44 | New Scientist | 18 April 2020
between wrong-handed molecules would
be akin to an awkward handshake between
a left and a right hand.
The question of how life became
homochiral, and why it continued that way,
has never been met with a satisfactory answer.
Pasteur sought clues in magnetism and light
before concluding he was on something of a
fool’s errand. More recently, some chemists
have invoked cosmic influences. They have
proposed that the initial bias could have
been imprinted onto organic molecules
by exposure to light in space, before they
hurtled to Earth’s surface as a meteorite.
This starlight idea relies on light also having
a property akin to handedness: its waves can
spiral clockwise or anti-clockwise as they
travel. The idea is that in regions of space where
the balance tips towards photons with one
twist, light could have transferred the same
skew onto organic starter molecules as it
fell on them. “This is what we use as a chiral
trigger,” says Uwe Meierhenrich, a chemist
at the University of Nice Sophia Antipolis in
France. The hypothesis has been bolstered
by experiments at the synchrotron particle
accelerator in Paris, where Meierhenrich and
his colleagues successfully imprinted the
asymmetry in photons onto amino acids
in simulated comet ice.
The team had ambitions for a real-world trial
with the European Space Agency’s Rosetta
mission, which sent a probe to comet 67P/
Churyumov-Gerasimenko in 2014. The plan
was to detect molecules and check for any bias
in their handedness. But the rover botched
its landing, settling the wrong way up so itmolecular “chirality” – from the Greek for
“handedness” – in 1848. Working with tartaric
acid crystals, he saw that some were mirror
images of each other. He sorted them into
left and right-handed crystals. And when
he shined polarised light through them, the
light emerged rotating in opposite directions.
“There is no doubt,” he wrote, “that there is a
grouping of the atoms of an asymmetric type
that is not superposable on its mirror image.”
Not all molecules are chiral but most of the
more complex ones are (see “Curious chirality”,
page 46). We now know the types of molecules
that make life possible tend to be exclusively
right- or left-handed: the sugars that form the
basis of RNA and DNA are always right-handed,
for instance, whereas the amino acids that
make proteins are only left-handed in nature.
We have also learned that this
“homochirality” has an essential role in
helping molecules to recognise one another.
When amino acids string together to make
proteins, say, or molecules need to recognise
each other to react or pass on a message within
a cell, their handedness determines how their
shapes fit together. In that sense, a meetingOn the one hand...
The molecules of life have a strange asymmetry
- and we finally have a grasp on why,
finds Hayley Bennett
L
IFE can be strange. Just look at narwhals,
and those stick insects that resemble
leaves on legs. Or consider the cockeyed
squid, with its bizarrely mismatched peepers:
one yellow and huge, the other tiny and blue.
And yet almost nothing about life is as baffling
as the lopsidedness at its core.
All biological molecules have an inherent
“handedness”: they can exist in two mirror-
image forms, just like your left and right
hands. But for each type of molecule it uses,
life on Earth prefers a single form. So much so,
in fact, that their opposite numbers are rarely
seen in living things.
How did life’s building blocks end up single-
handed? The short answer is we don’t know.
Some people have suggested that something
happened in space to seed the predilection for
left or right-handed molecules; others reckon
it happened in shallow prebiotic pools where
life is sometimes thought to have begun on
Earth. Now, one researcher is claiming to have
uncovered the first hints of a more convincing
answer – one that could explain not only
what broke life’s mirror in the first place,
but also give a richer understanding of why
the preference for one form of molecules has
persisted over billions of years of evolution.
The answer comes not from deep space or
deep time, but from the quantum nature of
matter. And if the latest discoveries are
anything to go by, its unexpected influence
on the building blocks of life could solve the
mirror mystery once and for all by revealing in
fine detail why some of the most fundamental
processes in biology work so beautifully.
French chemist Louis Pasteur discovered >“ Was life’s bias initially
imprinted onto organic
molecules in space,
before they hurtled to
Earth’s surface?”
Features