Nachliel E, Finkelstein Y, Gutman M
Department of Biochemistry, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Israel.
Biochim Biophys Acta. 1996 Dec 4;1285(2):131-45. doi: 10.1016/s0005-2736(96)00149-6.
Monensin is an ionophore that supports an electroneutral ion exchange across the lipid bilayer. Because of this, under steady-state conditions, no electric signals accompany its reactions. Using the Laser Induced Proton Pulse as a synchronizing event we selectively acidify one face of a black lipid membrane impregnated by monensin. The short perturbation temporarily upsets the acid-base equilibrium on one face of the membrane, causing a transient cycle of ion exchange. Under such conditions the molecular events could be discerned as a transient electric polarization of the membrane lasting approx. 200 microseconds. The proton-driven chemical reactions that lead to the electric signals had been reconstructed by numeric integration of differential rate equations which constitute a maximalistic description of the multi equilibria nature of the system (Gutman, M. and Nachliel, E. (1989) Electrochim. Acta 34, 1801-1806). The analysis of the reactions reveals that the ionic selectivity of the monensin (H+ > Na+ > K+) is due to more than one term. Besides the well established different affinity for the various cations, the selectivity is also derived from a large difference in the rates of cross membranal diffusivities (MoH > MoNa > MoK), which have never been detected before. (v) Quantitative analysis of the membrane's crossing rates of the three neutral complexes reveals a major role of the membranal dipolar field in regulating ion transport. The diffusion of MoH, which has no dipole moment, is hindered only by the viscose drag. On the other hand, the dipolar complexes (MoNa and MoK) are delayed by dipole-dipole interaction with the membrane. (vi) Comparison of the calculated dipoles with those estimated for the crystalline conformation of the [MoNa(H2O)2] and [MoK(H2O)2] complexes reveals that the MoNa may exist in the membrane at its crystal configuration, while the MoK definitely attains a structure having a dipole moment larger than in the crystal.
莫能菌素是一种离子载体，可支持跨脂质双层的电中性离子交换。因此，在稳态条件下，其反应不会伴随电信号。利用激光诱导质子脉冲作为同步事件，我们选择性地酸化了被莫能菌素浸渍的黑色脂质膜的一侧。短暂的扰动暂时打破了膜一侧的酸碱平衡，导致离子交换的瞬态循环。在这种条件下，分子事件可被识别为膜持续约200微秒的瞬态电极化。导致电信号的质子驱动化学反应已通过对微分速率方程的数值积分进行了重构，这些方程构成了对系统多平衡性质的最大化描述（古特曼，M.和纳赫利尔，E.（1989年）《电化学学报》34卷，1801 - 1806页）。对反应的分析表明，莫能菌素的离子选择性（H⁺ > Na⁺ > K⁺）是由多个因素造成的。除了对各种阳离子已确定的不同亲和力外，选择性还源于跨膜扩散率的巨大差异（MoH > MoNa > MoK），这是以前从未检测到的。（v）对三种中性复合物跨膜速率的定量分析揭示了膜偶极场在调节离子运输中的主要作用。没有偶极矩的MoH的扩散仅受到粘性阻力的阻碍。另一方面，偶极复合物（MoNa和MoK）因与膜的偶极 - 偶极相互作用而延迟。（vi）将计算出的偶极与为[MoNa(H₂O)₂]和[MoK(H₂O)₂]复合物的晶体构象估计的偶极进行比较，结果表明MoNa在膜中可能以其晶体构型存在，而MoK肯定获得了一种偶极矩比晶体中更大的结构。
