Origin of life in a digital microcosm

While all organisms on Earth descend from a common ancestor, there is no consensus on whether the origin of this ancestral self-replicator was a one-off event or whether it was only the final survivor of multiple origins. Here we use the digital evolution system Avida to study the origin of self-replicating computer programs. By using a computational system, we avoid many of the uncertainties inherent in any biochemical system of self-replicators (while running the risk of ignoring a fundamental aspect of biochemistry). We generated the exhaustive set of minimal-genome self-replicators and analyzed the network structure of this fitness landscape. We further examined the evolvability of these self-replicators and found that the evolvability of a self-replicator is dependent on its genomic architecture. We studied the differential ability of replicators to take over the population when competed against each other (akin to a primordial-soup model of biogenesis) and found that the probability of a self-replicator out-competing the others is not uniform. Instead, progenitor (most-recent common ancestor) genotypes are clustered in a small region of the replicator space. Our results demonstrate how computational systems can be used as test systems for hypotheses concerning the origin of life.Comment: 20 pages, 7 figures. To appear in special issue of Philosophical Transactions of the Royal Society A: Re-Conceptualizing the Origins of Life from a Physical Sciences Perspectiv

Synthesis of well-defined polyethylene-polydimethylsiloxane-polyethylene triblock copolymers by diimide-based hydrogenation of polybutadiene blocks

Polyethylene, PE, is a crystalline solid with a relatively high melting temperature, and it exhibits excellent solvent resistance at room temperature. In contrast, polydimethylsiloxane, PDMS, is a rubbery polymer with an ultralow glass transition temperature and poor solvent resistance. PE-PDMS block copolymers have the potential to synergistically combine these disparate properties. In spite of this potential, synthesis of PE-PDMS block copolymers has not been widely explored. We report a facile route for the synthesis of well-defined polyethylene-b-polydimethylsiloxane-b-polyethylene (EDE) triblock copolymers. Poly(1,4-butadiene)-b-polydimethylsiloxane-b-poly(1,4-butadiene) (BDB) copolymer precursors were synthesized by anionic polymerization, followed by diimide-based hydrogenation. Under the standard hydrogenation conditions established by the work of Hahn, the siloxane bond undergoes scission resulting into significant degradation of the PDMS block. Our main accomplishment is the discovery of reaction conditions that avoid PDMS degradation. We used mechanistic insight into arrive at the optimal hydrogenation conditions, and we established the efficacy of our approach by successfully synthesizing a wide variety of block copolymers with total molecular weights ranging from 124 to 340 kg/mol and PDMS volume fractions ranging from 0.22 to 0.77. © 2014 American Chemical Society

Influence of Bound Ion on the Morphology and Conductivity of Anion-Conducting Block Copolymers

International audienceAnion-conducting membranes are important for several applications including fuel cells and artificial photosynthesis. In this study such membranes were made by quaternizing polystyrene-block-polychloromethylstyrene (PS-b-PCMS) copolymers. PS-b-PCMS copolymers with molecular weights ranging from 4 to 60 kg/mol were synthesized by nitroxide-mediated controlled radical polymerization. Separate aliquots of the PS-b-PCMS samples were quaternized to transform the PCMS block. This resulted in block copolymers with ionizable blocks containing either trimethylammonium chloride or n-butylimidazolium chloride. We refer to ion-containing block copolymers synthesized from the same precursor as matched pairs: SAM (containing trimethylammonium chloride) and SIM (containing n-butylimidazolium chloride). The volume fraction of the ion-containing block, ϕ, ranges from 0.26 to 0.50 for the case of SAM and from 0.35 to 0.60 for the case of SIM. Self-assembly in these copolymers resulted in the formation of lamellar phases regardless of ϕ, chemical formula of the bound ion, and chain length. Chloride ion conductivity and water uptake measurements on one of the matched pairs led to similar results. Preliminary experiments wherein the chloride ions in this matched pair were replaced by hydroxide ions were performed, and the changes in conductivity due to this are reported

Relationship between Segregation Strength and Permeability of Ethanol/Water Mixtures through Block Copolymer Membranes

A series of poly(styrene-b-dimethylsiloxane-b-styrene) (SDS) triblock copolymers with molecular weights ranging from 55 to 150 kg/mol and polydimethylsiloxane (PDMS) volume fractions ranging from 0.59 to 0.83 were used to fabricate membranes for ethanol/water separation by pervaporation. The rigid polystyrene (PS) microphase provides the membrane with structural integrity, while the rubbery PDMS microphase provides nanoscale channels for ethanol transport. We use a simple model to study the effect of morphology and PDMS volume fraction on permeabilitites of ethanol and water through the block copolymer membranes. We defined normalized permeabilities of ethanol and water to account for differences in morphology and PDMS volume fraction. We found that the normalized ethanol permeability in SDS copolymers was independent of the total polymer molecular weight. This is qualitatively different from what was previously reported for poly(styrene-b-butadiene-b-styrene) (SBS) membranes, where the normalized ethanol permeability was found to be a sensitive function of total molecular weight [J. Membr. Sci. 2011, 373, 112]. We demonstrate that this is due to differences in the Flory-Huggins interaction parameter (χ) for the two systems. When χN is less than 100 (N is the number of segments per chain), the two microphases are weakly segregated, and the presence of glassy PS segments in the transporting microphase impedes ethanol transport. When χN exceeds 100, the two microphases are strongly segregated and the glassy PS segments do not mix with the transporting phase. We compare these results with normalized ionic conductivity data previously reported for mixtures of a lithium salt and polystyrene-b-poly(ethylene oxide) (SEO). Evidence suggests that the product χN governs the transport of widely different species such as ethanol and lithium salts through block copolymer membranes. Surprisingly, the normalized permeability of water is independent of total molecular weight for both SDS and SBS block copolymers. © 2013 American Chemical Society