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首页> 外文期刊>The journal of physical chemistry, B. Condensed matter, materials, surfaces, interfaces & biophysical >Chromophore/DNA interactions: Femto- to nanosecond spectroscopy, NMR structure, and electron transfer theory
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Chromophore/DNA interactions: Femto- to nanosecond spectroscopy, NMR structure, and electron transfer theory

机译：发色团/ DNA相互作用：飞秒至纳秒级光谱，NMR结构和电子转移理论

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The mechanism of photoinduced hole injection into DNA has been studied using an integrated approach that combines NMR structural analysis, time-resolved spectroscopy, and quantum-chemical calculations. A covalently linked acridinium derivative, the protonated 9-amino-6-chloro-2-methoxyacridine (X+), is replacing a thymine and separated from either guanine (G) or the easier to oxidize 7-deazaguanine (Z) by one adenine center dot thymine (A-T) base pair. The key features of this donor/acceptor system are the following: (i) In more than 95% of the duplexes, X+ is located in a central, coplanar position between the neighboring A-T base pairs with its long axis in parallel showing minimal twist and tilt angles (< 15 degrees). The complementary adenine base is turned out into the extrahelical space. In a minority of less than 5%, X+ is found to be still attached to the duplex. X+ is most probably associated with one of the phosphates, since it is neither intercalated between more remote base pairs nor bound to sugars or grooves. This minority characterized by an excited state lifetime > 10 ns gives rise to a small background signal in time-resolved measurements and contributes predominantly to steady-state fluorescence spectra. (ii) Although the intercalation mode of X+ is well defined, the NMR structure reveals that there are two conformations of X+ with respect to the arrangement of its methoxy substituent. In one conformation, the methoxy group is in the plane of the chromophore, while, in the other extraplanar conformation, the methoxy group forms an angle of 70 degrees with the acridinium ring. The fluorescence decay of 5'-ZAX and 5'-GAX tracts can be fitted to a biexponential function with similar amplitudes, reflecting the oxidation dynamics of G and Z, with the slower rate being determined by larger thermal activation energy. The attribution of biexponential electron transfer (ET) dynamics to the bimodal orientation of the methoxy group at the acridinium is supported by quantum-chemical calculations. These predict a larger free energy change for hole transfer in the nonplanar conformation as compared to the planar one, whereas the difference in the electronic couplings is negligible. (iii) Kinetic studies of the directionality of the (1)(X+)* induced hole injection reveal similarly fast decay components in both directions of the duplex, that is,in 5'-ZAX and 5'-XAZ, with the amplitude of the fast component being significantly reduced in 5'-XAZ. The NMR structure shows that local structural deviations from B-DNA are much more pronounced in the 3'-5' direction than in the 5'-3' direction. According to quantum-chemical calculations, the directionality of charge injection is not a universal feature of the DNA duplex but depends critically on the rotation angle of the aromatic plane of the acridinium within the pi stack. The arrangement of X+ in 5'-ZAX and 5'-XAZ corresponds to a conformation with weak directionality of the electronic couplings. The increased disorder in the 3'-5'direction favors slow hole transfer components at the expense of the fast ones. (iv) A comparison of the hole transfer in 5'-GAX and 5'-ZAG shows that classical Marcus theory can explain the ratio of the charge shift rates of more than 2 orders of magnitude on the basis of a free energy difference between G and Z of 0.3 eV. Both NMR structures and quantum-chemical calculations justify the appreciable neglect of differences of electronic couplings as well as in the reorganzation energy in 5'-GAX and 5'-ZAG. Despite the attractive concept for the behavior of floppy DNA oligonucleotides, in this acridinium/DNA system, there is no evidence for conformational gating, that is, for fluctuations in the electronic couplings that permit the ET to occur.
机译：已使用结合NMR结构分析，时间分辨光谱和量子化学计算的集成方法研究了光诱导空穴注入DNA的机理。共价连接的a啶衍生物质子化的9-氨基-6-氯-2-甲氧基ac啶（X +）正在取代胸腺嘧啶并与鸟嘌呤（G）或更容易被一个腺嘌呤中心氧化7-脱氮鸟嘌呤（Z）分离点胸腺嘧啶（AT）碱基对。该供体/受体系统的主要特征如下：（i）在超过95％的双链体中，X +位于相邻AT碱基对之间的中心共面位置，其长轴平行，显示出最小的扭曲和倾斜角度（<15度）。互补的腺嘌呤基变成了螺旋外空间。在少于5％的少数族裔中，发现X +仍附着在双链体上。 X +最有可能与一种磷酸酯相关，因为它既不插入更远的碱基对之间，也不与糖或凹槽结合。这种以激发态寿命> 10 ns为特征的少数在时间分辨测量中会产生小的背景信号，并且主要对稳态荧光光谱有所贡献。 （ii）尽管X +的插入模式被很好地定义，但NMR结构显示，关于其甲氧基取代基的排列，X +有两个构象。在一个构象中，甲氧基在生色团的平面内，而在另一种平面外构象中，甲氧基与a啶环形成70度角。 5'-ZAX和5'-GAX束的荧光衰减可以拟合为具有相似振幅的双指数函数，反映了G和Z的氧化动力学，而较慢的速率取决于较大的热活化能。量子化学计算支持双指数电子转移（ET）动力学归因于the啶基上甲氧基的双峰取向。与平面的相比，这些预测了在非平面的构象中空穴转移的更大的自由能变化，而电子耦合的差异可忽略不计。 （iii）对（1）（X +）*诱导的空穴注入的方向性的动力学研究表明，在双链体的两个方向上（即5'-ZAX和5'-XAZ中）的振幅都相似地具有快速衰减的成分快速成分在5'-XAZ中显着减少。 NMR结构表明，与B-DNA的局部结构偏差在3'-5'方向上比在5'-3'方向上更为明显。根据量子化学计算，电荷注入的方向性不是DNA双链体的普遍特征，而是主要取决于pi叠层内部the啶的芳族平面的旋转角度。 X'在5'-ZAX和5'-XAZ中的排列对应于电子耦合方向性较弱的构造。 3'-5'方向上增加的无序性有利于缓慢的空穴传输成分，而以快速成分为代价。 （iv）比较5'-GAX和5'-ZAG中的空穴传输表明，经典的马库斯理论可以基于G之间的自由能差来解释大于2个数量级的电荷转移率之比和Z为0.3 eV。 NMR结构和量子化学计算均证明了对5'-GAX和5'-ZAG中电子偶联以及重组能差异的明显忽略。尽管对于松散的DNA寡核苷酸的行为具有有吸引力的概念，但是在这种this啶/ DNA系统中，没有证据表明构象门控，即允许ET发生的电子偶联的波动。

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