Solvation of deoxynucleosides in aqueous mixtures of organic sovents probed through their intrinsic flourescence : implications for open base pair states in DNA

Because of the importance of solvation in the structure and function of DNA, there is considerable interest in understanding the salvation network of its constituent components. This is of particular importance in connection with the closing of base pairs that have been disrupted as a result of structural fluctuations. Open base pairs play a role in sequence recognition by proteins, DNA replication and transcription, interactions with ligands (e.g. drugs), and DNA photodamage. Following the opening of a base pair, the open base is exposed to a heterogeneous environment which includes, in addition to interactions with hydrating water, polar, nonpolar, and stacking interactions with adjacent bases and sugars. Toward the goal of understanding how the open bases interact with such a heterogeneous environment, we have studied the intrinsic fluorescence properties of the purine and pyrimidine deoxynucleosides: 2'-deoxyadenosine (dA), 2'-deoxyguanosine (dG), 2'-deoxythymidine (<.IT), and 2'-deoxycytidine (dC) in organic solvents in the presence of small amounts of water. Exposure of the nucleoside to water was made at three different stages during the process of solution preparation and the resulting three solutions were designated accordingly: (i) the "premixed" solution in which the water and the organic solvent were mixed first and then the nucleoside was dissolved in their mixture (i.e. initial partial exposure to water), (ii) the "carry its own water" solution in which the nucleoside was dissolved in water and then diluted in the organic solvent to form the solution with the desirable water content (i.e. initial full exposure to water), and (iii) the "injected water" solution in which the nucleoside was dissolved in the organic solvent and then a small amount of water was added to the solution (i.e. initial shielding from water). The organic solvents used in the present study were: n-butanol, acetonitrile, methanol, n-propanol, isopropanol, and isobutanol.