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.
Department of Chemistry and Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
Raanan
R
Carmieli
Amanda L
AL
Smeigh
Sarah M
SM
Mickley Conron
Arun K
AK
Thazhathveetil
Masaaki
M
Fuki
Yasuhiro
Y
Kobori
Frederick D
FD
Lewis
Michael R
MR
Wasielewski
D000880:Anthraquinones / Q000737:chemistry*; D020029:Base Pairing; D001483:Base Sequence; D004247:DNA / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D006147:Guanine / Q000737:chemistry; D007477:Ions / Q000737:chemistry; D008958:Models, Molecular; D010777:Photochemistry
D000880:Anthraquinones; D007477:Ions; D006147:Guanine; D004247:DNA