Mg ~（2+）反离子对噬菌体排出DNA的抑制作用
Inhibition of DNA ejection from bacteriophage by Mg+2 counterions
摘要：
研究了多价反离子，特别是Mg $^{+2}$反离子抑制病毒DNA从噬菌体中排出的问题。在实验上，已知MgSO $_4$盐对排出的DNA的量具有强的和非单调的影响。存在一个最佳浓度，在该浓度下，从病毒中排出的DNA量最少。在更低或更高的浓度下，更多的DNA从衣壳中排出。我们认为这种现象是Mg $^{+2}$多价反离子对DNA过充电的结果。随着Mg $^{+2}$浓度从零增加，DNA的净电荷从负变为正。最佳抑制对应于DNA为中性的Mg $^{+2}$浓度。在较低/较高浓度下，DNA基因组带电。它更喜欢在溶液中以降低其静电自能，从而导致DNA喷射的增加。通过将我们的理论与现有的实验数据拟合，发现由Mg $^{+2}$介导的DNA $-$ DNA短程吸引能的强度为$-$ 0.004 $k_BT$/核苷酸碱基。这个和其他拟合参数与其他实验和计算机模拟的已知值吻合得很好。这些参数也在aggreement定性值为三价和四价抗衡离子。
英文摘要：
The problem of inhibiting viral DNA ejection from bacteriophages by multivalent counterions, specifically Mg$^{+2}$ counterions, is studied. Experimentally, it is known that MgSO$_4$ salt has a strong and non-monotonic effect on the amount of DNA ejected. There exists an optimal concentration at which the minimum amount of DNA is ejected from the virus. At lower or higher concentrations, more DNA is ejected from the capsid. We propose that this phenomenon is the result of DNA overcharging by Mg$^{+2}$ multivalent counterions. As Mg$^{+2}$ concentration increases from zero, the net charge of DNA changes from negative to positive. The optimal inhibition corresponds to the Mg$^{+2}$ concentration where DNA is neutral. At lower/higher concentrations, DNA genome is charged. It prefers to be in solution to lower its electrostatic self-energy, which consequently leads to an increase in DNA ejection. By fitting our theory to available experimental data, the strength of DNA$-$DNA short range attraction energies, mediated by Mg$^{+2}$, is found to be $-$0.004 $k_BT$ per nucleotide base. This and other fitted parameters agree well with known values from other experiments and computer simulations. The parameters are also in aggreement qualitatively with values for tri- and tetra-valent counterions.
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