Triple oscillating field technique for accurate measurements of couplings in homonuclear spin systems
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We present a new concept for homonuclear dipolar recoupling in magic-angle-spinning (MAS) solid-state NMR experiments which avoids the problem of dipolar truncation. This is accomplished through the introduction of a new NMR pulse sequence design principle: the triple oscillating field technique. We demonstrate this technique as an efficient means to accomplish broadband dipolar recoupling of homonuclear spins, while decoupling heteronuclear dipolar couplings and anisotropic chemicals shifts and retaining influence from isotropic chemical shifts. In this manner, it possible to synthesize Ising interactions in homonuclear spin networks and thereby avoid dipolar truncation - a serious problem essentially all previous homonuclear dipolar recoupling experiments suffer from. Combination of this recoupling concept with rotor assisted dipolar refocusing enables easy readout of internuclear distances through comparison with analytical Fresnel curves. This forms the basis for a new class of solid-state NMR experiments with potential for structure analysis of uniformly carbon labelled proteins through accurate measurement of {13}C-{13}C internuclear distances. The concept is demonstrated experimentally by measurement of C_alpha-C', C_β-C', and C_γ-C' internuclear distances in powder samples of the amino acids \textit{L}-alanine and \textit{L}-threonine.