Drain C M, Mauzerall D C
Rockefeller University, New York, New York 10021.
Biophys J. 1992 Dec;63(6):1556-63. doi: 10.1016/S0006-3495(92)81739-3.
The photogating of hydrophobic ion currents across the lipid bilayer membrane allows the direct study of their kinetics by symmetrically forming charge within the membrane and across each interface, rather than across the membrane. We find that the photoinduced conductance continues to increase beyond the region where the tetraphenylboride charge density in the membrane exceeds the estimated porphyrin cation density. This photoconductance is proportional to the tetraphenylboride charge density raised to the second to third power. The risetime of the photogating effect increases with increasing concentration of tetraphenyl boride. The porphyrin cation mobility is increased when the tetraphenylboride anion is present, and low concentrations of tetraphenylphosphonium cation increase the dark conductivity while inhibiting the photoconductivity. The activation energy for both the porphyrin and phosphonium cation induced conductance is more positive than that of the tetraphenylboride conductance. From these results we conclude that in addition to some cancellation of space charge within the membrane, the mechanism of increased conductance involves the transport of these hydrophobic anions via an alternating anion-cation chain, analogous to the Grotthuss mechanism for excess proton conduction in water. This ion chain conductance can be viewed as an evolutionary prototype of an ion channel across the membrane. It also underscores the importance of the counter ion in the transport of large ions such as peptides across the lipid bilayer.
疏水性离子电流跨脂质双分子层膜的光门控作用，使得能够通过在膜内以及跨每个界面（而非跨膜）对称地形成电荷，直接研究其动力学。我们发现，光诱导电导在膜中四苯基硼化物电荷密度超过估计的卟啉阳离子密度的区域之后仍继续增加。这种光电导与四苯基硼化物电荷密度的二次方至三次方成正比。光门控效应的上升时间随四苯基硼化物浓度的增加而增加。当存在四苯基硼化物阴离子时，卟啉阳离子迁移率增加，低浓度的四苯基鏻阳离子会增加暗电导，同时抑制光电导。卟啉和鏻阳离子诱导电导的活化能比四苯基硼化物电导的活化能更正。从这些结果我们得出结论，除了膜内空间电荷的部分抵消外，电导增加的机制涉及这些疏水性阴离子通过交替的阴离子 - 阳离子链的传输，类似于水中过量质子传导的Grotthuss机制。这种离子链电导可被视为跨膜离子通道的进化原型。它还强调了抗衡离子在诸如肽等大离子跨脂质双分子层运输中的重要性。
