Long-range structural restraints in spin-labeled proteins probed by solid-state nuclear magnetic resonance spectroscopy

Magic-angle spinning NMR studies of C-13,N-15-enriched proteins containing a covalently attached nitroxide spin label are presented, with the goal of using paramagnetic relaxation enhancements in the solid state to obtain long-range structural restraints. A 56 aa protein, B1 immunoglobulin-binding domain of protein G (GB1), was used as a model system, with nitroxide spin labels incorporated at residues 28 (alpha-helix) or 53 (beta 4-strand) via site-directed spin labeling. The presence of nitroxide spin labels in the GB1 analogues results in negligible pseudocontact shifts and large enhancement of the transverse relaxation rates for a number of residues, as detected by 2D N-15-C-13(alpha) correlation spectroscopy. The experimentally observed relaxation rate enhancements are found to be highly correlated with the distance of H-1, N-15, and C-13(alpha) nuclei from the electron spin, with significant relaxation effects observed for nuclei up to similar to 20 A away, thus providing valuable information about the protein fold on length scales inaccessible to traditional solid-state NMR techniques.