Mitochondria‐Targeting Photocytotoxic Ferrocenyl Conjugates of N ‐Alkylpyridinium Salts

Ferrocenyl (Fc) conjugates ( 1 – 3 ) of alkylpyridinium cations ( E )‐ N ‐alkyl‐4‐[2‐(ferrocenyl)vinyl]pyridinium bromide (alkyl = n ‐butyl in 1 , N , N , N ‐triethylbutan‐1‐aminium bromide in 2 , and n ‐butyltriphenylphosphonium bromide in 3 ) were prepared and characterized, and their photocytotoxicities and cellular uptakes in HeLa cancer and 3T3 normal cells were studied. The species with a 4‐methoxyphenyl moiety ( 4 ) instead of Fc was used as a control. The triphenylphosphonium‐appended 3 was designed for specific delivery into the mitochondria of the cells. Compounds 1 – 3 showed metal‐to‐ligand charge‐transfer bands at λ ≈ 550 nm in phosphate buffered saline (PBS). The Fc + /Fc and pyridinium core redox couples were observed at 0.75 and –1.2 V versus a saturated calomel electrode (SCE) in CH 2 Cl 2 /0.1 M ( n Bu 4 N)ClO 4 . Conjugate 3 showed a significantly higher photocytotoxicity in HeLa cancer cells [IC 50 = (1.3 ± 0.2) μ M ] than in normal 3T3 cells [IC 50 = (27.5 ± 1.5) μ M ] in visible light (400–700 nm). The positive role of the Fc moiety in 3 was evident from the inactive nature of 4 . A JC‐1 dye (5,5,6,6‐tetrachloro‐1,1,3,3‐tetraethylbenzimidazolylcarbocyanine iodide) assay showed that 3 targets the mitochondria and induces apoptosis by the mitochondrial intrinsic pathway caused by reactive oxygen species (ROS). Annexin/propidium iodide studies showed that 3 induces apoptotic cell death in visible light by ROS generation, as evidenced from dichlorofluorescein diacetate assay. Compounds 1 – 3 exhibit DNA photocleavage activity through the formation of hydroxyl radicals.