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

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 (T1Q) 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 S0 of S{L} REDOR, REAPDOR, RIDER, or SPIDER experiments, due to T1Q-relaxation of the transiently generated SyLz coherence, reducing or even eliminating the observable dephasing ΔS. Pulse sequences for measuring an improved reference signal S00 with minimal heteronuclear recoupling but the same number of pulses as for S0 and S have been demonstrated. From the observed intensity ΔS0 = S00 − S0 and the SPIDER signal ΔS/S0, T1Q can be estimated. Accelerated decays analogous to the dipolar S0 curves will occur in T2 measurements for J-coupled S-L spin pairs. Even in the absence of recoupling pulses, fast T1Q relaxation of the unobserved nucleus shortens the transverse relaxation time T2S,MAS of the observed nucleus, in particular at low spinning frequencies, due to unavoidable heteronuclear dipolar evolution during a rotation period. The observed spinning-frequency dependence of T2S,MAS matches the theoretical prediction and may be used to estimate T1Q. 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.