Comparison of the stabilities and solution structures of metal ion complexes formed with 5'-di- and 5'-triphosphates of purine nucleosides

Nucleotides and metal ions are involved in the basic metabolic processes of
life, participating as substrates in enzymatic reactions in the form of metal ion complexes.
Hence, their acid–base and metal-ion coordinating properties were intensely
studied in the past years. However, these studies were mainly devoted to nucleoside
50-monophosphates, and to ATP. Only little information exists on nucleoside
50-diphosphates and GTP or ITP. Considering their importance in biochemical reactions,
and the recently growing interest on G-proteins, which utilize guanosine 50-
triphosphate, a systematic investigation on their coordinating properties is needed.
In this study, mainly based on potentiometric pH titrations in aqueous solution
at 25 �C and I = 0.1 M (NaNO3), the acidity constants of purine-nucleoside 50-
di- and 50-triphosphates (adenosine 50-diphosphate; guanosine and inosine 50-diand
50-triphosphates, here designated as Nu) and the stability constants of their 1:1
complexes formed with ten metal ions (M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+,
Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ ) have been determined. Nucleotides are composed of three units, each of them with potential binding
sites for protons and metal ions. In this work, only the properties of the phosphate
groups and the nucleobase residues have been investigated in detail, since the sugar
moiety interacts with metal ions or releases protons only under special conditions
or in the strongly alkaline pH range.
Analysis of the collected equilibrium constant data reveals that the stability of
all these complexes is largely governed by the metal ion affinity of the phosphate
residue. While for the complexes formed with the divalent ions of the second half of the 3d series, including Zn2+ and Cd2+, macrochelate formation is an important
property in all the M(Nu) species considered, with GDP, GTP and ITP, the
formation of macrochelated species is considerable with all the metal ions studied,
including the alkaline earth ions. These macrochelates are formed by the interaction
of the phosphate-coordinated metal ion with N7 of the purine moiety. Analysis
of the isomer ratio between the open and macrochelated species shows that the formation
degree of the latter increases in the series M(AP)cl < M(IP)cl < M(GP)cl
(where AP, IP, or GP represents the nucleoside phosphate, i.e., NMP2