Spin label side chain. (a) Cysteine bound spin label MTSSL. The molecular coordinate system (x, y, z) of a nitroxide with x along the N-O bond and z perpendicular to the plane of the nitroxide for z-axis anisotropic motion is depicted. The orientation of the tether with respect to the nitroxide axis system is determined by the angles α, β and γ (α and γ not shown). β is the angular deviation from the mode of the orientational distribution of the z-axis of the nitroxide. The nitrogen p orbital along the z-axis is shown in shaded color. For simplicity, the methyl substituents of the nitroxide ring are not shown. (b) All flexible bonds within the spin label side chain (shown in dotted box). The interaction between the Sγ and Cα proton limits the rotmers Cα-Cß, Cß-Sγ, Sγ-Sδ.