Mechanistic investigations of the photoinduced DNA damage by acridizinium salts (4a-azonia-anthracene derivatives) are presented. Irradiation of 9-bromoacridizinium in the presence of defined double-and single-stranded DNA oligomers under aerobic conditions leads to both frank strand breaks and alkalilabile sites as determined by polyacrylamide gel electrophoresis (PAGE). The extent of the DNA damage increases significantly in D2O and occurs selectively at guanosine residues. These observations reveal the formation of singlet oxygen (O-1(2)) as reactive species, which oxidizes the DNA bases, above all the guanine bases. Further evidence for O-1(2) formation was obtained from laser-flash spectroscopic investigations, which show intersystem crossing (S-1 to T-1) of the excited states of the parent acridizinium and of the 9-bromo- and 9-amino-substituted derivatives. The resulting triplet state is efficiently quenched by oxygen (k(q) > 10(9) s(-1)M(-1)) to yield O-1(2)- Under anaerobic conditions, no significant alkali-labile lesions are observed, but frank strand breaks are induced; however, to lesser extent than under aerobic conditions. The DNA damage is suppressed in the presence of a radical scavenger, namely t-BuOH, and hydroxyl radicals are shown to be the reactive intermediates by trapping experiments with terephthalic acid. Moreover, the intercalated acriclizinium molecules are not involved in the DNA damage reactions. The intercalated acriclizinium salt leads to a primary PET reaction with the DNA bases; however, a fast BET transfer is proposed that regains the dye and the DNA, so that the excited intercalated dye does not contribute significantly to the overall DNA damage.