REEXAMINATION OF ULTRASONIC RELAXATION KINETICS OF AQUEOUS-SOLUTIONS OF NUCLEOTIDES - EVIDENCE FOR FAST SYN-ANTI GLYCOSYL ISOMERIZATION IN ADENOSINE 5'-MONOPHOSPHATE AND ADENOSINE 5'-DIPHOSPHATE

Ultrasonic absorption coefficients in aqueous solutions of adenosine 5 '-monophosphate and adenosine 5'-diphosphate were measured at 25 degre es C as a function of the concentration and pH in the frequency range from 0.8 to 220 MHz. At pH near 5, there existed two relaxational abso rptions. One is observed at the frequency range <10 MHz with a large a mplitude of the ultrasonic relaxation and a relaxation frequency which is strongly dependent on both the nucleotide concentration and pH. Th e source of the relaxation was attributed to a perturbation of the che mical equilibrium associated with the proton transfer reaction: N-1-R- PO42- + +HN1-R-PO4H- half arrow right over half arrow left 2N(1)-R-PO4 H-. The forward and reverse rate constants were determined from the co ncentration dependence of the reactants and the solution pH. The forwa rd rate constant obtained was similar to that previously reported. Whe n the solution pH was increased to >11, the relaxation associated with the proton transfer reaction disappeared. A second relaxation is foun d at around 100 MHz, and its amplitude is smaller than that due to the proton transfer reaction. This relaxation is barely observed at neutr al pH, but it is clearly distinguishable at high pH because the absorp tion associated with the proton transfer reaction is no longer observa ble. The relaxation frequency of the second relaxation is independent of nucleotide concentration and the solution pH, and the maximum absor ption per wavelength increases linearly with concentration. It was con cluded that the source of this relaxation is an isomerization process, probably the syn-anti interconversion of nucleotides. The value of th e relaxation frequency in aqueous solutions of ADP was greater than th at in AMP solutions, which in turn is greater than that for adenosine. The results are discussed in relation to nucleotide molecular structu res and interactions.