F NMR-based solvent accessibility profiling reveals tryptophan ring-flip dynamics in a protein.

Aromatic ring flipping is a key component of protein conformational dynamics, particularly in facilitating breathing motions that involve cooperative rearrangements of hydrophobic cores. While phenylalanine and tyrosine flipping have been extensively studied, tryptophan flipping has often been overlooked due to assumptions about limited mobility arising from its bulkiness and the asymmetric structure of the indole ring. Here, we report a 19F NMR-based approach to probe the dynamic flipping of Trp23 in the DNA-binding domain (DBD) of heat shock factor 1 (Hsf1). We incorporated 5-fluorotryptophan into the Hsf1 DBD and monitored Trp23 flipping in solution using 19F NMR. Temperature-dependent analysis corroborated with relaxation dispersion experiments revealed interconversion between two conformers of Trp23, which correspond to the flip-in and flip-out states observed in crystal structures. Furthermore, D2O isotope shifts and solvent paramagnetic relaxation enhancement experiments revealed population shifts between the solvent-exposed flip-out state and the buried flip-in ground state. These results establish that Trp23 undergoes flipping between buried and solvent-exposed conformations in solution. This study represents the first application of 19F NMR-based solvent accessibility profiling to characterize aromatic ring flipping. The strategy presented here is broadly applicable to studies of aromatic dynamics in diverse protein systems.