The self-association of ATP: thermodynamics and geometry.

The concentration and temperature dependence of the self-association of ademosin-5'-triphosphate (ATP) in aqueous solution was studied by means of ultraviolet absorption spectroscopy and circular dichroism (CD). Of several possible models, a model was indefinite linear self-association, in which each step has the same equilibrium constant, describes the data best. The two different methods lead within experimental error to the same thermodynamic parameters. At pH 8.7, IN 1 M Tris and 0.5 M 7gCl-2, we find deltaH-0 equals -5.1 kcal/mole and deltaS-0 equals -13.0 e.u. These values do not differ much from those found for the self-association of uncharged bases and nucleosides in aqueous solution. The CD spectrum that results from the self-association is conservative and quite similar in shape to that observed for some stacked dinucleotides: it is interpreted as a first approximation within the framework of the exciton model.