Approaches to the assignment of (19)F resonances from 3-fluorophenylalanine labeled calmodulin using solution state NMR.

Traditional single site replacement mutations (in this case, phenylalanine to tyrosine) were compared with methods which exclusively employ (15)N and (19)F-edited two- and three-dimensional NMR experiments for purposes of assigning (19)F NMR resonances from calmodulin (CaM), biosynthetically labeled with 3-fluorophenylalanine (3-FPhe). The global substitution of 3-FPhe for native phenylalanine was tolerated in CaM as evidenced by a comparison of (1)H-(15)N HSQC spectra and calcium binding assays in the presence and absence of 3-FPhe. The (19)F NMR spectrum reveals six resolved resonances, one of which integrates to three 3-FPhe species, making for a total of eight fluorophenylalanines. Single phenylalanine to tyrosine mutants of five phenylalanine positions resulted in (19)F NMR spectra with significant chemical shift perturbations of the remaining resonances, and provided only a single definitive assignment. Although (1)H-(19)F heteronucleclear NOEs proved weak, (19)F-edited (1)H-(1)H NOESY connectivities were relatively easy to establish by making use of the (3)J(FH) coupling between the fluorine nucleus and the adjacent fluorophenylalanine delta proton. (19)F-edited NOESY connectivities between the delta protons and alpha and beta nuclei in addition to (15)N-edited (1)H, (1)H NOESY crosspeaks proved sufficient to assign 4 of 8 (19)F resonances. Controlled cleavage of the protein into two fragments using trypsin, and a repetition of the above 2D and 3D techniques resulted in unambiguous assignments of all 8 (19)F NMR resonances. Our studies suggest that (19)F-edited NOESY NMR spectra are generally adequate for complete assignment without the need to resort to mutational analysis.