(1-Oxyl-2,2,5,5-tetramethyl-∆3-pyrroline-3-methyl) Methanethiosulfonate-d12 is the labelled analog of a highly reactive thiol-specific spin-label, (1-Oxyl-2,2,5,5-tetramethyl-∆3-pyrroline-3-methyl) Methanethiosulfonate (O87500). A specific conformational probe of thiol site structure by virtue of its minimal rotational freedom and distance from the covalent disulfide linkage to the macromolecule under study.