Department of Chemistry and Biochemistry, 100 W. 18th Ave., Columbus, Ohio 43210, USA.
Departamento de Química, Facultad de Ciencias and Institute for Advanced Research in Chemistry (IADCHEM), Universidad Autónoma de Madrid, Campus de Excelencia UAM-CSIC, Cantoblanco, 28049 Madrid, Spain.
J Chem Phys. 2020 Sep 14;153(10):105104. doi: 10.1063/5.0020463.
To better understand the nexus between structure and photophysics in metallo-DNA assemblies, the parallel-stranded duplex formed by the all-cytosine oligonucleotide, dC, and silver nitrate was studied by circular dichroism (CD), femtosecond transient absorption spectroscopy, and time-dependent-density functional theory calculations. Silver(I) ions mediate Cytosine-Cytosine (CC) base pairs by coordinating to the N3 atoms of two cytosines. Although these silver(I) mediated CC base pairs resemble the proton-mediated CC base pairs found in i-motif DNA at first glance, a comparison of experimental and calculated CD spectra reveals that silver ion-mediated i-motif structures do not form. Instead, the parallel-stranded duplex formed between dC and silver ions is proposed to contain consecutive silver-mediated base pairs with high propeller twist-like ones seen in a recent crystal structure of an emissive, DNA-templated silver cluster. Femtosecond transient absorption measurements with broadband probing from the near UV to the near IR reveal an unusually long-lived (>10 ns) excited state in the dC silver ion complex that is not seen in dC in single-stranded or i-motif forms. This state is also absent in a concentrated solution of cytosine-silver ion complexes that are thought to assemble into planar ribbons or sheets that lack stacked silver(I) mediated CC base pairs. The large propeller twist angle present in metal-mediated base pairs may promote the formation of long-lived charged separated or triplet states in this metallo-DNA.
为了更好地理解金属-DNA 组装体中结构与光物理之间的关系，通过圆二色性（CD）、飞秒瞬态吸收光谱和含时密度泛函理论计算研究了由全胞嘧啶寡核苷酸 dC 和硝酸银形成的平行双链体。银（I）离子通过与两个胞嘧啶的 N3 原子配位来介导胞嘧啶-胞嘧啶（CC）碱基对。尽管这些银（I）介导的 CC 碱基对乍一看类似于 i-motif DNA 中发现的质子介导的 CC 碱基对，但实验和计算 CD 光谱的比较表明，银离子介导的 i-motif 结构不会形成。相反，提出在 dC 和银离子之间形成的平行双链体包含连续的银介导碱基对，其螺旋扭曲程度类似于最近发现的具有发射性质的 DNA 模板银团簇的晶体结构。宽带探测从近紫外到近红外的飞秒瞬态吸收测量揭示了在 dC 银离子配合物中存在异常长寿命（>10 ns）的激发态，而在单链或 i-motif 形式的 dC 中则没有观察到这种激发态。在认为组装成缺乏堆叠银（I）介导的 CC 碱基对的平面带或薄片的胞嘧啶-银离子配合物的浓溶液中，也不存在这种状态。金属介导碱基对中存在的大螺旋扭曲角可能促进这种金属-DNA 中长寿命的电荷分离或三重态的形成。
