THERMODYNAMIC AND CHEMICAL CONSTRAINTS ON THE RELATIVE STABILITIES OF NUCLEOTIDES, NUCLEOSIDES, AND NUCLEIC ACID BASES IN HYDROTHERMAL SYSTEMS

Although the taxonomy, metabolic requirements, growth rates, energetics, and genomics of hyperthermophilic microbes in hydrothermal systems have received considerable attention in recent years, little is known about the relative stabilities of the biomacromolecules that are essential to life at high temperatures. Recent efforts to remedy this situation have led to advances in the application of group additivity algorithms and equations of state for biomolecules to numerical analysis of low-temperature calorimetric and solubility data reported in the literature. As a consequence, it is now possible to quantify for the first time the relative stabilities in vivo and in vitro at both high and low temperatures of nucleotides, nucleosides, and nucleic acid bases, which are the constituents of DNA and RNA. Although the relative stabilities of the nucleotide, nucleoside, and nucleic acid base classes can be described in terms of the activities of phosphate species and ribose and deoxyribose, these activities are in turn a function of pH and log f_O2(g), respectively. Similarly, the relative stabilities of the various species within each class of these constituents are highly sensitive to both the fugacities of CO_2(g) and O_2(g) in hydrothermal systems and/or the cells of hyperthermophiles. For example, the logarithms of the equilibrium activities of the bases at 25ºC and 1 bar range from ~ -8 to ~ -2 at log f_O2(g)=-77 and log f_CO2(g)=-2.5 which are consistent with log a_CH3COOH(aq)=–3. The corresponding range at 100ºC and log f_O2(g)=-61.5, log f_CO2(g)=-2, and log a_CH3COOH(aq)=-2 is ~ -6 to ~ -1.5. In the absence of nucleotide triphosphates (NTPs) the activities of the bases exceed in both cases those of the nucleosides and nucleotides by ~3 and ~8 orders of magnitude, respectively, for log a_H2PO4^2-(aq) » log a_ribose(aq)=-3. Computer experiments are planned to document and quantify the extent to which these activities increase in response to NTP hydrolysis as a function of solution pH and temperature.