Assignment of the nonexchanging protons of the α-spectrin SH3 domain by two- and three-dimensional 1H-13C solid-state magic-angle spinning NMR and comparison of solution and solid-state proton chemical shifts

The assignment of nonexchanging protons of a small microcrystalline protein, the a-spectrin SH3 domain (7.2 kDa, 62 residues), was achieved by means of three-dimensional (3D) heteronuclear (H-1-C-13-C-13) magic-angle spinning (MAS) NMR dipolar correlation spectroscopy. With the favorable combination of a high B-0-field, a moderately high spinning frequency, and frequency-switched Lee-Goldburg irradiation applied during H-1 evolution, a proton linewidth less than or equal to 0.5 ppm at 17.6 Tesla was achieved for the particular protein preparation used. A comparison of the solid-state H-1 chemical shifts with the shifts found in solution shows a remarkable similarity, which reflects the identical protein structures in solution and in the solid. Significant differences between the MAS solid- and liquid-state H-1 chemical shifts are only observed for residues that ore located at the surface of the protein and that exhibit contacts between different SH3 molecules. In two cases, aromatic residues of neighboring SH3 molecules induce pronounced upfield ring-current shifts for protons in the contact area.