Topics in Astrobiology

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Topics in Astrobiology

1. What is and can be done
2. The problem of chirality

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The range of problems

• From dust to planets
• Exoplanets
• Water delivery
• Habitable worlds
• Archaen world
• Volcanos, Carbon cycle
• Impacts, panspermia
• Miller experiment
• Amino acids, chemistry

• Artificial life
• Origin of life
• RNA world, TNA, PNA
• homochirality
• Early life
• Extremophiles
• cyanobacteria

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Rough time line

• 104 yr 106 yr 108 yr 109
  yr

All gas gone Remaining dust settles
planetesimals
Sun ignites
First life
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Time line (later)
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Carbon cycle
Source of the atmosphere
Venus, Earth active volcanos thick atmosph.
Mars Volcanos extinct
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Water deliveryvia comets
Comet LINEAR same HDO ratio as on Earths oceans
cf. Hale-Bopp (too much)
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Evaporationof LINEARduring fly-by.Cloud
ofhydrogen
Obsered in Ly a
Estimated water content 3x109 kg (outgassed
during fly-by)
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Miller/Urey experiment
Wait 1 week
15 organic compounds 2 amino acids (11 different
ones)
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The RNA world
Walter Gilbert (1986)

• Central dogma of chemistry of life
• DNA ? RNA ? protein
• enzyme
• all enzymes are themselves proteins?

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RNA itself as enzyme
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pre-RNA worlds
Many problems stability of sugars
2-amino ethyl glycin (AEG)
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pre-RNA worlds
better alternative Nelson, Levi, Miller (2000)
PE Nielsen (1993)
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Photosynthesis
Requires chlorophyll As catalyst What about
chiralitry?
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Homochiralitymodels and results

• Axel Brandenburg, Anja Andersen, Susanne Höfner,
  Martin Nilsson

To appear in OLEB, q-bio.BM/0401036
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Aminoacids in protein left-handedSugars in DNA
and RNA right-handed
carboxyl group
animo group
Louis Pasteur (1822-1895)
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Racemic mixture
(from lecture of Antoine Weiss)
Racemization ?dating method
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Chirality and origin of life

• Dead stuff not chiral, so is chirality
• with racemic mixture structure fragile
• Importance
• Prerequisite of life (provides curvature and
  twist)
• Consequence of life (enzymatic reactions)
• Miller-Urey amino acids both chiralities
• in Murchison meteorite mostly left-handed!
• but contamination is debated
• Reasons discussed circularly polarized light,
  beta-decay (weak force), homochiral template

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Relevant experiments nucleotides
? Mononucleotides with wrong Chirality terminate
chain growth
ok
poisoned
template-directed oligomerization poly (CD) ?
oligo (GD)
(using HPLC)
guanine
cytosine
Joyce, et al. (incl. Orgel) (1984)
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Contergan was sold as racemic mixture
Cures morning sickness during pregnancy
causes misformations
(abundaned in December 1961)
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Relevant experiments crystals
Crystal growth, many different nucleation sites
racemic mixture
Crystal growth with stirring primary nucleation
suppressed
Autocatalytic self-amplification? Frank (1953),
Goldanskii Kuzmin (1989),
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Model by Saito Hyuga (Jan 2004)
Bimodal behavior
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Model by Sandars (Dec 2003, OLEB)
Reaction for left-handed monomers
Loss term for each constituent
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Combined equations
Loss term for each constituent
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Including enantiomeric cross-inhibition
Loss term for each constituent
Racemic solution 21-n
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Coupling to substrate S
Source of L1 monomers QL
QL comes from substrate acts as a sink of S S
sustained by source Q
Catalytic properties of substrate (depending on
how much L and R one has) ? QL QR(Ln,Rn)
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Self-catalytic effect
Form of QL QR(Ln,Rn)
Possible proposals for CL (similarly for CR)
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Birfurcation properties
? Mononucleotides with wrong Chirality terminate
chain growth
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Stability
Relative perturbation of racemic solution, 10-4
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Conservation law
Dependence on fidelity
where
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Differences to Sandars

• Coupling to substrate here proportional to EL
• in Sandars LN
• Outer boundary condition here open
• in Sandars prescribed damping term
• Future extensions
• Chain braking
• add spatial 3D dynamics

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Reduced equations
Quantitatively close to full model
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Initial bias
Effect in reality very weak
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Conclusions

• There are many astrobiology topics where
  astrophysicists can help out with their
  knowledge, as well as technical and computational
  tools
• Homochirality is a hot one with several papers
  this topic in each issue of OLEB