Fluorine NMR study of proline-rich sequences using fluoroprolines.

Proline homopolymer motifs are found in many proteins; their peculiar conformational and dynamic properties are often directly involved in those proteins' functions. However, the dynamics of proline homopolymers is hard to study by NMR due to a lack of amide protons and small chemical shift dispersion. Exploiting the spectroscopic properties of fluorinated prolines opens interesting perspectives to address these issues. Fluorinated prolines are already widely used in protein structure engineering - they introduce conformational and dynamical biases - but their use as ${}^{19}$F NMR reporters of proline conformation has not yet been explored. In this work, we look at model peptides where C$\gamma$-fluorinated prolines with opposite configurations of the chiral C$\gamma$ centre have been introduced at two positions in distinct polyproline segments. By looking at the effects of swapping these (4$R$)-fluoroproline and (4$S$)-fluoroproline within the polyproline segments, we were able to separate the intrinsic conformational properties of the polyproline sequence from the conformational alterations instilled by fluorination. We assess the fluoroproline ${}^{19}$F relaxation properties, and we exploit the latter in elucidating binding kinetics to the SH3 (Src homology 3) domain.