Structure and Dynamics of Photogenerated Triplet Radical Ion Pairs in DNA Hairpin Conjugates with Anthraquinone End Caps
Structure and Dynamics of Photogenerated Triplet Radical Ion Pairs in DNA Hairpin Conjugates with Anthraquinone End Caps
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DOI：
10.1021/ja303721j
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发表时间：
2012-07-11
影响因子：
15
通讯作者：
Wasielewski, Michael R.
中科院分区：
文献类型：
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作者：
Carmieli, Raanan;Smeigh, Amanda L.;Wasielewski, Michael R.
合成了一系列具有蒽醌（Aq）末端的DNA发夹（AqGn），通过改变Aq与鸟嘌呤-胞嘧啶（G-C）碱基对之间的腺嘌呤-胸腺嘧啶（A-T）碱基对数目（n - 1），系统地改变Aq与G-C碱基对之间的距离。利用纳秒瞬态吸收和时间分辨电子顺磁共振（TREPR）光谱研究了这些发夹的光物理和光化学。在光激发时，（1）*Aq经历快速的系统间交叉以产生（3）*Aq，其能够氧化嘌呤核碱基，导致形成（3）（Aq（-中心点）Gn（+中心点））。所有（3）（Aq（-中心点）Gn（+中心点））自由基离子对由于其不同的三重态自旋子能级布居而表现出具有吸收和增强发射（A/E）的电子自旋极化相位图案的不对称TREPR光谱，所述三重态自旋子能级布居源自（3）*Aq前体的相应非玻尔兹曼自旋子能级布居。（3）（Aq（-center dot）Gn（+center dot））自由基离子对的TREPR谱强依赖于它们的自旋-自旋偶极相互作用，弱依赖于它们的自旋-自旋交换耦合。（3）（Aq（-中心点）Gri（+中心点））的各向异性使得可以确定自由基离子对内的Aq（-中心点）和G（+中心点）的π系统彼此平行。长寿命（3）（Aq（-中心点）Gn（+中心点））自由基离子对的电荷重组显示出不寻常的双峰距离依赖性，这是由于自由基对系间交叉的电荷重组速率决定步骤的变化造成的n < 4 to coherent superexchange for n >4。
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, (l)*Aq undergoes rapid intersystem crossing to yield (3)*Aq, which is capable of oxidizing purine nucleobases resulting in the formation of (3)(Aq(-center dot)Gn(+center dot)). All (3)(Aq(-center dot)Gn(+center dot)) 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(-center dot)Gn(+center dot)) radical ion pairs depend strongly on their spin spin dipolar interaction and weakly on their spin spin exchange coupling. The anisotropy of (3)(Aq(-center dot)Gri(+center dot)) makes it possible to determine that the pi systems of Aq(-center dot) and G(+center dot) within the radical ion pair are parallel to one another. Charge recombination of the long-lived (3)(Aq(-center dot)Gn(+center dot)) 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.