One of the problems with trying to research the hypothetical ‘primordial soup’ from which all life may have emerged is how to separate the elements which led to the first living cells being formed. Now, a research team has found that evaporation could facilitate compartmentalisation of the relevant biochemical compounds and may provide an environment within which early evolution could have begun. Long-term collaborators Professor Anderson Shum from the Department of Mechanical Engineering and Professor Julian Tanner from the School of Biomedical Sciences were working on another project when the discovery came to light. “The aim of the original project was to develop new ways to identify nucleic acid sequence (often known as aptamers) that has selected properties such as for RNA imaging,” said Professor Shum. “Then in the process, our group observed the phenomenon induced by evaporation from droplets that we observed.” “When I put a droplet containing polyethylene glycol (PEG) and dextran under the microscope, an exciting phenomenon appeared,” explained Wei Guo, PhD student in Professor Shum’s group. “During the evaporation, hundreds of immiscible tiny daughter droplets emerged from the initially homogeneous, clear droplet. The decrease of the water content leads to an increase in polymer concentrations, resulting in phase separation that drives this fascinating evaporation-driven phenomenon.” “We discovered that the nucleation of new immiscible daughter droplets in the original droplets can be used for enrichment of nucleic acids,” added Professor Shum. “Our two teams discussed the phenomenon and we came up with the idea of using the system as a model and analogy for studying prebiotic compartmentalisation in droplets.” The work has been published in Nature Communications, in an article entitled ‘Non-associative phase separation in an evaporating droplet as a model for prebiotic compartmentalisation’. Potential mechanism Previously, scientists had hypothesised that liquid-liquid phase separation (LLPS), a means by which membraneless organelles form inside cells, might work as a potential mechanism for prebiotic compartmentalisation. But creating a suitable ‘primordial soup’ in laboratory settings had proved elusive, particularly since the concentration of ingredients involved in the LLPS needs to be adequate for the onset of phase separation. “Basically, if you add two water soluble additives, such as salt and sugar, to water, at low concentrations, you only get one homogenous aqueous solution containing both salt and sugar,” explained Professor Shum. “However, at high enough concentrations of salt and sugar, you can actually form two immiscible aqueous phases, one rich in salt, and the other one rich in sugar. “Such a phenomenon is known as aqueous liquid-liquid phase separation. This type of phase separation process depends a lot on factors, such as pH values, temperatures and salt concentrations. Once you have the aqueous daughter droplets in the larger aqueous droplet, the daughter droplets can preferentially enrich other macromolecules that prefer the daughter phase and are more soluble in it.” Enriched molecules Significantly, the research team found that through the evaporation of droplets, LLPS can be triggered significantly and the molecules arising from it are subsequently enriched. When evaporation occurs, the water content in a droplet decreases, leading to an increase in the polymer concentration, and it is this dissolution of the bigger droplet into smaller ones that could be used for further research into living cells. “The work can be used for enriching small molecules in the aqueous environment,” said Professor Shum. “This can be applied to disease biomarkers, enrichment of which will facilitate their detection and disease diagnosis.” He also welcomes feedback from other scientists on their discovery. “Science discoveries always lead to even more exciting questions,” he said. “We continue to explore collaborations with different partners, who may find uses with our approach to do work on enrichment of molecules. “We are also involved in a collaboration where we are using this approach for the enrichment of small particles that can exhibit structural colours as characteristics of the separation distance between the particles. This can have potential applications in optics.” The image shows the evaporation-induced phase separation process inside an all-aqueous sessile droplet (Scale bar: 500 microns). Upon evaporation, the concentration of Polyethylene glycol (PEG) and dextran increases and incompatibility arises, forming tiny dextran-rich droplets (green fluorescently labelled) dispersed in the continuous PEG-rich phase. These tiny dextran-rich droplets move towards the centre of the sessile droplet with the inward Marangoni flow. Compartmentalisation and localisation of biopolymers like nucleic acids (red fluorescently labelled) inside these dextran-rich droplets are achieved, with great potential in serving as all-aqueous reactors for a wide range of biochemical reactions. (From left) Professor Anderson Shum and Wei Guo from the Department of Mechanical Engineering, and Dr Andrew B Kinghorn and Professor Julian Tanner from the School of Biomedical Sciences. PROFESSOR ANDERSON SHUM We discovered that the nucleation of new immiscible daughter droplets in the original droplets can be used for enrichment of nucleic acids. Our two teams discussed the phenomenon and we came up with the idea of using the system as a model and analogy for studying prebiotic compartmentalisation in droplets. EVAPORATION REVELATION A collaborative study between mechanical engineering and biomedical sciences has found evaporation may have played an important role in the origins of life and could provide a new path of exploration into how living things are formed. 34 RESEARCH The University of Hong Kong Bulletin | May 2022 35
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