An Efficient, Robust New Scheme for Establishing Broadband Homonuclear Correlations in Biomolecular Solid State NMR

An efficient mixing scheme is introduced for establishing two‐dimensional (2D) homonuclear correlations based on dipolar couplings. This mixing scheme achieves broadband dipolar recoupling using remarkably low powers even under ultrafast magic‐angle spinning (MAS) rates. This Adiabatic Linearly FREquency Swept reCOupling (AL FRESCO) method applies a series of weak frequency‐chirped pluses on the 1H channel, for performing efficient 13C−13C magnetization transfers leading to cross peaks between sites separated over small or large chemical shift differences. The mixing scheme is nearly free from dipolar truncation effects, and thanks to the low RF powers it involves it can act over long mixing times (≥1.5 sec). Key considerations required for optimizing this chirped pulse mixing scheme are discussed, and the new kind of correlations that can emerge from this method are demonstrated using uniformly 13C‐labeled Barstar as test protein sample. Your Mixer – AL FRESCO! This study presents a methodology based on frequency‐swept pulses, for imparting 2D homonuclear solid NMR correlations using unusually weak pulses. The potential of the ensuing 2D AL FRESCO experiment is demonstrated with fast 13C MAS NMR results on the protein barstar, established by 1H irradiations lasting over 1 sec mixing times.