Baumgart Alexander, Le Do Trang, Cranfield Charles G, Bridge Samara, Zerlotti Rocco, Palchetti Ilaria, Tadini-Buoninsegni Francesco, Clarke Ronald J
School of Chemistry, University of Sydney, Camperdown, New South Wales 2006, Australia.
School of Life Sciences, University of Technology Sydney, Ultimo, New South Wales 2007, Australia.
Langmuir. 2025 Apr 1;41(12):8081-8091. doi: 10.1021/acs.langmuir.4c04779. Epub 2025 Mar 18.
Understanding membrane transport processes such as ion occlusion reactions of ion pumps and transporters and the ion gating of channels requires knowledge of lipid bilayer electrostatics. A simple example of the effect of membrane electrostatics on ion transport is the much higher permeability of the membrane to hydrophobic anions, such as tetraphenylborate (TPB), compared to hydrophobic cations, such as tetraphenylphosphonium (TPP) or tetraphenylarsonium (TPA). This has been attributed to the membrane dipole potential, of which a major contributor has been determined to be oriented water dipoles in the lipid headgroup region of the membrane. From the ratio of the TPB to TPP or TPA conductances, the magnitude and polarity of the dipole potential can be estimated. Using the voltage-sensitive dye RH421 in conjunction with the stopped-flow technique and solid-supported membrane electrophysiology here we show that the transport of these ions is not simply a diffusion through the membrane but rather occurs in jumps between discrete binding sites within the membrane. The hydrophobic anion TPB causes much greater RH421 spectral changes than TPA. This could be explained by a combination of a stronger interaction of TPB with RH421 and a deeper binding of TPB within the membrane compared to TPA. The experimental methods, used here for the first time to study the kinetics of ion transport across membranes, are potentially applicable to investigations of the membrane permeability of charged drug molecules, in particular anticancer agents.
理解诸如离子泵和转运体的离子封闭反应以及通道的离子门控等膜运输过程需要了解脂质双分子层的静电学知识。膜静电学对离子运输影响的一个简单例子是，与疏水性阳离子（如四苯基鏻（TPP）或四苯基砷（TPA））相比，膜对疏水性阴离子（如四苯硼酸盐（TPB））的通透性要高得多。这归因于膜偶极电势，其中一个主要贡献因素已被确定为膜脂质头部区域中定向排列的水偶极子。根据TPB与TPP或TPA电导率的比值，可以估算偶极电势的大小和极性。在此，我们结合使用电压敏感染料RH421、停流技术和固体支持膜电生理学方法，证明这些离子的运输并非简单地通过膜扩散，而是在膜内离散结合位点之间跳跃发生。疏水性阴离子TPB引起的RH421光谱变化比TPA大得多。这可以通过TPB与RH421之间更强的相互作用以及与TPA相比TPB在膜内更深的结合来解释。本文首次用于研究离子跨膜运输动力学的实验方法，有可能应用于带电药物分子，特别是抗癌药物的膜通透性研究。
