Broadband heteronuclear solid-state NMR experiments by exponentially modulated dipolar recoupling without decoupling

We present a novel solid-state NMR method for heteronuclear dipolar recoupling without decoupling. The method, which introduces the concept of exponentially modulated rf fields, provides efficient broadband recoupling with large flexibility with respect to hetero- or homonuclear applications, sample spinning frequency, and operation without the need for high-power ¹H decoupling. For previous methods, the latter has been a severe source of sample heating, which may cause detoriation of costly samples. The so-called EXPonentially mOdulated Recoupling experimenT (EXPORT) is described analytically and numerically and demonstrated experimentally by 1D ¹³C spectra and 2D ¹³C-¹⁵N correlation spectra of ¹³C, ¹⁵N-labeled samples of GB1, ubiquitin, and fibrils of the SNNFGAILSS fragment of amylin. Through its flexible operation, robustness, and strong performance, it is anticipated that EXPORT will find immediate application for both hetero- and homonuclear dipolar recoupling in solid-state NMR of ¹³C,¹⁵N-labeled proteins, and compounds of relevance in chemistry.