Fatty-acid liposomes compartmentalize inside a clay vesicle. Credit: Anand Bala Subramaniam.
A team of applied physicists at Harvard’s School of Engineering
and Applied Sciences (SEAS), Princeton, and
Brandeis have demonstrated the formation of semipermeable vesicles from
The research, published online in the
journal Soft Matter,
shows that clay vesicles provide an ideal container for the
compartmentalization of complex organic molecules.
The authors say the discovery opens the
possibility that primitive cells might have formed inside inorganic clay
“A lot of work, dating back several
decades, explores the role of air bubbles in concentrating molecules and
nanoparticles to allow interesting chemistry to occur,” says lead author Anand
Bala Subramaniam, a doctoral candidate at SEAS.
“We have now provided a complete
physical mechanism for the transition from a two-phase clay–air bubble system,
which precludes any aqueous-phase chemistry, to a single aqueous-phase clay
vesicle system,” Subramaniam says, “creating a semipermeable vesicle
from materials that are readily available in the environment.”
“Clay-armored bubbles” form
naturally when platelike particles of montmorillonite collect on the outer
surface of air bubbles under water.
When the clay bubbles come into contact
with simple organic liquids like ethanol and methanol, which have a lower
surface tension than water, the liquid wets the overlapping plates. As the
inner surface of the clay shell becomes wet, the disturbed air bubble inside
The authors’ schematic of clay vesicle formation, showing a cut-away view of the clay shell and dissolving bubble at the top, and a view of the water-air interface at the bottom. Credit: Anand Bala Subramaniam.
resulting clay vesicle is a strong, spherical shell that creates a physical
boundary between the water inside and the water outside. The translucent,
cell-like vesicles are robust enough to protect their contents in a dynamic,
aquatic environment such as the ocean.
pores in the vesicle walls create a semipermeable membrane that allows chemical
building blocks to enter the “cell,” while preventing larger structures from
Scientists have studied montmorillonite,
an abundant clay, for hundreds of years, and the mineral is known to serve as a
chemical catalyst, encouraging lipids to form membranes and single nucleotides
to join into strands of RNA.
Because liposomes and RNA would have been
essential precursors to primordial life, Subramaniam and his coauthors suggest
that the pores in the clay vesicles could do double duty as both selective
entry points and catalytic sites.
This SEM image shows the exterior surface of a clay vesicle. Credit: Anand Bala Subramaniam.
“The conclusion here is that small fatty
acid molecules go in and self-assemble into larger structures, and then they
can’t come out,” says principal investigator Howard A. Stone, the Dixon
Professor in Mechanical and Aerospace Engineering at Princeton,
and a former Harvard faculty member. “If there is a benefit to being protected
in a clay vesicle, this is a natural way to favor and select for molecules that
Future research will explore the physical
interactions between the platelike clay particles, and between the liquids and
the clay. The researchers are also interested to see whether these clay
vesicles can, indeed, be found in the natural environment today.
“Whether clay vesicles played a
significant role in the origins of life is of course unknown,” says
Subramaniam, “but the fact that they are so robust, along with the
well-known catalytic properties of clay, suggests that they may have had some
part to play.”
Subramaniam and Stone’s coauthors include
Jiandi Wan, of Princeton Univ., and Arvind Gopinath, of Brandeis Univ.