Ultrasonic Absorption and Kinetics of Proton Exchange in Aqueous Solutions of 5′ AMP

Abstract

The excess ultrasonic absorption of aqueous solutions of 5′ adenosine monophosphate (5′ AMP) goes through a maximum at a pH of about 5.05 which is independent of frequency, concentration (between 0.06 and 0.148M), and ionic strength (between 0 and 0.15M KCl). The variation and ion excess absorption at pH = 5.05 with frequency in the range 1–115 MHz and for 5′ AMP concentration between 0.016 and 0.148M can be fitted by relaxation curves with a single relaxation time. This excess absorption has been assigned to a bimolecular proton exchange between the protonable nitrogen atom of the base moiety and the secondary phosphoric acid function, coupled with the stacking of the nucleotide. The assumption that stacking equilibrates much faster than proton exchange enabled us to obtain the values of the rate constants for the proton exchange and for the ionization of the secondary phosphoric acid function, as well as the volume changes upon stacking and proton exchange and the association constant for stacking (dimerization).