Iron(III) catecholates for cellular imaging and photocytotoxicity in red light

Iron(III) complexes [Fe(L)(L')(NO3)]--in which L is phenyl-N,N-bis[(pyridin-2-yl)methyl]methanamine (1), (anthracen-9-yl)-N,N-bis[(pyridin-2-yl)methyl]methanamine (2), (pyreny-1-yl)-N,N-bis[(pyridin-2-yl)methyl]methanamine (3-5), and L' is catecholate (1-3), 4-tert-butyl catecholate (4), and 4-(2-aminoethyl)-benzene-1,2-diolate (5)--were synthesized and their photocytotoxic properties examined. The five electron-paramagnetic complexes displayed a Fe(III)/Fe(II) redox couple near -0.4 V versus a saturated calomel electrode (SCE) in DMF/0.1 M tetrabutylammonium perchlorate (TBAP). They showed unprecedented photocytotoxicity in red light (600-720 nm) to give IC50≈15 μM in various cell lines by means of apoptosis to generate reactive oxygen species. They were ingested in the nucleus of HeLa and HaCaT cells in 4 h, thereby interacting favorably with calf thymus (ct)-DNA and photocleaving pUC19 DNA in red light of 785 nm to form hydroxyl radicals.