Ionization constants and thermal stabilities of uracil and adenine under hydrothermal conditions as measured by in situ UV–visible spectroscopy

UV–visible spectra for aqueous uracil and adenine were measured in a high-pressure platinum flow cell with sapphire windows at temperatures up to 250°C at a constant pressure of 7.2MPa. Ionization constants were determined from pH-dependent spectra in the buffer solutions NH3/NH4Cl, NaHCO3/Na2CO3, HCOOH/NaHCOO and NaH2PO4/Na2HPO4. Variations in the spectra with pH and temperature at constant flow rate were used to determine the first ionization constant of uracil, K1a (uracil), and the first and second acid ionization constants of adenine, K1a (adenine) and K2a (adenine), at ionic strength I=0.2molkg−1 and temperatures up to 200°C. Time-dependent spectra, obtained by operating the cell as a stopped flow reactor, were used to examine the rate of thermal decomposition as a function of temperature and pH from 200 to 250°C. Reaction pathways and rate constants were determined by singular value decomposition methods. Uracil and adenine decomposition occurred by one-step and two-step processes, respectively. The kinetic results in the NaH2PO4/Na2HPO4 buffer solution are in quantitative agreement with the less-extensive pioneering study by White (Nature 310, 430–432 (1984)), but differ from results in buffers that do not contain phosphate, suggesting that phosphate enhances the thermal stability of nucleic acid bases.