Polycyclic aromatic hydrocarbons: primitive pigment systems in the prebiotic environment

Area of Science:

• Astrobiology and Geochemistry
• Origin of Life studies
• Biochemistry and Biophysics

Background:

• Polycyclic aromatic hydrocarbons (PAHs) constitute over 90% of organic material in carbonaceous chondrites.
• PAHs are likely major components of the prebiotic Earth's organic inventory due to survival during late accretion.
• The nature of primitive pigments for early light energy transduction is a key question in chemical evolution.

Purpose of the Study:

• To investigate the potential of polycyclic aromatic hydrocarbons (PAHs) and their derivatives as primitive pigments for early life.
• To explore the photochemical reactions driven by PAHs under simulated prebiotic conditions.
• To model the transduction of light energy into chemical energy via proton gradients.

Main Methods:

• Model system: mixtures of pyrene, fluoranthene, and their derivatives with hexadecane in dilute salt solutions.
• Illumination of the model system to observe photochemical oxidation of hexadecane and pH changes.
• Incorporation of PAH derivatives into lipid bilayer membranes (liposomes) to assess proton gradient formation.

Main Results:

• Photochemical oxidation of hexadecane to long-chain amphiphiles (2-hexadecanone, 2-hexadecanol) occurred under anaerobic conditions, with oxygen from water.
• Illumination induced acid pH shifts, indicating photochemical production of hydrogen ions.
• PAH derivatives in liposomes accumulated protons inside vesicles, establishing significant pH gradients upon illumination.

Conclusions:

• PAHs dissolved in hydrocarbons absorb light energy, oxidizing the hydrocarbon and releasing protons.
• PAH derivatives within lipid vesicles can form proton gradients, modeling primitive light energy transduction.
• This system serves as a model for early photochemical reactions converting light energy into usable forms.