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. BoydLawrence Livermore National Laboratory

and

Toshitaka Kajino and Takashi Onaka

OMEG InstituteNovember 18, 2011

OutlineThis 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 AcidsAmino 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 left-circularly-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 handedLevrorotary—left handedEnantiomer—a chemical with a preferred chirality L D Racemic—equal parts D & LEnantiomeric excess = ee = (D - L)/(D + L)x100%

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 MeteoriteOn 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 MeteoriteFrom 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 - ICore-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.

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)

Neutrino spectrum (Gava et al.) for two assumptions of processing

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.

Since B will also align molecular angular momenta via the molecular magnetic moment, the anti-neutrinospin will also be aligned therewith on one side and antialigned on the other.

Neutron StarBB

A Little Nuclear PhysicsElectron 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.

When the νe spin and the 14N spin (=1 Ћ) are aligned, their total spin must be ½ + 1 = 3/2. When they are anti-aligned, the total spin can be either 3/2 or ½; the fractions are well defined ( ½, 2/3 of time).

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.

0.16 0.00

2.31

3.95

5.14

2mec2

14N

14O

14C100%

99.3%

0.06%

0.61%

β-decay scheme for mass 14 nuclei. Energies are in MeV.

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 14NHow 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 x

(N) (odd under time reversal, parity even) to induce a molecular electric dipole moment y

(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 chiralityHowever, 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):

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.

No Magnetic Field

Magnetic Field, LH Molecules

Magnetic Field, RH Molecules

m = +3/2

m = +1/2

m = -1/2

m = -3/2

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?• 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 11 12 13 14 log r (cm)

Solar radius

Radius of Type Ib or Ic progenitor

Red Giant radius range

Maximum processing distance

Earth orbit

lo

g

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 right-handed 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 (k1

L) A+B MD (k1D)

A+B+ML 2ML (k2L/k-2

L) A+B+MD 2MD (k2D)/k-2

D)

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 GalaxyHaving 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, wind- blown 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 StatisticsEarthly 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 othernon-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.

Supernovae synthesize C, N, O, etc.

Racemic mixture of amino acids forms in grains and meteoroids in Supernova nebulae (same number of and )

Neutrinos from other (massive) Supernovae convert racemic to enantiomeric mixture via selec-tive destruction of one chirality of 14N-based molecules

“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 mole-cules via more amplification when they arrive on planets

To Summarize the SNAAP Model:

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)