Water
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Earth is teeming with life — and water makes
it all possible. But elsewhere in the cosmos,
life might be built from different chemicals that
dissolve and assemble in some other liquid: perhaps
methane, kerosene, or even chloroform. For now,
it’s not feasible for humans to set foot on those
worlds and see what’s there, but researchers are
exploring some possibilities in labs here on Earth.
The idea of a strange parallel form of life, whose
cells do the same basic things as ours using a
completely different chemistry, isn’t new to sci-
ence. Isaac Asimov first broached the subject in
his 1962 essay “Not as We Know It: The Chemistry
of Life.” And in 2004, the same year the Cassini
spacecraft entered Saturn’s orbit, biochemist Steven
Benner proposed in a paper in Current Opinion in
Biological Chemistry that on a world like Saturn’s
moon Titan, life might use liquid hydrocarbons as a
solvent (a liquid that can dissolve other substances),
the way water is used on Earth.
With new exoplanets joining the roster of known
worlds every day, it’s likely that some of them have
oceans (or at least warm puddles) of hexane, ethers,
chloroform, or other exotic liquids that might serve
as the basis for life as we don’t yet know it.
GATHERING THE PIECES
In such alien oceans, the chemistry of life on Earth
just wouldn’t work. Water is a polar molecule; its
oxygen end has a slight negative charge, while its
hydrogen end has a slight positive charge. Those
charges affect the kinds of chemical bonds that
can happen in water. The structure of molecules
like DNA and proteins depends on water’s polar
hydrogen bonds.
Most hydrocarbons (compounds made of hydro-
gen and carbon, such as methane and ethane) are
nonpolar — there’s no charge at either end of the
molecule. So it’s impossible to form the same kinds
of bonds in these chemicals as in water. That’s why
if you want to create life in Titan’s methane lakes,
you’re going to have use a different set of building
blocks altogether.
Chemists and biologists from across the United
States — led by organic chemist Paul Bracher at
Saint Louis University and funded by a three-year
grant from the National Science Foundation —
have formed a team to explore what the building
blocks of truly alien life might be made of. Using
computer simulations and hands-on lab work,
they’re exploring how molecules bond in liquid
hydrocarbons such as hexane, ethers, and chloro-
form. Their work falls right on the border between
sciences, where chemistry becomes biology.
“It’s like trying to build a car in your backyard out
of lawn mower parts, versus having the Maserati
factory build a supercar. Life as we know it is the
supercar, and we are trying to hack together some-
thing that looks like it, out of a different set of parts,
to see what we can learn about putting it together,”
says Chris Butch of the Earth Life Science Institute.
Butch, a computational chemist, will use digital
simulations to help understand the details of the
chemistry his colleagues will observe in the lab.
So which parts do you need to scavenge from
your metaphorical lawn mower to build some-
thing that looks like a Maserati? The two most
important components you need to build a cell
are a molecule that can carry information, like
DNA or RNA, and some combination of mol-
ecules that can form a membrane. The team’s
goal is to see what other molecules might interact
in ways that mimic the basic processes of life,
but using different machinery. To accomplish
this, they’re using shorter, simpler versions of the
complex polymers that come together to carry
out the chemical processes of life. Their experi-
ments include coaxing the building blocks of life
as we know it into oil “membranes,” modifying
the structure of DNA and RNA, and creating
their own genetic molecules from scratch.
THE BOUNDARIES OF LIFE
Picture an alien sea, where droplets of hydrocar-
bons float suspended in water. “As long as there
are waves crashing on a shore, or some kind
of weather, then if oil floats to the top, it’s
going to be consistently mixed into the
water phase in a droplet form,” says
Connecticut State University bio-
chemist Sarah Maurer, whose area
of interest is cell membranes and
containers.
If you want life, you’ve got to con-
tain all the chemistry that happens
in a cell, separating it from the outside
environment. In the cells of every living
thing on Earth, oily membranes provide that
container. Membranes play an important role in
powering the cell’s functions; they’re involved in
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JULY/AUGUST 2019. DISCOVER 81