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Directly coupled HPLC−NMR spectroscopy has now become a standard, commercially available technique for mixture characterization. Here the extension of the technique to chiral HPLC separation is reported and it is shown that HPLC−NMR together with HPLC−CD provide complementary approaches for the identification of structural isomers and enantiomers. The general approach has been exemplified using the neuromuscular blocking agent atracurium besylate, which comprises a mixture of 10 isomers in various proportions as four racemic pairs and two meso compounds. Diagnostic reporter resonances in the 1H NMR spectrum of atracurium besylate were assigned using a combination of one-dimensional and two-dimensional NMR experiments at 750 MHz. Stop-flow 750-MHz 1H NMR spectroscopy was used on-line after chiral column HPLC separation to identify the enantiomeric pairs, to distinguish the meso compounds, and to identify key configurational features of the isomers. The parallel HPLC−CD experiments served to assign the enantiomers based upon the known CD and absolute stereochemistry of (R)-laudanosine hydrochloride, an analogue with the same tetrahydroisoquinoline structural unit as atracurium. It is thereby demonstrated that high-field HPLC−NMR and HPLC−CD is a powerful combination of techniques which could be combined on-line for mixture characterization.

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The use of microbial cultures as a complementary model for mammalian drug metabolism has been well established previously. Here is a preliminary investigation into the potential of 19F NMR spectroscopy as a rapid screening tool to quantify the biotransformations of fluorine-containing model drugs. Biotransformations of three model drugs in 48 taxonomically diverse organisms were measured by acquiring 19F NMR spectra at 376 MHz. The presence of fluorine in the molecules allowed rapid, simultaneous detection of over 20 biotransformation products without sample pretreatment, chromatography, mass spectrometric techniques or the use of radiolabelled substrates. The detection limit at 376 MHz using 5 mm NMR tubes was ca. 0.3 microg ml(-1) using a typical analysis time of 20 min per sample. With the recent advent of flow injection NMR technology, analysis time of 5 min could be achieved with less sample. This approach may be used to develop fast small-scale microbial screens for the biosynthesis of metabolite standards and production of novel drug analogues, whilst also having a role in reducing animal experiments needed to identify animal and human metabolites of fluorinated xenobiotics.