Migration and trapping of photoinjected excess electrons in double-stranded B-form DNA but not in single-stranded DNA

Photoexcitation of a pyrene derivative covalently bound to double-stranded calf thymus DNA with 355 nm Nd:YAG laser pulses (fwhm = 24 ps, 50 mJ/cm²/pulse) results in the efficient two-photon-induced ionization of the pyrenyl residues. By use of nanosecond transient absorption techniques, it is shown that the excess electrons injected into the DNA can reduce methylviologen cations (MV²⁺) that are noncovalently bound to the DNA but not MV²⁺ in the outer aqueous solution phase. In double-stranded DNA, this reduction of MV²⁺ to MV^•+ occurs via two kinetic phases, a rapid one that is complete within ≤7 ns and a slower one (200-300 ns) due to the diffusive reduction of MV²⁺ by hydrated electrons. The appearance of the first, rapid reduction phase of MV²⁺ depends on the secondary structure of the DNA, since it is observed only in the double-stranded form but not in the denatured, single-stranded form. This rapid reduction phase is entirely eliminated upon the addition of magnesium ions, which displace the positively charged MV²⁺ cations from the double-stranded DNA molecules. By variation of the concentration of MV²⁺ cations at a constant distribution of covalently bound pyrenyl residues (60 base pairs per pyrenyl residue), a mean distance of migration of excess electrons in double-stranded DNA of ca. 40 Å is estimated.