Homochirality through enantiomeric crossinhibition

1
Homochirality through enantiomeric
cross-inhibition

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

To appear in Orig. Life Evol. Biosph.,
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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PNA world prior to RNA world
Nielsen (1993) Nelson, Levi, Miller (2000)
NH2
NH2
NH
CH2
CH2
CH2
carboxyl group
C00H
C0
CH2
PE Nielsen (1993)
amino group
NH2
NH
Base
CH2
C0
N
NH2
CH2
CH2
CH2
CH
CH3
C00H
Peptide nucleotide
C00H
C0
C00H
alanine
dipeptide
achiral
chiral
glycine
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Chirality and origin of life

• Life plausible with left/right handed
  nucleotides
• Origin of life possibly achiral (e.g. PNA world)
• chiral nucleotides preferred structurally more
  stable
• Source of chirality
• Polarized light, electroweak interaction
• auto-catalytic (enzymatic) reactions during
  polymerization
• ? chirality as a consequence of life

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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)
? enantiomeric cross-inhibition
guanine
cytosine
Joyce, et al. (1984)
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Relevant experiments crystals
Crystal growth, many different nucleation sites
racemic mixture
Crystal growth with stirring primary nucleation
suppressed
Kondepudi et al. (1990)
? competition important
Alkanol with 2 e.e. treated with carboxylaldehyde
? autocatalytic self-amplification Frank (1953),
Goldanskii Kuzmin (1989),
Soai et al. (1995)
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Model by Saito Hyuga (2004)
non-autocatalytic
linearly autocatalytic
nonlinearly autocatalytic
nonlinautocat. with backreaction
Frank (1953)
Can the right model be found by trial/error?
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Polymerization model of Sandars
Orig. Life Evol. Biosph. (Dec 2003)
Reaction for left-handed monomers
Loss term for each constituent
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Combined equations traveling wave
Loss term for each constituent
(if QL0)
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Including enantiomeric cross-inhibition
Loss term for each constituent
Racemic solution 21-n
Stability
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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
fidelity
Form of QL QR(Ln,Rn)
Possible proposals for CL (similarly for CR)
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Birfurcation properties
exponential growth ? growth rate l
Degree of homochirality
Red line source Q from fragmented polymers
(waste)
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Reduced equations
Quantitatively close to full model
Initial bias ?
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Conclusions

• Polymerization model
• Based on measurable processes
• Predicts wavelike chromatograms (HPLC)
• Reduction to accurate simplified model
• Homochirality in space (earth, interstellar, etc)