Sumikama Takashi, Saito Shinji, Ohmine Iwao
Department of Chemistry, Faculty of Science, Nagoya University, Nagoya, Aichi 464-8602, Japan.
J Phys Chem B. 2006 Oct 19;110(41):20671-7. doi: 10.1021/jp062547r.
The mechanism of the ion permeation is investigated for an anion-doped carbon nanotube, as a model of the K+ channel, by analyzing the free energy surface and the dynamics of the ion permeation through the model channel. It is found that the main rate-determining step is how an ion enters the channel. The entrance of the ion is mostly blocked by a water molecule located at this entrance. Only about 10% of K+ ions which reach the mouth of the channel can really enter the channel. The rejection rate sensitively depends on the location of this water molecule, which is easily controlled by the charge of the carbon nanotube; for example, the maximum permeation is obtained when the anion charge is at a certain value, -5.4e in the present model. At this charge, the facile translocation of the ion inside the channel is also induced due to the number of fluctuations of the ions inside the channel. Therefore, the so-called "Newton's balls", a toy model, combined with a simple ion diffusion model for explaining the fast ion permeation should be modified. The present analysis thus suggests that there exists an optimum combination of the length and the charge of the carbon nanotube for the most efficient ion permeation.
作为钾离子通道的模型，通过分析自由能表面以及离子通过该模型通道的渗透动力学，对阴离子掺杂的碳纳米管的离子渗透机制进行了研究。研究发现，主要的速率决定步骤是离子如何进入通道。离子的入口大多被位于该入口处的一个水分子所阻挡。到达通道口的钾离子中只有约10%能够真正进入通道。排斥率敏感地取决于这个水分子的位置，而该位置很容易通过碳纳米管的电荷来控制；例如，在本模型中当阴离子电荷为某个值-5.4e时可获得最大渗透率。在这个电荷值下，由于通道内离子的波动数量，也会诱导离子在通道内的容易转运。因此，应该修改用于解释快速离子渗透的所谓“牛顿摆”（一种玩具模型）与简单离子扩散模型的组合。目前的分析因此表明，为了实现最有效的离子渗透，碳纳米管的长度和电荷存在一个最佳组合。
