Application of 13C nuclear magnetic resonance spectroscopy to the analysis and structural investigation of tetracycline antibiotics and their common impurities.

The spectral assignations of all the main resonances in the 13C nuclear magnetic resonance spectra in DMSO-d6 or D2O are presented and reviewed for ten of the principal tetracyclines in therapeutic use: tetracycline, chlortetracycline, 6-demethylchlortetracycline, minocycline, oxytetracycline, methacycline, meclocycline, doxycycline, rolitetracycline and lymecycline. NMR techniques employed include: proton noise-decoupled spectra, off-resonance decoupled spectra, nuclear Overhauser enhancement and the INEPT technique (insensitive nuclei enhanced by polarization transfer). 13C NMR is also examined as a means of detecting and identifying the principal degradation and isomerization products of tetracycline antibiotics. The analytical potential of 13C NMR is discussed with respect to quantitative analysis of impurities, studies on sites of protonation and metal-binding studies.