MTSL is bifunctional, consisting of the nitroxide and the thiosulfonate ester functional groups. The nitroxide label is sterically protected, so it is relatively unreactive.
MTSL is a highly reactive, thiol-specific spin-label. MTSL is a specific conformational probe of thiol site structure by its minimal rotational freedom and distance from the covalent disulfide linkage to the macromolecule under study.