Silin Vitalii I, Hoogerheide David P
University of Maryland, Institute for Bioscience and Biotechnology Research, Rockville MD 20850, USA.
Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
J Colloid Interface Sci. 2021 Jul 15;594:279-289. doi: 10.1016/j.jcis.2021.03.016. Epub 2021 Mar 10.
Composition and asymmetry of lipid membranes provide a means for regulation of trans-membrane permeability of ions and small molecules. The pH dependence of these processes plays an important role in the functioning and survival of cells. In this work, we study the pH dependence of membrane electrical resistance and capacitance using electrochemical impedance spectroscopy (EIS), surface plasmon resonance (SPR) and neutron reflectometry (NR) measurements of biomimetic tethered bilayer lipid membranes (tBLMs). tBLMs were prepared with single-component phospholipid compositions, as well as mixtures of phospholipids (phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, sphingomyelin and cholesterol) that mimic the inner- and outer- leaflets of plasma cell membranes. We found that all studied tBLMs have a resistance maximum at pHs near the pKs of the phospholipids. SPR and NR indicated that surface concentration of phospholipids and the thickness of the hydrophobic part of the membrane did not change versus pH. We postulate that these maxima are the result of protonation of the phosphate oxygen of the phospholipids and that hydronium ions play a major role in the conductance at pHs < pKs while sodium ions play the major role at pHs > pKs. An additional sharp resistance maximum of the PE tBLMs found at pH 5.9 and most likely represents the phosphatidylethanolamine's isoelectric point. The data show the key roles of the characteristic parts of phospholipid molecules: terminal group (choline, carboxyl, amine), phosphate, glycerol and ester oxygens on the permeability and selectivity of ions through the membrane. The interactions between these groups lead to significant differences in the electrical properties of biomimetic models of inner- and outer- leaflets of the plasma cell membranes.
脂质膜的组成和不对称性为调节离子和小分子的跨膜通透性提供了一种方式。这些过程对pH的依赖性在细胞的功能和存活中起着重要作用。在这项工作中，我们使用电化学阻抗谱（EIS）、表面等离子体共振（SPR）和中子反射测量（NR）对仿生拴系双层脂质膜（tBLM）进行研究，以探讨膜电阻和电容对pH的依赖性。tBLM由单组分磷脂组合物以及模拟质膜内、外小叶的磷脂混合物（磷脂酰胆碱、磷脂酰丝氨酸、磷脂酰乙醇胺、鞘磷脂和胆固醇）制备而成。我们发现，所有研究的tBLM在接近磷脂pK值的pH下都有一个电阻最大值。SPR和NR表明，磷脂的表面浓度和膜疏水部分的厚度不会随pH变化。我们推测，这些最大值是磷脂磷酸氧质子化的结果，并且在pH < pK时水合氢离子在电导中起主要作用，而在pH > pK时钠离子起主要作用。在pH 5.9时发现PE tBLM还有一个额外的尖锐电阻最大值，很可能代表磷脂酰乙醇胺的等电点。数据显示了磷脂分子特征部分：末端基团（胆碱、羧基、胺）、磷酸、甘油和酯氧对离子通过膜的通透性和选择性的关键作用。这些基团之间的相互作用导致了质膜内、外小叶仿生模型电性质的显著差异。
