School of Polymer Science and Polymer Engineering, University of Akron, Akron, Ohio, USA.
Integrated Biosciences Program, University of Akron, Akron, Ohio, USA.
Astrobiology. 2023 Mar;23(3):327-343. doi: 10.1089/ast.2022.0056. Epub 2023 Feb 1.
Primitive cells are believed to have been self-assembled vesicular structures with minimal metabolic components, that were capable of self-maintenance and self-propagation in early Earth geological settings. The coevolution and self-assembly of biomolecules, such as amphiphiles, peptides, and nucleic acids, or their precursors, were essential for protocell emergence. Here, we present a novel class of amphiphiles-amino acid-fatty alcohol esters-that self-assemble into stable primitive membrane compartments under a wide range of geochemical conditions. Glycine -octyl ester (GOE) and isoleucine -octyl ester (IOE), the condensation ester products of glycine or isoleucine with octanol (OcOH), are expected to form at a mild temperature by wet-dry cycles. The GOE forms micelles in acidic aqueous solutions (pH 2-7) and vesicles at intermediate pH (pH 7.3-8.2). When mixed with cosurfactants (octanoic acid [OcA]; OcOH, or decanol) in different mole fractions [ = 0.1-0.5], the vesicle stability range expands significantly to span the extremely acidic to mildly alkaline (pH 2-8) and extremely alkaline (pH 10-11) regions. Only a small mole fraction of cosurfactant [ = 0.1] is needed to make stable vesicular structures. Notably, these GOE-based vesicles are also stable in the presence of high concentrations of divalent cations, even at low pHs and in simulated Hadean seawater composition (without sulfate). To better understand the self-assembly behavior of GOE-based systems, we devised complementary molecular dynamics computer simulations for a series of mixed GOE/OcA systems under simulated acidic pHs. The resulting calculated critical packing parameter values and self-assembly behavior were consistent with our experimental findings. The IOE is expected to show similar self-assembly behavior. Thus, amino acid-fatty alcohol esters, a novel chimeric amphiphile class composed of an amino acid head group and a fatty alcohol tail, may have aided in building protocell membranes, which were stable in a wide variety of geochemical circumstances and were conducive to supporting replication and self-maintenance. The present work contributes to our body of work supporting our hypothesis for synergism and coevolution of (proto)biomolecules on early Earth.
原始细胞被认为是具有最小代谢成分的自组装囊泡结构，能够在早期地球地质环境中自我维持和自我繁殖。生物分子（如两亲分子、肽和核酸或其前体）的共进化和自组装对于原生细胞的出现至关重要。在这里，我们提出了一类新的两亲体——氨基酸-脂肪醇酯，它们在广泛的地球化学条件下自组装成稳定的原始膜隔室。甘氨酸-辛基酯（GOE）和异亮氨酸-辛基酯（IOE）是甘氨酸或异亮氨酸与辛醇（OcOH）缩合的缩合酯产物，预计在干湿循环过程中在温和的温度下形成。GOE 在酸性水溶液（pH 2-7）中形成胶束，在中间 pH（pH 7.3-8.2）下形成囊泡。当与不同摩尔分数的助表面活性剂（辛酸[OcA]；OcOH 或癸醇）混合时[=0.1-0.5]，囊泡稳定性范围显著扩大，跨越极酸到微碱性（pH 2-8）和极碱性（pH 10-11）区域。只需少量助表面活性剂[=0.1]即可形成稳定的囊泡结构。值得注意的是，即使在低 pH 值和模拟原始海洋海水组成（不含硫酸盐）下，基于 GOE 的这些囊泡在存在高浓度二价阳离子时也是稳定的。为了更好地理解基于 GOE 的系统的自组装行为，我们设计了一系列混合 GOE/OcA 系统在模拟酸性 pH 下的互补分子动力学计算机模拟。所得计算的临界堆积参数值和自组装行为与我们的实验结果一致。异亮氨酸-辛基酯预计会表现出类似的自组装行为。因此，氨基酸-脂肪醇酯，一种由氨基酸头基和脂肪醇尾基组成的新型嵌合两亲体类，可能有助于构建原始细胞膜，该膜在各种地球化学环境中稳定，并有利于支持复制和自我维持。本工作为支持我们关于早期地球上（原）生物分子协同进化的假说提供了依据。
