Mao Xi-an, Gmeiner William H
Biochemistry Department, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157-1016, USA.
Biophys Chem. 2005 Feb 1;113(2):155-60. doi: 10.1016/j.bpc.2004.09.003.
Hydrogen exchange rates of the imino protons of the thrombin-binding 15 mer DNA aptamer d(G(1)G(2)T(3)T(4)G(5)G(6)T(7)G(8)T(9)G(10)G(11)T(12)T(13)G(14)G(15)) in the presence of Sr(2+) were measured. In the temperature range 15-35 degrees C, the exchange rates of the eight iminos in the quadruplex core were not uniform, with the G(2), G(11) and G(15) iminos exchanging faster, the G(1), G(5), G(10) and G(14) iminos exchanging slower, and the G(6) imino exchanging at a medium rate. In the quadruplex G(1), G(5), G(10) and G(14) adopted syn glycosidic conformation, while G(2), G(6), G(11) and G(15) adopted anti-conformation. It was found that the four slowly exchanging iminos, which were all the syn-iminos, happened to be located in the TT loops that were not easy to open to the solvent. The anti-iminos exchanged faster, but the G(6) imino exchanged slower than other anti-iminos, because its hydrogen bond with the G(10)O6 was stabilized by the TGT loop. The fact that the G(6) imino exchanged at a faster rate than those syn-iminos in the TT loops suggested that the TGT loop was less stable than the TT loops. Unfolding mechanism for the quadruplex was thus proposed: The quadruplex first uncoupled the three base pairs: G(1)-G(15), G(2)-G(14) and G(5)-G(11), which were not protected by any loops. Then it opened the TGT loop. Finally, it opened the TT loops and the sequence became an unstructured random coil that exchanged with the quadruplex conformation. The conformational exchange between the quadruplex and random coil had been detected.
测定了凝血酶结合15聚体DNA适配体d(G(1)G(2)T(3)T(4)G(5)G(6)T(7)G(8)T(9)G(10)G(11)T(12)T(13)G(14)G(15))在Sr(2+)存在下亚氨基质子的氢交换速率。在15至35摄氏度的温度范围内，四链体核心中八个亚氨基的交换速率并不均匀，其中G(2)、G(11)和G(15)亚氨基交换较快，G(1)、G(5)、G(10)和G(14)亚氨基交换较慢，G(6)亚氨基以中等速率交换。在四链体中，G(1)、G(5)、G(10)和G(14)采用顺式糖苷构象，而G(2)、G(6)、G(11)和G(15)采用反式构象。发现四个交换缓慢的亚氨基，即所有顺式亚氨基，恰好位于不易向溶剂开放的TT环中。反式亚氨基交换较快，但G(6)亚氨基比其他反式亚氨基交换慢，因为其与G(10)O6的氢键被TGT环稳定。G(6)亚氨基比TT环中的顺式亚氨基交换速率快这一事实表明TGT环比TT环稳定性差。因此提出了四链体的解折叠机制：四链体首先解开未被任何环保护的三对碱基：G(1)-G(15)、G(2)-G(14)和G(5)-G(11)。然后打开TGT环。最后，打开TT环，序列变成与四链体构象相互交换的无结构随机卷曲。已经检测到四链体与随机卷曲之间的构象交换。
