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Homochirality: models and results

This article explores the concept of homochirality and its significance in the origin of life. It discusses the importance of chiral molecules in various biological processes and presents relevant experiments and theories. The RNA world hypothesis is also examined.

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Homochirality: models and results

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  1. Homochirality:models and results Axel Brandenburg, Anja Andersen, Susanne Höfner, Martin Nilsson To appear in Orig. Life Evol. Biosph., q-bio.BM/0401036

  2. Photosynthesis Requires chlorophyll as catalyst What about chiralitry?

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

  4. (from lecture of Antoine Weiss) Racemic mixture Racemization dating method

  5. Miller/Urey experiment Wait 1 week 15 organic compounds 2% amino acids (11 different ones) racemic mixture

  6. 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

  7. 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)

  8. Contergan: was sold as racemic mixture Cures morning sickness during pregnancy causes misformations (abundaned in December 1961)

  9. 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), …

  10. Auto-catalytic effect in dead matter Alkanol with 2% e.e. Treated with carboxylaldehyde

  11. Model by Saito & Hyuga (Jan 2004) Bimodal behavior

  12. Model by Sandars (Dec 2003, OLEB) Reaction for left-handed monomers Loss term for each constituent

  13. Combined equations Loss term for each constituent

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

  15. 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)

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

  17. Birfurcation properties  Mononucleotides with wrong Chirality terminate chain growth

  18. Stability Relative perturbation of racemic solution, 10-4

  19. Dependence on fidelity Conservation law where

  20. 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

  21. Reduced equations Quantitatively close to full model

  22. Initial bias Effect in reality very weak

  23. Spatially extended model with Tuomas Multamäki Reaction-diffusion equation Proto type: Fisher’s equation Propagating front solutions wave speed

  24. 1D model (reaction-diffusion equation) Propagation into racemic environment

  25. 2D model (reaction-diffusion equation) Short run

  26. 2D model (reaction-diffusion equation) Time scale longer than for simple fronts

  27. P polymerization olymerization in 1D chain growth, Rn and Ln in different places

  28. Walter Gilbert (1986) The RNA world • Central dogma of chemistry of life • DNA  RNA  protein enzyme • all enzymes are themselves proteins?

  29. RNA itself as enzyme

  30. pre-RNA worlds Many problems: stability of sugars 2-amino ethyl glycin (AEG)

  31. pre-RNA worlds better alternative: Nelson, Levi, Miller (2000) PE Nielsen (1993)

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