A Mutagenesis-Free Approach to Assignment of 19F NMR Resonances in Biosynthetically Labeled Proteins

Solution NMR studies of protein structure and dynamics using fluorinated amino acid probes are a valuable addition to the repertoire of existing C-13, N-15, and H-1 experiments. Despite the numerous advantages of the F-19 nucleus in NMR, protein studies are complicated by the dependence of resonance assignments on site-directed mutagenesis methods which are laborious and often problematic. Here we report an NMR-based route to the assignment of fluorine resonances in C-13,N-15-3-fluoro-L-tyrosine tabeled calmodulin. The assignment begins with the correlation of the fluorine nucleus to the delta proton in the novel C-13, N-15-enriched probe which is achieved using a CT-HCCF-COSY experiment. Connection to the backbone is made through two additional solution NMR experiments, namely the (H-B)C-B(C gamma C delta)H-delta and HNCACB. Assignments are completed using either previously published backbone chemical shift data or obtained experimentally provided uniform C-13,N-15 labeling procedures are employed during protein expression. Additional benefits of the C-13,N-15-3-fluoro-L-tyrosine probe include the reduction of spectral overlap through ((CF)-C-13-F-19) CT-HSQCs, as welt as the ability to monitor side chain dynamics using F-19 T-1, T-2, and the C-13-F-19 NOE.