This plot provides a master curve which is seen to provide a consistent measure of the necessary additional scaling. As it can be seen, fMAS2 gives a correction related to the non-trivial shape of the modulation curve when it is not matched by the AW approximation. Indeed the accuracy of the approximation can be quantified by the Reduced χ2χ2 value extracted from the fit. As depicted in Fig. 3, by taking a lower threshold of 0.001 for the Reduced χ2χ2, one can establish that the limit for using the AW approximation in the DIPSHIFT experiments is M2/ωr2<1, which gives a good parameter for deciding the minimum MAS rate that the experiments should be run. Indeed, in

the limit M2/ωr2<1 the fMAS2 vs. Hydroxychloroquine in vitro M2/ωr2 curve is well reproduced by a second order polynomial. For practical use, the figure caption indicates the polynomial used to fit and this predict this universal dependence. Alternatively,

the experimental curve measured at low (rigid limit) and high temperatures (fast limit) can of course be fitted with Eq. (4) in order to directly extract the scaled second moments. As shown in Ref. [27], the AW approximation for evaluating DIPSHIFT NMR signals only holds for evolution periods shorter than the inverse of the dipolar coupling. This is primarily due to the failure of the second-moment approximation for HKI272 the local field at longer evolution periods, where the particularities of the distribution (higher moments) become important. Besides, the typical T2T2 decay in the DIPSHIFT experiment may not be reproduced by an AW-based approximation. In this respect, the tCtC-recDIPSHIFT behaves differently, since the sensitivity to the rate of motion arises only from the apparent averaging effect of the dipolar coupling. For a demonstration, in Fig. 4a–c we

compare 2tr-tC-recDIPSHIFT2tr-tC-recDIPSHIFT curves calculated via full dynamic spin dynamics simulations (symbols) with those obtained using the AW approximation (lines), Eq. (4), considering several motional rates. In the curves calculated using Eq. (4), the scaled second moments s×M2HT and s×M2LT were obtained by fitting the fast-limit and rigid curves, i.e., they are actually the second moment multiplied by fMASHT×fLG2 and fMASLT×fLG2, respectively. Note HA-1077 molecular weight that because of the different dipolar couplings fMASLT≠fMASHT. Fig. 4a shows CHCH spin pair simulations, mimicking a two-site jump with a reorientation angle of 120°120°, as indicated in the inset. Clearly, the AW approximation holds in this case, being valid for the whole evolution time window of the experiment. The possibility of reproducing the complete tCtC-recDIPSHIFT curve with the proposed AW-based fitting function is an intrinsic advantage over the T2T2-dependent DIPSHIFT variants [27] and [33]. However, increasing the number of 1H attached to the carbon, the AW approach fails to describe the 2tr-tC-recDIPSHIFT2tr-tC-recDIPSHIFT data. This is demonstrated in Fig.