DNA PHOTOCLEAVAGE BY NOVEL INTERCALATING 6-(2-PYRIDINIUM)PHENANTHRIDINIUM VIOLOGENS

A new type of DNA-intercalating viologen dications, derived from the N ,N'-dialkyl-6-(2-pyridyl)phenanthridine structure (in which dialkyl is -CH2CH2-, -CH2CH2CH2, or (-CH3)(2), abbreviated dq2pyp, dq3pyp, and M e2pyp, respectively), are able to produce frank strand breaks in super coiled plasmid DNA upon irradiation with visible light, The amount of photocleavage is similar for the three drugs. The observed DNA photose nsitization appears to follow a single-strand cleavage model, as shown by a kinetic analysis of the reaction with dq2pyp, The photodynamic a ction of the drugs seems to be initiated by a light-induced electron t ransfer reaction from the nucleobases, given the singlet excited-state redox potentials (ca. + 2.1 V vs, SHE) and the low quantum yields of singlet molecular oxygen production of the drugs (0.1-0.2 in aerated D 2O).