Determination of Long-Range Distances by Fast Magic-Angle-Spinning Radiofrequency-Driven F-F Dipolar Recoupling NMR.

Nanometer-range distances are important for restraining the three-dimensional structure and oligomeric assembly of proteins and other biological molecules. Solid-state NMR determination of protein structures typically utilizes C-C and C-N distance restraints, which can only be measured up to ∼7 Å because of the low gyromagnetic ratios of these nuclear spins. To extend the distance reach of NMR, one can harvest the power of F, whose large gyromagnetic ratio in principle allows distances up to 2 nm to be measured. However, F possesses large chemical shift anisotropies (CSAs) as well as large isotropic chemical shift dispersions, which pose challenges to dipolar coupling measurements. Here, we demonstrate F-F distance measurements at high magnetic fields under fast magic-angle spinning (MAS) using radiofrequency-driven dipolar recoupling (RFDR). We show that F-F cross-peaks for distances up to 1 nm can be readily observed in two-dimensional F-F correlation spectra using less than 5 ms of RFDR mixing. This efficient F-F dipolar recoupling is achieved using practically accessible MAS frequencies of 15-55 kHz, moderate F radio frequency field strengths, and no H decoupling. Experiments and simulations show that the fastest polarization transfer for aromatic fluorines with the highest distance accuracy is achieved using either fast MAS (e.g., 60 kHz) with large pulse duty cycles (>50%) or slow MAS with strong F pulses. Fast MAS considerably reduces relaxation losses during the RFDR π-pulse train, making finite-pulse RFDR under fast-MAS the method of choice. Under intermediate MAS frequencies (25-40 kHz) and intermediate pulse duty cycles (15-30%), the F CSA tensor orientation has a quantifiable effect on the polarization transfer rate; thus, the RFDR buildup curves encode both distance and orientation information. At fast MAS, the impact of CSA orientation is minimized, allowing pure distance restraints to be extracted. We further investigate how relayed transfer and dipolar truncation in multifluorine environments affect polarization transfer. This fast-MAS F RFDR approach is complementary to F spin diffusion for distance measurements and will be the method of choice under high-field fast-MAS conditions that are increasingly important for protein structure determination by solid-state NMR.