Department of Chemistry and Biochemistry, Montana State University, P.O. Box 173400, Bozeman, Montana 59717, United States.
J Am Chem Soc. 2013 Jul 17;135(28):10290-3. doi: 10.1021/ja4049459. Epub 2013 Jul 9.
Double-stranded DNA conjugates with the sequence (dA)10·(dT)10 and hexaethylene glycol linkers at one end (hairpin) or both ends (dumbbell) were studied in buffer solution by deep UV femtosecond transient absorption spectroscopy. These covalently constrained duplexes have greatly enhanced thermal stability compared to A·T duplex oligonucleotides that lack linkers. The conjugates eliminate the slipped-strand and end-frayed structures that form readily in unlinked (dA)n·(dT)n sequences, allowing the excited-state dynamics of stacked A·T base pairs to be observed without interference from structures with stacking or pairing defects. Transient absorption signals show that subpicosecond internal conversion to the electronic ground state takes place in addition to the formation of long-lived excited states having lifetimes of approximately 70 ps. Watson-Crick base-pairing slows the rate of vibrational cooling compared to monomeric bases or single-stranded DNA, possibly by reducing the total number of solute-solvent hydrogen bonds. Long-lived excited states in intact A·T base pairs decay several times more quickly than long-lived excited states observed in single-stranded (dA)n sequences. These results show that base-pairing can measurably affect nonradiative decay pathways in A·T duplexes.
具有序列 (dA)10·(dT)10 和六亚乙基二醇接头的双链 DNA 缀合物在一端（发夹）或两端（哑铃）被研究了在缓冲溶液中通过深紫外飞秒瞬态吸收光谱。与缺乏接头的 A·T 双链寡核苷酸相比，这些共价约束的双链体具有大大增强的热稳定性。这些缀合物消除了在未连接的 (dA)n·(dT)n 序列中容易形成的滑链和末端磨损结构，从而可以观察到堆叠的 A·T 碱基对的激发态动力学，而不受具有堆叠或配对缺陷的结构的干扰。瞬态吸收信号表明，除了形成寿命约为 70 ps 的长寿命激发态之外，还发生了亚皮秒内的电子基态内部转换。与单体碱基或单链 DNA 相比，沃森-克里克碱基配对会降低振动冷却的速度，这可能是通过减少溶质-溶剂氢键的总数来实现的。完整的 A·T 碱基对中的长寿命激发态的衰减速度比在单链 (dA)n 序列中观察到的长寿命激发态快几倍。这些结果表明，碱基配对可以在 A·T 双链体中显著影响非辐射衰减途径。
