²H-¹³C HETCOR MAS NMR for indirect detection of ²H quadrupole patterns and spin-Lattice relaxation rates

Two-dimensional (2D) cross-polarization magic angle spinning (CP-MAS) ²H-¹³C heteronuclear correlation (HETCOR) experiments were utilized to indirectly detect site-specific deuterium MAS powder patterns. The ²H-¹³C cross-polarization efficiency is orientation-dependent and non-uniform for all crystallites. This leads to difficulty in extracting the correct ²H MAS quadrupole powder patterns. In order to obtain accurate deuterium line shapes, ¹³C spin lock rf field, spin lock rf ramp and CP contact time were carefully calibrated with the assistance of theoretical simulations. The extracted quadrupole patterns for U-[²H/¹³C/¹⁵N]-alanine indicate that the methyl deuterium undergoes classic, three-site jumping in the fast motion regime (10^-8-10^-12 s) and the methine deuterium has a rigid deuterium powder pattern. For U-[²H/¹³C/ ¹⁵N]-phenylalanine, indirectly detected deuterium line shapes illustrate that the aromatic ring undergoes 180° flips in the fast motion regime while ²Hβ and ²Hα are completely rigid. The experimental deuterium line shapes for U-[²H/¹³C/ ¹⁵N]-proline reflect that ²Hβ, ²Hγ and ²Hδ are subjected to fast, two-site reorientations at an angle of (15 ± 5)°, (30 ± 5)° and (25 ± 10)° respectively. In addition, an approach that combines a composite inversion pulse with ²H-¹³C CP-MAS is applied to measure ²H spin-lattice relaxation times in a site-specific, ¹³C-detected fashion.