Report for the Joint Use/Research of the Institute for Planetary Materials,
Okayama University
2022 fiscal year first term
1/18/2023
Category:

General Joint Research

Name of the research project: Composition of “Extraterrestrial” Polyester Gel Microdroplets
Principal applicant: Tony Z. Jia
Affiliated institution and department: Earth-Life Science Institute, Tokyo Institute of Technology
Collaborator
Name: (1) Chen Chen, Ruiqin Yi, Rehana Afrin; (2) Tak Kunihiro, Eizo Nakamura, Katsura
Kobayashi, Christian Potiszil
Affiliated institution and department: (1) Earth-Life Science Institute, Tokyo Institute ofTechnology; (2) IPM
Research report:
We first subjected a series of alpha hydroxyacids to dehydration synthesis to form polyesters, whichwere then subsequently screened for microdroplet assembly (forming protocells). These included
pure phenyllactic acid (PA), and mixtures of PA with 4-amino-2-hydroxybutyric acid (4a2h) ormalic acid (Malic). We then devised a novel ICP-MS analytical approach and applied it to polyester
microdroplets, to determine the cation distribution within them following incubation with four
different chloride salts (NaCl, KCl, MgCl2 and CaCl2). We thus quantified the salt ion concentration
inside and outside of the droplets after incubation with the different salts as a measure of salt uptake.
ICP-MS was able to detect each of the four cations of the chloride salts (Na+, K+, Mg2+ and Ca2+),
but was not able to detect chloride anions. We observed that in all cases, cations were detected
within the droplets, suggesting that protocells uptake all salt cations. For all cases, theconcentrations of salt cations detected within the droplets as compared to the supernatant were alldifferent, suggesting that the salt uptake process is likely selective and differential.
K+ was generally uptaken by the droplets to the greatest degree compared to the other cations, withMg2+ uptaken the least. The relative amount of cations in solution uptaken was greater at lower
overall salt concentrations; at low concentrations (1 mM), K+ could be concentrated within thedroplets. Finally, we observed that poly(PA) microdroplets tend to uptake salt ions with differential
affinity and in order of decreasing affinity of (K+ > Ca2+ > Na+ > Mg2+). The exact mechanism of
this difference is not clear, and may be due to a number of factors such as differences in salt cation

structure, size and/or affinity with poly(PA). However, the order of decreasing affinity of cation
uptake matches roughly with the order of decreasing ionic radius (K+ > Ca2+ = Na+ > Mg2+). If
we consider just the divalent ions, Mg2+ is more strongly hydrated and with a larger hydration shellthan Ca2+, resulting in the formation of a larger hydration sphere around Mg2+ compared to Ca2+.
The larger hydration sphere could potentially result in a weakening of the interaction between
Mg2+ with the apolar polyester droplet (compared to Ca2+) due to inhibition of interaction of thecation interior with the negatively charged polyester microdroplet surface, resulting in lower Mg2+
uptake compared to Ca2+. When comparing just the monovalent ions, Na+ and K+ also follow asimilar relative trend as Mg2+ and Ca2+, respectively (i.e., the hydration shell of Na+ is larger thanthat of K+), which could modulate the relative propensity of Na+ and K+ to be uptaken.
If there are indeed some prebiotic evolutionary advantages to selective incorporation of K+ over theother salts tested, such as for generation or maintenance of protocell boundary chemical potential,
then polyester microdroplets could have been able to utilize its ability to selectively uptake K+ tosupport some type of primitive type of evolution. We make this conjecture considering that
intracellular K+ concentration is much higher than that of Na+, Mg2+ and Ca2+ in plant and animalcells, likely due to the action of potassium channels or the potassium-sodium pump, respectively.
For instance, plants maintain a relatively high K+ (100–200 mM) and low Na+ concentration (1–10mM) in their cytosol; animals also maintain a high K+ (~150 mM) and low Na+ concentration (~15mM). Additionally, the modern ocean contains far more Na+ than K+, while the ancient ocean also
likely contained much more Na+ than K+. Polyester microdroplets in ancient oceans which could
selectively incorporate low-abundance K+, while simultaneously excluding highly abundant Na+,
could have been one mechanism to achieve a local microenvironment containing a K+/Na+ ratio
more on par with modern biology.
These studies form the basis of a manuscript currently in preparation, to be submitted within thisfiscal year, with all of the listed collaborators (and another IPM staff member, Chie Sakaguchi, who
was instrumental in developing the ICP-MS technique) as co-authors; after publication, we will
inform you of the article details. We plan to continue this work in the second term of FY2023, likely
through another general joint research proposal.