Analysis of the\nMODIST Sequence for Selective Proton–Proton\nRecoupling

Theoretical and simulated analyses of selective homonuclear\ndipolar\nrecoupling sequences serve as primary tools for understanding and\ndetermining the robustness of these sequences under various conditions.\nIn this article, we investigate the recently proposed first-order\ndipolar recoupling sequence known as MODIST (Modest Offset Difference\nInternuclear Selective Transfer). We evaluate the MODIST transfer\nefficiency, assessing its dependence on rf-field strengths and the\nnumber of simulated spins, extending up to 10 spins. This helps to\nidentify conditions that enhance polarization transfer among spins\nthat are nearby in frequency, particularly among aliphatic protons.\nThe exploration uncovers a novel effect for first-order selective\nrecoupling sequences that we term “facilitated dipolar recoupling”.\nThis effect amplifies the recoupled dipolar interaction between distant\nspins due to the presence of additional strongly dipolar-coupled spins.\nUnlike the third spin-assisted recoupling mechanism, facilitated dipolar\nrecoupling only requires a coupling to one of the two distant spins\nof interest. Experimental demonstration of MODIST, including at different\nrf-field strengths, was carried out with the membrane protein influenza\nA M2 in lipid bilayers using 55 kHz magic-angle spinning (MAS). Reducing\nMODIST rf-field strength by a factor of 2 unveils possibilities for\ndetecting Hα–Hα and H<sup>Meth</sup>–H<sup>Meth</sup> correlations with a 3D (H)C(H)(H)CH experiment under fast\nMAS rates, all achievable without specific spin labeling.