Trend B L, Knoll D A, Ueno M, Evans D F, Bloomfield V A
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis 55455.
Biophys J. 1990 Apr;57(4):829-34. doi: 10.1016/S0006-3495(90)82602-3.
Most polyelectrolyte theories of the effect of ions on the thermal melting of DNA assume that the predominant influence of the cations comes through their charge. Ion size and structure are treated, for analytic convenience, as negligible variables. We have examined the validity of this assumption by measuring the melting temperature of calf thymus DNA as a function of salt concentration with four univalent cations of different hydrated radii. These are K+ (3.3 A), (n-Pr)4N+ (4.5 A), (EtOH)4N+ (4.5 A), and C222-K+ (5 A). C222-K+ is a complex of cryptand C222 with K+. With K+ as the sole cation, Tm varies linearly with the log of ionic strength over the range 0.001-0.1 M. With all the K+ sequestered by an equimolar amount of C222, Tm is depressed by 10-20 degrees C and the slope of Tm vs. ionic strength is lower. At low ionic strength, an even greater reduction in Tm is achieved with (n-Pr)4N+; but the similar-sized (EtOH)4N+ gives a curve more similar to K+. Theoretical modeling, taking into account cation size through the Poisson-Boltzmann equation for cylindrical polyelectrolytes, predicts that larger cations should be less effective in stabilizing the double helix; but the calculated effect is less than observed experimentally. These results show that valence, cation size, and specific solvation effects are all important in determining the stability of the double-helical form of DNA.
大多数关于离子对DNA热解链影响的聚电解质理论都假定阳离子的主要影响是通过其电荷来实现的。为了便于分析，离子大小和结构被视为可忽略不计的变量。我们通过测量小牛胸腺DNA的解链温度作为盐浓度的函数，使用四种具有不同水合半径的单价阳离子，检验了这一假设的有效性。这四种阳离子分别是K⁺（3.3 Å）、四正丙基铵离子（4.5 Å）、四乙醇铵离子（4.5 Å）和穴状配体C222与K⁺形成的配合物C222-K⁺（5 Å）。以K⁺作为唯一阳离子时，在0.001 - 0.1 M的离子强度范围内，解链温度（Tm）随离子强度的对数呈线性变化。当所有的K⁺被等摩尔量的C222螯合时，Tm降低了10 - 20℃，并且Tm与离子强度的斜率变小。在低离子强度下，四正丙基铵离子使Tm降低得更多；但大小相似的四乙醇铵离子给出的曲线更类似于K⁺的曲线。通过考虑圆柱状聚电解质的泊松 - 玻尔兹曼方程来纳入阳离子大小的理论模型预测，较大的阳离子在稳定双螺旋结构方面应该效果较差；但计算出的效应小于实验观察到的结果。这些结果表明，化合价、阳离子大小和特定溶剂化效应在决定DNA双螺旋形式的稳定性方面都很重要。
