Protocell Self-Assembly Driven by Sodium Trimetaphosphate.

The co-evolution of peptide formation and membrane self-assembly is considered an essential step in the origin of life. However, more research is required on both processes, particularly on the interaction between prebiotic simple fatty-acid membranes and peptide synthesis. In this study, the sodium trimetaphosphate (P3 m)-activated peptide formation reaction of phenylalanine (Phe) in an alkaline decanoic acid-decanol vesicle system was systematically investigated. The experimental results showed that peptide formation could competitively occur with N-acyl amino acid (NAA) formation. NAA formation did not follow the traditional P3 m-activated peptide formation reaction involving the intermediate cyclic acylphosphoramidate (CAPA). Decanoic acid was activated by P3 m to form a mixed anhydride, which then reacted with an amino acid to form the amide NAA. As a kind of membrane-forming amphiphile, NAA can form vesicles independently and reduce the critical vesicle concentration of the fatty-acid vesicles. Moreover, 11 other representative amino acids, namely alanine (Ala), aspartic acid (Asp), glutamic acid (Glu), glycine (Gly), isoleucine (Ile), leucine (Leu), proline (Pro), serine (Ser), threonine (Thr), valine (Val), and arginine (Arg), were selected for investigation. All of them reacted with decanoic acid to form NAA via the activation effect of P3 m. The abovementioned mechanism involving P3 m-activated carboxylic acid has not been reported in the literature. Our experimental results indicate that the participation of decanoic acid in the P3 m activation-based peptide formation reaction system plays a significant role in the emergence of functionalized protocells. The P3 m activation effect can provide diversified raw membrane materials to form and stabilize protocell membranes; moreover, the small peptides, such as Phe-Leu, formed in the same reaction system can induce the amplification of primitive cells. This implies that synergistic symbiosis between membrane and peptide can be realized via the P3 m activation effect.

A Self-Controlled and Self-Healing Model of Bacterial Cells.

A new kind of self-assembly model, morphogenetic (M) systems, assembles spatial units into larger structures through local interactions of simpler components and enables discovery of new principles for cellular membrane assembly, development, and its interface function. The model is based on interactions among three kinds of constitutive objects such as tiles and protein-like elements in discrete time and continuous 3D space. It was motivated by achieving a balance between three conflicting goals: biological, physical-chemical, and computational realism. A recent example is a unified model of morphogenesis of a single biological cell, its membrane and cytoskeleton formation, and finally, its self-reproduction. Here, a family of dynamic M systems (Mbac) is described with similar characteristics, modeling the process of bacterial cell formation and division that exhibits bacterial behaviors of living cells at the macro-level (including cell growth that is self-controlled and sensitive to the presence/absence of nutrients transported through membranes), as well as self-healing properties. Remarkably, it consists of only 20 or so developmental rules. Furthermore, since the model exhibits membrane formation and septic mitosis, it affords more rigorous definitions of concepts such as injury and self-healing that enable quantitative analyses of these kinds of properties. Mbac shows that self-assembly and interactions of living organisms with their environments and membrane interfaces are critical for self-healing, and that these properties can be defined and quantified more rigorously and precisely, despite their complexity.

Synergetics of the Membrane Self-Assembly: A Micelle-to-Vesicle Transition.

A model approach is developed to study intermediate steps and transientstructures in a course of the membrane self-assembly. The approach isbased on investigation of mixed lipid/protein-detergent systems capable ofthe temperature induced transformation from a solubilized micellar stateto closed membrane vesicles. We performed a theoretical analysis ofself-assembling molecular structures formed in binary mixtures ofdimyristoylphosphatidylcholine (DMPC) and sodium cholate (NaC). Thetheoretical model is based on the Helfrich theory of curvature elasticity,which relates geometrical shapes of the structures to their free energy inthe Ginzburg-Landau approximation. The driving force for the shapetransformation is spontaneous curvature of amphiphilic aggregates which isnonlinearly dependent on the lipid/detergent composition. An analysis ofthe free energy in the regular solution approximation shows that theformation of mixed structures of different shapes (discoidal micelles,rod-like micelles, multilayer membrane structures and vesicles) ispossible in a certain range of detergent/lipid ratios. A transition fromthe flat discoidal micelles to the rod-like cylindrical micelles isinduced by curvature instabilities resulting from acyl chain melting andinsertion of detergent molecules into the lipid phase. Nonideal mixing ofthe NaC and DMPC molecules results in formation of nonideal cylindricalaggregates with elliptical cross section. Further dissolution of NaCmolecules in DMPC may be accompanied with a change of their orientation inthe lipid phase and leads to temperature-induced curvature instabilitiesin the highly curved cylindrical geometry. As a result the rod-likemicelles fuse into less curved bilayer structures which transformeventually to the unilamellar and multilamellar membrane vesicles. Thetheoretical analysis performed shows that a sequence of shapetransformations in the DMPC/NaC mixed systems is determined by thesynergism of four major factors: detergent/lipid ratio, temperature (acylchain melting), DMPC and NaC mixing, and reorientation of NaC molecules inmixed aggregates.