J.C. Sturm

Electrocatalytic activity of vitamin B12 adsorbed on graphite electrode for the oxidation of cysteine and glutathione and the reduction of cystine

Abstract We have investigated the electrocatalytic activity of vitamin B 12 adsorbed on ordinary pyrolytic graphite (OPG) for the oxidation of cysteine and glutathione and for the reduction of cystine, using cyclic voltammetry and rotating-disc techniques. The activity of the bare graphite substrate is very low for all reactions studied. The presence of monolayer levels of vitamin B 12 on OPG substantially decreases the overpotential of these reactions. Tafel plots of slope RT/F are obtained for the oxidation of cysteine, suggesting a mechanism controlled by the Co(II/III) couple in the vitamin. Similar slopes are obtained for the oxidation of glutathione, but they gradually change to 2 RT/F for acid pH. No linear Tafel regions are obtained for cysteine reduction and the process occurs on Co(I).

Simultaneous determination of melatonin and pyridoxine in tablets by gas chromatography-mass spectrometry

A gas chromatographic-mass spectrometric (GC-MS) method for the qualitative and quantitative determination of melatonin plus pyridoxine commercial tablets is described. Melatonin and pyridoxine were simultaneously determined by GC-MS after extraction from ground tablets with methanol and derivatization with N-methyl-N-N-trimethlylsilyltrifluoroacetamide (MSTFA). The mass chromatograms were generated using 232 m/z ion for melatonin and 280 m/z ion for pyridoxine, respectively. Splitless injection offers good reproducibility with a standard deviation of 2%. The developed method was applied to analyze the melatonin and pyridoxine content from two different tablet formulations. Also, recovery, detection and quantification limits are reported.

Nitrosobenzene: electrochemical, UV-visible and EPR spectroscopic studies on the nitrosobenzene free radical generation and its interaction with glutathione

Abstract This paper reports both the electrochemical characterization and the reactivity of the nitroso radical anion from nitrosobenzene with glutathione. The reduction of nitrosobenzene to the corresponding nitroso radical anion was kinetically characterized in acetonitrile. Free radicals exhibited a natural decay of second order, with a constant value of k 2 =15 555±321 M −1 s −1 . Also, the radicals were characterized by UV-Visible and EPR spectroscopy. Data obtained with these two independent techniques clearly substantiated the formation of the nitrosobenzene radical under our experimental conditions. Furthermore, we have unambiguously demonstrated that glutathione (GSH) scavenged the nitroso radical anion electrochemically generated from nitrosobenzene. The scavenging effect of GSH is supported by the following experimental facts: 1. The parallel decrease of the anodic peak current in the cyclic voltammograms, corresponding to the nitroso radical anion concomitantly with the addition of GSH. 2. The significant decrease of the visible band at 560 nm corresponds to the radical after the addition of GSH. 3. The drop of the EPR signal intensity of the nitroso radical after the addition of GSH. By using a spin trapping technique, thiyl radical (GS . ) was detected during the reaction between the nitroso radical anion with GSH.

Electrochemical Oxidation of 4‐Methyl‐1,4‐dihydropyridines in Protic and Aprotic Media. Spin Trapping Studies

This work reports the electrochemical oxidation of a new series of 4‐methyl‐1,4‐dihydropyridine derivatives with platelet activation factor antagonistic activity. Differential pulse and cyclic voltammetry studies on a glassy carbon electrode showed an irreversible single peak due to the oxidation of the dihydropyridine ring via two electrons. Rotating disk electrode studies show a linear dependence between the current and the rotating rate, indicating that the oxidation process is diffusion‐controlled. Calculated diffusion coefficients did not exhibit significant differences between the derivatives. Voltammetric apparent values for the protonation‐deprotonation equilibrium of the N‐heterocyclic nitrogen are compared with 4‐phenyl substituted 4‐aryl‐1,4‐dihydropyridine (1,4-DHP) derivatives. Based on voltammetric experiments on glassy carbon electrodes, it is seen that the potential peak values are directly related to the electron density on the dihydropyridine ring, wherein C-5 substituent with electron‐withdrawing character produces the most oxidizable 1,4-DHP derivative. The oxidation mechanism follows the general pathway: electron, , electron, , with formation of an unstable cation radical in the first step. A one‐electron oxidation intermediate was confirmed with controlled potential electrolysis and electron spin resonance experiments. On applying N‐benzilydene‐t‐butylamine‐N‐oxide as the spin trap, the unstable radical intermediates from the oxidation of 4‐methyl‐1,4‐dihydropyridine derivatives were intercepted. Comparison of the electron spin resonance spectra of the nitroxide spin adducts revealed no significant differences in the splitting constants of the radicals.

Synthesis, electrochemical, and spectroscopic studies of novel S-nitrosothiols

Abstract A series of S-nitrosothiol compounds, structurally related to the NO-donor S-nitroso-N-acetyl- d , l -penicillamine (SNAP) that contain amidin groups were synthesised by S-nitrosation of the corresponding thiols and characterised. The kinetics of decomposition were investigated and showed that the two adenine-based thionitrites exhibited an unusual stability in aqueous solution compared to SNAP, suggesting that these compounds may complex the traces of free copper ions present in solution, which is known to catalyze the decomposition of thionitrites. The electrochemical behaviour of these compounds and their nitric oxide-releasing potential were studied by means of cyclic voltammetry techniques on mercury and glassy carbon electrodes.

