Pearling and helical nanostructures of model protocell membranes

The diversity of protocell membrane structures is crucial for the regulation of cell activities and indispensable to the origin of life. Prior to the evolution of complex cellular machinery, spontaneous protocell membrane evolution results from the intrinsic physicochemical properties of simple molecules under specific environmental conditions. Here, we report the evolution of the morphology of cell-sized model protocell membranes from giant vesicles to pearling and helical nanostructures, resembling morphologies of eukaryocytes, nostoc, and spirilla. This evolution occurs in a single binary aqueous system composed of an achiral single-chain amphiphile and a biogenic polyamine (spermidine or spermine) upon evaporating water, feeding amphiphiles, or increasing pH in response to various primitive fluctuating conditions. In contrast, nonbiogenic polyamines (triamine, triethylenetetramine, and hexamethyltriethylenetetramine) with slight differences in the number of methylene groups or protonated amine groups do not induce such a kind of evolution. The evolution of the shape transformation strongly relies on the balance between electrostatic attraction and hydrogen bonding, attributed to the odd/even effect of polyamines in the assembly. Strikingly, both pearling and helical structures emerge from multilamellar vesicles undergoing different processes, where the helix shows stronger permeability and encapsulation capability due to its multicompartmentalized structure. Thus, subtle adjustment of weak intramolecular interactions not only yields significant changes in the morphological evolution of protocell membranes but also brings new insights into the natural inevitability of biogenic small molecules.