Fig. 1. Proposed mechanisms of different types of photoaffinity labeling: (A) azide (AZ), (B) benzophenone (BP), (C) diazirine (DZ), (D) 2-aryl-5-carboxytetrazole (ACT), and two “more natural” types, namely, (E) pyrimidone, and (F) pyrone. X = C, N, O, or S.