The interaction between pyrimidine nucleosides and benzene in aqueous solution, studied by proton magnetic resonance.

Two types of interaction yield the main contributions to the stability of the secondary structure of DNA and RNA molecules in aqueous solution: 1. Specific hydrogen bonding with stringent geometric requirements between the complementary base pairs in the two strands of the double helix of DNA or in helical regions of the RNA chain. 2. Unspecific hydrophobic interactions between the adjacent bases of the nucleosides, with less rigid geometrical requirements. The second type also occurs in aqueous solutions of the monomeric nucleosides and nucleotides, and is responsible for the solubility enhancement of uncharged polycyclic aromatic compounds in such solutions as compared to pure water 2. The self association has been investigated previously by PMR 3 _ 8 . The shielding of the ring protons is influenced by the ^-electron-clouds of the adjacent heterocyclic ring structure. A neighbouring aromatic molecule wil cause similar effects and it should be possible to decide via changes in the chemical shifts, whether an aromatic molecule will be found preferably in the vicinity of other nucleosides or if it is distributed at random in the bulk solution. The limited sensitivity of high resolution PMR combined with the low solubility in water of most aromatic compounds containing no polar groups, restricted the investigation of the stacking properties to aromatic ring systems solubilized by an attached polar or even charged group as for instance ethidium bromide9. In these cases the attraction between the nucleoside and the aromatic molecule could be caused by electrostatic or dipoledipole interactions as well. However, many of the