High-Sensitivity Detection of Nanometer H-F Distances for Protein Structure Determination by H-Detected Fast MAS NMR.

Protein structure determination by solid-state NMR requires the measurement of many interatomic distances through dipole-dipole couplings. To obtain multiple long-range distance restraints rapidly and with high sensitivity, here we demonstrate a new H-detected fast magic-angle-spinning NMR technique that yields many long distances in a two-dimensional (2D)-resolved fashion. The distances are measured up to ∼15 Å, with an accuracy of better than 10%, between H and F, two nuclear spins that have the highest gyromagnetic ratios. Exogenous fluorines are sparsely introduced into the aromatic residues of the protein, which is perdeuterated and back-exchanged to give amide protons. This H-F distance experiment, termed 2D heteronuclear single-quantum coherence rotational-echo double-resonance (HSQC-REDOR), is demonstrated on the singly fluorinated model protein, GB1. We extracted 33 distances between 5-F-Trp43 and backbone amide protons, using 2D spectral series that were measured in less than 3 days. Combining these H-F distance restraints with C-F distances and chemical shifts, we calculated a GB1 structure with a backbone root-mean-square deviation of 1.73 Å from the high-resolution structure. This H-detected H-F distance technique promises to provide a highly efficient tool for constraining the three-dimensional structures of proteins and protein-ligand complexes, with not only precise and fast measurements but also access to truly long-range distances.