Department of Cell biology, Microbiology and Molecular Biology, University of South Florida, Tampa, FL 33620, United States of America.
Department of Physics, University of South Florida, Tampa, FL 33620, United States of America.
Biochim Biophys Acta Biomembr. 2019 May 1;1861(5):907-915. doi: 10.1016/j.bbamem.2019.01.016. Epub 2019 Feb 8.
A distinguishing feature of Archaeal plasma membranes is that their phospholipids contain ether-links, as opposed to bacterial and eukaryotic plasma membranes where phospholipids primarily contain ester-links. Experiments show that this chemical difference in headgroup-tail linkage does produce distinct differences in model bilayer properties. Here we examine the effects of salt on bilayer structure in the case of an ether-linked lipid bilayer. We use molecular dynamics simulations and compare equilibrium properties of two model lipid bilayers in NaCl salt solution - POPC and its ether-linked analog that we refer to as HOPC. We make the following key observations. The headgroup region of HOPC "adsorbs" fewer ions compared to the headgroup region of POPC. Consistent with this, we note that the Debye screening length in the HOPC system is ∼ 10% shorter than that in the POPC system. Herein, we introduce a protocol to identify the lipid-water interfacial boundary that reproduces the bulk salt distribution consistent with Gouy-Chapman theory. We also note that the HOPC bilayer has excess solvent in the headgroup region when compared to POPC, coinciding with a trough in the electrostatic potential. Waters in this region have longer autocorrelation times and smaller lateral diffusion rates compared to the corresponding region in the POPC bilayer, suggesting that the waters in HOPC are more strongly coordinated to the lipid headgroups. Furthermore, we note that it is this region of tightly coordinated waters in the HOPC system that has a lower density of Na ions. Based on these observations we conclude that an ether-linked lipid bilayer has a lower binding affinity for Na compared to an ester-linked lipid bilayer.
古菌质膜的一个显著特点是，它们的磷脂含有醚键，而细菌和真核质膜中的磷脂主要含有酯键。实验表明，这种头基-尾基连接的化学差异确实会导致模型双层性质的明显差异。在这里，我们研究了盐对醚键连接的双层结构的影响。我们使用分子动力学模拟，并比较了两种模型脂质双层在 NaCl 盐溶液中的平衡性质 - POPC 及其醚键类似物 HOPC。我们得出了以下关键观察结果。与 POPC 相比，HOPC 的头基区域吸附的离子更少。与此一致，我们注意到 HOPC 系统中的德拜屏蔽长度比 POPC 系统短约 10%。在此，我们引入了一种识别脂质-水界面边界的方案，该方案可重现与古伊-查普曼理论一致的体相盐分布。我们还注意到，与 POPC 相比，HOPC 双层在头基区域有多余的溶剂，与静电势中的低谷相对应。与 POPC 双层中相应区域相比，该区域的水具有更长的自相关时间和更小的横向扩散率，这表明 HOPC 中的水与脂质头基的协调性更强。此外，我们注意到，正是 HOPC 系统中这种紧密协调的水分子区域的钠离子密度较低。基于这些观察结果，我们得出结论，与酯键连接的脂质双层相比，醚键连接的脂质双层对 Na 的结合亲和力较低。
