Charge separation and charge delocalization identified in long-living states of photoexcited DNA
Charge separation and charge delocalization identified in long-living states of photoexcited DNA
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DOI：
10.1073/pnas.1323700111
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发表时间：
2014-03-25
影响因子：
11.1
通讯作者：
Zinth, Wolfgang
中科院分区：
文献类型：
--
作者：
Bucher, Dominik B.;Pilles, Bert M.;Zinth, Wolfgang
DNA中的碱基堆积与长寿命激发态有关，其分子性质仍在争论中。为了阐明分子背景，我们研究了具有天然碱基的定义明确的寡核苷酸，其允许选择性UV激发链中的一个单碱基。红外探测在皮秒制度使我们能够剖析不同的单基地的激发态的贡献。所有研究的寡核苷酸显示长寿命状态的100 ps的时间尺度上，这是不可观察到的单碱基的混合物。这些状态的分数与堆叠概率相关，并达到0.4的值。长寿命状态显示特征吸收带，可以分配给电荷转移状态，通过比较它们的标记带的自由基阳离子和阴离子光谱。电荷分离是由所涉及的碱的氧化还原电位指导的，因此由序列控制。在较长的寡核苷酸中研究了这种电荷分离的空间维度，其中桥接序列将激发的碱基与具有特征标记带的传感器碱基分开。激发后，我们观察到所有相关碱基的漂白。即使电桥由多个基座组成，传感器基座的贡献也是可观察的。这一结果可以解释为电荷离域沿着一个良好的堆叠域的链。光吸收后DNA链中带电自由基的存在可能会引起氧化或还原损伤反应，目前在DNA光化学中没有考虑。
Base stacking in DNA is related to long-living excited states whose molecular nature is still under debate. To elucidate the molecular background we study well-defined oligonucleotides with natural bases, which allow selective UV excitation of one single base in the strand. IR probing in the picosecond regime enables us to dissect the contribution of different single bases to the excited state. All investigated oligonucleotides show long-living states on the 100-ps time scale, which are not observable in a mixture of single bases. The fraction of these states is well correlated with the stacking probabilities and reaches values up to 0.4. The long-living states show characteristic absorbance bands that can be assigned to charge-transfer states by comparing them to marker bands of radical cation and anion spectra. The charge separation is directed by the redox potential of the involved bases and thus controlled by the sequence. The spatial dimension of this charge separation was investigated in longer oligonucleotides, where bridging sequences separate the excited base from a sensor base with a characteristic marker band. After excitation we observe a bleach of all involved bases. The contribution of the sensor base is observable even if the bridge is composed of several bases. This result can be explained by a charge delocalization along a well-stacked domain in the strand. The presence of charged radicals in DNA strands after light absorption may cause reactions-oxidative or reductive damage-currently not considered in DNA photochemistry.