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Stardust, Supernovae, and the Molecules of Life
(Why are the amino acids all left-handed,
and what does that tell us about their origin?)
Richard N. Boyd
Lawrence Livermore National Laboratory
and
Toshitaka Kajino and Takashi Onaka
OMEG Institute
November 18, 2011
Outline
This talk covers a lot of different areas of science, so it will be
pretty basic on all of them!
• A little bit of history of the subject.
• What are amino acids and why do we care about them?
• What is chirality and how does this affect the amino acids?
• Why/how do we think complex molecules are formed in the
interstellar medium?
• What are core-collapse supernovae, and how might they affect
the amino acids formed in space?
• Magnetic fields
• Neutrinos
• A little nuclear physics
• A model for molecular chiral selection
• Amplification of amino acids formed in space and spreading
them throughout the Galaxy
• Conclusions
Past Explanations of Amino Acid Origin/Chirality
•
•
Pasteur first discovered a “demarcation line” between life and nonlife: the
“mirror dissymmetry of organisms.”
Miller-Urey: Amino acids were produced by lightning in an appropriate
environment on Earth (didn’t know about chirality)
– Then they’re made chiral by circularly polarized light from the Sun (difficult)
•
Panspermia hypothesis—”seeds” of life have an
extraterrestrial origin
– Seeds have always been here—revise big bang
scenario (Not likely!)
– Seeds brought to Earth by an advanced civilization
•
Chiral amino acids produced by circularly polarized
uv light in space?
– This is the currently favored model, but must destroy many molecules to create
chirality, will make both LH and RH molecules, etc.
•
Weak interactions
– -β-decays of 14C would produce chiral Bremsstrahlung, which would selectively
destroy molecules of one chirality (Effects are very small: ~10-12)
– -Cline; neutrinos from Sne on 1H could do the processing (Effects are even smaller)
– -Boyd, Kajino, Onaka; SN neutrinos selectively destroy 14N, which is coupled to
molecular chirality, so this destroys one chirality, thus selecting the other.
– Supernova Neutrino Amino Acid Processing Model: SNAAP Model
Amino Acids
Amino acids are organic molecules
containing an amino group (NH2 ), a
carboxylic acid group (COOH), and any of
various side groups. Amino acids are
critical to life, as they are the building
blocks of proteins, which are linear chains
of amino acids. The proteins are created
inside the cells from the existing bath of
amino acids according to instructions
contained in our DNA.
Alanine & Valine, two
naturally occurring
amino acids.
Humans need ~20 amino acids to make all the proteins we require;
we can produce half; the other half has to be supplied by our food.
And there are many more amino acids besides these 20, more than
200 are known; most do not naturally exist on Earth.
Chirality (handedness)
An object or a system is chiral if it cannot be superimposed on its mirror
image. Chirality of a material can also be defined by the way in which it
affects circularly polarized light; the effect will be different for leftcircularly-polarized light than it will for right-circularly-polarized light.
The amino acids that appear in nature are left-handed, although if
produced in the laboratory they are equally likely to be right-handed.
This is curious!
Some definitions:
Dextrorotary—right handed
Levrorotary—left handed
Enantiomer—a chemical with a preferred
chirality
L
Racemic—equal parts D & L
Enantiomeric excess = ee = (D - L)/(D + L)x100%
D
Basic Question: Where Did the Amino Acids
Come From?
This is a question of the origin of life: where did the
molecules of life originate?
Were they produced in some
Earthly cauldron? (Miller-Urey
experiment)
Or were they brought here by a
cosmic stork? (Panspermia
hypothesis)
And are the molecules of life on Earth the same as
those in other parts of the Universe?
Are there molecules of life elsewhere in the Universe?!
Murchison Meteorite
On 28 September 1969 at about 10:58 AM,
near the town of Murchison, Victoria,
Australia, a bright fireball was observed
to separate into three fragments before
disappearing. Many specimens were
found over an area larger than 13 km²,
with individual masses up to 7 kg. The
total collected mass exceeded 100 kg.
The Murchison Meteorite contained more
than 70 amino acids; they ARE made in
outer space. Panspermia lives! (sort of,
anyway)
MM amino acids are either left-handed,
racemic, or non-chiral. How/why did
they get that way?
