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.