Formation of Adenine by Electron Irradiation of Methane, Ammonia, and Water.

Several reports have described the effect of different forms of energy on gaseous mixtures of methane, ammonia, hydrogen, and water-the kind of mixture that probably comprised the atmosphere of the prebiotic earth.'-3 While the formation of amino acids in such experiments has been clearly demonstrated, there is little evidence for the formation of the heterocyclic bases which are major constituents of the nucleic acids. Apart from Or6's synthesis of adenine by the action of heat on a concentrated solution of ammonium cyanide4 and Fox's synthesis of uracil by a thermal reaction between malic acid and urea,5 there appears to be no report in the literature of the formation of a purine or pyrimidine under simulated prebiotic earth conditions. Palm and Calvin suggested that adenine was a probable product of the electron irradiation of a mixture of methane, ammonia, hydrogen, and water.' They had preliminary but not conclusive evidence for the formation of adenine. The primary object of the present investigation was to examine the possibility of the synthesis of heterocyclic bases from mixtures of primitive gases. We have established that (1) adenine is indeed a product of electron irradiation of a mixture of methane, ammonia, and water, (2) there is an inverse relationship between the amount of adenine synthesis and the amount of hydrogen gas present, and (3) of the five nucleic acid bases, adenine is the one most readily synthesized under prebiotic conditions. Materials and Methods.-Mixtures of methane-C'4, ammonium hydroxide (4 N), and, in some experiments, hydrogen were irradiated with electrons in the glass apparatus (volume approx. 750 ml) shown in Figure 1. The source of ionizing radiation used in these experiments was, simply as a matter of convenience, a linear electron accelerator. Four separate experiments were performed. Twenty ml of 4 N NH40H and two or three boiling chips (SiC) were introduced into the irradiation tube. The flask B was cooled to -78° and the tube evacuated. One-quarter mM of C14H4 (containing 0.5 mc; sources: Tracerlab, Inc. and New England Nuclear Corp.) was introduced, followed by nonlabeled methane (12 mM) until the pressure in the tube was 300 mm. During the methane addition, care was exercised to exclude all air from inlet tubes. In two experiments, no H2 was used. In a third experiment, 50 mm of H2 was added; in a fourth experiment, 100 mm of H2 was introduced into the irradiation tube. During the irradiations the tube was kept in a horizontal position. The electrons entered the tube through the concave end window, which was larger in diameter than the cross section of the electron beam. The electrons had an energy of 4.5 Mev and were delivered in 60 pulses per sec, each pulse lasting 6 gsec. The integrated dose rate was 18 samps, and the time of irradiation was 45 min. The current delivered during this time was 0.0486 coulombs. Cobalt glass dosimetry at the center of a similar irradiation tube indicated that 1.5 X 104 rads were absorbed per microcoulomb. The total energy absorption was therefore about 7 X 108 rads, or 7 X 1010 ergs/ gram. The liquid in flask B (Fig. 1) was boiled during the irradiations; heat was supplied by two infrared lamps. The irradiations took place in both the gas and liquid (on the cold-finger surface)