Omeprazole determination using HPLC with coulometric detection.

A sensitive high performance liquid chromatography (HPLC) method for the determination of omeprazole and three related benzimidazoles is reported. Coulometric detection was carried out at +800 mV using a porous carbon electrode. The linear range is 0.01-10 microg/ml. The method has a high degree of precision; the relative standard deviation of omeprazole at a concentration of 1.06 microg/ml was 0.7% (n=4). The cyclic voltammogram of omeprazole is consistent with the hydrodynamic voltammogram exhibiting a single major irreversible oxidative wave with a peak potential at +1105 mV. The response factors for the four compounds are similar indicating that the oxidative process does not involve the sulfur moiety exclusively. The data are most consistent with oxidation primarily of the benzimidazole groups. The method was applied successfully to the determination of omeprazole in a paste formulation.

Meropenem exists in equilibrium with a carbon dioxide adduct in bicarbonate solution.

Evidence is provided for the existence of a second discrete chemical form of meropenem at clinically relevant concentrations prepared from the marketed formulation of the drug. Proton and carbon-13 NMR spectra in D2O, coupled with tandem mass spectroscopy (MS/MS) and cross polarization/magic angle spinning (CP-MAS) NMR experiments, allow structural assignment of the compound as a covalent carbon dioxide adduct of meropenem. This carbon dioxide adduct exists in equilibrium with the free drug in solution and can be observed in the solid state following lyophilization. The equilibrium constant of formation of the adduct (Keq = 20 +/- 8 M-1) was estimated in D2O at 25 degreesC.

Identification of the secondary degradates of L-648,548 in an animal health formulation.

L-648,548 is a semisynthetic analog of avermectin. During stability investigations of this compound in an animal health formulation, two new degradates were discovered. These degradates (L-648,548 phenol and its 8,9-Z isomer) were identified as the reaction products of 5-oxo-L-648,548 formed by oxidation of L-648,548. Addition of base to the reaction medium containing 5-oxo-L-648,548 was found to catalyze the formation of L-648,548 phenol via a postulated dehydration by an E1cb elimination followed by the rapid tautomerization of the C5 carbonyl. Photolysis of L-648,548 phenol with visible light (including ambient laboratory lighting) was found to readily produce 8,9-Z-L-648,548 phenol. This transformation was confirmed to be exclusively a photoinduced process.

Radical-induced oxidation of oxahydrindene (hexahydrobenzofuran) portion of 22,23-dihydroavermectin B1a: identification of autooxidation products.

Three products resulting from free-radical-induced oxidation of the oxahydrindene portion of 22,23-dihydroavermectin B1a (H2B1a) are 5-oxo-H2B1a, 8a-oxo-H2B1a, and 5,8a-bisoxo-H2B1a. The last of these compounds has not been reported previously.

A computer-controlled potentiometric/spectrophotometric titrator.

A laboratory computer controlled potentiometric titrator interfaced to a diode array spectrophotometer is described. The titrator consists of widely used, commercially available components; therefore, major attention is given to modes of interconnection and software implementation in data format and system control. Replicate potentiometric titrations of glycines gave a relative standard deviation in titre of 1.035% and a relative standard deviation in pH of 0.745%. Replicate spectrophotometric titrations of bromophenol blue were analysed at three wavelengths to yield pKa= 3.898 +/- 0.075 (1.9% rsd).Methods of data presentation and manipulation are presented.

A robotic sample preparation scheme for the high performance liquid chromatographic determination of ivermectin in animal plasma.

A lengthy sample preparation scheme for the high performance liquid chromatographic determination of the antiparasitic agent ivermectin at ppb concentrations in animal plasma is adapted to a laboratory robotic system. Sample treatment involves both liquid-liquid partitioning and solid phase extraction. Specific modifications to the manual procedure include the use of serial vortex mixings in place of batchwise lateral shaking and the substitution of small (6 mL), disposable solid phase extraction columns driven by compressed gas for large (25 mL), gravity-fed, reusable glass columns. Coupling these columns to an automated solvent dispensing device simplifies handling of the large solvent volumes prescribed in the manual procedure. A productivity gain over the manual procedure is realized when operating the robotic system in the single sample mode, and an additional gain is achieved by integrating the system for multiple sample handling. Equivalence to the manual procedure is demonstrated.

Picosecond resonance Raman spectroscopic evidence for excited-state spin conversion in carbonmonoxy-hemoglobin photolysis.

The structure of the carbonmonoxy-hemoglobin (COHb) photoproduct has been studied on the picosecond time scale with resonance Raman spectroscopy, by tightly focusing the 30-ps pulses of a synchronously pumped mode-locked cavitydumped dye laser on a jet stream of COHb solution. The spectrum of the photoproduct is similar to that of deoxy Hb, but the frequencies 1603 cm(-1) (depolarized), 1552 cm(-1) (anomalously polarized), and 1542 cm(-1) (depolarized) are 2-4 cm(-1) lower than those of deoxy Hb. Similar low frequencies are observed for a species believed to be the bis-tetrahydrofuran adduct of Fe(II) octaethylporphyrin, containing in-plane high-spin Fe(II). These results indicate that in the COHb photoproduct the Fe(II) is already high-spin but is closer to the heme plane than in deoxy Hb. Photodissociation from a quintet ligand-field excited state of COHb is suggested. The frequency shifts relative to deoxy Hb persist when the laser pulses are lengthened to 20 ns. The apparently slow relaxation to the fully out-of-plane heme conformation of deoxy Hb is suggested to be associated with change of the globin tertiary structure.

Electronic and resonance Raman spectra of iron(III) complexes of enterobactin, catechol, and N-methyl-2,3-dihydroxybenzamide.

Resonance Raman electronic absorption and circular dichroism spectra and pH titration curves are reported for the trianionic ferric complexes of enterobactin, catechol, and N-methyl-2,3-dihydroxybenzamide (MDHB). The spectral signatures of the enterobactin and MDHB complexes are virtually identical and differ from those of the catechol complex in ways that reflect the influence of the amide group on the electronic structure. Excitation in either the visible charge-transfer bands or the near-ultraviolet pi-pi* bands enhances Raman bands associated with benzene ring modes, although the relative enhancements differ markedly in the two regions. The data stronly support a structural model in which iron is bound exclusively to the phenolate oxygen atoms in all three complexes.

Heme models. I. Solution behavior of a water soluble iron porphyrin.

A well-behaved water soluble iron-porphyrin system, meso-tetra-(4-carboxyphenyl) porphinato iron (III) was synthesized. Its solution behavior is described using visable and electron paramagnetic resonance (EPR) spectroscopy. The complex exists in solution as three distinct forms of bridged dimers, oxo, hydroxo and aquo, with the following pK's: oxo + H+ in equilibrium hydroxo, pK = 9.58; hydroxo + H+ in equilibrium aquo, pK = 6.72. In the presence of excess imidazole the second pK is found to be 7.05. Detailed analysis of the interaction of the hydroxo-bridged form with imidazole is presented. It is found that one dimer unit simultaneously binds two imidazole molecules, with an over-all equilibrium constant log Keq = -1.22. EPR spectra are presented for the various forms of iron-porphyrin discussed.