Dielectric studies on self-associating nucleosides and bases in aqueous solution

Measurments have been made of the dielectric properties of aqueous solutions in which aggregates are formed by stacking. The nucleosides cytidine, uridine and thymidine, and the bases purine, pyrimidine and 6-methylamino-9-methyl-purine (N6,N9-dimethyladenine) were investigated at around 1 MHz, where the static increments can be determined, and for cytidine, dimethyladenine, uridine and pyrimidine measurements were also made in the 100-2000 MHz range where the main relaxation of the solute dipoles is found. Whereas cytidine and purine show a positive static dielectric increment increasing linearly with concentration, dimethyladenine, uridine, thymidine and pyrimidine show a similar negative effect. Also, within the experimental accuracy, single relaxation times are found for the solute dispersions investigated. It is suggested that these relaxations correspond to the effects of free rotation of individual polar molecules in the plane of stacking. This phenomenon would also account for the linear variation of the dielectric increments with concentration. These increments are thought to be positive or negative due to the varying balance in the solutions between the loss of polarization due to displaced and "bound" water and the corresponding gain due to the polarity of the solute molecules.