Role of the solvent on the stability of cycloserine under ESI-MS conditions

Determination of d-Cycloserine Impurities in Pharmaceutical Dosage Forms: Comparison of the International Pharmacopoeia HPLC-UV Method and the DOSY NMR Method

d-cycloserine is a broad-spectrum antibiotic that is currently being used as a secondary choice in the treatment of tuberculosis. In recent years, it has become more popular, due to its effect on the nervous system. In this current study, we provide evidence that The International Pharmacopoeia HPLC–UV method for d-cycloserine impurity profiling is not repeatable due to the variable response of cycloserine dimer, one of d-cycloserine impurities. Therefore, we introduced the DOSY (diffusion ordered spectroscopy) NMR (nuclear magnetic resonance) technique to determine the levels of d-cycloserine impurities in pharmaceutical dosage forms. The DOSY NMR technique allowed separation of d-cycloserine, its degradation products, and key process impurities in concentrations below pharmacopoeial specification limits. The proposed DOSY NMR method allowed accurate identification and quantification of the cycloserine dimer, which was not possible through the use of the pharmacopoeial HPLC method. The current method has the potential for practical use in analytical laboratories of the pharmaceutical industry.

Benzoxazine Formation Mechanism Evaluation by Direct Observation of Reaction Intermediates

Benzoxazine formation is a fundamental step in the preparation of polybenzoxazine resins, and a detailed description of the mechanism governing the formation of benzoxazine and side products is vital for improving the properties and performance of these resins. Determination of the nature and properties of reaction intermediates is not trivial. Therefore, a Mannich-type condensation of aniline, formaldehyde, and phenol was evaluated as a potential method to form benzoxazine. Coupling positive mode electrospray ionization mass spectrometry (ESI(+)-MS) with infrared multiple photon dissociation (IRMPD) spectroscopy allowed unambiguous determination of an iminium-based mechanism and the direct observation of iminium intermediates. The benzoxazine formation mechanism was indirectly confirmed by the observation of side products that are relevant to the polymerization step, and directly confirmed by the identification of four distinct reaction intermediates that were completely characterized by IRMPD spectroscopy. The benzoxazine monomer was also shown to undergo isomerization under standard ESI-MS analysis conditions, suggesting the presence of a mixture of three isomers during their usual ESI-MS analysis.

Ribeiro FW, Cervi G, C. Correra T. Evaluation of Common Theoretical Methods for Predicting Infrared Multiphotonic Dissociation Vibrational Spectra of Intramolecular Hydrogen-Bonded Ions

Infrared photodissociation analyses are supported by theoretical calculations that allow a trustworthy interpretation of experimental spectra of gaseous ions. B3LYP calculations are the most prominent method used to model IR spectra, as detailed in our bibliographic survey. However, this and other commonly used methods are known to provide inaccurate energy values and geometries, especially when it comes to long-range interactions, such as intramolecular H-bonds, which show increased anharmonicity. Therefore, we evaluated some of the most commonly used density functional theory methods (B3LYP, CAM-B3LYP, and M06-2X) and basis sets (6-31+G(d,p), 6-311++G(d,p), 6-311++G(3df,2pd), aug-cc-pVDZ, and aug-cc-pVTZ), including anharmonicity and dispersion corrections. The results were compared to MP2 calculations and to experimental high-frequency (2000-4000 cm-1) IR multiphotonic dissociation (IRMPD) spectra of two protonated model molecules containing intramolecular hydrogen bonds: biotin and tryptophan. M06-2X/6-31+G(d,p) was shown to be the most cost-effective level of theory, whereas CAM-B3LYP was the most efficient method to describe the van der Waals interactions. The use of the dispersion correction D3, proposed by Grimme, improved the description of O-H vibrations involved in H-bonding but worsened the description of N-H stretches. Anharmonic calculations were shown to be extremely expensive when compared to other approaches. The efficiencies of well-established scaling factors (SFs) in opposition to sample-dependent SFs were also discussed and the use of fitted SFs were shown to be the most cost-effective approach to predict IRMPD spectra. M06-2X/6-31+G(d,p) and CAM-B3LYP/aug-cc-pVDZ were also tested against the fingerprint region. Our results suggest that these methods can also be used for analysis in this lower frequency range and should be regarded as the methods of choice for cost-effective IRMPD simulations rather than the ubiquitous B3LYP method, especially when further molecular properties are needed.

