Thermodynamic properties of nucleic acid bases and nucleosides under hydrothermal conditions

Mann, Vanessa
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University of Guelph

This is an investigation of thermodynamic properties of nucleic acid bases and nucleosides under hydrothermal conditions. Standard partial molar volumes have been determined for neutral nucleic acid bases and some nucleosides at temperatures between 15 °C and 90 °C using an Anton Parr DMA 5000 densitometer. Additionally, standard partial molar volumes of neutral uridine, cytidine, and thymidine were calculated from densities measured at temperatures up to 200 °C and pressures up to 6 bar using a custom-built platinum vibrating tube densitometer. Standard partial molar heat capacities were determined at temperatures up to 135 °C using a NDSC-III scanning nanocalorimeter. Standard partial molar properties of nucleic acid base and nucleoside non-electrolyte solutions become progressively more positive as the critical point is approached. Standard partial molar volumes and heat capacities for chloride salts of nucleic acid bases and nucleosides, BH +Cl-(aq) were also determined at temperatures between 15 °C and 90 °C. Partial molar properties of solutions of charged nucleic acid components become increasingly negative as the critical point is reached. These experimental results are in stark contrast with the previous assertions made by LaRowe and Helgeson (2006). It was also observed that the standard partial molar properties of both neutral and ionic nucleosides had much larger, positive values than their corresponding nucleic acid bases. This difference was attributed to contributions of the ribose group to standard partial molar properties, and was modeled and used to estimate the standard partial molar properties of neutral and positively charged guanine in water. Additionally, the first ionization constant of adenosine was determined at a pressure of 95 bar and temperatures up to 175 °C using a platinum flow-through UV-visible cell. At near-ambient temperatures, measured ionization constants agreed well with values predicted by the Van't Hoff equation, but deviated from this model as temperature increased. The thermal stabilities of adenosine in acidic, neutral, formic acid buffered and phosphate buffered solutions were determined using the UV-visible cell as a stopped flow system at 95 bar between 150 °C and 250 °C. Adenosine formed decomposition products with overlapping spectra, and this necessitated factor analysis techniques to resolve different species. As temperature increased, the number of coloured decomposition products formed increased, indicating multiple decomposition pathways and steps. Above 225 °C, decomposition products were unstable, and decomposed into colourless products quickly. Spectral deconvolution of kinetic measurements using SpecFit/32© indicated that different reaction products were formed in the phosphate buffered system than the formic acid buffered system. It is believed that the phosphate buffered system produced a phosphate-adenosine complex that was unstable at ambient temperatures, and could not be isolated.

Nucleic acid bases, Nucleosides, Thermodynamic properties, Hydrothermal conditions, Molar volumes