388548 Expanding the Goldilocks Zone: Cell Membrane Alternatives in Cryogenic Nonpolar Solvents
All known life requires the presence of liquid water. This fact has driven astronomers to search each star for planets in the "Goldilocks Zone," a narrow band in which liquid water can exist. But can there be life, centered around other solvents, at temperatures liquid water can never reach? We take an important step toward answering that question by demonstrating a new type of membrane, the "azotosome," which thrives in liquid methane at 180°C below zero. Made from simple molecules including those known to exist in the cryogenic seas of Saturn’s moon Titan, the azotosome shows the stability and mechanical properties to support life under hitherto inconceivable conditions.
Our means of studying the azotosome is molecular simulation via the well-tested Optimized Potentials for Liquid Simulations (OPLS) force field. We confirmed OPLS for this use by comparing the structures and binding energies it generates against quantum mechanics results at the M062X/aug-cc-pVDZ level of theory. We then performed molecular dynamics simulation of azotosomes made from six nitriles and four amines. Using umbrella sampling and thermodynamic integration we pulled a molecule free from each azotosome's surface and derived potential energy barriers, forces, and free energies, demonstrating resistance to dissolution, flexibility, and thermodynamic stability.
Our results show that compounds observable in the atmosphere of Saturn's moon Titan are able to form a vesicle in which life could exist in cryogenic nonpolar solvents. This increases the area of the Goldilocks Zone around a sunlike star by more than four hundred times. It also provides evidence that even in our own solar system we may not be as unique as we believed.