JTW's Evolutionary Origins - Author: Wachtershauser, Gunter

Oxidative Pyrite Formation as an Energy Source

[pp. 91-93]

Oxidative pyrite formation
provides the high energy electrons
needed to drive the reductive fixation of carbon dioxide,
and therby is able to circumvent
the potential problem of an abscence of a reducing atmosphere on the prebiotic earth.
Pyrite formation, under anaerobic conditions,
provides both the energy and the reducing units
necessary for cabon fixation.

"Pyrite has a surface with positive surface charges,
capable of bonding not only inorganic anions
(e.g. HS^-, HCO₃^-, HPO₃^2-),
but also organic molecules with anionic groups,
notably carboxylate groups (-C00^-),
but also thiolate groups (-S^-) and phosphate groups (-OPO₃^2-)."

"The theory of a pyrite-pulled chemo-auto-origin
is based on the possibility of an oxidative pyrite formation,
with protons and carbon dioxide competing
as terminal electron acceptors."

Pyrite Formation from Iron Sulphide and Hydrogen Sulphide

FeS + HS^- --> FeS2 + H⁺ + 2e^-

[Fe(II) --> Fe(IV) + 2e^-]

Standard Free Energies of Formation

@ pH 0, 298 K

FeS + H₂S --> FeS₂ + H₂

[Fe(II) --> Fe(IV) + 2e^-]

d(G') = -38.4 kJ/mol

Theoretical Model Rxns to Show Thermodynamic Suitability of Proposed Energy

Source. I.e. For Reduction of Carbon by Oxidative Formation of Pyrite

@ pH 7, 298 K

H₂CO₃ + H₂O --> HCO₃^- + H₃O⁺

HCO₃^- + FeS + H₂S --> HCOO^- + H₂O + FeS₂

HCOO^- + FeS + H₂S + H₃O⁺ --> H₂CO + H₂O + FeS₂

HC0₃^- + 2(FeS) + 2(H₂S) --> H₂CO + 2(H₂O) + 2(FeS₂)

d(G') Rxn 2 = -37.1 kJ/mol
[Fe(II) --> Fe(IV) + 2e^-]

d(G') Rxn 3 = -15. kJ/mol
[Fe(II) --> Fe(IV) + 2e^-]

The net result is a direct linear flow of electrons from Hydrogen Sulphide to Carbon Dioxide.

• Wachtershauser, Gunter
  • Groundworks for an Evolutionary Biochemistry: The Iron-Sulphur World
  • Progress in Biophysics and Molecular Biology: Vol. 58, No. 2, pp.85-202
  • 1992
  • [Pubmed]

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