The heterobifunctional, photoreactive crossslinker PEAS (N-((2-pyridyldithio)ethyl)-4-azidosalicylamide) enables one to covalently link two target groups. Following a mild reduction, the dithiol can react form a disulfide with other thiols and upon UV illumination, the photoreactive component reacts with nucleophiles or forms C-H insertion products. Crosslinking reagents are useful for probing the spatial relationships and interactions within and between biomolecules.