Deprotonation of the adenine (A) base in both mononucleotide and oligonucleotide (ODN) was measured by nanosecond pulse radiolysis. The cation radical (A(+*)) of deoxyadenosine (dA), produced by oxidation with SO(4)(-*), rapidly deprotonated to form the neutral A radical (A(- H)(*)) with a rate constant of 2.0 x 10(7) s(-1) and a pK(a) value of 4.2, as determined by transient spectroscopy. A similar process was observed in experiments performed on a variety of double-stranded ODNs containing adenine x thymine (A x T) base pairs. The transient spectrum of A(+)(*) in an ODN composed of alternating A x T pairs was essentially identical to that of free dA and differed from the spectra of ODNs containing AA and AAA. In contrast, the spectra of A(- H)(*) were not affected by the sequence. These results suggest that the positive charge on A(+)(*) in ODNs is delocalized as the dimer is stabilized by pi-orbital stacking between adjacent A's. The rate constants for deprotonation of A(+)(*) in ODNs containing AA and AAA (0.9-1.1 x 10(7) s(-1)) were a factor of 2 smaller than the rate constants for deprotonation of A(+)(*) in ODNs containing alternating A x T and dA (2.0 x 10(7) s(-1)). This suggests that the formation of a charge resonance stabilized dimer AA(+)(*) in DNA produced a significant barrier to deprotonation.