Intramolecular cation-pi interactions as the driving force to restrict the conformation of certain nucleosides.

Despite the well-established importance of intermolecular cation-pi interactions in molecular recognition, intramolecular cation-pi interactions have been less studied. Here we describe how the simultaneous presence of an aromatic ring at the 5'-position of an inosine derivative and a positively charged imidazolium ring in the purine base drive the conformation of the nucleoside toward a very major conformer in solution that is stabilized by an intramolecular cation-pi interaction. Therefore, the cation-pi interaction between imidazolium ions and aromatic rings can also be proposed in the design of small molecules where this type of interaction is desirable. The imidazolium ion can be obtained by a simple acidification of the pH of the media. So a simple change in pH can shift the conformational equilibrium from a random to a restricted conformation stabilized by an intramolecular cation-pi interaction. Thus the here described nucleosides can be considered as a new class of pH-dependent conformationally switchable molecules.