The heterobifunctional, photoreactive crossslinker 4-benzoylbenzoic acid, succinimidyl ester enables one to covalently link two target groups. The succinimidyl ester chemically reacts with amines 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.