Polarographic determination of loratadine in pharmaceutical preparations

Loratadine, a potent antihistamine drug, is not directly electroreducible at a dropping mercury electrode; however, by means of a nitration procedure it is possible to obtain a nitro-loratadine derivative which has been identified as 4-(8-chloro-7-nitro-5,6-dihydro-11 H-benzo-[5,6]-cyclohepta-[l,2-b]-pyridin-l l-ylidene)-1-piperidine carboxylic acid ethyl ester. The electrochemical reduction of this derivative at different pHs and concentrations using polarography and cyclic voltammetry was studied. The derivative exhibits a differential pulse polarographic peak due to the reduction of the nitro group. This peak was used in order to develop an analytical procedure for determining loratadine in pharmaceutical dosage forms. The recovery study shows adequate accuracy and precision for the developed assay and the excipients do not interfere in the determination.

Polarographic and Spectrophotometric Determination of Nimodipine in Tablets

Abstract Nimodipine is a calcium-channel blocker primarily used for its cerebral vasodilatory effect. The compound produces a d.p. polarographic peak due to the reduction of the nitro to the hydroxylamine group in a four electron process. The electroactivity of the nitro group was used in order to develop a new polarographic method. Furthermore, the UV absorption band at 360 nm of nimodipine was used as a comparative UV spectrophotometric method. The polarographic assay was applied to commercial samples [tablets] of two different pharmaceutical laboratories. Adequate precision and accuracy were obtained, thefore, the developed method is strongly recommended for uniformity content studies and stability test of nimodipine in routine analysis.

Electrochemical, UV-Visible and EPR studies on nitrofurantoin: Nitro radical anion generation and its interaction with glutathione

This paper deals with the reactivity of the nitro radical anion electrochemically generated from nitrofurantoin with glutathione. Cyclic voltammetry (CV) and controlled potential electrolysis were used to generate the nitro radical anion in situ and in bulk solution, respectively and cyclic voltammetry, UV-Visible and EPR spectroscopy were used to characterize the electrochemically formed radical and to study its interaction with GSH. By cyclic voltammetry on a hanging mercury drop electrode, the formation of the nitro radical anion was possible in mixed media (0.015M aqueous citrate/DMF, 40/60, pH 9) and in aprotic media. A second order decay of the radicals was determined, with a k2 value of 201 and 111M-1 s-1, respectively. Controlled potential electrolysis generated the radical and its detection by cyclic voltammetry, UV-Visible and EPR spectroscopy was possible. When glutathione (GSH) was added to the solution, an unambiguous decay in the signals corresponding to a nitro radical anion were observed and using a spin trapping technique, a thiyl radical was detected. Electrochemical and spectroscopic data indicated that it is possible to generate the nitro radical anion from nitrofurantoin in solution and that GSH scavenged this reactive species, in contrast with other authors, which previously reported no interaction between them.

Hydrolytic degradation of nitrendipine and nisoldipine

The development of a new HPLC-UV diode array procedure applied to follow the hydrolytic degradation of two well-known 4-nitrophenyl-1,4-dihydropyridine derivatives, nitrendipine and nisoldipine is reported. Hydrolysis of each drug were carried out in ethanol/Britton-Robinson buffer at different pHs, stored into amber vials at controlled temperatures of 40, 60 and 80 degrees C and periodically sampled and assayed by HPLC. Nitrendipine degradation in different parenteral solutions was also evaluated. The HPLC procedure exhibited an adequate selectivity, repeatability (<1%) and reproducibility (<2%). The recoveries were higher than 98% with CV of 1.13 and 1.54% for nitrendipine and nisoldipine, respectively. A significant degradation was observed at alkaline pH (>pH 8) with a first order kinetic for both drugs. At pH 12, 80 degrees C, k values of 3.56x10(-2) x h(-1) and 2.22x10(-2) for nitrendipine and nisoldipine, respectively were obtained. Also, activation energies of 16.8 and 14.7 kcal x mol(-1) for nitrendipine and nisoldipine, respectively, were calculated. Furthermore, from the results obtained from hydrolytic degradation in different solutions for parenteral use, we can affirm that solutions significantly increased the degradation of nitrendipine. In conclusion, the HPLC proposed procedure exhibited adequate analytical requirements to be applied to the hydrolytic degradation studies of nitrendipine and nisoldipine. Furthermore, all tested parenteral solutions significantly increased the hydrolytic degradation of nitrendipine, the composition of solution being a relevant factor.

Electrochemical study of β-nitrostyrene derivatives: steric and electronic effects on their electroreduction

The electrochemical reduction of a series of β-nitrostyrene and β-methyl-β-nitrostyrene derivatives by tast and differential pulse polarography and cyclic voltammetry over a wide pH range was studied. The reduction potentials are sensitive to the electronic properties of the para-substituent and to the substitution at Cβ. An increase in the electron-donor properties of the substituent at the para position makes the reduction potential more negative. On the other hand, the reduction potential shifts several tens of millivolts towards more negative potentials on going from β-nitrostyrene to β-methyl-β-nitrostyrene derivatives, due to the decrease in conjugation with the increase in the C1–Cα torsion angle. A linear correlation between the calculated electronic barrier and the half-wave potential was observed. Furthermore, a linear correlation between the Hammett σp substituent constant and the half-wave potential also was observed, demonstrating that the electrochemical behaviour of these derivatives depends primarily on molecular structure and electron density distribution in a way similar to rates and equilibrium of homogeneous chemical reaction. The β-nitrostyrenes studied illustrate nicely the effects of steric and electronic effects on electrochemical reactions.