13C-detected through-bond correlation experiments for protein resonance assignment by ultra-fast MAS solid-state NMR.

We present two sequences which combine (H-1,N-15) and (N-15,C-13) selective cross-polarization steps with an efficient variant of the J-based homonuclear transfer scheme, in which a spin-state-selective ((SE)-E-3) block is incorporated to improve both resolution and sensitivity in the direct C-13 dimension. We propose these two sequences as a part of a suite of four N-C correlation experiments allowing for the assignment of protein backbone resonances in the solid state. We illustrate these experiments under ultra-fast magic angle spinning conditions on two samples of microcrystalline dimeric human superoxide dismutase (SOD, 153x2 amino acids), in its diamagnetic (empty, Zn-II) and paramagnetic (Cu-II, Zn-II) states.