DNA平衡离子凝聚的动态光散射分析

利用动态光散射技术系统地研究了不同化合价的平衡离子氯化钠 (Na⁺)、氯化镁 (Mg²⁺)、三氯六氨络合钴 ([Co(NH₃)₆]³⁺) 和精胺 ([C₁₀N₄H₃₀]⁴⁺) 与DNA之间的相互作用. 实验结果显示, 当缓冲液中只包含Na⁺ 或Mg²⁺, 且浓度c≥ 5 mM时, DNA电泳迁移率之比约为2:1; 当缓冲液单独包含Na⁺或[Co(NH₃)₆]³⁺, 且浓度c≥ 5 mM时, DNA 的电泳迁移率之比约为4.5:1. 而当平衡离子的化合价为4时, 观察到DNA的电泳迁移率由负变正, 意味着发生了电荷反转. 在平衡离子浓度c<5 mM时, 随着离子浓度的增大, 迁移率之比逐渐增大. 对于一价或二价平衡离子情形, 实验结果与Manning的平衡离子凝聚理论的预言相符. 对于三价平衡离子, 实验结果与理论值有明显偏离. 而对于四价离子, 由于发生了DNA电荷逆转, 基于平均场的平衡离子凝聚理论失效. 另外, 通过原子力显微镜观察到当平衡离子的化合价大于等于3 时, DNA分子的构型发生变化. 因此, 自由溶液中的聚电解质的构型和离子关联效应在聚电解质迁移过程中起重要作用.关键词：电泳迁移率平衡离子动态光散射DNA

A dynamic light scattering study of counter-ions condensation on DNA

The interaction between DNA and counter-ions of different valence, including sodium chloride (Na⁺), magnesium chloride (Mg²⁺), hexammine cobalt III ([Co(NH₃)₆]³⁺), and spermine ([C₁₀N₄H₃₀]⁴⁺), is investigated by dynamic light scattering. It is found that the ratio of electrophoretic motilities of DNA in a buffer containing Na⁺ and Mg²⁺ is about 2:1, when the concentration of counter-ions c≥ 5 mM. But the ratio of DNA motilities in a buffer containing Na⁺ and [Co(NH₃)₆]³⁺ is about 4.5:1. When c<5 mM, the ratio grows with increasing concentration of counter-ions. DNA charge reversal can be observed in the case of quadrivalent counter-ion. The experimental results are in good agreement with the Manning counter-ions condensation theory for cases of monovalent or bivalent counter-ions. However, when the valency of counter-ions is equal to three, the experimental data deviates from the expectation of the theory significantly. For the quadrivalent counter-ions, the counter-ions condensation theory, which is based on the average field, fails. Furthermore, through the atomic force microscopy, it is found that DNA molecules will condense into compact structures when the valency of counter-ions is equal to or greater than three. Thus, the conformation of polyelectrolyte in free solution and the ion correlation play an important role in the migration process of polyelectrolyte.
Keywords：electrophoretic mobilitycounter-ionsdynamic light scatteringDNA