Effect of ß-cyclodextrin on intra and intermolecular Michael addition of some catechol derivatives.

The oxidation reactions of catechol, dopamine and epinephrine have been studied in the absence and presence of N-methylaniline by UV-Vis. Spectrophotometry. A variety of reaction pathways (inter and intramolecular reactions) that followed by this oxidation have been observed depending on the nature of catechol derivatives. The observed homogeneous rate constants of the reactions were estimated by fitting the absorption time profiles for each reaction. The effect of ß-cyclodextrin and its inclusion complex has also been studied on the chosen reactions. The formation constants of the complexes of catechol, dopamine and epinephrine with ß-cyclodextrin as well as the rate constants of the reactions of free and complexed forms have been obtained by fitting the absorption-time spectra to a proposed kinetic-equilibrium model.

Investigation of the inclusion complex of ß-cyclodextrin with mycophenolate mofetil.

The acid-base equilibrium of mycophenolate mofetil is studied in the absence and presence of ß-cyclodextrin (ß-CD). The conditional acidity constants are obtained by rank annihilation factor analysis (RAFA) as a function of ß-CD concentrations. Also the stability constants for inclusion complexes of ß-CD with both acidic and basic forms are calculated. The conditional acidity constant decreases by increasing ß-CD concentration. The calculated stability constants show that the acidic form of mycophenolate mofetil forms more stable inclusion complex (552±7 M(-1)) than its basic anionic form (158±2 M(-1)).

Effect of inclusion complex on nitrous acid reaction with flavonoids.

The kinetic of the nitrous acid reactions with quercetin and catechin has been studied using spectrophotometric method in aqueous solution. The results show that these antioxidants participate in oxidation reactions with nitrous acid which is derived from protonation of nitrite ion in mild acidic conditions. Corresponding o-quinones as relatively stable products were detected by spectrophotometric techniques. pH dependence of the reactions has been examined and the rate constants of reactions were obtained by non-linear fitting of kinetic profiles. The effect of ß-cyclodextrin on the oxidation pathway was another object of this study. It is shown that ß-cyclodextrin has an inhibitory effect on the oxidation reaction. The rate constants of oxidation reactions for complexed forms and their stability constants were obtained based on changes in the reaction rates as a function of ß-cyclodextrin concentration.

Kinetic study of the oxidation and nitration of catechols in the presence of nitrous acid ionization equilibria.

Conversion of catechols to corresponding nitro derivatives in the presence of nitrous acid dissociation is studied using voltammetry and UV-vis spectrophotometry. The results indicate that the quinones derived from oxidation of catechols by nitrous acid participate in Michael addition reaction with nitrite ion in very mild acidic solutions. Rank annihilation factor analysis RAFA is applied to resolve the two-way kinetic spectra data measured from spectroscopic reactions. The rank of the original data matrix is reduced by one by annihilating the information of each component. It is shown that both reactions are drastically depends on pH and nitrous acid or nitrite ion percentage. The rate constants of oxidation and nitration reactions of catechol derivatives are obtained at the pHs around pK(a) of nitrous acid.

Spectrophotometric investigation of the effect of beta-cyclodextrin on the intramolecular cyclization reaction of catecholamines using rank annihilation factor analysis.

The catalytic effect of beta-cyclodextrin (beta-CD) on the intramolecular cyclization reaction of catecholamines dopamine, methyldopa and levodopa after their oxidation with periodate was investigated spectrophotometrically using rank annihilation factor analysis (RAFA). The method was based on the effect of beta-CD concentration on the intramolecular cyclization reaction of the investigated catecholamines after their oxidation with potassium periodate. In order to perform RAFA, concentration profiles were calculated by optimizing the value of formation constant. The rank of original data matrix can be reduced by one by annihilating the information of the cyclization reaction in the absence of beta-CD. The rate constant for the reactions in the presence and absence of beta-CD was also determined. The best estimation of rate constant in the presence of beta-CD reduces the rank of the system to zero (noise level). The method was evaluated using synthetic data as well as experimental data and good results were obtained.

Spectrophotometric determination of fluoxetine by batch and flow injection methods.

A rapid, simple, and accurate spectrophotometric method is presented for the determination of fluoxetine by batch and flow injection analysis methods. The method is based on fluoxetine competitive complexation reaction with phenolphthalein-beta-cyclodextrin (PHP-beta-CD) inclusion complex. The increase in the absorbance of the solution at 554 nm by the addition of fluoxetine was measured. The formation constant for fluoxetin-beta-CD was calculated by non-linear least squares fitting. Fluoxetine can be determined in the range 7.0 x 10(-6)-2.4 x 10(-4) mol l(-1) and 5.0 x 10(-5)-1.0 x 10(-2) mol l(-1) by batch and flow methods, respectively. The limit of detection and limit of quantification were respectively 4.13 x 10(-6) mol l(-1) and 1.38 x 10(-5) mol l(-1) for batch and 2.46 x 10(-5) mol l(-1) and 8.22 x 10(-5) mol l(-1) for flow method. The sampling rate in flow injection analysis method was 80+/-5 samples h(-1). The method was applied to the determination of fluoxetine in pharmaceutical formulations and after addition to human urine samples.