Devarajan S, Shafer R H
Nucleic Acids Res. 1986 Jun 25;14(12):5099-109. doi: 10.1093/nar/14.12.5099.
We have examined the conformational properties of poly(dG-m5dC) under a variety of low salt conditions and sample preparations. Extensive dialysis against 0.5 mM Na-cacodylate resulted in a left-handed polynucleotide conformation as determined by circular dichroism, in agreement with recently reported results. Similarly, extensive dialysis against Tris-EGTA also led to a left-handed conformation. Dilution of these samples led to a transition to the right-handed conformation. More stringent treatments such as dialysis followed by passage over an ion exchange column also resulted in a right-handed conformation. When these various solutions were examined using atomic absorption spectroscopy, significant levels of Mg+2 were observed (greater than or equal to 190 per 1000 nucleotides) in all samples showing a left-handed form, while much lower levels (less than or equal to 45 per 1000 nucleotides) were found in the low salt samples displaying a right-handed conformation. Addition of MgCl2 to samples in which divalent cations had been almost completely removed led to the reformation of the left-handed form. These results indicate that the left-handed form seen under certain low salt conditions is due to the presence of Mg+2 ions that remain bound to the polynucleotide, even in the presence of EDTA.
我们在多种低盐条件和样品制备情况下研究了聚（dG-m5dC）的构象特性。用0.5 mM 二甲胂酸钠进行充分透析后，通过圆二色性测定得到一种左手多核苷酸构象，这与最近报道的结果一致。类似地，用Tris-EGTA进行充分透析也会导致左手构象。这些样品的稀释会导致向右手构象转变。更严格的处理，如透析后再通过离子交换柱，也会产生右手构象。当使用原子吸收光谱法检测这些不同溶液时，在所有呈现左手形式的样品中均观察到显著水平的Mg+2（每1000个核苷酸大于或等于190），而在呈现右手构象的低盐样品中发现的水平则低得多（每1000个核苷酸小于或等于45）。向几乎已完全去除二价阳离子的样品中添加MgCl2会导致左手形式重新形成。这些结果表明，在某些低盐条件下看到的左手形式是由于即使存在EDTA，仍有Mg+2离子与多核苷酸结合。
