Residue-Specific High-Resolution O-17 Solid-State Nuclear Magnetic Resonance of Peptides: Multidimensional Indirect H-1 Detection and Magic-Angle Spinning

Oxygen is an integral component of proteins but remains sparsely studied because its only NMR active isotope, O-17, has low sensitivity, low resolution, and large quadrupolar couplings. These issues are addressed here with efficient isotopic labeling, high magnetic fields, fast sample spinning, and H-1 detection in conjunction with multidimensional experiments to observe oxygen sites specific to each amino acid residue. Notably, cross-polarization at high sample spinning frequencies provides efficient C-13 <-> O-17 polarization transfer. The use of O-17 for initial polarization is found to provide better sensitivity per unit time compared to H-1. Sharp isotropic O-17 peaks are obtained by using a low-power multiple-quantum sequence, which in turn allows extraction of quadrupolar parameters for each oxygen site. Finally, the potential to determine sequential assignments and long-range distance restraints is demonstrated by using 3D H-1/C-13/O-17 experiments, suggesting that such methods can become an essential tool for biomolecular structure determination.