Oxygen-17 NMR of nucleosides. 2. Hydration and self-association of uridine derivatives

The first use of oxygen-17 NMR spectroscopy to obtain detailed information on water-nucleoside hydrogen-bonding equilibria is described. By monitoring the super(17)O chemical shifts in water-acetonitrile solvent mixtures of 2'-3'-O-isopropylideneuridine (IPU) specifically enriched at either O(4) or O(2) with oxygen-17 by unequivocal chemical synthesis, hydration at the C(4)=O(4) carbonyl group was shown to be significantly higher than at the C(2)=O(2) moiety. Detailed super(17)O chemical shift measurements on the (4- super(17)O)-IPU as a function of water concentration and temperature enabled equilibrium constants for the hydration process to be calculated. The super(17)O data, together with super(13)C NMR shift measurements, show a self-association of IPU to form a cyclic dimer.