The acidity constants of guanylyl(3'-->5')guanosine (GpG(-)) and 2'-deoxyguanylyl(3'-->5')-2'-deoxyguanosine [d(GpG)(-)] for the deprotonation of their (N1)H sites were measured by potentiometric pH titrations in aqueous solution (25 degreesC; / = 0.1 K NaNO3). The same method was used for the determination of the stability constants of the 1:1 complexes formed between Mg2+, Ni2+, or Cd2+ (= M2+) and (GG-H)(2-), and in the case of Mg2+ also of (GG2H)(3-), where GG(-) = GpG(-) or d(GpG)(-). The stability constants of the M(GG)(+) complexes were estimated. The acidity constants of the H(dGuo)+ and dGuo species (dGuo = 2'-deoxyguanosine) and the stability constants of the corresponding M(dGuo)(2+) and M(dGuo-H)(+) complexes were also measured. Comparison of these and related data allows the conclusion that N7 of the 5'G unit in GG- is somewhat more basic than the one in the 3'G moiety; the same holds for the (N1)- sites. On the basis of comparisons with the stability constants measured for the dGuo complexes, it is concluded that M2+ binding of the GG dinucleoside monophosphates occurs predominantly in a mono-site fashion, meaning that macrochelate formation is not very pronounced. Indeed, it was a surprise to find that the stabilities of the complexes of dGuo or (dGuo-H)- and the corresponding ones derived from GG- are so similar. Consequently, it is suggested that in the M(GG)(+) and M(GG-H) complexes the metal ion is mainly located at N7 of the 5'G unit since this is the more basic site allowing also an outer-sphere interaction with the C6 carbonyl oxygen and because this coordination mode is also favorable for an electrostatic interaction with the negatively charged phosphodiester bridge. It is further suggested that Mg2+ binding (which is rather weak compared to that of Ni2+ and Cd2+) occurs mainly in an outer-sphere mode, and on the basis of the so-called Stability Puler it is concluded that the binding properties of Zn2+ to the GG species are similar to those of Ni2+ and W.