Using pure shift HSQC to characterize microgram samples of drug metabolites

Difficulties in isolating samples from complex biological matrices and sensitivity limitations have long stymied the utilization of heteronuclear 2D NMR for the characterization of drug metabolites. Small diameter cryogenic NMR probes have largely ameliorated sensitivity limitations and the recently reported pure shift HSQC 2D NMR pulse sequence offers a further and marked improvement in both resolution and sensitivity. Using a 7.4 μg sample of the commercially available metabolite 3-hydroxy carbamazepine dissolved in 30 μL of deuterated solvent and a 600 MHz NMR equipped with a 1.7 mm cryogenic NMR probe, it was possible to acquire high signal-to-noise pure shift HSQC data in just over 30 min. A conventional HSQC spectrum acquired with identical parameters had approximately half the signal-to-noise of the pure shift HSQC spectrum. Collapsing the vicinal homonuclear couplings in the pure shift HSQC spectrum also significantly improves resolution. A practical, real world application of the technique is illustrated with the chromatographically isolated metabolite 3-hydroxy amiodarone from incubation with CYP2J2 recombinant enzyme. High quality pure shift HSQC data were recorded in slightly over 14 h for a 3 μg sample of the metabolite.