Structure and dynamics of an imidazoline nitroxide side chain with strongly hindered internal motion in proteins

A disulfide-linked imidazoline nitroxide side chain (V1) has a similar and highly constrained internal motion at diverse topological sites in a protein, unlike that for the disulfide-linked pyrroline nitroxide side chain (R1) widely used in site directed spin labeling EPR. Crystal structures of V1 at two positions in a helix of T4 Lysozyme and quantum mechanical calculations suggest the source of the constraints as intra-side chain interactions of the disulfide sulfur atoms with both the protein backbone and the 3-nitrogen in the imidazoline ring. These interactions apparently limit the conformation of the side chain to one of only three possible rotamers, two of which are observed in the crystal structure. An inter-spin distance measurement in frozen solution using double electron-electron resonance (DEER) gives a value essentially identical to that determined from the crystal structure of the protein containing two copies of V1, indicating that lattice forces do not dictate the rotamers observed. Collectively, the results suggest the possibility of predetermining a unique rotamer of V1 in helical structures. In general, the reduced rotameric space of V1 compared to R1 should simplify interpretation of inter-spin distance information in terms of protein structure, while the highly constrained internal motion is expected to extend the dynamic range for characterizing large amplitude nanosecond backbone fluctuations.