But were the samples contaminated?
A Murchison meteorite specimen
at the National Museum of Natural
History (Washington)
Chirality of the Amino Acids-Murchison Meteorite
From Cronin and Pizzarello, Science Magazine, 14 Feb., 1997
Structure of 2-a-2,3-dmpa, a not naturally
occurring amino acid. It has two chiral centers
&, thus, four stereoisomers: the D & L forms of
-methylisoleucine & -methylalloisoleucine.
These gave ee’s = ~7%
A) Isovaline, B) α-methylnorvaline,
C) α-amino-n-butyric acid, D)
norvaline. A and B gave ee’s 8.4%
(& 18%; Glavin and Dworkin!) &
2.8%, but C & D (“unmethylated
versions” of A & B) gave zero. A
does not occur naturally, & B has
“restricted distribution”.
Original MM analyses gave ee’s of naturally occurring amino acids,
but were those just Earthly? Cronin and Pizzarallo showed that at
least some nonzero ee’s must have been produced in outer space.
Other Meteorites?
Another meteorite: Orgueil. It also had amino acids; Dworkin and
Glavin found an ee for L-isovaline in Orgueil of 15%. Also Tagish Lake,
Murray, and Allende—same result.
Some Questions
• Might amino acids be produced in cosmic dust grains?
Bernstein et al. (2002) showed that (non-chirally selected) amino
acids (glycine, alanine, serine) could be produced in the lab via uv
photolysis of interstellar ice analogs (H2O, NH3, CH3OH, HCN). Chiral
selection could come later. This class of experiments was summarized
by Allamandola (2008).
• Why are some meteoritic amino acid ee’s = 0?
Amino acids might have gotten thermalized during their transit
through the Earth’s atmosphere, or made racemic when they hit the
Earth; some molecules would be more heat resistant than others.
And, of course, the 10% ees might have been much higher before the
meteorites hit the Earth’s atmosphere!
Running Summary
• So amino acids are definitely made in outer space, and
they have the correct chirality, which is not so easily
selected if the amino acids are made on Earth
Cas A; Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI;
Infrared: NASA/JPL-Caltech/Steward/O.Krause et al.
Core-collapse Supernovae, magnetic fields, and neutrinos - I
Core-collapse Supernovae?
Massive stars (M > 8 solar masses) will
complete all their stages of evolution
and then collapse to a black hole or
neutron star. They expel nearly all of
their core energy (resulting from
gravity) by emitting 1057 (1053 ergs of)
neutrinos in ~10 seconds.
Neutrino spectrum (Gava et al.)
for two assumptions of processing
Some neutrino properties:
• Neutrinos are nearly massless particles, have zero charge, and
interact through the weak interaction.
• There are 3 flavors: electron, muon, and tau, and each has its
corresponding antiparticle.
• Because they have such small masses, they are relativistic, so their
spin (1/2)ħ will either align or anti-align with their momentum:
• Electron neutrinos have negative “helicity” (anti-aligned)
• Electron antineutrinos have positive “helicity” (aligned)
Core-collapse Supernovae, magnetic fields, and neutrinos - II
The (dipolar) magnetic field B from a
collapsing neutron star or black hole
can be very strong—more than 1014
G at the surface of a neutron star
(radius = 10 km)
On one side of the neutron star B
and the anti-neutrino spin will be
aligned, but on the other side
they’ll be antialigned.
B
Neutron Star
B
Since B will also align molecular
angular momenta via the molecular
magnetic moment, the anti-neutrino
spin will also be aligned therewith on one side and antialigned on the
other.
A Little Nuclear Physics
Electron neutrinos (νe) and antineutrinos (νe) interact with 14N:
νe + 14N → e+ + 14C, Q = - 1.18 MeV (G-T transition)
νe + 14N → e- + 14O, Q = - 5.14 MeV (G-T transition)
The first reaction (0.16 MeV + 2mec2) is more likely to go than
the second, given the Eυ’s (~12 MeV). Also, σ ~ (Eυ – |Q|)2,
enhancing the effect.
