Prebiotic Pyrite Chemistry Molecular Scaffold & Catalyst 1 21

The Prebiotic Chemistry of Pyrite: Molecular Scaffold & Catalyst ABIOL 570 November 21, 2004

Introduction
Protocellular Scaffold
Framboidal Pyrite
A self-organizing system
Synthesized with or without oxygen
Physical properties of astrobiological significance
Catalysis
Adsorption of 5’-AMP onto FeS 2
Modulation by acetate
Readily synthesized under prebiotic conditions
Common component of metabolic pathways
Simulated prebiotic environment
5’-AMP inihibitor (DMF)
Interactions on mineral surface

Phase Separation
Adsorption on a surface
Mineral-H 2 O
Air-H 2 O
FeS 2 -H 2 O
Trapping in a container
Oil droplets
Proteinoids
Amphiphile vesicles
http://tycho.bgsu.edu/~laird/ast305/class/IVC-5.html

Framboidal Pyrite
Self-organization: “the autonomous passage of a system from an unpatterned to a patterned state without the intervention of an external template”.

Framboidal Pyrite
Closely-packed, spheroidal clusters of 100-100,000 pyrite microcrystals
May be synthesized in 1 of 2 ways:
FeS (ppt) + S (aq)  Fe 3 S 4  FeS 2 (Low [O 2 ])
Greigite = magnetic thiospinel; formation determines rxn rate in the presence of oxygen
FeS (mk) + H 2 S (aq)  FeS 2(py) + H 2(aq) (no O 2 )
Most rapid rxn

Framboidal Pyrite
Forms instantaneously in anoxic sediments
Texture is result of rapid nucleation where pyrite is supersaturated
Normal saturation: single crystals form

A porous, catalytic scaffold…
Fatty Acid vesicles can be forced to divide by extrusion through porous substances…
Liposomes range in size from 50-60 nm up to Giant Vesicles of 30-100 um
Framboidal or weathered FeS 2 fits the bill!

The First “Membranes”
Spontaneously form proteinoid microspheres (electrostatic interactions)
Able to take up molecules & have electrical potentials across “membranes”
Respond to changes in osmotic pressure
http://www.biologie.uni-hamburg.de/b-online/e41/3.htm

Fatty Acid Vesicles
Phosphates + Glycerol + Fatty Acids
 Phospholipids
Clumped together
Phospholipid Bilayers
 Liposomes
Acquire many different solutes while drying
Preferred size in range of living cells!
www.bio.davidson.edu/Courses/Molbio/MolStudents/.../Favorite_Molecular_Tool.html

Creation of Protocells
Microsphere can pick up an ything … even liposomes
ATP + nucleotides  oligonucleotides (inside ingested liposome)
Began to base pair with itself?
Liposomes + hollow proteins  “membrane” pores
http://www.stc.uniroma2.it/cfmacro/cfmacroindex.htm

Primitive Protocell Metabolism
Precursors needed to maintain “membranes”
Proteins, lipids & carbohydrates
First chemoorganotroph (popular; simple metabolism)
Protocells died when starved, became toxic, got too big or in wrong environment
Some grew faster than others, made products that facilitated growth, etc.
http://www.funhousefilms.com/sciencpg.htm

Origin of Heredity & Metabolism
RNA most likely not first genetic system
TNA suggested but linking bases, sugars & PO 4 ’s has not been demonstrated
Mineral catalysis?
http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=189

A T NA World?
(L)- α –threofuranosyl-(3 1  2 1 ) oligonucleotide
Threose is the sugar
Simplest nucleic acid alternative
Possible ancestor of RNA
Possible protector/regulator of RNA (binds to it)
Forms base pairs
G = C & T/U=A
Informational in anti-parallel
Cross-pairs with RNA & DNA
A & T nucleobase analogs:
2’-amino-(2’-NH2 TNA)
3’-amino-(3’-NH2 TNA)
Bst PolI, bacteriophage T7 DNA Pol (exo-) & MMLV-RT
Chaput, J.C., Ichida, J.K & Szostak, J.W. (2002) DNA Polymerase-Mediated DNA Synthesis on a TNA Template . J. AM. CHEM. SOC. 125, 856-857.

A T NA World?
Easily forms hairpins
much more stable to hydrolytic cleavage than are RNAs and may be as stable as DNAs
TNA strands can be synthesized by template-controlled ligation with either complementary TNA or RNA strands as templates
corresponding formation of RNA sequences by ligation on a TNA template does also occur, although with less efficiency
http://www.scripps.edu/research/sr2001/chm03.html

Mineral Catalysis… remember the framboidal pyrite ?
A multifunctional surface!
Implicated in:
Reverse Citric Acid Cycle
LPS of bacteria in bioleaching
CO 2 fixation (+ H 2 S)
Purine can adsorb to uncharged sites on FeS 2 surface!

Pyrite Catalysis
Bases arranged in planar arrangement
Adsorbed purines (attached by van der Waals interactions) may have paired with pyrimidines (H-bonding)
Enclosure by vesicles act as reaction vessels
Wachtershauser: 2-D surface ↑ organization

Pyrite Catalysis
FeS+ H 2 S  FeS 2
Reducing power that could convert CO 2  C-containing metabolites
Directly to CO 2 failed but successful from CO (Stetter et al.)
CO 2 + FeS + 2 H 2 S  FeS 2 + 2 H 2 O + C
FeS needs higher reductive power to fix CO 2
Possible with additional energy input

Pyrite Catalysis
Fe implicated in e- transfer
Light-driven generation of H 2 gas
Oxidative/Reductive rxns catalyzed by Fe-S minerals (FeS 2 )
Adoption of Fe-S clusters in:
Ferredoxines
N-fixing enzymes
Many other cofactors
Solubilization of FeS 2 by Cys  dissolved chemical energy
Semiconducting Properties
Adsorption Constant of α = 6E5 cm -1 for h v >1.8 eV)
High quantum efficiencies (up to 90% of adsorpbed photons generate e- hole pairs in the sulfide)

A tantalizing possibility!
The first life form may have been photosynthetic! You’re kidding, right?
http://www.bact.wisc.edu/bact330/lecturestaph

Earliest Photosynthesizers
Anaerobic environment
1 st photosynthesizers used H 2 or H 2 S as substrates
Microbes still do this (H 2 S  H 2 + S)
Purple & green sulfur bacteria
http://bio.winona.msus.edu/bates/Bio241/images/figure-08-12-2.jpg

Questions?

Prebiotic Pyrite Chemistry Molecular Scaffold & Catalyst 1 21

by Heather Jordan

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