Residue-specific high-resolution 17O solid-state NMR of peptides: multidimensional indirect 1H detection and magic-angle spinning

Oxygen is an integral component of proteins and nucleic acids, but remains sparsely studied in such samples because its only NMR active isotope, 17O, is a low sensitivity and resolution species. These properties are a consequence of its low natural abundance (0.039%) and the fact that 17O is a S = 5/2 nuclide with large quadrupolar couplings (6-11 MHz). In this work, we address these issues with efficient isotopic labeling, high magnetic fields, fast magic-angle spinning and indirect 1H detection. This combination of refinements, in conjunction with multidimensional heteronuclear correlation experiments, improves sensitivity and permits observation of oxygen sites specific to each amino acid residue in a model dipeptide sample in a manner consistent with the goal of high resolution. In particular, double-quantum cross-polarization at high sample spinning frequencies is found to provide efficient polarization transfer between 13C and 17O nuclei. Notably, the use of 17O as the initial source of polarization for experiments, as opposed to 1H, is found to be advantageous in terms of sensitivity per unit time due to the short 17O T1 relaxation. Additionally, the second-order quadrupolar broadening in the 17O dimension is averaged by incorporation of a low-power multiple-quantum sequence to yield sharp isotropic peaks. Comparison of isotropic and anisotropic 17O spectra allows extraction of quadrupolar parameters for each oxygen site. Finally, the high 17O resolution obtained is used in 3D experiments in combination with 13C polarization transfers and subsequent 1H detection to demonstrate the potential to determine sequential assignments and long range distance restraints. Collectively, these results suggest that 17O correlation spectroscopy can become an essential tool in the repertoire of techniques for biomolecular structure determination with the backbone 17O that has not yet been fully utilized.