[Study of intermolecular interactions and self-organization of adenylic nucleotides by the spin label method]

A spin labeling method for obtaining thermodynamic parameters of nucleotide association is proposed. The method is based on the dependence of ESR parameters of the spin-labelled derivative on concentration of the nonlabelled compound due to formation of associates involving both spin-labelled and unmodified molecules. It is found that at pH 7.5 the constant of adenylic nucleotide association practically does not depend on the number of phosphate groups and is equal to 9.7 +/- 0.3 M-1 for AMP, ADP and ATP in 0.1 M NaCl at 28 degrees C. In acidic medium the value of the association constant increases by a factor two. Base stacking is shown to make the main contribution to stability of the associates of adenylic nucleotides at neutral pH, whereas upon base protonation the key role is played, apparently, by base-phosphate interaction. It is thought, that an increase in the solvent entropy is essential for stabilisation of the associates, this factor being more important in the case of nucleotide association as compared to the association of nucleosides. A possible role of nucleotide association in the processes of intracellular regulation is discussed.