Ion-electrode study of magnesium(II)-ATP and manganese(II)-ATP association

Thermodynamic formation constants for the magnesium (II)-ATP and manganese (II)-ATP species have been evaluated by direct potentiometry with ion-selective liquid membrane electrodes responsive to Mg²⁺ and Mn²⁺, respectively. The existence of the second complex Mg₂ATP has also been demonstrated; an evaluation of its formation constant is provided. It is shown that the ion electrode method offers advantages over earlier methods because it permits direct measurement of ion activities and, further, enables the ion of interest to be measured selectively in the presence of other ions needed to control pH and ionic strength.