Padmanabhan S, Richey B, Anderson C F, Record M T
Department of Chemistry, University of Wisconsin, Madison 53706.
Biochemistry. 1988 Jun 14;27(12):4367-76. doi: 10.1021/bi00412a025.
The interactions of the divalent hexamethonium (Hex2+) cation with double-helical calf thymus DNA are investigated by means of 14N NMR and, indirectly, by means of 23Na NMR. During a titration of NaDNA with HexBr2, the displacement of Na+ from DNA by Hex2+ is monitored by concurrent measurements of the Lorentzian 14N signals and the bi-Lorentzian 23Na signals. The variations in the quadrupolar relaxation rates of 14N and 23Na are analyzed according to a simple two-state model for the competition between Hex2+ and Na+ associated with DNA. From this analysis parameters characterizing the exchange process are evaluated, and the following conclusions are drawn: (1) The association of one Hex2+ displaces 1.7-2.0 sodium ions from the vicinity of the DNA. (2) Cation accumulation near DNA neutralizes approximately half of the phosphate charge at all points in the titration. (3) The exchange coefficient characterizing the displacement of Na+ by Hex2+ is of the same order of magnitude as the exchange coefficients determined by NMR for other divalent cations such as Mg2+ and putrescine. These findings imply that the interaction of Hex2+ with DNA is primarily electrostatic in character. The transverse and longitudinal relaxation rates observed for 14N are analyzed under the assumption that the quadrupolar relaxation processes of 14N in Hex2+ associated with DNA can be characterized by a single-exponential correlation function with correlation time tau NB. The resulting value of tau NB, 7.8 +/- 0.8 ns, is 3 orders of magnitude greater than that estimated for Hex2+ in the absence of DNA and is only 3-4 times greater than correlation times reported for 23Na and other quadrupolar cations near DNA. These comparisons indicate that the observed enhancements in the relaxation rates of 14N are due mainly to slowing of the motions that modulate its quadrupolar interactions in Hex2+ near DNA. The magnitudes of tau NB and of the quadrupolar coupling constant of Hex2+ associated with DNA are consistent with the conclusion that this association is primarily electrostatic.
通过14N核磁共振以及间接通过23Na核磁共振研究了二价六甲铵（Hex2+）阳离子与双螺旋小牛胸腺DNA的相互作用。在用HexBr2滴定NaDNA的过程中，通过同时测量洛伦兹14N信号和双洛伦兹23Na信号来监测Hex2+将Na+从DNA中置换出来的情况。根据Hex2+与与DNA相关的Na+之间竞争的简单双态模型，分析了14N和23Na的四极弛豫率的变化。通过该分析评估了表征交换过程的参数，并得出以下结论：（1）一个Hex2+的结合会从DNA附近置换出1.7 - 2.0个钠离子。（2）在滴定的所有点，DNA附近的阳离子积累使大约一半的磷酸电荷中和。（3）表征Hex2+置换Na+的交换系数与通过核磁共振测定的其他二价阳离子（如Mg2+和腐胺）的交换系数处于同一数量级。这些发现表明Hex2+与DNA的相互作用主要是静电性质的。在假设与DNA相关的Hex2+中14N的四极弛豫过程可以用具有相关时间tau NB的单指数相关函数来表征的前提下，分析了观察到的14N的横向和纵向弛豫率。得到的tau NB值为7.8±0.8纳秒，比在没有DNA的情况下对Hex2+估计的值大3个数量级，并且仅比报道的与DNA附近的23Na和其他四极阳离子相关的相关时间大3 - 4倍。这些比较表明，观察到的14N弛豫率的增强主要是由于在DNA附近的Hex2+中调节其四极相互作用的运动减慢所致。与DNA相关的Hex2+的tau NB值和四极耦合常数的大小与该结合主要是静电性质的结论一致。
