Slonitskiĭ S V, Kuptsov V Iu
Mol Biol (Mosk). 1988 May-Jun;22(3):741-59.
The influence of the ionic strength of solution on the DNA molecule protonation was studied by means of circular dichroism (CD), spectrophotometric and potentiometric titration methods over a wide range of the supporting electrolyte concentrations [( NaCl] = 0.0005 divided by 4 M). Consideration of the obtained CD spectra shown that the acidation of the solution induces two cooperative structural transitions in the double stranded DNA molecule in the pre-denaturation pH region. Further decrease in the solution pH results in acidic melting of the DNA molecule. Analysis of the potentiometric data shows that diluted DNA solutions exhibit marked buffer capacity at pH greater than 4.2. A concept of local pH dependent on the electrostatic potential in the vicinity of the polyion was used for interpreting the obtained results. A phase diagram, which describes the polymorphic transformations of the protonated macromolecule, was constructed in terms of pHloc and -log[Na+]. Consideration of this phase diagram allows to hypothesize that: 1) in the neutral diluted DNA solution with a very low supporting electrolyte content the macromolecule exists in a polymorphic state; 2) at [NaCl] greater than or equal to 0.001 M the acid-base equilibrium in the DNA molecule is invariant in respect to the ionic strength of the solution.
通过圆二色性（CD）、分光光度法和电位滴定法，在广泛的支持电解质浓度范围（[NaCl]=0.0005至4M）内研究了溶液离子强度对DNA分子质子化的影响。对获得的CD光谱的分析表明，溶液的酸化在变性前的pH区域中诱导双链DNA分子发生两种协同结构转变。溶液pH的进一步降低导致DNA分子的酸性解链。对电位数据的分析表明，稀释的DNA溶液在pH大于4.2时表现出显著的缓冲能力。使用依赖于聚离子附近静电势的局部pH概念来解释所得结果。根据pHloc和-log[Na+]构建了描述质子化大分子多晶型转变的相图。对该相图的考虑允许假设：1）在具有非常低支持电解质含量的中性稀释DNA溶液中，大分子以多晶型状态存在；2）当[NaCl]大于或等于0.001M时，DNA分子中的酸碱平衡相对于溶液的离子强度是不变的。
