Pörschke D
Biophys Chem. 1976 Jul;4(4):383-94. doi: 10.1016/0301-4622(76)80018-x.
The coupling of ion binding to the single strand helix-coil transition in poly (A) and poly (C) is used to obtain information about both processes by ion titration and field-jump relaxation methods. Characterisation of the field-jump relaxation in poly (C) at various concentrations of monovalent ions leads to the evaluation of a stability constant K = 71 M(-1) for the ion binding to the polymer. the rate constant of helix formation is found to be 1.3 X 10(7)s(-1), whereas the dissociation rate is 1.0 X 10(6)s(-1). Similar data are preseented for poly (A) and poly (dA). The interaction of Mg(++) and Ca(++) with poly (A) and poly (C) is measured by a titration method using the polymer absorbance for the indication of binding. The data can be represented by a model with independent binding "sites". The stability constants increase with decreasing salt concentration from 2.7 X 10(4) M(-1) at medium ionic strengths up to 2.7 X 10(7) M(-1) at low ionic strength. The number of ions bound per nucleotide residue is in the range 0.2 to 0.3. Relaxation time constants associated with Mg(++) binding are characterised over a broad range of Mg(++) concentrations form 5 muM to 500 muM. The observed concentration dependence supports the conclusion on the number of binding places inferred from equilibrium titrations. The rate of Mg(++) and Ca(++) association to the polymer is close to the limit of diffusion control kR =1 X 10(10) to 2 X 10(10) M(-1) s(-1)). This high rate demonstrates that Mg(++) and Ca(++) ions do not form inner-sphere complexes with the polynucleotides. Apparently the distance between two adjacent phosphates is too large for a simultaneous site binding of Mg(++) or Ca(++), and inner sphere complexation at a single phosphate seems to be too weak. The data support the view that the ions like Mg(++) and Ca(++) surround the polynucleotides in the form of a mobile ion cloud without site binding.
通过离子滴定和场跃弛豫方法，利用离子结合与聚（A）和聚（C）中单链螺旋 - 线圈转变的耦合来获取有关这两个过程的信息。在不同浓度的单价离子下对聚（C）中场跃弛豫进行表征，得出离子与聚合物结合的稳定常数K = 71 M⁻¹。发现螺旋形成的速率常数为1.3×10⁷ s⁻¹，而解离速率为1.0×10⁶ s⁻¹。给出了聚（A）和聚（dA）的类似数据。通过使用聚合物吸光度指示结合的滴定法测量Mg²⁺和Ca²⁺与聚（A）和聚（C）的相互作用。数据可以用具有独立结合“位点”的模型表示。稳定常数随着盐浓度降低而增加，从中等离子强度下的2.7×10⁴ M⁻¹增加到低离子强度下的2.7×10⁷ M⁻¹。每个核苷酸残基结合的离子数在0.2至0.3范围内。在5 μM至500 μM的宽Mg²⁺浓度范围内表征了与Mg²⁺结合相关的弛豫时间常数。观察到的浓度依赖性支持了从平衡滴定推断出的结合位点数量的结论。Mg²⁺和Ca²⁺与聚合物结合的速率接近扩散控制极限kR = 1×10¹⁰至2×10¹⁰ M⁻¹ s⁻¹。这个高速率表明Mg²⁺和Ca²⁺离子不与多核苷酸形成内球络合物。显然，两个相邻磷酸盐之间的距离太大，无法使Mg²⁺或Ca²⁺同时进行位点结合，并且单个磷酸盐处的内球络合似乎太弱。数据支持这样的观点，即Mg²⁺和Ca²⁺等离子以移动离子云的形式围绕多核苷酸，而没有位点结合。
