Recent femtosecond transient absorption experiments have shown that UV excitation of stacked nucleobases in aqueous solution induces interbase electron transfer with high efficiency. In single strands, the resulting radicals decay by back electron transfer on a time scale of 10 � 200 ps. In double-stranded DNA, the radicals produced by intrastrand charge separation are hydrogen bonded to bases in the other strand. This raises the intriguing possibility of photoinduced proton-coupled electron transfer in the DNA double helix because barriers to proton transfer are predicted to be lower in oxidized and reduced base pairs than in neutral base pairs. Kinetic isotope effects on excited-state dynamics support this paradigm, but direct evidence is needed. In this work, a femtosecond UV pulse excites a dA18�dT18 homo-duplex or an alternating d(AT)9�d(AT)9 duplex and a mid-IR probe pulse is used to monitor vibrations in the double-bond stretching region. The frequencies of the carbonyl and ring stretching modes monitored in these experiments are sensitive to the location of protons within the base pairs. At 7 �C, hydrogen bonds between adenine and thymine are intact, and a number of positive bands are observed in the transient absorption difference spectra, which decay in 100 � 200 ps. At 77 �C, base pairing is eliminated, but the separated single strands retain a significant degree of base stacking. Long-lived bleach signals are still seen for ground-state fundamentals, but the positive features seen in the intact duplexes vanish completely. Although the homo- and alternating duplexes contain identical AT base pairs, the positive bands differ for the two systems, indicating that distinct transient states are formed, which reflect complex couplings between intra- and interstrand charge transfer in stacked base pairs.

Photoinduced proton-coupled electron transfer in model DNA helices

248th National Meeting (2014)

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