Asynchronising five-fold symmetry sequence for better homonuclear polarisation transfer in magic-angle-spinning solid-state NMR

Recoupling, decoupling, and multidimensional correlation experiments in magic-angle-spinning (MAS) solid-state NMR can be designed by exploiting the symmetry of internal spin interactions. One such scheme, namely, C5(2)(1), and its supercycled version SPC5(2)(1), notated as a five-fold symmetry sequence, is widely used for double-quantum dipole-dipole recoupling. Such schemes are generally rotor synchronised by design. We demonstrate an asynchronous implementation of the SPC5(2)(1) sequence leading to higher double-quantum homonuclear polarisation transfer efficiency compared to the normal synchronous implementation. Rotor-synchronisation is broken in two different ways: lengthening the duration of one of the pulses, denoted as pulse-width variation (PWV), and mismatching the MAS frequency denoted as MAS variation (MASV). The application of this asynchronous sequence is shown on three different samples, namely, U-C-13-alanine and 1,4-C-13-labelled ammonium phthalate which include C-13(alpha)-C-13(beta), C-13(alpha)-C-13(o), and C-13(o)-C-13(o) spin systems, and adenosine 5'- triphosphate disodium salt trihydrate (ATP center dot 3H(2)O). We show that the asynchronous version performs better for spin pairs with small dipole-dipole couplings and large chemical-shift anisotropies, for example, C-13(o)-C-13(o). Simulations and experiments are shown to corroborate the results.