5.14
3.95
0.06%
2mec2
14O
99.3%
2.31
0.61%
When the νe spin and the 14N spin (=1 Ћ) are aligned,
0.16 100% 0.00
their total spin must be ½ + 1 = 3/2. When they are
14C
14N
anti-aligned, the total spin can be either 3/2 or ½; the
β-decay scheme for mass 14
fractions are well defined ( ½, 2/3 of time).
nuclei. Energies are in MeV.
But 14C is a spin zero nucleus, so the final state total spin will be ½ (= ½ + 0).
Total angular momentum must be conserved: one additional unit of orbital angular
momentum transfer is required (from the νe or e+ wave function) in the aligned
case, but none in the anti-aligned case. This inhibits that transition by ~ x10.
Thus the transition will be less probable for the aligned case, so destruction of 14N,
hence the molecule to which it is attached, is more likely for the anti-aligned case.
Chiral Molecules and 14N
How does this affect molecules?
The molecules will have some total angular momentum that will
interact with the magnetic field. Buckingham has developed a model
(for NMR) for these molecules that applies also to our case.
Chiral selectivity requires a net odd parity operator; time reversal
symmetry allows a rotating nuclear magnetic moment mx(N) (odd
under time reversal, parity even) to induce a molecular electric dipole
moment my(M) (even under time reversal, parity odd), in an external
Bz; the resulting effect is opposite for D and L enantiomers.
THIS REQUIRES A NON-ZERO SPIN NUCLEUS.
For example, assume the total angular momentum
of electron antineutrino + 14N is 3ħ/2, which will
have the projections along Bz as shown.
In thermal equilibrium, and with Bz = 0, these four
levels will be equally populated.
m/ħ
3/2
1/2
-1/2
-3/2
Neutrinos, magnetic fields, and molecular chirality
However, the effect of the Buckingham effect is to drive the populations away from
equality, as suggested in the figure below (where the width of the line indicates
the relative population):
m = +3/2
m = +1/2
m = -1/2
m = -3/2
No
Magnetic
Field
Magnetic
Field, LH
Molecules
Magnetic
Field, RH
Molecules
This will do a chiral selection at each throat; the
SNL that aligns with Sν at the LHS will let more
NL than NR live there. But the exact opposite
effect occurs at the RHS; there NR prevails by
the same amount, if nothing else happens.
But (Horowitz &Li, Arras & Lai, Lai & Qian, and
Maruyama et al.): B affects the neutrino absorption
cross sections, so that the neutrino luminosity, hence the ee, might be 20-30%
higher at one neutron star throat than at the other. This WILL give an overall ee.
The 14N’s that live will give their molecules a preferred chirality.
Running Summary
• So amino acids are definitely made in outer space, and
they have the correct chirality, which is not so easily
selected if the amino acids are made on Earth
• Combination of magnetic field and electron antineutrinos
from core-collapse supernovae, together with the spin
alignment of 14N, would select a preferred amino acid
chirality
What About all those Supernova Photons?
But wouldn’t the photons from the supernovae destroy all the chiral
molecules just processed by the neutrinos? There aren’t as many
of them as there are neutrinos, but their interaction cross sections
are >>> than those of the neutrinos.
If the end state of the supernova is a black hole, the collapse to the
black hole might result before any of the photons could escape
(since it takes them ~hours to get out). The neutrinos only need
~seconds to get out! And black holes aren’t rare; they occur a
reasonable fraction (~20%) of the time.
And the progenitor star can’t be too large; a red
giant (progenitor of core collapse supernovae)
would encompass the entire region that
might be processed. So progenitors need
to be “Wolf-Rayet” stars, which shed their
H and/or He shells, creating clouds, and
ultimately produce Type Ib or Ic supernovae
(which DO produce black holes).
How Large a Region Gets Processed by the Neutron Star?
-6 -4 -2
Maximum processing
distance
-12 -10 -8
log sn Fn
• Assume N-star’s magnetic field orients out to radius at which it is
equal to the ambient galactic magnetic field, ~10-6 gauss.
• Assuming it’s 1014 gauss at N-star’s surface (10 km), that’s 5x1012 cm.
