Studies on Functional Role of Archaeal Etherial Lipids

古细菌包括产甲烷菌、极端嗜盐菌、极端嗜热菌和热嗜酸菌，它们在相当特殊的条件下生长，具有结构独特的膜脂以适应极端环境。脂质由疏水类异戊二烯链组成，通过醚键与甘油相连，这与真核和真细菌膜脂的酯键形成鲜明对比。古菌醚脂质最显著的特征是其大环（36元或72元）环结构。这些不寻常的脂质与基于脂质双层结构的物理化学性质有关。我们对这些脂质的兴趣集中在大环分子结构的生化意义上。由于很难从天然来源获得大量纯古菌脂质，因此开发大环脂质的合成方法是前提。我们首次利用McMurry偶联法合成了古细菌的36元和72元大环脂质。此外，近年来还发现了一种利用烯烃复分解制备大环脂质的新方法。在Grubbs钌-烷基烯配合物RuCl_2(=CHPh)(PCy_3)_2存在下，在高稀释条件下，α， ω-二烯的闭合环复分解（RCM）以79%的收率有效地得到36元脂质。通过改变反应条件，同一底物主要生成无环二烯复分解（ADM）产物。无环产物随后在高稀释条件下进行RCM反应，以提供46%的72元脂质。因此，只要改变复分解反应的顺序和条件，就可以从相同的原料中随意得到36元脂质和72元脂质。研究了36元大环磷脂的物理化学特性，并与相应的非环磷脂在脂质体的流动性、热稳定性和渗透性方面进行了比较。热分析和单层研究表明，由于36元大环磷脂在烷基链区域的运动自由度较小，因此与相应的非环磷脂相比，它可以聚集成更紧密的排列结构。此外，我们发现大环结构有助于热稳定膜的形成。这些趋势似乎与嗜热古甲烷球菌对高温的耐受性有关。少

Archaea including methanogens, extreme halophiles, extreme thermophiles and thermoacidophiles grow under rather extraordinary conditions and have structurally unique membrane lipids to adapt the extreme environments. The lipids are consisted of hydrophobic isoprenoid chains linked to glycerol with the ether bonds, which are well contrast to the ester linkage of the eubacterial and eukaryotic membrane lipids. The most striking feature of the archaeal ether lipid is found in the macrocyclic (36- or 72-membered) ring structures. These unusual lipids have been interested in connection with the physicochemical properties based on the lipid bilayer structure. Our interest in these lipids focuses on the biochemical significance of the macrocyclic molecular structures. Prerequisite is to develop synthetic methods of the macrocyclic lipids, because it is difficult to obtain significant amount of the archaeal lipids in pure form from natural sources.We were the first in synthesizing the archaeal … More 36- and 72-membered macrocyclic lipids using McMurry coupling. In addition, a new and efficient approach to the macrocyclic lipids using olefin metathesis has been accomplished recently. In the presence of a Grubbs' ruthenium-alkylidene complex, RuCl_2(=CHPh)(PCy_3)_2, a ring closing metathesis (RCM) of α, ω-diene efficiently proceeded in 79% yield under high dilution conditions to give 36-membered lipid. By changing the reaction conditions, an acyclic diene metathesis (ADM) product was predominantly formed from the same substrate. The acyclic product was subsequently subjected to the RCM reaction under high dilution conditions to provide 72-membered lipid in 46%. Thus, both 36- and 72- membered lipids can be obtained at will from the same starting material only by changing the order and conditions of the metathesis reaction.The physicochemical features of the 36-membered macrocyclic phospholipid were investigated in comparison with the corresponding acyclic counterpart in terms of fluidity, thermostability and permeability of the liposomes. Thermal analyses and monolayer studies indicated that the 36-membered macrocyclic phospholipid can aggregate to more tightly packed structures than the corresponding acyclic counterpart due to its less motional freedom at the alkyl chain region.Furthermore, we found that the macrocyclic structure contributed to the formation of thermally stable membrane. These tendencies appear to be related to the tolerance of thermophilic archaea Methanococcus jannaschii against high temperature. Less