Structure and dynamics of photogenerated triplet radical ion pairs in DNA hairpin conjugates with anthraquinone end caps

A series of DNA hairpins (AqGn) possessing a tethered anthraquinone (Aq) end-capping group were synthesized in which the distance between the Aq and a guanine-cytosine (G-C) base pair was systematically varied by changing the number (n - 1) of adenine-thymine (A-T) base pairs between them. The photophysics and photochemistry of these hairpins were investigated using nanosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Upon photoexcitation, ^1*Aq undergoes rapid intersystem crossing to yield ^3*Aq, which is capable of oxidizing purine nucleobases resulting in the formation of ³(Aq ^-•Gn ^+•). All ³(Aq ^-•Gn ^+•) radical ion pairs exhibit asymmetric TREPR spectra with an electron spin polarization phase pattern of absorption and enhanced emission (A/E) due to their different triplet spin sublevel populations, which are derived from the corresponding non-Boltzmann spin sublevel populations of the ^3*Aq precursor. The TREPR spectra of the ³(Aq ^-•Gn ^+•) radical ion pairs depend strongly on their spin-spin dipolar interaction and weakly on their spin-spin exchange coupling. The anisotropy of ³(Aq ^-•Gn ^+•) makes it possible to determine that the π systems of Aq ^-• and G ^+• within the radical ion pair are parallel to one another. Charge recombination of the long-lived ³(Aq ^-•Gn ^+•) radical ion pair displays an unusual bimodal distance dependence that results from a change in the rate-determining step for charge recombination from radical pair intersystem crossing for n < 4 to coherent superexchange for n > 4.