Finkelstein A, Andersen O S
J Membr Biol. 1981 Apr 30;59(3):155-71. doi: 10.1007/BF01875422.
Gramicidin A forms univalent cation-selective channels of approximately 4 A diameter in phospholipid bilayer membranes. The transport of ions and water throughout most of the channel length is by a single-file process; that is, cations and water molecules cannot pass each other within the channel. The implications of this single-file mode of transport for ion movement are considered. In particular, we show that there is no significant electrostatic barrier to ion movement between the energy wells at the two ends of the channel. The rate of ion translocation (e.g., Na+ or Cs+) through the channel between these wells is limited by the necessity for an ion to move six water molecules in single file along with it; this also limits the maximum possible value for channel conductance. At all attainable concentrations of NaCl, the gramicidin A channel never contains more than one sodium ion, whereas even at 0.1 M CsCl, some channels contain two cesium ions. There is no necessity to postulate more than two ion-binding sites in the channel or occupancy of the channel by more than two ions at any time.
短杆菌肽A在磷脂双分子层膜中形成直径约为4埃的单价阳离子选择性通道。在通道的大部分长度范围内，离子和水的运输是通过单排过程进行的；也就是说，阳离子和水分子在通道内不能相互通过。本文考虑了这种单排运输模式对离子移动的影响。特别是，我们表明，在通道两端的能量阱之间，离子移动不存在显著的静电势垒。离子（如Na+或Cs+）在这些阱之间通过通道的转运速率受到离子必须携带六个水分子单排移动的限制；这也限制了通道电导的最大可能值。在所有可达到的NaCl浓度下，短杆菌肽A通道中钠离子的数量从不超过一个，而即使在0.1M CsCl溶液中，一些通道中也含有两个铯离子。无需假设通道中存在超过两个离子结合位点，也无需假设通道在任何时候被超过两个离子占据。
