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Homochirality through enantiomeric cross-inhibition

This paper explores the concept of homochirality in relation to the origin of life, focusing on enantiomeric cross-inhibition. Topics such as the preference for chiral nucleotides, sources of chirality, and relevant experiments are discussed. Through polymerization models and experiments on nucleotide chain growth and crystal formations, the significance of chirality is highlighted. Possible proposals for chirality levels and the role of catalytic properties in substrate reactions are considered. The paper concludes with insights on the polymerization model's predictions and the implications of homochirality in various environments, ranging from Earth to interstellar space.

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Homochirality through enantiomeric cross-inhibition

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  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

  2. Aminoacids in protein: left-handedSugars in DNA and RNA: right-handed carboxyl group animo group Louis Pasteur (1822-1895)

  3. 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 N C0 CH2 NH2 CH2 CH2 CH2 CH3 CH C00H Peptide nucleotide C00H C0 alanine C00H dipeptide achiral chiral glycine

  4. 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

  5. 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)

  6. 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)

  7. Model by Saito & Hyuga (2004) non-autocatalytic linearly autocatalytic nonlinearly autocatalytic nonlin+autocat. with backreaction Frank (1953) Can the right model be found by trial/error?

  8. Polymerization model of Sandars Orig. Life Evol. Biosph. (Dec 2003) Reaction for left-handed monomers Loss term for each constituent

  9. Combined equations: traveling wave Loss term for each constituent (if QL=0)

  10. Including enantiomeric cross-inhibition Loss term for each constituent Racemic solution ~21-n Stability

  11. 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)

  12. Self-catalytic effect fidelity Form of QL = QR(Ln,Rn) Possible proposals for CL(similarly for CR)

  13. Birfurcation properties exponential growth  growth rate l Degree of homochirality Red line: source Q from fragmented polymers (“waste”)

  14. Reduced equations Quantitatively close to full model Initial bias 

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

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