• Also assume that the processed volume has to be appreciably larger
than the N-star’s progenitor:
• The Sun’s radius is 7x1010 cm
• A Red Giant’s radius is 1.4 to 5.6x1013 cm
• But massive star (Type Ib or Ic) supernovae (which collapse to
black holes) have progenitors with radii < than that of the Sun.
• For reference, Earth’s orbit is 1.5x1013 cm.
10
Radius of
Type Ib or Ic
progenitor
11
Solar
radius
12
log r (cm)
13
Earth
orbit
14
Red Giant
radius range
Running Summary
• So amino acids are definitely made in outer space, and
they have the correct chirality, which is not so easily
selected if the amino acids are made on Earth
• Combination of magnetic field and electron antineutrinos
from core-collapse supernovae, together with the spin
alignment of 14N, would select a preferred amino acid
chirality
• A volume surrounding a Wolf-Rayet star does exist in
which molecular processing (of meteoroids) would occur,
and the meteoroids wouldn’t be destroyed
Might the SNAAP Model Produce RH Molecules?
We ignored the possibility of νe+14N → 14O+e-; this can still occur. And it
would have the same spin alignment features that the νe+14N → 14C+e+
reaction had, i.e., selective destruction, but this would favor righthanded amino acids! However, it occurs in the same space as the other
reaction, so could not produce net RH molecules.
However, RH molecules would be produced preferentially at the “other”
throat of the neutron star. So if the two regions were not well mixed,
there could be pockets of RH molecules. But this seems highly unlikely.
So this is an interesting prediction of the SNAAP model! But we clearly need
better statistics from meteoroids, or comets (HAYABUSA might provide a
test; ROSETTA should provide a crucial test!).
So far everything seems LH, but that’s only 5 out of 5! Or perhaps it’s 6 out
of 6, if we add in the seven billion humanoids (we may only count for 1
datum!)
Mostly Likely Competing Model: Chirality from Circularly
Polarized Light—in Outer Space—1
• Experiments have shown that CP light can produce enantiomerism
• CP light has been detected from outer space, usually at the <1% level. Also
from the Sun. Both sources are multiply scattered light.
• CP light might chirally select amino acids in outer space, then have them
transported to Earth via meteorites
• Cross sections for this process are >>> than those for the neutrinos
• Bailey model (1):
–
–
–
–
Amino acid chirality established from CPL on (small) dust grains in reflection nebulae
Polarization can be high (Gledhill & McCall); scattering from aligned grains can give > 50%
After ee is established, grains clump, then agglomerate to form meteoroids
This would be the scenario if life in the Universe is common
• Critique:
– Processing small dust grains, then letting them agglomerate into larger entities gets
around problem of producing chirality throughout the volume of objects large enough to
get through planetary atmospheres
– Timing is very tricky; amino acids in such environments may only live several hundred
years, processing to large ee may take thousands of years, and grains may form too
rapidly or not rapidly enough
Mostly Likely Competing Model: Chirality from Circularly
Polarized Light—in Outer Space—2
• Bailey model (2):
– Amino acid chirality could be established from CPL from a magnetized white dwarf
from matter accreting onto the poles
– This has high polarization—as high as 50% has been observed
– But this is a rare event; it might prevail if life in the Universe is rare
• Critique:
– Same problem as for model #1 for timing of the relevant processes for getting
processed amino acids to planetary surfaces.
• For both models, CP light has to destroy most of the molecules to create
any ee—100% polarized CPL gets 10% ee after destroying 99.6% of amino
acids
• And CP light would produce both LH and RH molecules, although in
different places. But if we ultimately see both LH and RH amino acids,
this model wins.
To Circumvent the RH here, LH there Issue …
Since the only data
we have, probably
for a long time, are
from the solar
system, this might
provide a plausible
explanation. But
questions remain
unanswered.
We need a
hyperbolic comet!
Creation/Amplification of Chirality in the Interstellar Medium—I
The ee’s produced in any existing model are really small.
In the SNAPP model, the volume each SN would process times the number
of SNe occurring in the lifetime of a “giant molecular cloud”, in which
complex molecules are thought to be made, is <<< the size of the cloud.
But once created, the molecules will replicate, possibly on warmed (more
than 20 K) dust grains that exist in the clouds or on comets.