Real time evolution of unprotected protonated galactosamine probed by IRMPD spectroscopy

Conformational characterization of single α- and β-epimers of galactosamine in solution still remains an intriguing task because of their flexibility and ability to interconvert. This difficulty was circumvented by recording several "snapshots" of the epimerization process by means of fast ESI vaporization of a galactosamine·HCl sample solution at different times. Consequently, the so generated gaseous mixtures were spectroscopically investigated and the specific conformational features of both α- and β-epimers were assigned, despite the overlapping of several IR signals. Interestingly, from a comparison with time-resolved 1H-NMR data obtained for the same solutions, the catalyzing effect of the applied ESI technique in the anomerization process clearly emerges. Finally, the experimental data were supported using both the Density Functional Theory (DFT) and Block-Localized Wavefunction (BLW) approaches: the latter method was applied here for the first time for the investigation of charged species.

Determination of cycloserine in microdialysis samples using liquid chromatography-tandem mass spectrometry with benzoyl chloride derivatization

A new method for the analysis of cycloserine (4-amino-3-isoxazolidinone, CYC) in rat microdialysis samples has been developed. This method consists of derivatizing the CYC with benzoyl chloride, which transforms primary amines into highly stable derivatives. An attractive feature of this method was that the derivatization reaction is straightforward and can be completed within 10 min. The formed derivative, in contrast to the non-derivatized analyte, exhibited increased chromatographic retention and decreased matrix effects resulting from the co-elution of other components using reversed-phase liquid chromatography and on-line switching. Detection on a quadrupole-linear ion trap mass spectrometer (AB3200 Q-Trap) was performed using electrospray tandem mass spectrometry in multiple reaction monitoring mode. Various derivatization parameters were optimized in order to improve chromatographic separation and minimize ion suppression. In particular, the benzoylation reaction was improved to enhance the reproducibility and sensitivity of the chromatographic method. The transition m/z 207.1 → 105.1 was acquired to monitor the CYC derivatization products. The method was fully validated for its sensitivity, selectivity, matrix effect and stability. A good linearity over the selected range (r > 0.99, range = 22-2200 mg/L), as well as accuracy and precision within ±7% of the target values, was obtained. The assay described herein was successfully applied to quantitatively measure CYC in the lung and blood of anesthetized rats.

A simplified LC-MS/MS method for rapid determination of cycloserine in small-volume human plasma using protein precipitation coupled with dilution techniques to overcome matrix effects and its application to a pharmacokinetic study

Matrix effects have been a major concern when developing LC-MS/MS methods for quantitative bioanalysis of cycloserine. Sample handling procedures including solid phase extraction or derivatization have been reported previously by researchers to overcome matrix effects of cycloserine. In the present study, the possibility of reducing matrix effects of cycloserine using protein precipitation coupled with dilution techniques was investigated. Plasma samples were pretreated by protein precipitation with methanol followed by a 40-fold dilution with methanol-water (50:50, v/v). The analyte and the internal standard (mildronate) were chromatographed on a Shim-pack XR-ODS (100 mm × 2.0 mm, 2.2 μm) column using methanol-0.01% formic acid (70:30, v/v) as mobile phase and detected by multiple reaction monitoring mode via positive electrospray ionization. The total run time was only 2 min per sample. The suppression of cycloserine response was reduced with the matrix effects ranging between 80.5 and 87.9%. A lower limit of quantification (LLOQ) of 0.300 μg/mL was achieved using only 10 μL of plasma. The intra- and inter-day precisions were less than 4.8% and the accuracy ranged from -2.6 to 6.6%. The method was successfully applied to a pharmacokinetic study of cycloserine in 30 healthy Chinese male subjects after oral administration of a single dose of cycloserine at 250, 500 and 750 mg under fasting conditions. The newly developed method is simpler, faster, cost-effective, and more robust than previously reported LC-MS/MS methods.