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

Oxygen is an integral component of proteins but remains sparsely studied because its only NMR active isotope, O, 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 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 ↔ O polarization transfer. The use of O for initial polarization is found to provide better sensitivity per unit time compared to H. Sharp isotropic O 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/C/O experiments, suggesting that such methods can become an essential tool for biomolecular structure determination.