optical absorption and RLS spectroscopies, and thermal denaturation experiments. The appearance of hypochromicity of less than 10% and bathochromicity shift of Δλ ≤ 2 nm in UV-vis spectra of ([ Cu(II) 2,3- tmtppa ]4+), increasing of thermal melting point of DNA, and no change in RLS spectra of porphyrazine due to interaction with DNA, represent the minor outside groove binding mode without any stack aggregate formation. The thermodynamic of the binding of ([ Cu(II) 2,3- tmtppa ]4+)-DNA also has been studied. The binding constant (K) was obtained by analysis of the optical absorption spectra of the complex at various DNA concentrations using SQUAD software. The value of K was estimated to be 2.34 × 105 ± 0.06 M -1 at 25 °C. The thermodynamic parameters were calculated by van't Hoff equation. The enthalpy and entropy changes were 41.83 ± 3.28 kJ/mol and 242.08 ± 9.88 J/mol.K at 25 °C, respectively. The results indicate that the process is entropy-driven and suggest that hydrophobic interactions are the main driving forces for the complex formation. Increasing of the ionic strength due to addition of NaCl destabilized porphyrazine-DNA complexes, indicating the competition of Na + ions with porphyrazine complexes for occupation of minor groove binding sites.

Ultrathin Carbon Film Electrodes from Vacuum‐Carbonised Cellulose Nanofibril Composite

AbstractA novel way to produce ultrathin transparent carbon layers on tin‐doped indium oxide (ITO) substrates is developed. The ITO surface is coated with cellulose nanofibrils (from sisal) via layer‐by‐layer electrostatic binding with poly(diallyldimethylammonium chloride) or PDDAC acting as the binder. The cellulose nanofibril‐PDDAC composite film is then vacuum‐carbonised at 500 °C. The resulting carbon films are characterised by atomic force microscopy (AFM), small angle X‐ray scattering (SAXS), wide‐angle X‐ray scattering (WAXS), and Raman methods. Smooth carbon films with good adhesion to the ITO substrate are formed. The electrochemical characterisation of the carbon films is based on the oxidation of hydroquinone and the reduction of benzoquinone in aqueous phosphate buffer media. A modest effect of the cellulose nanofibril‐PDDAC film on the rate of electron transfer is observed. The effect of the film on the rate of electron transfer after carbonisation is more dramatic. For a 40‐layer cellulose nanofibril‐PDDAC film after carbonisation a two‐order of magnitude change in the rate of electron transfer occurs presumably due to a better interaction of the hydroquinone/benzoquinone system with the electrode surface.

Synthesis, chemical and enzymatic hydrolysis, and bioavailability evaluation in rabbits of metronidazole amino acid ester prodrugs with enhanced water solubility.

A series of amino acid esters (3a-e) have been synthesized and evaluated as potential prodrugs of metronidazole with the aim of improving aqueous solubility and therapeutic efficacy. The aqueous solubility and the lipophilicity (expressed as the log P value) of metronidazole and its esters were investigated. In general the prodrugs revealed enhanced water solubility compared with metronidazole. N,N-diethylglycinate hydrochloride (3a) and 4-ethylpiperazinoacetate (3e) derivatives displayed higher aqueous solubility, which exceeded that of the parent drug by factors of approximately 140 and 100, respectively. All the esters revealed lower log P values than metronidazole except for the 4-phenylpiperazinoacetate derivative (3f), which was 6.5-times more lipophilic than metronidazole. The hydrolysis kinetics of the esters were studied in aqueous phosphate buffer (pH 7.4) and 80% human plasma at 37 degrees C. In all cases the hydrolysis followed pseudo-first-order kinetics and resulted in a quantitative reversion to metronidazole as evidenced by HPLC analysis. The prodrugs exhibited adequate chemical stability (half-life, t1/2, 4-16 h) in aqueous phosphate solution of pH 7.4. In 80% human plasma they were hydrolysed within a few minutesto metronidazole. The esters 3d (methylpiperazinoacetate derivative) and 3f were exempted since their t1/2 values were approximately 2.5 and 8.5 h, respectively. A comparative pH-rate profile study of N,N-diethylglycinate hydrochloride (3a) and 4-ethyl-piperazinoacetate (3e) derivatives in aqueous buffer solution over the pH range 2.2-10 was investigated. The results indicated that 3a showed marked stability at pH 2-6 followed by accelerated hydrolysis at pH 7.4. The basic ester 3e was found to be less stable at lower pH values but exhibited comparative stability at physiological pH. Moreover, in-vivo experiments in rabbits revealed a higher metronidazole plasma level with sustained release characteristics within the prodrug-treated animals (10- and 2.5-fold) as compared with the parent drug-treated group. In conclusion, the designed amino acid esters 3a and 3c-e might be considered as good candidates for water-soluble prodrug forms of metronidazole.