Photoelectron circular
dichroism on gas phase
alanine: a possible
photophysical process
at the origin of life’s
homochirality?
Gas phase pure enantiomers of alanine,
the simplest chiral amino-acid, have been
photoionized by VUV circularly-polarized
light (CPL) at the Lyman
α
wavelength,
giving rise to a measured large asymmetry
(4%) of the electron angular distribution,
and because of momentum conservation,
of the corresponding parent alanine ion.
This asymmetric photophysical process
could be linked to the origin of life’s
homochirality, the fact that only L-amino
acids, the chiral building blocks of proteins,
are found in the biosphere.
Within a bottom/up approach
of biomolecular complexity, the gas
phase study of elementary sub-units
of key macro-biomolecules allows one
to probe, in a solvent-free environment,
electronic and structural characteristics
that will be involved in the formation
of larger molecular structures. Amino
acids, the building blocks of proteins,
which are also chiral may provide a clear
dichroic signature when probed, here
for the first time, via PhotoElectron Circular
Dichroism (PECD), a chiroptical, orbital-
specific and conformer-sensitive effect
giving rise to large forward/backward
asymmetries in the photoelectron angular
distribution upon photoionization
by circularly polarized light (CPL).
Two complementary vaporization
techniques (resistive heating followed
by supersonic expansion and aerosol
thermodesorption) were used in order
to produce gas phase pure enantiomers
of alanine, a fragile thermolabile
biomolecule (Figure
➊
). They were then
photoionized by the VUV CPL from DESIRS
and the corresponding electrons and
ions detected by the DELICIOUS3 double
imaging e
-
/ion coincidence spectrometer.
Electron imaging measurements made
at the Lyman
α
radiation photon energy
(10.2 eV), the most intense radiation
in the interstellar medium, revealed
a strong overall asymmetry for the
outermost orbital (Figure
➋
), of 4 %
as obtained by filtering on the alanine
parent ion. Despite the presence
of different conformers, this electron
angular distribution asymmetry appears
independent of sample temperature
(and hence of conformer population)
and may have some linked with
the origin of life’s asymmetry.
Indeed, since its discovery by Pasteur
more than 150 years ago, the origin
of biomolecular asymmetry remains
a puzzling mystery, which traces back
probably to the origin of life itself. Amino
acids were discovered in carbonaceous
meteorites, with enantiomeric excesses
(e.e) and isotopic composition indicating
an extra-terrestrial origin. Were there
to be an interstellar origin of elementary
building blocks of life, one should look
for an asymmetric bias applied before
this organic matter was delivered on our
planet, inducing a significant e.e, which
was probably a necessary condition
for the development of life on Earth.
Among the deterministic scenarios are
those based upon asymmetric photon-
induced processes involving CPL, for
which astronomical sources have indeed
been reported. Several asymmetric
photochemical
processes were proposed
and simulated in the condensed matter,
by using UV CPL, leading to significant e.e,
in the few % range.
Our findings on alanine asymmetric
electron emission suggest that PECD
could be an alternative
photophysical
asymmetric process, acting on gas
phase amino-acids in the interstellar
medium. Indeed, because of momentum
conservation, the recoil motion
of the corresponding produced chiral
ions should exhibit an opposite
asymmetric flux, leading to a significant
and spatially resolved e.e (up to 4 %
for alanine) in a given line of sight.
PECD could therefore produce
an enantio-enriched gas phase ion cloud
of a given handedness, separating from
its counterpart (Figure
➌
), which would
then be captured, neutralized and
embedded into comets and meteorites
seeding Earth with an exogenous organic
matter presenting an initial e.e. It might
therefore, in combination with other
processes, have triggered the appearance
of biomolecular asymmetry and
consequently could have assisted
the emergence of life.
ATOMIC AND MOLECULAR PHYSICS, DILUTE MATTER, UNIVERSE SCIENCE
88
SOLEIL
HIGHLIGHTS
2013
