Dipolar filtered 1H-13C heteronuclear correlation spectroscopy for resonance assignment of proteins

Resonance assignment is necessary for the comprehensive structure determination of insoluble proteins by solid-state NMR spectroscopy. While various 2D and 3D correlation techniques involving C-13 and N-15 spins have been developed for this purpose, H-1 chemical shift has not been exploited sufficiently. We demonstrate the combination of the regular H-1-C-13 heteronuclear correlation (HETCOR) experiment and a dipolar filtered HETCOR technique to obtain better resolved H-1 chemical shift spectra. The dipolar filtered experiment, MELODI-HETCOR, simplifies the H-1 spectra by suppressing the directly bonded C-H correlation peaks and retaining only the medium- and long-range cross peaks. We apply this MELODI-HETCOR technique to several amino acids and proteins with various isotopic labeling patterns. The enhanced H-1 chemical shift resolution allows the assignment of overlapping H alpha and H beta resonances in Ser, identifies the H-1 chemical shift differences between neutral and cationic imidazole rings of His, and permits the assignment of residues with side chain nitrogen atoms in ubiquitin. The potential utility of this dipolar filtered HETCOR technique to resonance assignment of extensively labeled proteins is discussed.