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 (²H $R_{1\rho }$) are important tools in quantitative studies of molecular dynamics. We demonstrate how ²H to ¹³C cross-polarization (CP) approaches under 10–40 kHz magic angle spinning rates can be combined with the ²H $R_{1\rho }$ blocks to allow for extension of deuterium rotating frame relaxation studies to methyl groups in biomolecules. This extension permits detection on the ¹³C 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 −¹³CD₃ 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 ²H and ¹³C channels can be combined with the ²H $R_{1\rho }$ relaxation rates measurement. Off-resonance ²H $R_{1\rho }$ measurements are investigated in addition to the on-resonance condition, as they extent the range of effective spin-locking field.