April 2, 2012
Why (Not) Do Fluorine NMR?
Lipitor, Prevacid, and an estimated 20% of all drugs on the market contain one or more fluorine atoms. The relative lack of fluorine in the biological environment allows NMR spectroscopists to easily detect fluorine atoms on drugs, lead candidates and other small molecules when mixed with proteins. But does anybody actually use 19F- NMR?
Advantages:
- Strong signal (high gyromagnetic ratio);
- No isotopic enrichment (normal fluorine is NMR-active);
- Wide chemical shift range (200 ppm versus 10 ppm for proton);
- Zero background (proteins, DNA, RNA, etc. don't have fluorine).
The first use of 19F-NMR for biophysics dates back to the 1970s, using a Varian XL-100 (that's 100 MHz!) spectrometer. Bruce Dunlap and co-workers detected covalent AND non-covalent binding between the age-old cancer chemotherapeutic 5-fluoro uracil (5FU) and its enzymatic target, thymidylate synthase[1, 2]. 19F-NMR has also been successfully used as a protease inhibitor assay; tagging a target peptide with a single flurorine atom allows easy detection of cleaved peptide - and uncleaved peptide in the presence of a caspase inhibitor (see image)[3]. Now, fluorine NMR is being revisited for the purposes of fragment screening. Taking a lead from Claudio Dalvit[4], researchers at Amgen have retooled their systems and sorted their screening collection to conduct fluorine-based NMR fragment screening[5].
With such a range of applications, why is 19F not more widely used? Is it just instrumentation? Most X-tuneable probes "don't go to fluorine", requiring purchase of a dedicated 19F, 1H-decoupling probe and the amplifiers to power them. But are such capital expenditures worth the investment in discovery science?
Do you use 19F-NMR? If so, how often? And to what purpose?
References 
- Byrd RA, Dawson WH, Ellis PD, & Dunlap RB (1977). 19F nuclear magnetic resonance investigation of the ternary complex formed between native thymidylate synthetase, 5-fluoro-2'-deoxyuridylate, and 5,10-methylenetetrahydrofolate. Journal of the American Chemical Society, 99 (18), 6139-41 PMID: 893883
- Byrd, R., Dawson, W., Ellis, P., & Dunlap, R. (1978). Elucidation of the detailed structures of the native and denatured ternary complexes of thymidylate synthetase via fluorine-19 NMR Journal of the American Chemical Society, 100 (24), 7478-7486 DOI: 10.1021/ja00492a007 Fattorusso, R., Jung, D., Crowell, K., Forino, M., & Pellecchia, M. (2005). Discovery of a Novel Class of Reversible Non-Peptide Caspase Inhibitors via a Structure-Based Approach Journal of Medicinal Chemistry, 48 (5), 1649-1656 DOI: 10.1021/jm0493212
- Vulpetti A, Hommel U, Landrum G, Lewis R, & Dalvit C (2009). Design and NMR-based screening of LEF, a library of chemical fragments with different local environment of fluorine. Journal of the American Chemical Society, 131 (36), 12949-59 PMID: 19702332
- Jordan JB, Poppe L, Xia X, Cheng AC, Sun Y, Michelsen K, Eastwood H, Schnier PD, Nixey T, & Zhong W (2012). Fragment based drug discovery: practical implementation based on ¹⁹F NMR spectroscopy. Journal of medicinal chemistry, 55 (2), 678-87 PMID: 22165820