Origin of homochirality in an early peptide world

1
Origin of homochirality in an early peptide world

• Axel Brandenburg, Harry Lehto and Kirsi Lehto
  (Nordita)

Some advantages of peptide world Plasson et al.
model is non-autocatalytic Is this true? How
come? Connection with earlier work Plasson model
is auto-catalytic in some sense!
Int. J. Astrobio. 3, 209 (2004), 4, 75
(2005), Orig. Life Evol. Biosph. 35, 225 (2005),
35, 507 (2005)
2
Producing enantiomeric excess (e.e.)

• Life only develops in homochiral environment
• Isotatic chains more stable
• Homochirality is a prerequisite
• Amino acids in dead organisms lose e.e.
• However no mechanism produces 100 e.e.
• only minute partial excess possible (electroweak)
• slightly bigger for large Z, goes like Z5 ? Cu
  (II) ions
• Still need amplification mechanism
• Frank (1953) model (plus extensions) nucleotides
• Autocatalysis and enantiomeric cross inhibition
• Plasson, Bersini, Commeyras (2004) peptides

3
Chirality selection during polymerizationof the
first replicating molecule?
lipid world?
Peptide world, PNA world
dual world
RNA world
achiral
RNA DNA
proteins
RNA
chiral
Rasmussen et al (2003 Artif. Life 9, 269-316)
Isotactic polymer (same chirality)
R
Monomers to dimers
waste (enantiomeric cross-inhibition)
L
4
RNA versus peptide worlds
peptide world
RNA world

• Activation D ? D, L ? L
• Polymerization DD? DD, LL ? LL
• Epimerization DL ? LL, LD ? DD
• Depolymerization DD ? DD, LL ? LL

• Template-directed (PCR-like)
• Enantiomeric cross-inhibition
• Autocatalytic production of new mono-nucleotides
  (? but uncertain)

APED model (Plasson et al.)
5
Recap of Frank-type model (Frank 1953)
auto-catalysis alone
kinetic equation
exponential growth
enantiomeric excess
catalyst
6
Need mutual antagonism
add cross-inhibition
kinetic equation
unspecific antagonism
7
APED model (Plasson et al. 2004)
a activation p polymer. a inhibition h
depolym.
e epimerization on N-terminal DL?LL not DL? DD
8
APED model equations
Consider limit of large h and e
9
APED model equations
limit of large h and e
corresponding reaction
? D catalyzes conversion of both D and L into D
10
Reduced APED model
a one way round-about
For nearly racemic state
11
Exponential evolution
For nearly racemic state
12
Conclusions

• Plasson et al. model is simple
• No explicit autocatalysis necessary
• Autocatalytic (autoinductive) effects can be
  identified
• Experiments now necessary

13
Initial bias not necessary

• Exponential amplification
• Initial excess can be virtually zero
• Initial bias may not change this
• depends on strength
• Second genesis other chirality?
• would be cool
• but is not necessary

Bifurcation diagram
with Andersen, Höfner, Nilsson Orig. Life Evol.
Biosph. 35, 225 (2005)
fidelity