The self-organization of adenosine 5′-triphosphate and adenosine 5′-diphosphate in aqueous solution as determined from ultraviolet hypochromic effects

The self-association of adenosine 5'-triphosphate (ATP) and of adenosine 5'-diphosphate (ADP) was studied in aqueous solution at different pH values, over the concentration range from 5 x 10(-6) to 5 x 10(-2) M, by ultraviolet spectroscopy. Measures of the molar absorptivity of the ultraviolet bands of these compounds with increasing concentration have shown two hypochromic effects, at concentrations below 10(-3) and above 10(-3) M, respectively These results can be interpreted in terms of self-association processes involving the formation of dimers and of polymers. From the fitting of the experimental curves of hypochromic effects, self-association constants for dimerization and polymerization were calculated. The results obtained are discussed in relation to the values reported in the literature and indicate the influence of the concentration range not only on the numerical value but also on the order of magnitude of the association constants. Comparison of ATP with ADP shows that the length of the phosphate chain may be a relevant feature in the nature of the self-organization processes in these adenine nucleotides in aqueous solution. (C) 2000 Elsevier Science B.V. All rights reserved.