Kinetic and thermodynamic study of syn-anti glycosyl isomerization in aqueous solutions of AMP, ADP, and ATP by ultrasonic relaxation methods

Ultrasonic absorption coefficients in aqueous solutions of adenosine 5'-tri phosphate (ATP) were measured at: 25 degrees C as a function of concentrati on and pH in the frequency range from 0.8 to 220 MHz. An ultrasonic relaxat ional absorption with two relaxation frequencies was observed near pH 3. Ho wever, above pH 12, only a single relaxational absorption was detected at a round 100 MHz, and this relaxation is the focus of this report. The relaxat ion frequency is independent of the concentration and pH at around pH 12, a nd the maximum absorption per wavelength increases linearly with concentrat ion. From these results, the cause of the relaxation was attributed to a sy n-anti glycosyl isomerization reaction of ATP. The race of the rotational m otion in ATP was found to be smaller than that for ADP and greater than tho se for AMP and adenosine;These results are discussed in relation to the cha rge distribution and the effect of the size of the phosphate group of the n ucleotides. To examine the reaction mechanism further, ultrasonic absorptio n measurements were carried out for aqueous solutions of adenosine 5'-monop hosphate (AMP), adenosine S-diphosphate (ADP), and ATP at 15, 20, and 25 de grees C above pH 12 from which activation enthalpies were determined. Furth ermore, the detailed temperature dependence of the absorption in AMP was in vestigated at several concentrations in order to determine the standard Gib bs free energy, enthalpy, entropy, and volume changes of the reaction, reve aling the thermodynamic properties of the syn-anti conformational isomeriza tion process of the nucleotides.