Bauer Benjamin, Sharma Rahul, Chergui Majed, Oppermann Malte
Laboratory of Ultrafast Spectroscopy (LSU), Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne, ISIC-FSB CH-1015 Lausanne Switzerland
Laboratory for Computation and Visualization in Mathematics and Mechanics, École Polytechnique Fédérale de Lausanne, MATH-FSB CH-1015 Lausanne Switzerland.
Chem Sci. 2022 Apr 19;13(18):5230-5242. doi: 10.1039/d1sc06450a. eCollection 2022 May 11.
The photochemistry of DNA systems is characterized by the ultraviolet (UV) absorption of π-stacked nucleobases, resulting in exciton states delocalized over several bases. As their relaxation sensitively depends on local stacking conformations, disentangling the ensuing electronic and structural dynamics has remained an experimental challenge, despite their fundamental role in protecting the genome from potentially harmful UV radiation. Here we use transient absorption and transient absorption anisotropy spectroscopy with broadband femtosecond deep-UV pulses (250-360 nm) to resolve the exciton dynamics of UV-excited adenosine single strands under physiological conditions. Due to the exceptional deep-UV bandwidth and polarization sensitivity of our experimental approach, we simultaneously resolve the population dynamics, charge-transfer (CT) character and conformational changes encoded in the UV transition dipoles of the π-stacked nucleotides. Whilst UV excitation forms fully charge-separated CT excitons in less than 0.3 ps, we find that most decay back to the ground state a back-electron transfer. Based on the anisotropy measurements, we propose that this mechanism is accompanied by a structural relaxation of the photoexcited base-stack, involving an inter-base rotation of the nucleotides. Our results finally complete the exciton relaxation mechanism for adenosine single strands and offer a direct view into the coupling of electronic and structural dynamics in aggregated photochemical systems.
DNA系统的光化学以π堆积核碱基的紫外（UV）吸收为特征，导致激子态在多个碱基上离域。由于它们的弛豫敏感地依赖于局部堆积构象，尽管其在保护基因组免受潜在有害紫外线辐射方面具有重要作用，但解开随之而来的电子和结构动力学问题仍然是一个实验挑战。在这里，我们使用宽带飞秒深紫外脉冲（250 - 360 nm）的瞬态吸收和瞬态吸收各向异性光谱来解析生理条件下紫外激发的腺苷单链的激子动力学。由于我们实验方法具有特殊的深紫外带宽和偏振敏感性，我们同时解析了π堆积核苷酸的紫外跃迁偶极子中编码的布居动力学、电荷转移（CT）特征和构象变化。虽然紫外激发在不到0.3皮秒的时间内形成了完全电荷分离的CT激子，但我们发现大多数激子通过反向电子转移衰变回基态。基于各向异性测量，我们提出这种机制伴随着光激发碱基堆积的结构弛豫，涉及核苷酸的碱基间旋转。我们的结果最终完善了腺苷单链的激子弛豫机制，并提供了对聚集光化学系统中电子和结构动力学耦合关系的直接观察。
