Oxidatively generated damage to DNA induced by a pyrenyl photosensitizer residue (Py) covalently attached to a guanine base in the DNA sequence context 5'-d(CAT[G(1)(Py)]CG(2)TCCTAC) in aerated solutions was monitored from the initial one-electron transfer, or hole injection step, to the formation of chemical end-products monitored by HPLC, mass spectrometry, and high-resolution gel electrophoresis. Hole injection into the DNA was initiated by two-photon excitation of the Py residue with 355 nm laser pulses, thus producing the radical cation Py center dot+ and hydrated electrons; the latter are trapped by O-2, thus forming the superoxide anion O-2(center dot-). The decay of the Py center dot+ radical is correlated with the appearance of the G(center dot+)/G(-H)(center dot) radical on microsecond time scales, and O-2(center dot-) combines with guanine radicals at G(1) to form alkali-labile 2,5-diamino-4H-imidazolone lesions (Iz(1)(Py)). Product formation in the modified strand is smaller by a factor of 2.4 in double-stranded than in single-stranded DNA. In double-stranded DNA, hot piperidine-mediated cleavage at G(2) occurs only after G(1)(Py), an efficient hole trap, is oxidized thus generating tandem lesions. An upper limit of hole hopping rates, k(hh) < 5 x 10(3) s(-1) from G(1)(center dot+)-Py to G(2) can be estimated from the known rates of the combination reaction of the G(-H)(center dot) and O-2(center dot-) radicals. The formation of Iz products in the unmodified complementary strand compared to the modified strand in the duplex is similar to 10 times smaller. The formation of tandem lesions is observed even at low levels of irradiation corresponding to "single-hit" conditions when less than similar to 10% of the oligonucleotide strands are damaged. A plausible mechanism for this observation is discussed.