Highly reactive, thiol-specific spin-label. Specific conformational probe of thiol site structure by its minimal rotational freedom and distance from the covalent disulfide linkage to the macromolecule under study. Used to label cysteine residues in proteins (site-directed labeling, SDS-labeling). Allows protein structure and protein dynamics determination as well as the study of protein-protein and protein-oligonucleotide interactions.