Figure 1. Definition of the principal magnetic axes (x_m, y_m, z_m) oriented relative to the nitroxide molecular frame in a TOAC-labeled α-helical peptide.

The y_m axis is perpendicular to the others, forming a right-handed coordinate system. The rotational diffusion axes (x_R, y_R, z_R) are taken to coincide with the magnetic frame (see text). The local director, z_D, is defined as the average orientation of the z_R axis over the course of its motion. In the rotational diffusion frame, where z_R is fixed, z_D traces a trajectory around z_R. The orienting potential is then expressed as a function of the polar angles of z_D in the rotational diffusion frame. The TOAC spin label was inserted at position 13 according to Ghimire and collaborators [32] using MolMol program [33].