Site-directed spin labeling and DEER. (A) Model showing the MTSSL attached to an α-helix at a cysteine residue. (B) Models showing spin labels at extracellular sites of two helices of an integral membrane protein in two different conformational states. (C) The expected distance distributions for the OO (red) and OC (black) states. (D) The corresponding intramolecular DEER signals.