Understanding the combined effect of Magic Angle Spinning (MAS) and rf irradiation on quadrupolar nuclei is not only important in the context of high resolution MQMAS methods but also for the design of homo - and heteronuclear correlation experiments. Biophysical systems in which th e latter aspect is of particular interest include nucleic acids and membrane proteins where cations can be found in close spatial proximity and/or in chemical exchange. In this work, we have shown that it is possible to establish homonuclear through-space correlations among quadrupolar nuclei (spin I = 3/2). We compare our experimental findings with theoretical predictions using an effective Hamiltonian under MAS and rf irradiation. In accordance with this description, we demonstrate that double-quantum coherence can be created between dipolar coupled I = 3/2 spins. Applied in a two-dimensional correlation experiment, this technique can be used to discriminate between through-space couplings and chemical exchange in immobilized systems. We anticipate that this new methodology will be useful for structural and dynamical studies in membrane proteins such as ion channels and DNA complexes such as G-rich oligonucleotides, and in general to any system containing ions in spatial proximity.
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