Ultrasonic absorption and kinetics of proton exchange in solutions of 5′ ADP and 5′ ATP

Abstract

The variation of α/f2 (α = ultrasonic absorption coefficient, f = frequency) versus the pH for aqueous solutions of adenosine 5′-diphosphate (5′ ADP), goes through two maxima at pH 3 and 5, respectively. For aqueous solutions of adenosine 5′-triphosphate (5′ ATP) the excess absorption shows a maximum at pH 3 and a shoulder at around pH 5. The excess absorption observed for 5′ ADP and 5′ ATP at around pH 5 seems to be due to a bimolecular proton exchange reaction between the protonatable nitrogen atom of the base moiety and the secondary phosphoric acid function. The excess absorption observed at pH 3 for both nucleotides seems partly due to a bimolecular proton exchange between the protonatable nitrogen atom of the base and the primary phosphoric acid function. At above pH 6, 5′ ATP solutions show an excess of ultrasonic absorption relatively to 5′ ADP solutions. This excess of absorption is attributed to the binding of the Na+ counterion to the triphosphate moiety of 5′ ATP.