Structural determinants of nitroxide motion in spin-labeled proteins: solvent-exposed sites in helix B of T4 lysozyme.

Site-directed spin labeling provides a means for exploring structure and dynamics in proteins. To interpret the complex EPR spectra that often arise, it is necessary to characterize the rotamers of the spin-labeled side chain and the interactions they make with the local environment in proteins of known structure. For this purpose, crystal structures have been determined for T4 lysozyme bearing a nitroxide side chain (R1) at the solvent-exposed helical sites 41 and 44 in the B helix. These sites are of particular interest in that the corresponding EPR spectra reveal two dynamic states of R1, one of which is relatively immobilized suggesting interactions of the nitroxide with the environment. The crystal structures together with the effect of mutagenesis of nearest neighbors on the motion of R1 suggest intrahelical interactions of 41R1 with the i + 4 residue and of 44R1 with the i + 1 residue. Such interactions appear to be specific to particular rotamers of the R1 side chain.