Self-organization of tetrapyrrole constituents to give a photoactive protocell

A tandem combinatorial reaction of four acyclic, colorless compounds (two alpha-aminoketones and two diones; one polar and one nonpolar for each) in aqueous solution (30 mM each, 60 degrees C, pH 7, 24 h) containing lipid vesicles and with or without quinones and solar illumination affords a distribution of up to 538 porphyrins in 1.6-3.9% overall yield. The reactions leading to the porphyrins can occur in either or both the aqueous phase and the hydrophobic membrane of the vesicles. The porphyrins encompass a broad range of polarity and partition in the aqueous-lipid medium. Two fractions obtained by size-exclusion chromatography include porphyrins associated with the lipid vesicles and porphyrins in the aqueous phase. The porphyrins in both phases are photoactive as demonstrated by fluorescence quantum yield measurements (Phi(f) similar to 0.07-0.08) and by the Krasnovsky reaction (a photosynthetic-like process). The constituents of the Krasnovsky reaction employed here are methyl red, ascorbic acid, and 2,6-dichlorophenolindophenol. Illumination of the two porphyrin-containing samples (aqueous phase or vesicles) in the presence of the Krasnovsky constituents results in the reduction of methyl red and oxidation of ascorbic acid. Thus, the overall process transforms a colorless aqueous suspension via four stages (two combinatorial reactions, oxidation, physical partitioning) to photoactive porphyrins in distinct venues. The process may provide a model for the origin of pigments that enable proto-photosynthesis.