The heterobifunctional, crossslinker succinimidyl 3-(2-pyridyldithio)propionate (SPDP) enables one to covalently link two target groups. The succinimidyl ester chemically reacts with primary amines and after mild reduction of the dithiol, it can react form a disulfide with other thiols. Crosslinking reagents are useful for probing the spatial relationships and interactions within and between biomolecules.