Gol’danskii and Kuz’min: autocatalysis; two effects could contribute:
- “Advantage,” e.g., via circularly polarized light, weak interaction
(or supernova neutrinos!) to initiate ee.
-Kinetic, i.e., through reactions, originally due (sort of) to Frank
(1953) (where the k’s indicate the reaction rates):
A+B ML (k1L)
A+B MD (k1D)
A+B+ML
2ML (k2L/k-2L) A+B+MD
2MD (k2D)/k-2D)
ML+MD A’ (ks)
These reactions probably could not produce ee’s via thermal fluctuations;
the result depends on the relationship between the different rates. But
these can definitely amplify ee’s initiated by some other mechanism.
Spreading Chirality Throughout the Galaxy
Having established left-handed amino
acids in blotches within a giant
molecular cloud, might this get
propagated throughout the Galaxy?
Probably—there are several processes
by which this can occur. They include
“many types of astronomical sources,
including planetary nebulae, windblown bubbles, supernova remnants,
starburst superwinds, and the intercluster medium” (from Pittard
(http://adsabs.harvard.edu/abs/2007dmsf.book..245P)
Some indication of the mixing time for this is given by the rotation time of
the Milky Way; it is thought to have rotated several times during its ~12
Gy lifetime.
Poor Statistics
Earthly amino acids are all LH, but perhaps
all of them should be LH if one is?
The only extensive extra-terrestrial samples
of amino acids are from the Meteorites.
But those are either zero or LH.
Another possible test: Hayabusa. It returned
to Earth in 2010 with samples from asteroid
Itokawa; they’re being analyzed.
Then, ROSETTA will sample amino acids on
comet 67P/Churyumov-Gerasimenko in
2014.
Hayabusa and Rosetta will test these
models; only LH amino acids would
support the SNAAP model.
A mixture of LH and RH amino acids would support the circularly
polarized light model, although all LH wouldn’t rule it out.
So far, the SNAAP model looks good, but on limited statistics!
When/How Did Life on Earth Begin?
• Once slightly chiral species got to a planet, the above experiments
suggest they could be driven to homochirality in whatever chirality had
the edge before they got to the planet. And there were probably lots of
meteoroids that got to Earth, or any other planet, so the chirality that
dominated in outer space would win every time.
• The Moon, and therefore Earth, was bombarded by tremendously
intense meteor showers until 3.8 billion years ago. (Jupiter swept the
inner solar system clean)
• And life is thought to have begun on Earth shortly after that. Is that a
coincidence??
How Were the Other Molecules of Life Made—DNA/RNA?
• One possibility: Once the amino acids got to Earth (via meteorites)
they could have made peptides, which may be able to evolve into
nucleobases, the constituents of DNA and RNA.
• Another possibility: the nucleobases were also made in space and
transported to earth by meteorites.
• Callahan et al. (2011) searched for nucleobases in meteorites, and
found adenine and guanine (mostly Murchison), two of the five
bases (also cytosine and thimine/uracil). They also found other
non-naturally occurring chemicals
that are made via the same
processes that make nucleobases.
• Earlier studies also claimed
detections of nucleobases in MM,
but samples may have been
contaminated.
• Martins et al. (2008) was probably
also correct; they found uracil in
MM samples.
So all the basic ingredients for life
appear to be made in outer space.
To Summarize the SNAAP Model:
Neutrinos from other (massive)
Supernovae convert racemic to
enantiomeric mixture via selective destruction of one chirality of
14N-based molecules
Racemic mixture of amino acids
forms in grains and meteoroids
in Supernova nebulae (same
number of
and )
Supernovae
synthesize
C, N, O, etc.
“Rapid” chemical evolution
amplifies the enantiomerism as
the material in the molecular
clouds “slowly” expands to fill
the galaxy (more
than )
Subsequent generations of
stars form, along with planets
and biological forms, and
evolve to homochiral molecules via more amplification
when they arrive on planets
So our origins may lie in outer space!
Supernovae,
Neutrinos
and the
Chirality of
Amino Acids,
R.N. Boyd,
T. Kajino, and
T. Onaka,
Int. J. Mol. Sci.
12, 3432-3444
(2011)