Snoussi Karim, Halle Bertil
Department of Biophysical Chemistry, Center for Molecular Protein Science, Lund University, SE-22100 Lund, Sweden.
Biochemistry. 2008 Nov 18;47(46):12219-29. doi: 10.1021/bi801657s. Epub 2008 Oct 25.
The structural stability of guanine quadruplexes depends critically on an unusual configuration of dehydrated Na (+) or K (+) ions, closely spaced along the central axis of the quadruplex. Crystallography and NMR spectroscopy indicate that these internal ions can be located between the G-quartet planes as well as in the thymine loops, but the precise ion coordination has been firmly established in only a few cases. Here, we examine the bimolecular diagonal-looped foldback quadruplexes [d(G 3T 4G 3)] 2 (Q3) and [d(G 4T 4G 4)] 2 (Q4) by (2)H, (17)O, and (23)Na magnetic relaxation dispersion (MRD). The MRD data indicate that both quadruplexes contain Na (+) ions between the T 4 loops and the terminal G-quartets and that these ions have one water ligand. These ions exchange with external ions on a time scale of 10-60 mus at 27 degrees C, while their highly ordered water ligands have residence times in the range 10 (-8)-10 (-6) s. The MRD data indicate that Q4 contains three Na (+) ions in the stem sites, in agreement with previous solid-state (23)Na NMR findings but contrary to the only crystal structure of this quadruplex. For Q3, the MRD data suggest a less symmetric coordination of the two stem ions. In both quadruplexes, the stem ions have residence times of 0.6-1.0 ms at 27 degrees C. The equilibrium constant for Na (+) --> K (+) exchange is approximately 4 for both loop and stem sites in Q3, in agreement with previous (1)H NMR findings.
鸟嘌呤四链体的结构稳定性关键取决于脱水的Na(+)或K(+)离子的特殊构型，这些离子沿四链体的中心轴紧密排列。晶体学和核磁共振光谱表明，这些内部离子可位于G-四联体平面之间以及胸腺嘧啶环中，但仅在少数情况下精确的离子配位才得以确定。在此，我们通过(2)H、(17)O和(23)Na磁弛豫色散(MRD)研究了双分子对角环回四链体[d(G3T4G3)]2(Q3)和[d(G4T4G4)]2(Q4)。MRD数据表明，两种四链体在T4环和末端G-四联体之间都含有Na(+)离子，且这些离子有一个水配体。在27℃时，这些离子与外部离子在10 - 60微秒的时间尺度上进行交换，而它们高度有序的水配体的停留时间在10(-8)-10(-6)秒范围内。MRD数据表明，Q4在茎位点含有三个Na(+)离子，这与之前的固态(23)Na NMR结果一致，但与该四链体唯一的晶体结构相反。对于Q3，MRD数据表明两个茎离子的配位对称性较低。在两种四链体中，茎离子在27℃时的停留时间为0.6 - 1.0毫秒。Q3中Na(+)→K(+)交换的平衡常数在环位点和茎位点均约为4，这与之前的(1)H NMR结果一致。
