Polycyclic aromatic hydrocarbons: Primitive pigment systems in the prebiotic environment

Polycyclic aromatic hydrocarbons (PAH) in the form of polymerized derivatives represent over 90% of the organic material of carbonaceous chondrites. It now appears likely that there was substantial survival of the organic content of meteoritic and cometary infall during late accretion, so that PAH would presumably be major components of the organic inventory present on the prebiotic Earth. An important question relative to chemical evolution and energy transduction is the nature of pigments which could be available to make light energy available to the earliest cellular forms of life. PAH and their derivatives all absorb light in the near UV and blue wavelengths, and are candidates for primitive pigments. We have explored this possibility in a model system consisting of mixtures of pyrene, fluoranthene and pyrene derivatives with hexadecane, dispersed in dilute salt solutions. Upon illumination, photochemical oxidation of the hexadecane occurs, with long-chain amphiphiles such as 2-hexadecanone and 2-hexadecanol as products. Because the reaction proceeds under strictly anaerobic conditions, the source of oxygen is apparently water. We also observed acid pH shifts during illumination. Photochemical production of hydrogen ion is significant, in that chemiosmotic proton gradients across membranes are used by all contemporary cells as a source of energy for ATP synthesis and nutrient transport. To test whether the protons could be used to transduce light energy into a useful form, PAH derivatives were included in lipid bilayer membranes (liposomes). Upon illumination, protons (or acidic products) were produced and accumulated inside the vesicles, so that substantial pH gradients were established across the membranes, acid inside. We conclude that PAH dissolved in aliphatic hydrocarbons absorb light energy and use it to oxidize the hydrocarbon to long-chain amphiphilic molecules. The oxidation is accompanied by release of protons. If PAH derivatives are included in the bilayer membrane of lipid vesicles, protons accumulate within the membrane-bounded volumes to form proton gradients. This system provides a useful model of a primitive photochemical reaction in which light energy is transduced into potentially useable forms.