A new study conducted by the Earth-Life Science Institute (ELSI) at the Institute of Science Tokyo expands our understanding of one such candidate for protocells: polyester microdroplets. The research demonstrates that these structures can form under a broader range of prebiotic conditions than previously assumed, shedding light on how early Earth may have supported the emergence of life.

The study, led by PhD student Mahendran Sithamparam from the Space Science Center (ANGKASA) at the National University of Malaysia, was co-supervised by ELSI's Specially Appointed Associate Professor Tony Z. Jia and ANGKASA Research Scientist Kuhan Chandru.

Their research explored the formation of polyester microdroplets under conditions more reflective of the ancient Earth's environment. The findings indicate that these structures could assemble in salt-rich settings, at lower concentrations of alpha-hydroxy acids (aHAs), and within small reaction volumes-suggesting that protocells were likely more widespread than previously thought, forming in diverse locations such as confined rock pores and briny pools.

Polyester microdroplets were first observed in 2019 when researchers discovered that heating phenyllactic acid (PA), a type of aHA, to 80C resulted in a gel-like substance that, upon rehydration, formed membraneless droplets.

In their latest work, the team sought to determine whether these droplets could form under more diluted conditions, as might have been common on early Earth. "Many previous laboratory experiments used high concentrations and volumes of aHAs, often in the hundreds-of-millimolar or microliter range, but such conditions may not have been realistic in the prebiotic world. This study aimed to test whether protocell formation could still occur under more limited conditions," explained Jia.

The researchers reduced the initial PA concentration and volume in their experiments, finding that polyester synthesis and droplet formation were still possible with as little as 500 uL of 1 mM PA or 5 uL of 500 mM PA.

This suggests that polyester microdroplets could have naturally emerged even in environments with limited water availability, such as rock crevices or areas experiencing intermittent hydration through flooding or precipitation.

To further examine real-world conditions, the team simulated the salinity levels of ancient oceans, adding NaCl, KCl, and MgCl2 to the reaction mixtures. The results revealed that polyester microdroplets could form in solutions containing NaCl and KCl but not in those with MgCl2. This finding implies that early Earth environments rich in NaCl and KCl, but low in MgCl2, would have been more conducive to polyester protocell formation.

"Our results highlight that polyester-based protocells were likely more common than previously believed and provide critical insights for future experimental studies," noted Chandru. "A variety of primitive environments-including marine, freshwater, briny, and confined settings-may have facilitated the development of these protocells on Earth and potentially on other planets."

The research was conducted at ELSI and was made possible through the ELSI Visitor Program, which supports international collaboration. Sithamparam participated in two visits to ELSI in 2023, and graduate student Ming-Jing He from National Central University also contributed to the experiments as part of her master's thesis. The findings have been published in the ACS Bio and Med Chem Au Special Issue, 2024 Rising Stars in Biological, Medicinal, and Pharmaceutical Chemistry, where Jia is an awardee.

Research Report:Probing the Limits of Reactant Concentration and Volume in Primitive Polyphenyllactate Synthesis and Microdroplet Assembly Processes

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