Isotopic 13C NMR spectrometry to assess counterfeiting of active pharmaceutical ingredients: Site-specific 13C content of aspirin and paracetamol

A 13C NMR spectrometric method for the determination of intramolecular δ13C values in fructose from plant sucrose samples

Recent developments in (13) C NMR spectrometry have allowed the determination of intramolecular (13) C/(12) C ratios with high precision. However, the analysis of carbohydrates requires their derivatization to constrain the anomeric carbon. Fructose has proved to be particularly problematic because of a byproduct occurring during derivatization and the complexity of the NMR spectrum of the derivative. Here, we describe a method to determine the intramolecular (13) C/(12) C ratios in fructose by (13) C NMR analysis of the acetyl-isopropylidene derivative. We have applied this method to measure the intramolecular (13) C/(12) C distribution in the fructosyl moiety of sucrose and have compared this with that in the glucosyl moiety. Three prominent features stand out. First, in sucrose from both C(3) and C(4) plants, the C-1 and C-2 positions of the glucosyl and fructosyl moieties are markedly different. Second, these positions in C(3) and C(4) plants show a similar profile. Third, the glucosyl and fructosyl moieties of sucrose from Crassulacean acid metabolism (CAM) metabolism have a different profile. These contrasting values can be interpreted as a result of the isotopic selectivity of enzymes that break or make covalent bonds in glucose metabolism, whereas the distinctive (13) C pattern in CAM sucrose probably indicates a substantial contribution of gluconeogenesis to glucose synthesis.

Improved characterization of the botanical origin of sugar by carbon-13 SNIF-NMR applied to ethanol

Until now, no analytical method, not even isotopic ones, had been able to differentiate between sugars coming from C4-metabolism plants (cane, maize, etc.) and some crassulacean acid metabolism plants (e.g., pineapple, agave) because in both cases the isotope distributions of the overall carbon-13/carbon-12 and site-specific deuterium/hydrogen isotope ratios are very similar. Following recent advances in the field of quantitative isotopic carbon-13 NMR measurements, a procedure for the analysis of the positional carbon-13/carbon-12 isotope ratios of ethanol derived from the sugars of pineapples and agave using the site-specific natural isotopic fractionation-nuclear magnetic resonance (SNIF-NMR) method is presented. It is shown that reproducible results can be obtained when appropriate analytical conditions are used. When applied to pineapple juice, this new method demonstrates a unique ability to detect cane and maize sugar, which are major potential adulterants, with a detection limit in the order of 15% of the total sugars, which provides an efficient mean of controlling the authenticity of juices made from this specific fruit. When applied to tequila products, this new method demonstrates a unique ability to unambiguously differentiate authentic 100% agave tequila, as well as misto tequila (made from at least 51% agave), from products made from a larger proportion of cane or maize sugar and therefore not complying with the legal definition of tequila.

Performance evaluation of quantitative adiabatic (13)C NMR pulse sequences for site-specific isotopic measurements

(2)H/(1)H and (13)C/(12)C site-specific isotope ratios determined by NMR spectroscopy may be used to discriminate pharmaceutically active ingredients based on the synthetic process used in production. Extending the Site-specific Natural Isotope Fractionation NMR (SNIF-NMR) method to (13)C is highly beneficial for complex organic molecules when measurements of (2)H/(1)H ratios lead to poorly defined molecular fingerprints. The current NMR methodology to determine (13)C/(12)C site-specific isotope ratios suffers from poor sensitivity and long experimental times. In this work, several NMR pulse sequences based on polarization transfer were evaluated and optimized to measure precise quantitative (13)C NMR spectra within a short time. Adiabatic 180 degrees (1)H and (13)C pulses were incorporated into distortionless enhancement by polarization transfer (DEPT) and refocused insensitive nuclei enhanced by polarization transfer (INEPT) to minimize the influence of 180 degrees pulse imperfections and of off-resonance effects on the precision of the measured (13)C peak areas. The adiabatic DEPT sequence was applied to draw up a precise site-specific (13)C isotope profile of ibuprofen. A modified heteronuclear cross-polarization (HCP) experiment featuring (1)H and (13)C spin-locks with adiabatic 180 degrees pulses is also introduced. This sequence enables efficient magnetization transfer across a wide (13)C frequency range although not enough for an application in quantitative (13)C isotopic analysis.