The stability of A to hydrolysis previously has been cited as a major factor that would limit its availability for use in the first genetic material (25). These data, however, are based on the rate of hydrolysis of A in acid (26) where the 
H
is much smaller than at neutral pH. To establish the stability of adenine at neutral pH we have measured the pH rate profile at several temperatures. The pH rate profiles for the decomposition of adenine under anaerobic conditions at 100°, 120°, and 145°C are shown in Fig. 1, Upper. The profiles exhibit a flat region from pH 5-10, and there appear to be at least four reactions leading to decomposition. These include (AH+)(H+), (AH+)(H2O), (A)(H2O), (A)(OH
), or their kinetic equivalents.
The pH rate profiles for the decomposition of guanine at 100°, 120°, and 145°C are shown in Fig. 1, Lower. Like the pH rate profile for adenine, the pH rate profile for guanine exhibits a relatively flat region from pH 5-10. There also appear to be at least four reactions leading to the decomposition. These include (GH+)(H+), (GH+)(H2O), (G)(H2O), (G)(OH), or their kinetic equivalents.
The stability of cytosine previously has been cited as a factor that would limit its availability on the early Earth (27). The pH rate profile for the hydrolysis of cytosine has been reported previously by Garret and Tsau (28) and also exhibits a flat region from pH 4-9.
Fig. 2 shows the Arrhenius curves and equations for the decomposition of A, U, G, C, and T at pH 7. The Arrhenius curves and equations for the alternative bases xanthine, hypoxanthine, and diaminopyrimidine are shown in Fig. 3. At pH 7 and 100°C, the rate of hydrolysis of diaminopurine is 1.01 × 108 s1, isoguanine is 4.1 × 107 s1, 5-methylcytosine is 8.9 × 107 s1, and 5-hydroxymethylcytosine is 5.8 × 107 s1.
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