Effect of the ribose versus 2′-deoxyribose residue on the metal ion-binding properties of purine nucleotides

The interaction between metal ions and nucleotides is well characterized, as is their importance for metabolic processes, e.g. in the synthesis of nucleic acids. Hence, it is surprising to find that no detailed comparison is available of the metal ion-binding properties between nucleoside 5′-phosphates and 2′-deoxynucleoside 5′-phosphates. Therefore, we have measured here by potentiometric pH titrations the stabilities of several metal ion complexes formed with 2′-deoxyadenosine 5′-monophosphate (dAMP²⁻), 2′-deoxyadenosine 5′-diphosphate (dADP³⁻) and 2′-deoxyadenosine 5′-triphosphate (dATP⁴⁻). These results are compared with previous data measured under the same conditions and available in the literature for the adenosine 5′-phosphates, AMP²⁻, ADP³⁻ and ATP⁴⁻, as well as guanosine 5′-monophosphate (GMP²⁻) and 2′-deoxyguanosine 5′-monophosphate (dGMP²⁻). Hence, in total four nucleotide pairs, GMP²⁻/dGMP²⁻, AMP²⁻/dAMP²⁻, ADP³⁻/dADP³⁻ and ATP⁴⁻/dATP⁴⁻ (= NP/dNP), could be compared for the four metal ions Mg²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ (= M²⁺). The comparisons show that complex stability and extent of macrochelate formation between the phosphate-coordinated metal ion and N7 of the purine residue is very similar (or even identical) for the AMP²⁻/dAMP²⁻ and ADP³⁻/dADP³⁻ pairs. In the case of the complexes formed with ATP⁴⁻/dATP⁴⁻ the 2′-deoxy complexes are somewhat more stable and show also a slightly enhanced tendency for macrochelate formation. This is different for guanine nucleotides: the stabilities of the M(dGMP) complexes are clearly higher, as are the formation degrees of their macrochelates, than is the case with the M(GMP) complexes. This enhanced complex stability and greater tendency to form macrochelates can be attributed to the enhanced basicity (ΔpK_aca. 0.2) of N7 in the 2′-deoxy compound. These results allow general conclusions regarding nucleic acids to be made.