Frankel Erica A, Bevilacqua Philip C, Keating Christine D
Department of Chemistry, ‡Center for RNA Molecular Biology, and §Department of Biochemistry and Molecular Biology, Pennsylvania State University , University Park, Pennsylvania 16802, United States.
Langmuir. 2016 Mar 1;32(8):2041-9. doi: 10.1021/acs.langmuir.5b04462. Epub 2016 Feb 16.
Phase separation of aqueous solutions containing polyelectrolytes can lead to formation of dense, solute-rich liquid droplets referred to as coacervates, surrounded by a dilute continuous phase of much larger volume. This type of liquid-liquid phase separation is thought to help explain the appearance of polyelectrolyte-rich intracellular droplets in the cytoplasm and nucleoplasm of extant biological cells and may be relevant to protocellular compartmentalization of nucleic acids on the early Earth. Here we describe complex coacervates formed upon mixing the polycation poly(allylamine) (PAH, 15 kDa) with the anionic nucleotides adenosine 5'-mono-, di-, and triphosphate (AMP, ADP, and ATP). Droplet formation was observed over a wide range of pH and MgCl2 concentrations. The nucleotides themselves as well as Mg(2+) and RNA oligonucleotides were all extremely concentrated within the coacervates. Nucleotides present at just 2.5 mM in bulk solution had concentrations greater than 1 M inside the coacervate droplets. A solution with a total Mg(2+) concentration of 10 mM had 1-5 M Mg(2+) in the coacervates, and RNA random sequence (N54) partitioned ∼10,000-fold into the coacervates. Coacervate droplets are thus rich in nucleotides, Mg(2+), and RNA, providing a medium favorable for generating functional RNAs. Compartmentalization of nucleotides at high concentrations could have facilitated their polymerization to form oligonucleotides, which preferentially accumulate in the droplets. Locally high Mg(2+) concentrations could have aided folding and catalysis in an RNA world, making coacervate droplets an appealing platform for exploring protocellular environments.
含有聚电解质的水溶液发生相分离可导致形成致密的、溶质丰富的液滴，即凝聚层，其周围是体积大得多的稀连续相。这种液-液相分离被认为有助于解释现存生物细胞的细胞质和核质中富含聚电解质的细胞内液滴的出现，并且可能与早期地球上核酸的原始细胞区室化有关。在此，我们描述了将聚阳离子聚（烯丙胺）（PAH，15 kDa）与阴离子核苷酸5'-单磷酸、二磷酸和三磷酸腺苷（AMP、ADP和ATP）混合后形成的复合凝聚层。在很宽的pH和MgCl2浓度范围内都观察到了液滴的形成。核苷酸本身以及Mg2+和RNA寡核苷酸在凝聚层中都高度浓缩。本体溶液中仅2.5 mM的核苷酸在凝聚层液滴中的浓度大于1 M。总Mg2+浓度为10 mM的溶液在凝聚层中有1-5 M的Mg2+，RNA随机序列（N54）在凝聚层中的分配系数约为10000倍。因此，凝聚层液滴富含核苷酸、Mg2+和RNA，为生成功能性RNA提供了有利的介质。高浓度核苷酸的区室化可能促进了它们聚合成寡核苷酸，而寡核苷酸优先在液滴中积累。局部高浓度的Mg2+可能有助于RNA世界中的折叠和催化作用，使得凝聚层液滴成为探索原始细胞环境的一个有吸引力的平台。
