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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 degrees C; I = 0.1 M, NaNO(3)). The same method was used for the determination of the stability constants of the 1:1 complexes formed between Mg(2+), Ni(2+), or Cd(2+) (= M(2+)) and (GG-H)(2-), and in the case of Mg(2+) also of (GG-2H)(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 M(2+) 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 Mg(2+) binding (which is rather weak compared to that of Ni(2+) and Cd(2+)) occurs mainly in an outer-sphere mode, and on the basis of the so-called Stability Ruler it is concluded that the binding properties of Zn(2+) to the GG species are similar to those of Ni(2+) and Cd(2+).

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The acidity constants of H 2 (AcP) and H 2 (AnP), where AcP 2-) acetyl phosphate and AnP 2-) acetonylphosphonate, as well as the stability constants of the 1:1 complexes formed between Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , or Cd 2+ and AcP 2-or AnP 2-were determined by potentiometric pH titrations in aqueous solution (25°C; I ) 0.1 M, NaNO 3 ). On the basis of previously established log K M(R-PO 3 ) M versus pK H(R-PO 3 ) H straight-line plots for phosph(on)ate ligands (R-PO 3 2-), which allow only a simple -PO 3 2-coordination (Sigel, H.; et al. HelV. Chim. Acta 1992, 75, 2634), it is concluded that the carbonyl oxygen atom of AcP 2-and AnP 2-participates in complex formation by giving rise to six-membered chelates for all of the mentioned systems except for Ba(AnP) and Sr(AnP), the stability of which corresponds to a simple -PO 3 2-coordination. The formation degree of the chelates formed by the alkaline earth ions and AcP 2-is quite pronounced; it amounts, for example, for Mg(AcP) and Ca(AcP) to 41% ( 5% and 48% ( 8%, respectively. The corresponding results for Zn(AcP) and Cu(AcP) are 59% ( 6% and 76% ( 4%, respectively. The formation degree of the six-membered Cu(AnP) chelate increases in water containing 30% or 50% (v/v) 1,4-dioxane. This may also be surmised for the other divalent metal ions under conditions of lower solvent polarity and poorer solvating properties than water. Such conditions exist in active-site cavities of enzymes, and the expected effects are shortly discussed. The indicated measurements with AcP have only become possible after the stabilities of the corresponding M(HPO 4 ) complexes had been determined (Saha, A.; et al. J. Biol. Inorg. Chem. 1996, 1, 231) because AcP contains as an impurity always some phosphate; this phosphate content had to be quantified and its effect was carefully considered in the evaluations of the experimental data. Here is most probably also the main reason why previously published results regarding the stabilities of the Mg(AcP) and Ca(AcP) complexes vary widely; stability constants of the other M(AcP) complexes have not been determined previously.