WAS FERROCYANIDE A PREBIOTIC REAGENT

Hydrogen cyanide is the starting material for a diverse array of prebi otic syntheses, including those of amino acids and purines. Hydrogen c yanide also reacts with ferrous ions to give ferrocyanide, and so it i s possible that ferrocyanide was common in the early ocean. This can o nly be true if the hydrogen cyanide concentration was high enough and the rate of reaction of cyanide with ferrous ions was fast enough. We show experimentally that the rate of formation of ferrocyanide is rapi d even at low concentrations of hydrogen cyanide in the pH range 6-8, and therefore an equilibrium calculation is valid. The equilibrium con centrations of ferrocyanide are calculated as a function of hydrogen c yanide concentration, pH and temperature. The steady state concentrati on of hydrogen cyanide depends on the rate of synthesis by electric di scharges and ultraviolet light and the rate of hydrolysis, which depen ds on pH and temperature. Our conclusions show that ferrocyanide was a major species in the prebiotic ocean only at the highest production r ates of hydrogen cyanide in a strongly reducing atmosphere and at temp eratures of 0 degrees C or less, although small amounts would have bee n present at lower hydrogen cyanide production rates. The prebiotic ap plication of ferrocyanide as a source of hydrated electrons, as a phot ochemical replication process, and in semi-permeable membranes is disc ussed.