Figure 1. Spin labels used in the covalent modification of proteins: (a) maleimide; (b) methyl thiosulfonate; (c) ATP spin label.

Figure 2. Energy level diagram of the Zeeman and hyperfine interactions for a nitroxide (S_z = ±½, I_z = 0, ±1). The vertical arrows denote EPR transitions with the resulting first-derivative spectrum below. The g-factor defines the centre of the resonance split by A due to hyperfine interactions with the nucleus.

Figure 3. Orientational sensitivity of EPR spectra. (a) Splitting of the spectrum changes when the z-axis of the nitroxide rotates with respect to the magnetic field. The centre of the spectrum changes when the nitroxide rotates about any axis. (b) Broadening of the spectra with increasing disorder: top, Gaussian disorder of ±1°; centre, ±5°; bottom, isotropic distribution.

Figure 4. Orientation of the catalytic domain of the myosin head in the intermediate stages of the contractile cycle. Left to right: The myosin heads are dynamically disordered when detached from actin; initial binding in the presence of ATP provides little orientational and dynamic restraint; prepower stroke, actomyosin×ADP×P_i are still disordered but less mobile, postpower stroke, heads are rigid and stereospecifically bound to actin.

Figure 5. (a) Sensitivity of the conventional EPR spectra to rotational motion and (b) ST-EPR spectra. Note how the sharp resonances of the motionally averaged spectra progressively broaden, with the effect greatest in the high-field region. In ST-EPR spectra, decreasing motion increases the intensity at L², C¢ and H² positions.

Figure 6. Patterns of solvent accessibility for (a) strands (2-residue periodicity) and (b) an helix (3.6-residue periodicity).