Hexane
HydrogenHydrogenCarbonHydrogenJULY/AUGUST 2019. DISCOVER 83
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negatively charged molecules are less
likely to interact with oil than with water,
so our familiar genetic molecules may not
move into the oil droplets very readily.
However, other molecules might pair
with the nucleic acids and help sneak
them across the boundary. Biochemical
companies have developed molecules
called transection agents, which help
move nucleic acids through membranes,
and one of the team’s goals is to look for
combinations of DNA and transection
agents that can migrate into the hydrocar-
bon droplets and remain stable.
“That’s basically saying, ‘Can we tease
our biology into functioning at some
level in these strange environments?’ ”
says Georgia Tech molecular biologist
Loren Williams, who is especially inter-
ested in how polymers behave in liquid
chloroform.
The team also wants to swap
out the nucleic acids’ phos-
phorus for silicon, cre-
ating a molecule
that dissolves
more easily in
hydrocarbons.
They also plan
to test an alterna-
tive genetic molecule
using so-called “non-canonical
nucleotides,” substituting other chemicals
for the familiar adenine, thymine, gua-
nine, and cytosine.
And then there are the really exotic
ideas. Bracher’s group at Saint Louis
University is working on a completely
synthetic molecule that will work like
DNA but be made of completely different
molecules that form a different type of
chemical bond. Instead of the hydrogen
bonds that link DNA base pairs, Bracher’s
version would use base pairs that share
molecules called thioesters.
There’s reason to think it could
work. In 2015, Benner, who founded
the Foundation for Applied Molecular
Evolution in 2001 and is not involved
in Bracher’s project, tested a version of
DNA with an ether backbone in a solvent
of kerosene. He found that this combina-
tion wouldn’t work to form life on a place
astrophysics, plans to simulate that
process on the team’s array of molecules.
In this case, selection will be based on
whether or not a molecule, or set of mol-
ecules, can move back and forth across the
barrier between oil and water. The results
will tell the team which types of molecular
structures or bonds are best suited to the
simulated environment. “That’s sort of the
stuff of natural selection,” says Travisano.
Maurer is interested in whether the
molecules will be able to support basic
chemical reactions inside the hydrocar-
bon droplets. The base pairs in DNA,
for example, “recognize” each other by
finding which base “fits” well enough to
form a bond with another — adenine
to thymine and cytosine to guanine. If
alternative versions of those molecules
can recognize each other and form bonds
in other solvents, that’s an encouraging
sign that basic biochemistry could be
possible in alien seas.
Of course, such a system is not even
close to a working cell, and it would still
be missing some important building
blocks for life. In cells on Earth, proteins
nearly as cold as Titan, but on some so-
called “warm Titan” exoplanets, it might
be a good option.
“Given how many exoplanets we’re
finding around distant stars, chances
are there are going to be other worlds
like Titan that could have something
interesting going on,” says Bracher.
PLAYING WITH THE
BUILDING BLOCKS
From there, one of the next steps is to
see how the building blocks of alien life
might evolve. Chemistry is subject to
natural selection: Systems and structures
that are better at replicating themselves
tend to outcompete others. That process
may be how molecular systems evolve
toward greater complexity, eventually
producing the specialized systems of
molecules and reactions that form cells.
Michael Travisano, an evolution-
ary biologist with a background in
Ice made of hydrocarbons, rather than water,
floats on the surface of a lake on Titan in this
artist’s concept. Using Cassini data, scientists
have confirmed the presence of ethane in lakes
on the cloudy moon, which contains the only
known surface liquid in the solar system, aside
from Earth.