Effects of L-spin longitudinal quadrupolar relaxation in S{L} heteronuclear recoupling and S-spin magic-angle spinning NMR

摘要

In experiments on S-L heteronuclear spin systems with evolution of the S-spin magnetization under the influence of a quadrupolar nucleus (L-spin), effects of longitudinal quadrupolar (T_1Q) relaxation of the L-spin coherence on the sub-millisecond time scale have been documented and explored, and methods for minimizing their effect have been demonstrated. The longitudinal relaxation results in heteronuclear dephasing even in the reference signal So of S{L} REDOR, REAPDOR, RIDER, or SPIDER experiments, due to T_1Q-relaxation of the transiently generated S_yL_z coherence, reducing or even eliminating the observable dephasing AS. Pulse sequences for measuring an improved reference signal S_00 with minimal heteronu_clear recoupling but the same number of pulses as for S_0 and S have been demonstrated. From the observed intensity ΔS_0 = S_00 - S_0 and the SPIDER signal ΔS/S_0, T_1Q can be estimated. Accelerated decays analogous to the dipolar S_0 curves will occur in T_2 measurements for J-coupled S-L spin pairs. Even in the absence of recoupling pulses, fast T_1Q relaxation of the unobserved nucleus shortens the transverse relaxation time T_2S,MAS of the observed nucleus, in particular at low spinning frequencies, due to unavoid_able heteronuclear dipolar evolution during a rotation period. The observed spinning-frequency depen_dence of T_2S,MAS matches the theoretical prediction and may be used to estimate T_1Q. The effects are demonstrated on several ~13C{~14N} spin systems, including an arginine derivative, the natural N-acetylated polysaccharide chitin, and a model peptide, (POG)_10.