Prebiotic porphyrin genesis: porphyrins from electric discharge in methane, ammonia, and water vapor.

Considerable evidence is emerging to show the possibility of the prebiotic genesis of biological organic compounds from the primitive atmosphere. Notable among these compounds, following the synthesis of amino acids by Miller,' are dipeptides, polypeptides, proteinoids, adenine, guanine, pentose sugars, deoxyadenosine, adenosine 5'-triphosphate (ATP), nucleotides, and fatty acids, as recently summarized by Fox,2 and Ponnamperuma and Gabel.3 A major class of compounds missing from the foregoing group is that of the porphyrins, although these were sought in the classical Miller-Urey experiments.' Compounds with absorption features consistent with those for metal complexes of porphyrins were reported by Miller but identification as porphyrins was rejected on the basis of lack of fluorescence. Porphyrins are synthesized in the laboratory principally through condensation reactions involving pyrrole compounds. Early methods for the preparation of porphine (unsubstituted porphyrin) and tetraphenylporphine employ the condensation of pyrrole with formaldehyde and benzaldehyde, respectively.4 This synthesis, particularly the step involving benzaldehyde, has been further developed in a number of interesting ways. For example, Calvin, Ball, and Aronoff6 used zinc salts to promote the condensation reaction. Similarly, Szutka et al used zinc acetate, and Szutka (1963)3 carried out the reaction with gamma radiation. Later, the reaction of pyrrole with benzaldehyde was found to be little affected by visible and ultraviolet light, but promoted to some degree by dispersing the reactants in water.8 A similar approach was used recently for the reaction between pyrrole and formaldehyde. The latter is more likely to occur in primitive environments. In these experiments simulating geochemically plausible conditions, porphyrins have been synthesized in dilute aqueous systems,9 confirming similar experiments carried out by Krashnovskii and Umrikhina.'0 In a condensation reaction, 8-aminolevulinic acid (ALA) has been used as starting material. This reaction is analogous to the biogenic pathway recently reviewed by Lascelles."I Thus ALA is condensed under alkaline conditions anaerobically to porphobilinogen, a porphyrin precursor.'2 ALA is also converted in a small yield directly to porphyrin pigments using ultraviolet irradiation.'8 The data for the ready reaction of pyrrole and formaldehyde under mild geochemical conditions increase the probability of porphyrin genesis in primitiveatmosphere conditions, since formaldehyde is known to be formed," 14, 16 and preliminary evidence in our laboratory indicates that pyrrole may be formed under such conditions.