Long-Range Structural Restraints in Spin-Labeled Proteins Probed by Solid-State Nuclear Magnetic Resonance Spectroscopy

Magic-angle spinning NMR studies of 13 C, 15 N-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 (α-helix) or 53 (β4-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 15 N- 13 C α correlation spectroscopy. The experimentally observed relaxation rate enhancements are found to be highly correlated with the distance of 1 H, 15 N, and 13 C α nuclei from the electron spin, with significant relaxation effects observed for nuclei up to ∼20 Å away, thus providing valuable information about the protein fold on length scales inaccessible to traditional solid-state NMR techniques.