Spectroscopic studies of solvation : Part 1, Solvation of thiols; Part 2, Hydration of deoxyribonucleic acid
The extent of hydrogen bonding in ethanethiol is determined using a correlation between the chemical shift of the sulphydryl proton and the stretching frequencies of the component bands of the vS-H infrared band The component bands having stretching frequencies at 2585 cm-1 and 2570 cm-1 for non hydrogen bonded and mono-hydrogen bonded thiol groups respectively. It is calculated that the pure liquid at 22°C. contains ca. 49% free SH bonds. A dimerisation constant of 0.038 dm3 mol-1 is estimated at 22°C. 2-Hydroxyethanethiol is used to investigate the strength of hydrogen bonding to the thiol group in aqueous solutions. The results of the IR and NIR studies are consistent with the formation of hydrogen bonds of S-H O type between thiol and water molecules. Salt and solvent effects upon the phosphate and thiophosphate groups of Sodium Dimethyphosphate (NaDMP) and Sodium Dimethylthiophosphate (NaDMSP) are investigated. Infrared spectra in the and v3P-O of DMP and DMSP suggest that the order of strength of binding of metal ion to phosphate/thiophosphate group is Na+ < Mg2+ < Ca2f. The results indicate that solvent-separated ion pairs dominate in aqueous solutions, whereas, mono- and bi-dentate contact ion pairs are favoured in DMSO and methanol solutions. The thiophosphate group is solvated by methanol and water, primarily at the oxygen site. No evidence was found to support hydrogen bond formation to the anionic sulphur site. Infrared spectra were also consistent with Mg2+ and Ca2+ interacting with the Oxygen atom, but not the Sulphur atom. DNA hydration is investigated using high field NMR spectrometry. Frozen aqueous DNA solutions were found to have a broad NMR signal due to hydration water. From this signal it is estimated that, at -12°C., the DNA samples are hydrated by ca. 25 water molecules per base pair of DNA.