Optimal-Control-Based Cβ Chemical Shift Encoding for Efficient Signal Assignment of Solid Proteins

Fast magic-angle spinning (MAS) solid-state NMR spectroscopy is a powerful tool for gaining structural and dynamics in-formation on solid proteins. To access such information site-specifically, the signal assignment process is unavoidable. In the assignment process, Cα and Cβ chemical shifts are of paramount importance in identifying the type of the amino acid residues. Conventionally, however, recording the Cβ chemical shift of solid proteins with relatively short transverse relaxa-tion time is often limited by the long delay required for the magnetization transfer to Cβ spins and its evolution, i.e., by the sensitivity drop. In this paper, we propose a new method that encodes the Cβ chemical shifts into the intensities of the sca-lar-coupled Cα signals, by combining an optimal-control-based spin manipulation pulse and a spin-state filter. This reduced the total required transverse evolution to less than half of that for the previously proposed method, opening up the concept of the Cβ-encoding nearest-neighbor NMR, for the first time, to solid proteins. Also, the total measurement time was shorter than that required for the explicit Cβ shift evolution. We demonstrate the sequential signal assignment for microcrystalline protein GB1, then discuss the prospects for more challenging proteins.