Asymmetric doublets in MAS NMR: coherent and incoherent mechanisms

It has been long noted that J-resolved doublets observed in solid-state MAS experiments are asymmetric. The asymmetry has been attributed to a coherent interference effect involving dipolar and CSA interactions. Recently, Bernd Reif and co-workers suggested that under fast MAS conditions the coherent portion of the effect is suppressed and it becomes possible to observe an incoherent mechanism reminiscent of TROSY. The researchers were able to observe the characteristic TROSY-type patterns in (15)N-(1)H(N) spectra of heavily deuterated protein samples (Chevlekov, Diehl, and Reif, previous article in this issue). In the present computer simulation study, we seek to obtain a unified picture of this phenomenon, including both coherent and incoherent aspects. The chosen model focuses on the (15)N-(1)H(N) pair from a polycrystalline sample subject to magic angle spinning. To mimic local dynamics, we assume that the corresponding peptide plane jumps between two orientations. The simulations demonstrate that this simple model reproduces both coherent and incoherent behavior, depending on the MAS speed and the time scale of local dynamics. Furthermore, semianalytical expressions can be derived for both coherent and incoherent (Redfield) limits. Of particular interest is the possibility to use solution-style Redfield results to probe internal protein motions, especially slower motions on the nanosecond time scale. Our simulations show that the differential relaxation measurement permits accurate determination of (15)N dipolar-CSA cross correlations already at moderately high MAS speed (ca 15 kHz).