title: Chain-length heterogeneity allows for the assembly of fatty acid vesicles in dilute solutions.
abstract: A requirement for concentrated and chemically homogeneous pools of molecular building blocks would severely restrict plausible scenarios for the origin of life. In the case of membrane self-assembly, models of prebiotic lipid synthesis yield primarily short, single-chain amphiphiles that can form bilayer vesicles only at very high concentrations. These high critical aggregation concentrations (cacs) pose significant obstacles for the self-assembly of single-chain lipid membranes. Here, we examine membrane self-assembly in mixtures of fatty acids with varying chain lengths, an expected feature of any abiotic lipid synthesis. We derive theoretical predictions for the cac of mixtures by adapting thermodynamic models developed for the analogous phenomenon of mixed micelle self-assembly. We then use several complementary methods to characterize aggregation experimentally, and find cac values in close agreement with our theoretical predictions. These measurements establish that the cac of fatty acid mixtures is dramatically lowered by minor fractions of long-chain species, thereby providing a plausible route for protocell membrane assembly. Using an NMR-based approach to monitor aggregation of isotopically labeled samples, we demonstrate the incorporation of individual components into mixed vesicles. These experiments suggest that vesicles assembled in dilute, mixed solutions are depleted of the shorter-chain-length lipid species, a finding that carries implications for the composition of primitive cell membranes.
authors.affiliation: Howard Hughes Medical Institute, Department of Molecular Biology and Center for Computational and Integrative Biology, Massachusetts General Hospital, Boston, Massachusetts; Miller Institute for Basic Research in Science, University of California, Berkeley, Berkeley, California.
authors.forename: Itay
authors.initials: I
authors.lastname: Budin
authors.affiliation: Howard Hughes Medical Institute, Department of Molecular Biology and Center for Computational and Integrative Biology, Massachusetts General Hospital, Boston, Massachusetts.
authors.forename: Noam
authors.initials: N
authors.lastname: Prwyes
authors.affiliation: Howard Hughes Medical Institute, Department of Molecular Biology and Center for Computational and Integrative Biology, Massachusetts General Hospital, Boston, Massachusetts; High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, P. R. China.
authors.forename: Na
authors.initials: N
authors.lastname: Zhang
authors.affiliation: Howard Hughes Medical Institute, Department of Molecular Biology and Center for Computational and Integrative Biology, Massachusetts General Hospital, Boston, Massachusetts. Electronic address: szostak@molbio.mgh.harvard.edu.
authors.forename: Jack W
authors.initials: JW
authors.lastname: Szostak
mesh_terms: D005227:Fatty Acids / Q000737:chemistry*; D008051:Lipid Bilayers / Q000737:chemistry*; D012996:Solutions
chemical_list: D005227:Fatty Acids; D008051:Lipid Bilayers; D012996:Solutions