Mauzerall D C, Drain C M
Rockefeller University, New York, New York 10021.
Biophys J. 1992 Dec;63(6):1544-55. doi: 10.1016/S0006-3495(92)81738-1.
The conductances of the lipophilic ions tetraphenylboride and tetraphenylphosphonium across a lipid bilayer can be increased or decreased, i.e., gated, by the photoformation of closed-shell metalloporphyrin cations within the bilayer. The gating can be effected by pulsed or continuous light or by chemical oxidants. At high concentrations of lipophilic anions where the dark conductance is saturated due to space charge in the bilayer, the photogated conductance can increase 15-fold. The formation of porphyrin cations allows the conductance to increase to its nonspace charge limited value. Conversely, the decrease of conductance in the light of phosphonium cations diminishes toward zero as the dark conductance becomes space charge limited. We present electrostatic models of the space charge limited conductance that accurately fit the data. One model includes an exponentially varying dielectric constant for the polar regions of the bilayer that allows an analytical solution to the electrostatic problem. The exponential variation of the dielectric constant effectively screens the potential and implies that the inside and outside of real dielectric interfaces can be electrically isolated from one another. The charge density, the distance into the membrane of the ions, about one-quarter of its thickness, and the dielectric constant at that position are determined by these models. These calculations indicate that there is insufficient porphyrin charge density to cancel the boride ion space charge and the following article proposes a novel ion chain mechanism to explain these effects. These models indicate that the positive potential arising from oriented carbonyl ester groups, previously used to explain the 10(3)-fold larger conductance of hydrophobic anions over cations, is smaller than previously estimated. However, the synergistic movement of the positive choline group into the membrane can account for the large positive potential.
亲脂性离子四苯基硼化物和四苯基鏻离子跨脂质双层的电导率可通过在双层内光形成闭壳金属卟啉阳离子而增加或降低，即被门控。门控可通过脉冲光或连续光或化学氧化剂实现。在高浓度亲脂性阴离子存在的情况下，由于双层中的空间电荷导致暗电导率饱和，光门控电导率可增加15倍。卟啉阳离子的形成使电导率增加到其非空间电荷限制值。相反，随着暗电导率变得受空间电荷限制，鏻阳离子光照下电导率的降低趋近于零。我们提出了空间电荷限制电导率的静电模型，该模型能准确拟合数据。其中一个模型包括双层极性区域的指数变化介电常数，这使得静电问题有解析解。介电常数的指数变化有效地屏蔽了电势，这意味着真实介电界面的内部和外部在电方面可相互隔离。这些模型确定了电荷密度、离子进入膜内的距离（约为膜厚度的四分之一）以及该位置的介电常数。这些计算表明，卟啉电荷密度不足以抵消硼化物离子的空间电荷，随后的文章提出了一种新颖的离子链机制来解释这些效应。这些模型表明，先前用于解释疏水阴离子电导率比阳离子大10³倍的由取向羰基酯基团产生的正电势比先前估计的要小。然而，正胆碱基团协同移入膜内可解释这种大的正电势。
