A group of researchers exploring the possible mechanisms behind spontaneous cell formation on the early Earth have used small angle X-ray scattering at Diamond to further understand the processes involved, and develop a new model to help explain how life might have begun on Earth.
Many scenarios have been proposed for processes leading to the formation of the protocells which are a pre-requisite to life on the early Earth. One theory is based on fatty acid vesicles, which form when naturally occurring fatty acids self-assemble in water to form a bilayer membrane. The hydrophobic bilayer separates water in the centre of the vesicle from the main aqueous solution. For the protocell to function, small molecules and macromolecules (for example proteins) must be transported from the aqueous solution into the centre of the cell and remain encapsulated. Cells are extremely complex systems and for life to form there are high demands for a chemically rich, highly structured environment in the centre of the cell.
An alternative theory, proposed over 50 years ago, suggests that protocell formation may occur when organic chemicals and naturally occurring polymers, present in the aqueous solution, spontaneously phase separate to form membrane-free, chemically rich liquid microdroplets called coacervates.
Combining these two approaches, researchers from the University of Bristol Centre for Protolife Research and Chemistry Department of Imperial College London have examined a hybrid protocell model based on the spontaneous self-assembly of a fatty acid membrane on coacervate microdroplets.
The team used small angle X-ray scattering experiments on the Small Angle X-ray Scattering & Diffraction beamline (I22) at Diamond, combined with Fluorescence Lifetime Imaging, to explore the microstructure of the coated coacervate droplets when prepared using a range of different chemical compositions. The small angle X-ray scattering (SAXS) experiments showed that the fatty acid has assembled at the surface of the coacervate droplets forming highly organised multilayers. No multilamellar structures were seen in SAXS patterns at low fatty acid concentrations. The very high intense X-rays available at Diamond coupled with I22’s extremely sensitive detector were necessary to determine the size and structure of the microcompartments, with some key features of interest between 4 and 6nm in size.
(a) Graphic of a lipid-coated coacervate droplet showing a molecularly crowded interior and fatty acid bilayer membrane. (b) Fluorescence lifetime imaging microscopy image of a fatty acid-coated coacervate droplet containing kiton red showing two different lifetimes and viscosities in the centre (coacervate matrix) and edge regions (lipid membrane); scale bar = 5 µm (c) Small angle X-ray scattering profile showing Bragg reflections corresponding to a multilamellar fatty acid membrane at the surface of the coacervate droplets.
The team also used confocal fluorescence microscopy to look at how positively and negatively charged dyes (and a range of other chemicals) were transported across the protocell membrane and into the microcompartments. The results were consistent with the presence of a semipermeable, negatively charged barrier at the surface of the microdroplets and indicated that concentration gradients, essential for cell function, could be established across the fatty acid membrane. These properties are essential for the uptake or exclusion of specific chemicals related to the function of the cell.
The work at Diamond along with other studies allowed us to characterise the structure of fatty acid multilayers on the surface of polymer-containing microdroplets. The work brought together alternative ideas on how cells might have formed on the early earth and will be the foundation for new developments in protolife research. The research highlights a breakthrough in bottom-up synthesis of artificial cells for a range of technologies.
Dr T. -Y. Dora Tang, Centre for Protolife Research, University of Bristol
The researchers have shown that their new hybrid model for the structure of the protocell is a plausible hypothesis for the origins of life and hope that this research will provide the groundwork for further developments in the design, construction, function and evolutionary potential of model protocells to better understand the how life on Earth began
Tang T.-Y. D, Hak C. R. C., Thompson A.J., Kuimova M. K., Williams D.S., Perriman A. W., Mann S. Fatty acid membrane assembly on coacervate microdroplets as a step towards a hybrid protocell model. Nature Chemistry 6, 527-533 (2014). DOI:10.1038/NCHEM.1921
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