The acidity constants of 3-fold protonated 9-[2-(2-phosphonoethoxy)ethyl]adenine, H3(PEEA)+, and of 2-fold protonated (2-phosphonoethoxy)ethane, H2(PEE), and the stability constants of the M(H;PEEA)+, M(PEEA), and M(PEE) complexes with M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ have been determined (potentiometric pH titrations; aqueous solution; 25 degrees C; I = 0.1 M, NaNO3). It is concluded that in the M(H;PEEA)+ species, the proton is at the phosphonate group and the metal ion at the adenine residue. The application of previously determined straight-line plots of log K(M(R-PO3))M versus pK(H(R-PO3))H for simple phosph(on)ate ligands, R-PO3(2-), where R represents a residue that does not affect metal-ion binding, proves that the M(PEEA) complexes of Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ as well as the M(PEE) complexes of Co2+, Cu2+, and Zn2+ have larger stabilities than is expected for a sole phosphonate coordination of M2+. For the M2+ complexes without an enhanced stability (e.g., Mg2+ or Mn2+), it is concluded that M2+ binds in a monodentate fashion to the phosphonate group of the two ligands. Combination of all of the results allows the following conclusions: (i) The increased stability of the Co(PEE), Cu(PEE), Zn(PEE), and Co(PEEA) complexes is due to the formation of six-membered chelates involving the ether-oxygen atom of the aliphatic residue (-CH2-O-CH2CH2-PO3(2-)) of the ligands with formation degrees of about 15-30%. (ii) Cd(PEEA) forms a macrochelate with N7 of the adenine residue (formation degree about 30%); Ni(PEEA) has similar properties. (iii) With Zn(PEEA), both mentioned types of chelates are observed, that is, Zn(PEEA)(cl/O) and Zn(PEEA)(cl/N7), with formation degrees of about 13 and 41%, respectively; the remaining 46% is due to the "open" isomer Zn(PEEA)(op) in which the metal ion binds only to the PO3(2-) group. (iv) Most remarkable is Cu(PEEA) because a fourth isomer, Cu(PEEA)(cl/O/N3), is formed that contains a six-membered ring involving the ether oxygen next to the phosphonate group and also a seven-membered ring involving N3 of the adenine residue with a very significant formation degree of about 50%. Hence, PEEA(2-) is a truly ambivalent ligand, its properties being strongly dependent on the kind of metal ion involved. Comparisons with M2+ complexes formed by the dianions of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) and related ligands reveal that five-membered chelates involving an ether-oxygen atom are considerably more stable than the corresponding six-membered ones. This observation offers an explanation of why PMEA is a nucleotide analogue with excellent antiviral properties and PEEA is not.

The synthesis of (1H-benzimidazol-2-yl-methyl)phosphonic acid, H2(Bimp)+/-, is described: 2-chloromethylbenzimidazole was reacted with ethylchloroformate to give 1-carboethoxy-2-chloromethylbenzimidazole which was treated with trimethyl phosphite and after hydrolysis with aqueous HBr H2(Bimp)+/- was obtained. In H2(Bimp)+/- one proton is at the N-3 site and the other at the phosphonate group; both acidity constants were determined in aqueous solution by potentiometric pH titrations (25 degrees C; I = 0.1 M, NaNO3) and this furnished the pKa values of 5.37 +/- 0.02 and 7.41 +/- 0.02, respectively. The acidity constant for the release of the primary proton from the P(O)(OH)2 group of H3(Bimp)+ was estimated: pKa = 1.5 +/- 0.2. Moreover, Bimp2- can be further deprotonated at its neutral (N-1/N-3)H site to give the benzimidazolate residue, but this reaction occurs only in strongly alkaline solution (KOH); application of the H_ scale developed by G. Yagil (J. Phys. Chem., 1967, 71, 1034) together with UV spectrophotometric measurements gave pKa = 14.65 +/- 0.12. Comparisons with acidity constants taken from the literature show that this latter pKa value is far too large and this allows the conclusion that an intramolecular hydrogen bond is formed between the (N-1/N-3)H site and the phosphonate group of Bimp2-; the formation degree of this hydrogen-bonded isomer is estimated to be 98 +/- 2%. The general relevance of this and the other results are shortly discussed and the species distribution for the Bimp system in dependence on pH is provided.

The acidity constants of the 2-fold protonated (1H-benzimidazol-2-yl-methyl)phosphonate, H2(Bimp)(+/-), are given, and the stability constants of the M(H;Bimp)+ and M(Bimp) complexes with the metal ions M2+ = Mg2+, Ca2+, Ba2+, Mn2+, Co2+, Cu2+, Zn2+, or Cd2+ have been determined by potentiometric pH titrations in aqueous solution at I = 0.1 M (NaNO3) and 25 degrees C. Application of previously determined straight-line plots of log KM(M(Bi-R)) versus pKH(H(Bi-R)) for benzimidazole-type ligands, Bi-R, where R represents a residue which does not affect metal ion binding, proves that the primary binding site in the M(H;Bimp)+ complexes is (mostly) N3 and that the proton is located at the phosphonate group; outersphere interactions seem to be important, and the degree of chelate formation is above 60% for all metal ion complexes studied, except for Zn(H;Bimp)+. A similar evaluation based on log KM(M(R-PO3)) versus pKH(H(R-PO3)) straight-line plots for simple phosph(on)ate ligands, R-, where R represents a residue which cannot participate in the coordination process, reveals that the primary binding site in the M(Bimp) complexes is (mostly) the phosphonate group with all metal ions studied. In this case, the formation degree of the chelates varies more widely in dependence on the kind of metal ion involved, i.e., from 17 +/- 11% to nearly 100% for Ba(Bimp) and Cu(Bimp), respectively. For all the M(H;Bimp)+ and M(Bimp) systems, the intramolecular equilibria between the isomeric complexes are evaluated in a quantitative manner. The fact that for Bimp2- the metal ion affinity of the two binding sites, N3 and PO3(2-), can be calculated independently, i.e., the corresponding micro stability constants become known, allows us to present for the first time a method for the quantification of the chelate effect solely based on comparisons of stability constants which carry the same dimensions. This effect is often ill defined in textbooks because equilibrium constants of different dimensions are compared, which is avoided in the present case. For the M(Bimp) complexes, it is shown that the chelate effect is close to zero for Ba(Bimp) whereas for Cu(Bimp) it amounts to about four log units. This method is also applicable to other chelating systems. Finally, considering that benzimidazole as well as phosphonate derivatives are employed as therapeutic agents, the potential biological properties of Bimp, especially regarding nucleic acid polymerases, are briefly discussed.

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.