Insider report: A fluoroleucine mutant of ubiquitin (cyan) with two strategically placed fluorine reporter groups in its core was synthesised. 1H NMR, crystallography, CD spectroscopy, biological assay and calorimetry indicate minimal structural and functional perturbation compared to the native protein (yellow; see picture). 19F NMR spectra indicate changes within the core during folding and unfolding.

Syntheses of (2S,4S)- and (2S,4R)-5-fluoroleucine, and, and of (2S,4S)-[5,5-(2)H(2)]-5-fluoroleucine, have been completed. The methodology allows these compounds to be prepared in sufficient quantities for incorporation by solid-state protein synthesis into strategic sites in proteins for folding studies. X-ray structures of the epimers and have been obtained and show the presence of conformational isomerism. The torsion angles between the F-C bond and the main chain are compared with values found in a mutant of the protein ubiquitin in which (2S,4S)-5-fluoroleucine replaces leucine residues 50 and 67 in the native protein.

The fluorinated amino acid, (2S,3S)-3'-fluoroisoleucine 3, has been synthesised by a route involving stereoselective cuprate or photochemical addition to the compound 4, derived in turn from (2S)-pyroglutamic acid. This provides a reporter group for the hydrophobic amino acid (2S)-isoleucine for use in protein studies.