P R E P R I N T – ICPWS XV Berlin, September 8–11, 2008 Introduction Knowledge of the thermochemical and kinetic stability of amino acids, nucleic acid bases, and other simple biomolecules under hydrothermal conditions is essential for postulating mechanisms that may have led to the creation of life on earth. Attempts to model possible geochemical pathways for the formation of pre-biotic molecules have been limited by a lack of quantitative experimental data [1]. In this work, we report the first results from a program to determine standard partial heat capacities and volumes, ionization constants and rate constants of nucleic acid bases, nucleosides and nucleotides as a function of pH at temperatures and pressures as high as 250 °C and 30 MPa. Experimental Apparent molar heat capacities and volumes were measured at temperatures up to 90 °C using a commercial nanocalorimeter and vibrating tube densimeter with techniques developed by Woolley [2], and our own high temperature densimeter [3]. Equilibrium constants and time-dependent thermal decomposition spectra were measured in the UV- visible spectrometer, high-pressure cell and flow injection system similar to that described by Trevani et al. [4]. However, a new cell and pre- heater made from a platinum/iridium alloy, developed by Bulemela [5], was used here. We have avoided the use of titanium because the titanium oxide passivation layer is a good catalyst for the decomposition of organic compounds when irradiated with UV light. Ionization constants are determined from pH- dependant spectra in buffer solutions of NH 3 /NH 4 Cl, NaHCO 3 /Na 2 CO 3 , HCOOH/NaHCOO (formic acid/ formate) and NaH 2 PO 4 /Na 2 HPO 4 . Rates of thermal decomposition were determined by operating the flow system as a stopped-flow reactor and tracking the spectral intensities as a function of time. Results and Discussion Equilibrium Constants for Uracil and Adenine Our first spectroscopic results are for the nucleic acid bases, uracil, and adenine. The choice of suitable pH buffers and optimum buffer ratios was based on the requirement that there be good overlap between the useful buffer range and the estimated pK a value of the nucleobase, so that both neutral and ionized species are present in significant concentrations. Typical spectra for uracil and adenine in a series of ammonium /ammonia buffer solutions are shown in Figures 1 and 2, respectively. Ionization Constants, Equation of State Parameters, and Thermal Decomposition Kinetics of Nucleic Acid Bases under Hydrothermal Conditions Erik Balodis, Vanessa Mann, Liliana Trevani, and Peter Tremaine Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1 Email: tremaine@uoguelph.ca UV-visible spectra for buffered solutions of uracil and adenine were determined under hydrothermal conditions in a high-pressure flow system. Variations in the spectra with pH and temperature at constant flow rate were used to determine the first ionization constant of uracil, and the first and second ionization constants of adenine, at ionic strength I = 0.2 mol≅kg -1 , 7.2 MPa of pressure and temperatures up to 200 ΕC. Time- dependant 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. Factor analysis was used for modeling the kinetics of thermal decomposition reactions. Apparent molar heat capacities and volumes of uracil, adenine thymine and their nucleosides have been measured up to 140 °C. The data are being used to develop improved equations of state for predicting the properties of pre-biotic molecules under hydrothermal conditions.