Kinetics of Base-Stacking Interactions and Proton Exchange of 6-Methylpurine Aqueous Solutions

Abstract

Acoustical relaxation properties of aqueous solutions of 6-methylpurine have been studied using broadband ultrasonic spectrometry in the frequency range 0.2−2000 MHz. Excess attenuation with relaxation characteristics was found near the lower limit of the measuring range and also around 100 MHz. The former relaxation process has been assigned to the proton exchange of the ampholytic 6-methylpurine solutions, yielding reasonable rate constants, e.g., 1.1 × 103 s-1 dm3 mol-1 for the ionization and 6 × 108 s-1 dm3 mol-1 for the neutralization at neutral pH. The high frequency relaxation process has been discussed in terms of a sequential and a random isodesmic reaction scheme of stack formation. Similar rate constants have been found for both models, e.g., 5 × 109 and 2.5 × 108 s-1 for the association and dissociation constant of the sequential reaction scheme, if cooperativity effects are neglected. There are, however, indications of weak cooperativity in the stacking.