Homochirality models and results
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Homochirality: models and results. Axel Brandenburg, Anja Andersen, Susanne H öfner , Martin Nilsson. To appear in Orig. Life Evol. Biosph., q-bio.BM/0401036. Photosynthesis. Requires chlorophyll as catalyst What about chiralitry?.

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Homochirality: models and results

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Homochirality:models and results

Axel Brandenburg, Anja Andersen, Susanne Höfner, Martin Nilsson

To appear in Orig. Life Evol. Biosph., q-bio.BM/0401036


Photosynthesis

Requires chlorophyll

as catalyst

What about chiralitry?


Aminoacids in protein: left-handedSugars in DNA and RNA: right-handed

carboxyl

group

animo

group

Louis Pasteur

(1822-1895)


(from lecture of

Antoine Weiss)

Racemic mixture

Racemization

dating method


Miller/Urey experiment

Wait 1 week

15 organic compounds

2% amino acids (11 different ones)

racemic mixture


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


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)


Contergan: was sold as racemic mixture

Cures morning sickness

during pregnancy

causes misformations

(abundaned in December 1961)


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), …


Auto-catalytic effect in dead matter

Alkanol with 2% e.e.

Treated with

carboxylaldehyde


Model by Saito & Hyuga (Jan 2004)

Bimodal behavior


Model by Sandars (Dec 2003, OLEB)

Reaction for left-handed monomers

Loss term for each constituent


Combined equations

Loss term for each constituent


Including enantiomeric cross-inhibition

Loss term for each constituent

Racemic solution ~21-n


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)


Self-catalytic effect

Form of QL = QR(Ln,Rn)

Possible proposals for CL(similarly for CR)


Birfurcation properties

 Mononucleotides with wrong

Chirality terminate chain growth


Stability

Relative perturbation of

racemic solution, 10-4


Dependence on fidelity

Conservation law

where


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


Reduced equations

Quantitatively close to full model


Initial bias

Effect in reality very weak


Spatially extended model

with Tuomas Multamäki

Reaction-diffusion equation

Proto type: Fisher’s equation

Propagating front solutions

wave speed


1D model (reaction-diffusion equation)

Propagation into racemic environment


2D model (reaction-diffusion equation)

Short run


2D model (reaction-diffusion equation)

Time scale longer than for simple fronts


P polymerization olymerization in 1D

chain growth, Rn and Ln in different places


Walter Gilbert

(1986)

The RNA world

  • Central dogma of chemistry of life

    • DNA  RNA  protein

      enzyme

  • all enzymes are themselves proteins?


RNA itself as enzyme


pre-RNA worlds

Many problems: stability of sugars

2-amino ethyl glycin

(AEG)


pre-RNA worlds

better alternative: Nelson, Levi, Miller (2000)

PE Nielsen (1993)


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