Optimal control based NCO and NCA experiments for spectral assignment in biological solid-state NMR spectroscopy

We present novel pulse sequences for magic-angle-spinning solid-state NMR structural studies of 13C,15N-isotope labeled proteins. The pulse sequences have been designed numerically using optimal control procedures and demonstrate superior performance relative to previous methods with respect to sensitivity, robustness to instrumental errors, and band-selective excitation profiles for typical biological solid-state NMR applications. Our study addresses specifically 15N to 13C coherence transfers being important elements in spectral assignment protocols for solid-state NMR structural characterization of uniformly 13C,15N-labeled proteins. The pulse sequences are analyzed in detail and their robustness towards spin system and external experimental parameters are illustrated numerically for typical 15N-13C spin systems under high-field solid-state NMR conditions. Experimentally the methods are demonstrated by 1D 15N → 13C coherence transfer experiments, as well as 2D and 3D 15N,13C and 15N,13C,13C chemical shift correlation experiments on uniformly 13C,15N-labeled ubiquitin.