The acidity constants of 3-fold protonated 9-[(2-phosphonomethoxy)ethyl]-2-aminopurine, H(3)(PME2AP)(+), and the stability constants of the M(H;PME2AP)(+) and M(PME2AP) complexes with M(2+) = Ca(2+), Mg(2+), Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+) or Cd(2+) have been determined by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.1 M, NaNO(3)). It is concluded that in the M(H;PME2AP)(+) species, the proton is at the phosphonate group and the metal ion at N7 of the purine residue. This "open" form allows macrochelate formation of M(2+) with the monoprotonated phosphonate residue. The formation degree of this macrochelate amounts on average to 64 +/- 13% (3sigma) for those metal ions for which an evaluation was possible (Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+)). The identity of this formation degree indicates that the M(2+)/P(O)(2)(-)(OH) interaction occurs in an outersphere manner. The application of previously determined straight-line plots of log K(M)(M(R-PO(3)))versus pK(H)(H(R-PO(3))) for simple phosph(on)ate ligands, R-PO(3)(2-), where R represents a residue that does not affect metal ion binding, proves that all the M(PME2AP) complexes have larger stabilities than is expected for a sole phosphonate coordination of M(2+). Combination with previous results allows the following conclusions: (i) The increased stability of the M(PME2AP) complexes of Ca(2+), Mg(2+) and Mn(2+) is due to the formation of 5-membered chelates involving the ether-oxygen atom of the -CH(2)-O-CH(2)-PO(3)(2-) residue; the formation degrees of these M(PME2AP)(cl/O) chelates for the mentioned metal ions vary between about 25% (Ca(2+)) to 40% (Mn(2+)). (ii) For the M(PME2AP) complexes of Co(2+), Ni(2+), Cu(2+), Zn(2+) or Cd(2+) next to the mentioned 5-membered chelates a further isomer is formed, namely a macrochelate involving N7, M(PME2AP)(cl/N7). The formation degrees of these macrochelates vary between about 30% (Cd(2+)) and 85% (Ni(2+)). (iii) The most remarkable observation of this study is that the shift of the NH(2) group from C6 to C2 facilitates very significantly macrochelate formation of a PO(3)(2-)-coordinated M(2+) with N7 due to the removal of steric hindrance in the M(PME2AP) complexes. However, any M(2+) interaction with N3 is completely suppressed, thus leading to significantly different coordination patterns than those observed previously with the antivirally active PMEA(2-) species.