Carbon-detected deuterium solid-state NMR rotating frame relaxation measurements for protein methyl groups under magic angle spinning.

Deuterium rotating frame solid-state NMR relaxation measurements (H R) are important tools in quantitative studies of molecular dynamics. We demonstrate how H to C cross-polarization (CP) approaches under 10-40 kHz magic angle spinning rates can be combined with the H R blocks to allow for extension of deuterium rotating frame relaxation studies to methyl groups in biomolecules. This extension permits detection on the C nuclei and, hence, for the achievement of site-specific resolution. The measurements are demonstrated using a nine-residue low complexity peptide with the sequence GGKGMGFGL, in which a single selective -CD label is placed at the methionine residue. Carbon-detected measurements are compared with the deuterium direct-detection results, which allows for fine-tuning of experimental approaches. In particular, we show how the adiabatic respiration CP scheme and the double adiabatic sweep on the H and C channels can be combined with the H R relaxation rates measurement. Off-resonance H R measurements are investigated in addition to the on-resonance condition, as they extent the range of effective spin-locking field.