Probing the Electrostatics of Lipid Bilayer Membranes

莱斯大学的Jason Hafner教授在分析和表面化学项目的支持下研究了膜界面的静电势。该项目的目标是了解脂质膜中的电荷分布以及电荷分布如何影响脂质聚集并最终影响脂质膜的结构。利用该小组开发的一种新型基于电荷的纳米级成像技术，正在测量和绘制脂质中离子结合和偶极子的电荷分布和贡献。扫描探针仪器被称为流体电磁力显微镜（FEFM），它是原子力显微镜（AFM）的一个版本，但使用AFM尖端的天然静电电荷来检测膜上的表面电荷。AFM首先对表面进行扫描，从范德华相互作用中获得地形信息，然后再次在相同的轨迹上运行，但现在是以“起飞”模式跟随前一个轨迹的地形，允许尖端现在捕获静电信息。测量技术已经发展起来，并表明它可以区分带正电荷、负电荷和中性电荷的表面。为了破译和分离膜内的多种力（例如，静电、偶极、疏水、粘弹性和范德华相互作用），这些力可以影响形貌和电荷映射测量，研究小组利用离子结合研究、通过德拜长度变化的偶极子密度和膜中的脂质相来表征膜的离子亲和力。他们通过建立探针尖端的有效电荷，控制溶液离子强度以确保电荷扫描运行在德拜长度以上，测量不同离子对不同脂质的离子结合常数，并确定脂质相的静电贡献来实现这一目标。最后，探讨了利用溶液离子强度控制脂质双层尺寸的方法。静电相互作用是控制膜界面分子相互作用的主要驱动力。它们在决定所有生物细胞表面发生的行为和化学反应方面起着关键作用。目前对静电电荷在细胞膜中的分布了解甚少。大多数注意力被吸引到特定的相互作用，如蛋白质-配体识别或通过特定的脂质组合形成筏。静电是典型的非特异性相互作用领域。然而，静电相互作用对脂质相变具有惊人的影响，脂质相变可能在细胞膜筏形形成、导致特异性宿主-客体络合的远距离识别特性、细胞凋亡的细胞信号传导以及细胞分裂或内吞/胞吐过程中膜形态变化的促进等方面发挥重要作用。已经进行了许多研究来表征膜电荷对这些不同现象的贡献，但它们大多是全局或宏观的测量。为了真正理解静电电荷对细胞膜甚至合成膜系统的贡献，必须在纳米水平上对系统进行表征。极有可能的是，纳米级结构域中的电荷聚集，如脂筏中神经节苷脂的分配，是膜中特定位点蛋白质结合的激活剂。这个项目使人们能够以前所未有的分辨率首次观察这些现象。在这个项目中正在发展的概念将对生物物理学界对细胞膜系统的理解产生非常广泛的影响。此外，脂质膜材料在药物递送载体、传感器材料、生物相容性界面、检测阵列平台以及细胞膜模型等科技前沿的研究也在不断发展。了解脂质组织是了解膜材料如何在这些系统中发挥作用的关键。这项研究的成功成果可能使我们能够，例如，调整材料，以选择性地从溶液中捕获毒素分子进行传感和分离，或切换离子通道用于燃料电池或水净化。

Professor Jason Hafner of Rice University is supported by the Analytical and Surface Chemistry Program to investigate electrostatic potentials at membrane interfaces. The goal of the project is to understand charge distribution in lipid membranes and how that affects lipid aggregation and ultimately the structure of lipid membranes. The charge distribution and contributions from ion binding and dipoles from the lipids are being measured and mapped out using a novel charge-based nanoscale imaging technique developed by this group. The scanning probe instrument is called a Fluid Electric Force Microscope (FEFM), which is a version of an atomic force microscope (AFM) but uses the native electrostatic charge of the AFM tip to detect surface charge on the membrane. The AFM scans over the surface first to obtain topographic information from van der Waals interactions, then runs over the same trace again but now in a "lift-off" mode following the topography from the previous trace allowing the tip to now capture electrostatic information. The measurement technique has already been developed and has shown that it can distinguish between positively, negatively, and neutrally charged surfaces. In order to decipher and separate the multiple forces within the membrane (e.g., electrostatic, dipolar, hydrophobic, viscoelastic, and van der Waals interactions) that can influence both the topographic and charge mapping measurement, the team is characterizing the membrane's ion affinity using ion binding studies, dipole density through changes in the Debye length, and lipid phases in the membrane. They are achieving this by establishing the effective charge of the probe tip, controlling the solution ionic strength to ensure that the charge scan is run above the Debye length, measuring ion binding constants for different lipids with various ions, and determining the electrostatic contribution from lipid phases. Finally, the control of lipid bilayer dimensions by using the solution ionic strength is being examined. Electrostatic interactions are primary driving forces controlling molecular interactions at membrane interfaces. These play a critical role in determining the behavior and chemistry that takes place at the surfaces of all biological cells. Understanding the electrostatic charge distribution in the cell membrane is currently poor. Most of the attention is drawn towards specific interactions, such as protein-ligand recognition or raft formation through specific lipid combinations. Electrostatics are typically the domain of non-specific interactions. However, electrostatic interactions have astonishing effects on lipid phase transitions that could play a large role in raft formation in cell membranes, long range recognition properties that lead to specific host-guest complexation, cellular signaling for apoptosis, and facilitation of changes in membrane morphology during cell division or endo/exocytosis. Many studies have been performed to characterize the contribution of membrane charge with regard to these various phenomena, but they are mostly global or macroscopic measurements. To truly understand the contributions of electrostatic charge on cellular membranes, or even synthetic membrane systems, it is imperative to characterize the system at the nanoscopic level. It is highly likely that charge aggregation in nanoscale domains, such as in the partitioning of gangliosides in lipid rafts, is the activator for protein binding at specific sites in membranes. This project enables a first look into such phenomena with unprecedented resolution. The concepts that are being developed in this project would have a very broad impact on the biophysics community in their understanding of cellular membrane systems. Furthermore, research on lipid membrane materials is growing both at the scientific and technological fronts for drug delivery vehicles, sensor materials, biocompatible interfaces, detection array platforms, and as models for cell membranes. Understanding lipid organization is key to understanding how membrane materials function in each of these systems. Successful outcomes of this research may enable us to, for example, tune materials to selectively capture toxin molecules from solution for sensing and separations, or toggle ion channels for fuel cells or water purification.