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

Abstract

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 (10−8–10−12s) 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.