Acid ionization constants and thermal decomposition kinetics of aqueous adenine and uracil under hydrothermal conditions
This thesis is an investigation of the thermodynamic properties of uracil and adenine as nucleobase model compounds, and their thermal stabilities under hydrothermal conditions. Standard partial molar volumes, 'V' 2°, were measured and modelled for uracil, uridine, adenine and adenosine over the temperature range 10°C to 90°C using a newly acquired Anton Parr DMA 5000 densimeter, extending the previously established temperature limit of 55°C. Acid ionization constants, 'K'1a and 'K'2a were determined for the uracil ionization reaction, and both adenine ionization reactions over the temperature range 25°C to 200°C and 72 bar 'in situ' using a platinum UV-visible flow-cell. Thermodynamic models were developed to describe the behaviour of the ionization constants over the entire temperature range studied. This is the first ionization constant data above 45°C has been reported. Thermal decomposition experiments were performed using the UV-visible flow-cell as a stopped-flow reactor over the temperature range 150°C to 250°C as a function of pH. Factor analysis of coupled reactions was required to analyse the decomposition data using the Specfit© software package. Spectral deconvolution of adenine decomposition data was performed to generate values for the rate constants of successive decomposition reactions forming species with overlapping UV spectra. Both adenine and uracil are only transiently stable at high temperatures, with half lives on the order of several minutes.