A Modification of γ-encoded RN Symmetry Pulses for Increasing the Scaling Factor and More Accurate Measurements of the Strong Heteronuclear Dipolar Couplings

Abstract Symmetry based γ-encoded RNnν elements are broadly used in magic-angle spinning solid-state NMR experiments to achieve selective recoupling of the heteronuclear dipolar interactions. The recoupled dipolar couplings in such experiments are scaled by a factor, Ksc, which theoretically depends on the chosen symmetry numbers N, n, and ν. However, the maximum theoretical value of Ksc for γ-encoded RNnν pulses is limited with value of ∼0.25, resulting in long RNnν experiment times. Also, the dependence of Ksc on the experimental parameters can result in systematic errors in the experimental determination of the dipolar couplings, especially at low and moderate MAS rates. In this Communication, we investigate the use of MODifiEd RNnν symmetry (MODERNnν(ϕM)) pulses that increase the dipolar scaling factor by at least 1.45 fold compared to γ-encoded RNnν. The second advantage of MODERNnν(ϕM) pulses with respect to traditional RNnν pulses is the reduced influence of experimental parameters on Ksc, which allows for more accurate measurement of short-range distances. The robustness of MODERNnν(ϕM) is compared with γ-encoded R1423 symmetry pulses. The enhanced performance is demonstrated on two uniformly-13C-enriched samples, N-acetyl valine and the microcrystalline protein GB1, at a 31.111 kHz MAS rate.